# Creating ecology-based random encounters: This Eats That

In part one of this series, I talked about the philosophical grounding of random encounters – the theoretical why’s and wherefore’s that underpin the encounters that result, and the ways and reasons why they matter. In this part, I’m going to discuss ways of creating better, smarter, encounter tables – ones that prompt you to extend your campaign, be creative, and generally enhance the world around the PCs, little by little.

At it’s heart, this approach is all about creating a simplified, summarized, ecology. What does that mean? It starts with the fundamental question of what your encounters eat when they can’t get PC to brunch on. Then we will work some population numbers magic, allow for the mobility and “personality” of the encounter, and combine it all into an encounter footprint.

Having defined that process, I’ll show you how to use it to define and construct a simple ecology, and how to turn that ecology into an encounter table.

### Basic Approaches

There are three basic approaches to ecology design. You can proceed from the bottom up, from the top-down, or from the middle-in. I’ve seen all three advocated here and there, and consider that each of those advocates has a point, but that they are all misinterpreting the significance *of* that point.

##### Top-Down

The most common approach advocates starting with the top of the food chain, the king dog. You then work down the this-eats-that chains compiling the ecology. This prioritizes the creatures that are most likely to be dangerous to the PCs, i.e. the creatures the GM will naturally find most interesting to use for encounters, ensures that they have an appropriately likely diet, and so on down the line. Sounds pretty good, doesn’t it?

But there are a couple of price tags. The first is that the ecological balance can be utterly unrealistic, to the point of being implausible. If that is resolved by increasing the area within which the encounter table applies, the results can quickly lose any realistic connection with maps and terrain. And there is no firm dividing line between the application of one table and the next, producing complications and unlikely situations.

##### Bottom-Up

Most of these problems can be solved by building an ecology from the bottom up; but this is not ideal, either. It foregoes a lot of the GMs ability to control and dictate the inhabitants of the ecology, restricting his choice at the end of the ecology of greatest interest, or yielding results that are improbable to the point of absurdity. It requires more work on that part of the ecology that is of the least interest to both GMs and Players, and that can all be considered wasted effort. It is therefore far less efficient. Again, the usefulness and credibility of the results suffer.

##### Middle-In

A third approach that is not often even considered is the middle-in approach, which takes as its starting point the ecological level with the greatest total mass of animal life, then proceeds both upwards and downwards from that point. This works especially well in a fantasy environment where man (or elves or whatever) occupy that central level, for example in farmland, by selecting a small number of choice creatures that prey apon the resulting community, but in any real wilderness, where something like field mice, lizards, and birds are likely to occupy that central position, it falls apart pretty quickly, being susceptible to the drawbacks of both bottom-up and top-down approaches simultaneously.

##### The Best Solution

The big mistakes that all these make is that they assume that each level of an ecological food-chain has to be completed before moving on to the next, and that you always have to proceed from one end to another in the same way. If the construction of an ecology is viewed as a series of processes defining one facet at a time of the overall ecology throughout the food chain before moving on to the next, and choosing the direction of procedure (up-to-down or down-to-up) that is best suited to resolving the issues at hand, all the problems can be avoided and a more realistic, robust, and creative ecology created – in a lot less time, and with a lot less work, than either of the solutions discussed previously.

We can go from the top down to define the members of the ecology; we can work from the bottom up to determine their relative density; we can use travel time and existing maps to convert those relative densities into actual numbers; we can make allowances for behavior to derive one or more encounter tables; and because we are populating those tables at the top with discrete individuals, we can make the encounter tables location-specific and hence dynamic, altering them to take account of the actions of the PCs and of other natural changes that may occur.

A series of simple operations, repeated a few times, carried out on a spreadsheet or a table drawn up on a scrap of paper, can create an ecology and bring it to life for the players, permitting genuine interaction with the environment through which their characters travel.

### Encounter Corridor

How big an area does an encounter table represent? Or, to put it another way, how many encounter tables should there be for a given area?

This is actually a somewhat more complicated question than it initially appears. The PCs (the subjects of an encounter table) have a given speed of travel in a specific direction. Anything on that line, or in a narrow band to either side of it, will be available for encounters. Each individual creature will have a certain range within which it moves around, and the chance of an encounter will therefore be a function of the degree of overlap between that range and the corridor – if the corridor covers 20% of the range, then there is an 80% chance that the creature will be elsewhere. Multiply that percentage by the population density of the species and you determine the number of encounters that can be expected.

If only it were that simple!

Some creatures are significant than others. PCs will pay more attention to any such creatures even if they lie some distance to one side or the other of their path. Some creatures are more obvious than others – which has the same consequence. In effect, significance relative to the PCs increases the width of the corridor, assuming that if the PCs become aware of a member of a species within that corridor, they will react to it in some fashion – turning aside, confronting it, hiding from it, or whatever. If they see or hear a T-Rex 3 miles away, they *are* going to react to it – and that means that they have effectively encountered the T-Rex.

Some creatures are better camouflaged than others, either by size, coloration, or ability, while others will stand out more. That also effectively narrows or widens the encounter corridor, while widening or narrowing the likely range between the creature and the PCs when an encounter *does* take place.

Creatures are mobile, what’s more. Some will tend to ignore PC-sized creatures, others will hide from them. This effectively diminishes the size of the corridor, reducing the likelyhood of a noteworthy encounter. Other creatures will be more aggressive, seeking out an encounter, in effect deliberately moving into the encounter corridor – which is the same thing as increasing its width to the detection range of the creature, allowing for its speed of travel.

Then, there are other behavioral attributes to take into account – nesting behaviors/dens, protecting the young, seeking mates, nocturnal vs diurnal, and so on and on. Everything that makes it more likely that a creature will come to the PCs attention increases the width of the corridor.

These increases and reductions do not stack – the effective size of the encounter corridor is the largest of them.

Any attempt at being realistic would have to assess all of these factors, together with things like the PCs perception abilities. Fortunately, we don’t have to be realistic, we can abstract the entire question into a single value which I call the “encounter footprint”. What’s more, we can assign generic values for the encounter footprint by ecological niche if we’re sufficiently clever in defining them, then simply vary them for exceptions.

Of course, the entire situation changes when the PCs are not moving – when they stop for lunch or set up camp. If the PCs are stationary, creatures will come out of hiding to go about their business after a while, it makes it easier for predators to target them, and so on.

That suggests that we need a set of four encounter tables for each location: mobile day, mobile night, stationary day, and stationary night. Fortunately, this suggestion is misleading; PCs will rarely be mobile at night, and will not be stationary by day for a great length of time; it might not be entirely realistic, but a night-based stationary table will be close enough for practical purposes. What’s more, the tables will not be all that different – the night table will be a variation on the day table. In fact, they will be so close that we can, for all practical purposes, define them as a single table.

### Encounter significance

Encounter tables should report only encounters of significance, that is to say, encounters that require the PCs to alter their behavior or make a decision of some kind. Anything else can be discarded, or – better yet – incorporated into the terrain description. At the same time, to properly assess the chances of a significant encounter, it is necessary to at least generalize the non-significant encounter population.

### Elementary Niches

Having identified the basic parameters of an ecological simulation that will yield an encounter table, I will now turn my attention to defining the ecological niches which must be assessed and populated in order to construct a realistic encounter table.

From bottom to top, these are:

- Foliage
- Insects & Little Critters
- Middle Critters (including the most common critters)
- Big & Dominant Critters
- Scavengers & Oddities

Many of these ecological niches will have subcategories to be considered, so each of these requires some discussion. Remember that we will in fact be populating them in the other direction.

### Foliage

Foliage comes in three varieties: Little plants like grasses, herbs, root vegetables, Middle plants like shrubs, vines, and cornfields, and big plants like trees. Each of these needs to be considered separately, because they each support different ecological niches higher up the food chain.

##### Little Plants

In general, small plants don’t get much beyond waist height (though some grains are exceptions). In general, these plants have a very small horizontal cross-section and a height that varies with species. We’re used to thinking of grasses as being lawns, but it doesn’t take very long for grass to grow to the substantial height of half a meter or more. Small plants are mostly water and impurities, the same as animals. The small cross-section means that you can get a lot of them growing in a small area. In fact, you can guesstimate the volume of plant matter for each square meter as about 3/4 of the average height times 1m x 1m – and water is roughly 1 tonne per cubic meter, which is about 1.1 short tons. So if the plants grow to 1m in height, which gives 0.75 tonnes (0.825 tons) per square meter. There’s about 10.7 square feet in a square meter, so that’s 0.07 tonnes per square foot or 0.08 tons per square foot. Assuming the encounter footprint to be about 30′ to each side of the line of travel, each 5′ of travel covers 300 square feet, or 210 tonnes (240 tons) of plant matter. Each mile of travel is 5280′, or 1056 of those 5′ steps, or almost 222 thousand tonnes (over 253 thousand tons) of plant matter. For consistency, we should also change the height measurement into feet, so dividing 253,000 by 3.28 gives the number per foot of plant height. The answer is a little under 77300 tons.

In other words:

- 222,000 x average height in meters gives the mass of plant matter “encountered” per mile of travel, in tonnes;
- 138,000 x average height in meters gives the mass of plant matter “encountered” per kilometer of travel, in tonnes;
- 77,300 x average height in feet gives the mass of plant matter “encountered” per mile of travel, in tons.

Small plants provide food for two varieties of creature: small, high-metabolism creatures that feed on the grains and seeds, and large creatures that graze on the stalks. Assuming that the seeds are about 0.5% of the mass of the plant (some will be more, some less) that yields 1110 tonnes per mile of seed. At a paltry 1% efficiency, that would give 11.1 tonnes of such creatures per mile – and if the average weight of such creatures is about half a kilo, that’s 22,200 such creatures per mile. If the efficiency is more like 3%, that’s 66,600 such creatures per mile travelled.

The other type of creature are low-metabolism grazing herbivores like cattle, horses, sheep, elephants, herbivorous dinosaurs, and so on. These tend to become quite large (compared to the small metabolism creatures) according to Kleiber’s Law, which relates metabolic rate to mass. Plant metabolic rates scale almost perfectly, but animals scale at an exponent rate of about 0.75 – so creatures grow larger faster than their metabolic rate increases, and the metabolic rate determines how much food they need. That’s why elephants and the big dinosaurs and so on are large.

Again assuming a 1% efficiency, that gives 773 tons of large herbivore per mile per foot of average height of the plants. That could be seven 100-ton grazers or 70 ten-ton grazers or 700 1-tonne grazers.

Heavy horses weigh an average of about 800kg and are about 8′ in length from nose to tail. So that 773 tons is a herd of 876 full-sized horses – or more probably, a herd of 1000 horses, many of them being colts and ponies.

If these numbers don’t seem right, it’s because they are misleading. Only in cultivated farmland would you get a mile of nothing but small plants; and if you had a herd of grazers around, the plants wouldn’t get to anything like their potential full size; the small creatures and the large would compete for the available food (every seed eaten by a small critter fails to become a plant for the large one to eat); and it’s most unlikely that there would be only the one type of animal dividing up this edible booty. In the wilderness, these factors would be roughly 1/10, 1/3, 1/2, and 1:3 respectively – so each mile-by-60′ area could support about 4 horses. To get a square mile value, multiply the corridor width by roughly 90 to get 270 horses.

##### Medium Plants

Leaf Mass and berry/fruit mass are what count when we’re talking about most medium plants, and that’s at best about 50% of the total mass of the plant (some vines), and perhaps as little as 1%. For a guesstimate, let’s use 5%. The rest can be considered inedible by most species. These plants grow about twice as high as small plants, but occupy a much larger horizontal area per plant – a small bush might be under half a meter (1 foot) in diameter, and a large one as much as 2m (roughly 3′). lets use 1m (3.3′) diameter as an average. In comparison, small plants take up about 1% of this space per plant. So, 5% x 2 x 0.5 squared x 0.01 would give the relative number of plants in a 1 square meter area: it works out to be about one four-thousandth as much food value per mile.

That’s a useful number, because it means we can simply divide our small-plant numbers by 4000 to get the medium-plant equivalents: 5 small, high-metabolism creatures per mile (bats, squirrels, etc), or 0.07 large, low-metabolism creatures – that weigh as much as horses. But deer and the like tend to be a lot smaller and lighter than horses, on average – perhaps half as much – which doubles the latter number to 0.14. In fact, they could more properly be considered large high-metabolism creatures, but the numbers work out about the same, at least within the very broad margin of error.

This shows quite clearly that the creatures that live off this sort of food will be rare, requiring a very large grazing area, except in areas where these plants are the dominant vegetation. Such areas multiple those numbers by 20, giving 100 bats/squirrels or 3 deer – per mile. In practice, because these consume different parts of the plant, you can have both.

##### Big Plants

There aren’t many creatures that can eat and digest tree branches and trunks. Again, we’re talking canopy mass. Depending on the variety of tree, that can conceivably be 50% of the total, or 30%, or 10%, or less. Trees, by their nature, are again much larger in surface footprint by virtue of the large canopy of leaves they bear, and some varieties grow to astonishing heights – but the size of the canopy doesn’t scale with height for all varieties of tree. In fact, the taller the tree, the less likely it is to scale.

All trees are not alike in the food value of their foliage, either. It takes specialized adaptions to be able to live on the trees with needles like pine trees, whereas other varieties of tree have foliage that is much easier to digest – if you can reach it. Climbing ability or some other means of reaching more than the lowermost leafs is an essential for a diet of foliage. The first limits animal size to the weight that a tree limb can support, especially toward the tips – because that’s where the food is, and that is also different from one species of tree to another.

The alternative approach involves a long neck, which in turn requires a massive heart to pump sufficient blood to the brain, all of which requires a relatively high metabolism, and disproportionately large food supply. The long-necked dinosaurs, it is believed, were capable of stripping an entire tree bare in one or two bites. Giraffes need to spend up to 75% of their day eating, depending on the season (other sites say up to 90%).

One of the best solutions is for insects to eat the leaves, concentrating the nutrients, and acting as a delivery system to a small carnivore. A lot of bat varieties, birds, lizards, and spiders exist predominantly on a diet of insects, and other species can quite happily supplement their diet with insects.

Nor do the complications stop there; some trees are green all year round, others have a specific growing season. If the food isn’t there for part of the year, a species that lives on it must have evolved to accommodate the tree’s growth cycle. Either it eats something else, or it reduces its nutritional requirements through hibernation or some other strategy.

The total nutrition that can be derived from tree foliage is therefore highly variable, depending on a whole slew of factors. While it would be possible to abstract some average values, for a change these would be of little benefit. Trees generally come in clumps (or forests) of like trees, interrupted only by the occasional interloper, save when intelligence has played an active hand in the plantings. That means that the averages that should be applied are those of the most common ‘breed’ of tree in the region, and not some overall general value.

Which leaves me in the position of pretending to rather more knowledge on the subject than I actually possess, and inventing a table or system of classification for the many possible varieties of tree that would suit our needs; or of taking the easy way out and crafting a system for determining the approach without getting too specific.

The fundamental principle when considering foliage volume (and hence mass) is to simplify the shape of the “typical” tree of the variety desired. Determine (roughly) the volume, then subtract the volume of everything that *isn’t* green matter. I find it easiest to work on the profile of the tree first. Most trees have foliage that can be considered either a triangle (in profile) or a circle, possibly with another circle as an occlusion. The diagram shows three triangles (A, B, & C) and the occluded circle (E). Note that the size of the trunk, which determines the height of the tree, is largely irrelevant (D). Once you have the cross-sectional area, it’s relatively easy to get the volume if the canopy were a solid mass (F); it isn’t so we simply have to determine the total amount that isn’t empty air (G).

**Triangle:**

The area is half the base x the height. Ignore the trunk, there will be bigger sources of error, so why make life complicated? The solid volume (F) is the triangular area (as shown) x 2/3 x the base x pi.

In A, if the base is 10, the height is about 15. The triangular area of one side is 0.5 x 10 x 15 = 75. The volume is 75 x 2/3 x 10 x 3.14 = 1570. If we’re talking meters (Big tree!) that would give a solid canopy of about 1570 tonnes. However, as shown by G, roughly half the volume is completely empty so that 1570 becomes 785 tonnes; and even where the tree appears solid, at least 75% of it (probably more) will be empty space, even on a tree with dense vegetation, leaving about 196 tonnes of green matter.

If the base measurement is feet (which is a more reasonable tree size), divide the volume by 35 to get cubic meters: 1570 / 35 = aprox 45 cubic m = aprox 45 tonnes (solid canopy), = roughly 5.6 tonnes (realistic thick canopy), or 6.1 short tons if you want to use imperial measurements.

**Occluded Ellipse**

The area is half the height of the ellipse x the maximum width x pi. The volume of a solid canopy works out to be area x 8/3 x the maximum width. To allow for an occlusion, simply work out the occluded area and subtract it from the total before converting to solid canopy.

In E, the height is 15, and the maximum width is 10 (no, that’s not a coincidence). The height of the occlusion is 7.5 and the maximum width is about 3.

- Area of the semicircle is 10 x 15 / 2 x pi = 236.
- Area if the occlusion is 3 x 7.5 / 2 x pi = 35.
- Area less occlusion is 236 – 35 = 201.
- Volume (solid canopy) is 8/3 x 201 x 10 = 5360. This also equals the weight in tonnes.
- If the measurements are in feet and not meters, divide by 35 = 153 cubic meters = 153 tonnes.
- To get tons, multiply by 1.1: 153 x 1.1 = 168.
- 50% empty space from above, times 25% empty space even in what looks solid, yields 19 tonnes (21 tons) of foliage.

The average giraffe weighs 1600kg, or 1.6 tonnes, and eats 63kg a day. One tree with 19 tonnes of foliage will feed one giraffe for 301 days – except that the trees they feed on have relatively sparse vegetation, and we got a figure of 19 tonnes for a thick canopy. One-quarter of that value is probably closer to the mark. So that’s one tree’s worth for about 75 days – or about 5 trees a year per giraffe. Except that without leaves, the tree will die, and a few other critters also eat the leaves – so multiply that by 8. That’s 40 trees per giraffe.

**Tree Density**

It’s possible to calculate how many trees can be found in a given area according to various factors using something called the Stand Density Index but the math is probably too complex for everyday use, and we don’t have a convenient table of the constants. I found another table that gives a tree population based on tree density and total area of all the tree trunks in a given area, but that’s not especially useful either without knowing what that total area should be in different environments. Besides, if you know the total area of the trunks and the number of trees, it’s relatively simple to calculate how many trees there are – divide the total by the number of trees and work out the radius of a circle with that area.

I was just about to give up when I found this website and, more specifically, the table at the bottom of the page, which gives the approximate number of trees in an acre based on the average number of feet separation between the trees. One acre is roughly 4000 square meters, or 43,500 square feet.

Let’s say that the PCs have an awareness corridor in terms of trees of about 10,000′ to either side of them if the trees are sparse, down to maybe 15′ if the trees are especially dense. That means that a 5′ step “encounters” an area of between 75 sqr feet (dense) and 50,000 sqr ft (sparse). Each mile is 1056 of those 5′ steps, or 79,200 sqr ft (dense) to 52,800,000 sqr ft (sparse). Dividing those numbers by 43,500 gives us acres: 1.8 acres (dense) to 1214 acre(sparse). Using the table linked to above, we get 43,560 trees per acre (dense) to 303 (sparse) – for totals of 78,408 trees (dense) to 367,842 (sparse).

Of course, these numbers would only apply to wooded terrain; environments where a tree stands alone in the middle of a plain need not apply.

But those numbers tell us everything we need to know.

If we’re talking about 5 tonnes of foliage per tree, at a metabolic efficiency of 1%, that’s roughly between 4000 and 18,400 tonnes of wildlife supported. Divide by about 8 for sustainability to get 500-2300 tonnes of animal flesh. At half a kilo each for small creatures, divided by half (because roughly half will be in bigger creatures), that’s 125,000-2,300,000 small creatures.

If we’re talking 19 tonnes of foliage per tree, the result is 475,000-8,800,000 small creatures.

### Insects and little critters

This category combines two ecological niches – the smallest herbivores (so small they are usually dealt with in D&D as “swarms”) and the smallest carnivores, who survive by munching on those small herbivores. Or should that be “lunching”? I’ve called them “Picnickers” for a reason….

##### Bugs & Insects

Either way, much of the work for this category has already been done. We know that per mile of travel, small plants will be encountered that can support about 22,200 small creatures; that medium plants can support about 1/4000th of this, or 5.55 small creatures; and that trees will be encountered that can support 125,000-8,800,000 small creatures depending on the species of tree.

Actually, we don’t. These are exclusive numbers. To get the number of small-sized creatures that can be supported, we need to assess the relative proportions of each type of foliage that will be encountered in that mile (or in as many miles as the encounter table is to cover). We also need to assess the tree density and type to nail down that contribution to the total.

This is actually fairly straightforward, at first glance:

- Let’s say that 1/10th of the terrain is covered by small, dense, stands of trees. That gives us a number toward the dense end (the 125,000 per mile). Multiply the chosen number by 0.1. Simple. Call it 15,000.
- Let’s say that another 1/10th of the terrain is covered by bushes and other medium plants. That gives us one tenth of 5.5, or 0.55.
- That leaves 8/10ths of the land being either bare ground or having small plants like grass. Call the bare ground 1/10th, the same as trees and the same as bushes; that means we multiply the small plant number by 7/10ths, and get 15,540.
- Add these up and we get 30,540.55 small creatures.

But first glances can be deceptive, and it’s not that simple. Those huge numbers are for the encounter footprint of the *plants*, not the insects and small critters – which tend to be small, hidden, and not especially noticeable. Don’t believe me? It’s estimated that there are 1.4 BILLION insects for every human now alive on earth. Now, I’m 50 years old, and I would be lucky to have seen (in person, not on TV) 10,000 *and noticed* insects in total throughout my life – okay, I saw a small locust horde once so maybe 40,000. That’s a whole 0.03% of my share of the total. Even if I live another 50 years, and continue seeing insects at the same rate, I won’t come anywhere close to seeing 1% of “my” 1.4 Billion insects!

We’re talking about a corridor maybe 2′ wide. Unless it’s poisonous, or a swarm of dangerous insects, outside that distance, it’s probably out-of-sight and out-of-mind – and even if one or both of those things are true, it’s not going to extend the corridor very much. Giant insects are a different story, of course.

The width of the small plant corridor was 30′ to each side, or 60′ total. The medium-plant corridor was double that, 60′ to a side, 120′ in total. We don’t know exactly how big a corridor the trees occupy because they are in stands, but we can work it out: about 1/10th of 1/10th of the dense end of the scale, or about 8000 trees, in an area that could contain 100 times as many trees. One percent of the dense population count is about 435, which gives an overall average of 10′ between trees, which is very definitely at the sparse end of the scale – which in turn suggests an average corridor width, based on the numbers we defined earlier, of maybe 9000′ overall, or 4,500′ to each side (a little under a mile).

To get our numbers down to the small critter corridor, we need to divide each share by the width of the half-corridor:

- trees: 15,000 / 4,500 = 3.333.
- brush/bushes: 0.55/60 = 0.009.
- small plants: 15,540 / 30 = 518.
- Total: 521.342.

This shows something else that’s really significant: at least when it comes to small life, the small plant contribution is so significant that we could ignore everything else and only be wrong by 0.64%. That is miniscule, swamped by other sources of error. However, the results would be very different if we had different ratios of the ecological foundations, so you can’t assume this will be the case every time.

##### The Picnickers

Preying apon the insects and little critters are a whole host of small-to-medium carnivores. Insects eat their body weight in plant matter each day; People eat their bodyweight in food every 6 months. This is a combination of two factors: meat is a more concentrated source of energy (just look at how much salad it takes to get the number of calories equivalent to a single steak), and larger creatures are more efficient at processing their food. I’m going to oversimplify again and combine both factors; and even though the relationship isn’t close to being a straight line, I’m going to simplify that, too, by assuming that it is; individual variations can then be applied as necessary.

That means that the average body weight of a beastie can be used to estimate how much it needs to eat. With the average weight of an insect at about 3 milligrams (0.0001 ounces), this is close enough to be considered zero for practical purposes. With some rounding, it works out that:

Food per day = aprox Body Wt x [1 – (1% x Wt)], or = Wt – 1%(Wt^2)]. (It works with Wt in Kg, within the limits stated – up to Abouit 100 kg. I can’t promisde that it will work with other units.

Let’s assume that the average critter in the group we’re currently discussing is up to about 35kg (77 pounds – call it 80 for convenience). Some will be larger, some smaller. Applying this weight to our formula gives 22.75 kilos of food per day. 521.342 small critters per 5′ step, (2′ wide corridor), weighing an average of maybe 12g each – from insects at 3 milligrams to small lizards etc weighing in at up to 25g – adds up to about 6.25kg. So, for the same corridor width, each of our middle-sized critters requires 5.6 of those 5′ steps, or 28′.

But these creatures will be a little more noticeable – the corridor is back to 30′ wide, not two. I’m going to be obscure, and use 28′ wide – which (quite neatly) drops us back to 2.5 potential encounters per 5′ step. For each mile travelled, there would be 2640 potential encounters with creatures of this size, given our terrain mix.

Whoops, we missed a step. These creatures need that much food *every day*. So we have to divide that number of potential critters by the number of days it takes for its food to grow, in days. The average lifespans of insects vary from 3 days to 3 years, and again this is roughly proportionate to body mass – fleas are 30-90 days, bees are 9-12 months, ladybugs manage 2-3 years, and termite queens rack up an average of about 15 years. Five hundred days seems to be right in the middle of the range, but there would be more on the lower side of this value than higher, so let’s halve that to be on the same side.

2.5 potential encounters per 5′ step drops to 0.01 encounters per 5′ step, or 1 per 500′, or 5.3 per mile.

### The Middle Critters

These come in three basic varieties, and generally, all three will be present in any given ecosystem. They are the Hunters, the Vegetarians, and the Packs. In a simplified ecology, you can choose to have one, the other, or both of the two carnivore varieties.

##### The Hunters

Living off these smaller critters are those of the next size up – the creatures that range from half human weight to six times human weight, and that live on meat. Lions average 250kg in body mass, for example. We can use double the average values given in the previous section for humans as our benchmarks. So 140kg critters average, which gives us an average food requirement of 56kg per day.

The average weight of the picnickers gives 35kg per 500′ of food. Most lizard species have an average lifespan of 2-3 years, the average snake 10-12 years, and the average mouse about 1.5 years. So, again weighting toward the shorter end, we get roughly 2.5 years. So the available food in a 30′ wide corridor (15′ to each side of the party) is about 0.04 kg per day per 500′. However, the corridor for a hunter will be *much* wider, maybe as much as 3 miles on average – for some creatures it will be smaller, for some, much larger. 3 miles wide is 528 times the width of the 30′ corridor – increasing the available food mass to 21.12 kg per 500′ travelled. To get our required 35kg, we need to cover about 830′. Or, to put it another way, there will be 6.36 such potential encounters per mile.

However, this is not necessarily the only source of food for creatures to consider. This works in the case of solitary hunters – if the Monster Manual suggests 1-2 or maybe 1-3 in an encounter.

Somewhere in between the Picnickers and the Hunters in average weight are the pack-hunters. And they live off the big herbivores, predominantly.

Actually, that’s another oversimplification. A solitary hunter can munch down on a big herbivore, and will do so quite happily. And pack-hunters may well supplement their diet with small critters that present themselves as snacks-of-opportunity. But for our needs, the more general statement is close enough; we can ignore the occasional solitary-hunter vs. big herbivore feast, and the occasional snack-of-opportunity, and assume that these cancel each other out.

##### The Vegetarians

We’ve already worked out some numbers for these critters – jumping the gun somewhat, but it seemed relevant at the time. We got values of 4 large herbivores per square mile of small plants, using horses as our basis; we determined that we could divide that by 4 to get the number of vegetarian creatures that lived off medium plants; and we got a count of 1 herbivore per 40 trees for those that ate leaves, using giraffes as the basis – and that the tress in a clump were about 10′ apart, and about 500′ x 500′ across; each clump contained about 4000 trees. What we didn’t do was convert these into corridor values.

These creatures tend to be fairly noticeable. The corridor should be miles wide, especially since they almost-universally operate in herds, making them even more visible. So let’s assume we’re talking about a 5-mile corridor – 2.5 miles to either side of the party. Some creatures won’t be noticeable at that distance, and some will be visible at a much greater distance, so that should work out about right. In order to get a square mile with such a wide corridor, we need only 2 fifths of a mile in the direction of travel, or 2112′ – per adult creature.

How many are in a herd (or a flock, if we’re talking sheep)? The larger the creatures individually, the smaller the herd size that can be supported, suggesting that rather than considering individuals, the total mass of the herd is relatively fixed according to the nutritional density of the plant life. Once again, this isn’t the whole story (not even close) but it’s near enough for our purposes. According to this article on feral horses, the proper collective name for feral or wild horses is a band, and a band is usually 3-5 individuals, with some containing as many as a dozen. If 1/10th of bands have 10+ members, that basically adds 1 to the average, so 4-6 – call it 5 on average, overall. Since they average about 450kg each, depending on breed and nutrition, we can suggest the average pack or herd is very roughly 2250kg (aprox 5000 pounds) in total body mass. This could be 5 horses or 33 deer (white-tailed deer average 68kg in weight, with adult males reaching as much as 300kg and adult females 125kg – numbers which tell you that a lot of the deer in a typical herd(?) are juvenile, which is only to be expected when you think about it.

So:

- small plant-eaters: 4 per square mile, divided by a 5-mile wide corridor, gives 0.8 miles or roughly 4200′ each; times the number in a herd, 5 (since we used horses to derive this number) = 1 herd per 21000′ (about 4 miles).
- medium plant-eaters: 1 per square mile, or 0.2 miles each with a 5-mile corridor, or 1 herd per 16 miles.
- tree-leaf eaters: 1 per 10 trees, yielding 400 per clump. Divide by 365 (the rate of replenishment of leaves on a tree) to get 1.1 giraffes per clump. 1.1 adult giraffes weighing 1600 kg each is quite a lot, but the average will be much lower because most won’t be adults – the reindeer adult male to average ratio is 300/68, or 4.4; with that as a rough guide, we get an average giraffe weight of 363 kilos. 1.1 x 1600 / 363 = 4.8 giraffes (or equivalent) per clump – call it 5. Five giraffes at 363 kilos is 1815kg, a large fraction of 2250kg per herd, leaving 435kg per clump. This gives some notion of our margin of error on all these calculations – about 20%. More to the point, it suggests that for every 5 clumps, there will be 6 giraffe herds – or possibly one larger herd that migrates from clump of trees to clump of trees. Also, since we estimated 15000 trees per square mile in clumps of 4000, we can say that there are about 3.75 clumps per square mile – so 5 clumps is 1 & 1/3 square miles. With a 5-mile wide corridor, that’s 6 herds (or one larger herd) every 1400′, or thereabouts, or 235′ per herd.

Note that of course the baseline creatures don’t have to be the species that are located in this terrain; we’re just after an idea of numbers that happen to fall within the corridors that we’re specifying, for the terrain that we’re discussing.

Applying the relative proportions of the plant distribution specified earlier (80%, 10%, and 10% respectively) and adding the results gives a total of 2.45 herds per mile of journey, with each herd weighing 2250 kg – 5513kg of meat on the hoof that’s visible for every mile that the party travels.

##### The Pack Hunters

If we assume an average of 1 pack per herd, the numbers spill out rather quickly – but how realistic is that assumption? Is it even close? Bet you it isn’t. Let’s do the math:

Most wolf packs have 6-7 members, though some can have as many as 15. If 1/10th have 15, that’s +1.5 to the average size, so 6.5+1.5=8 members per pack of wolves. The average wolf is a little under 40kg in weight for a full-grown adult, so the average over an entire population would be about half that, or 20kg each. 8 wolves at 20 kg is 160kg of wolf per pack.

Each member of the herd will, according to our formula for body weight vs. food, need 16kg of meat per day. Multiply by 8 to get the whole pack’s food requirements, and we get 128kg.

2250 kilos in a herd, divided by the average lifespan of the creatures in the herd – deer average 10 years, and can survive in captivity for up to 20 – gives 0.6kg of meat per day per herd, sustainable losses. Multiply by 2.45 herds per mile, gives 1.5 kilos per mile. Not even close to 128kg. In fact, at that rate, it takes 85 herds to sustain one wolf pack. Anything less and the herds will be hunted to extinction – eventually.

If there’s 5513kg of meat on the hoof visible in a 5-mile-wide corridor for every mile the party travels, they will have to travel (5513/2250)x85=209 miles to encounter one pack of carnivores.

But that has a false assumption in it – that 5-mile-wide corridor. To meet their needs, the pack will move – a lot. And that means that the corridor is going to be a LOT wider. A reasonable estimate is more like 25 miles – given that if the pack finds the party’s trail, it will eventually yield an encounter. And that means one pack encounter for every 209/5=42 miles.

It’s also worth noting that an adult African bull elephant weighs about 5500kg while an average lion weighs about 250kg. That’s a ratio of 22:1. Comparing the biggest herbivorous dinosaurs (believed to be Brachiosaurus, Weight estimated at 25 tonnes) with the biggest carnivorous dinosaur (believed to be Spinosaurus, estimated Wt 7-21 tonnes) gives a roughly 2:1 or 3:1 best-guess ratio. Tyrannosaurus Rex is currently thought to be about 6 tonnes in adult weight – a ratio of 4:1 relative to Brachiosuarus. Carnivores are generally smaller, sometimes a lot smaller, than their possible prey. This is especially true of pack hunters. Why? Because by cooperating, the pack can bring down a large animal – and get enough meat in one meal to feed the entire pack. It’s worth bearing these ratios in mind when thinking about the likelyhood of encounters and what the creatures encountered usually eat.

### Masters of all they survey: The Big and Dominant

That brings me to the Big Beasties. There aren’t many of these in our world, but they are routinely present in fantasy and sci-fi environments. Creatures like a T-Rex, or a Dragon. Assuming that the two weigh about the same, we might be able to get some idea of the dietary requirements per day – (NB: these creatures push our quickie formula beyond its limits of usefulness). Many websites don’t nominate a T-Rex diet per day, but point out that they could probably eat up to 230kg of meat in a single bite. Another answer I found was “up to four semi-small dinosaurs a week” – but how big is semi-small? Two tonnes? The best answer I was able to find was 500 pounds (227 kg) every couple of days – but does that assume it was a cold-blooded or hot-blooded creature (it makes a difference). T-Rex is surmised to have eaten practically the whole carcass, bones and all, suggesting that it took everything it could get (always room for one more pesky human) – seriously, it has teeth that would have been excellent at crushing and extracting marrow from bones, and some partially-digested bone has been recovered from fossils.

The largest modern carnivore is the Polar Bear, tipping the scales at 350-700kg, matched with the omnivorous Kodiak bear which is about the same size. But environmental adaptions make both useless as a basis, and they are an order of magnitude too small, anyway.

So let’s go back to first principles, and that’s where those ratios that I pointed out earlier come in. The average adult lion weighs about 250kg, and eats about 5-7 kg of food a day – call it 6 kilos or about two-and-a-half percent of it’s bodyweight (though it will usually eat a lot more and then fast for several days). If a T-Rex or a Dragon weighs in at about 6000kg, that suggests a value of about 145kg per day – maybe more for the dragon, flying is hard work, especially when you weigh that much. And that says that the likelyhood of encountering such a creature is inversely proportionate to its relative bodyweight, per square mile of territory. 160kg per wolf pack, or 250 kg of lion, says that these king predators will be encountered 160/6000th as often and 250/6000th as often, respectively – one encounter per 37.5 wolf packs. At one Wolf Pack per 42 miles of travel, and a five-mile window, that’s one Apex Predator per 1575 miles of travel. Even widening the corridor 10-fold to 50 miles on each side(because the Dragon can fly and might easily cover that much territory in a day) only gives one possible encounter per 157.5 miles.

### Scavengers and Oddities

This is a sort of grab-bag for everything that’s left. It includes the obvious, the exotic, and the intelligent – who some might consider as fitting both of the first two categories at the same time.

##### Scavengers

Carnivores tend to gorge themselves quickly and move off before the scent of blood draws potential rivals and enemies to the kill. That typically means that there will be quite a bit left when the carnivore is done – perhaps as much as half, perhaps more or less. For the sake of simplicity, I chose to ignore this when deriving carnivore and pack hunter populations, but there’s one group that live off whatever’s left – the scavengers. As a general rule of thumb, these tend to be between one half and 2/3 the weight of a carnivore – meaning that their dietary requirements are roughly one-half as much, or a little more. So if there’s one-half of the meat left by the carnivore, there will be enough left to sustain scavengers in equal number to the carnivores.

##### Exotica

Semi-finally, we have exotica – creatures that live off Mental Energies or Souls or whatever, or that don’t have to live off anything at all, like undead, or golems.

The latter don’t move around much, as a general rule, and they may (depending on the campaign and the subtype) only be active at night. The rest may move around more, but they do so with a purpose, and – unless they are a pre-scripted part of the plot – that purpose is not likely to be assisted by random encounters with PCs. For that reason, I would estimate – as a general rule – that these are no more likely to be encountered than is a Dragon. In a campaign setting with a lot of undead, maybe triple that, maybe more – special circumstances.

##### Sentient Citizens

Lastly, we come to Sentient citizens of the world. In any area in which these are dominant, they will have driven out or domesticated most of the non-plant encounters, skewing everything their way. In an area which is still wild, they would be extremely rare – probably as rare as Dragons, unless the PCs are travelling the only navigable pass or something, and even that would not increase the chances much. People don’t tend to be in such reasons unless they have a reason to be there.

The fun comes when you start to think about the regions that are in-between. According to Medieval Demographics Made Easy (available from lots of places – Google it), population densities in amenable lands would be about 120 per square mile, and would be full of small villages no more than a mile or two apart (Medieval France was about 100), while areas that are less amenable (like Medieval Germany) would have an average of 30 people per square mile. Medieval England was somewhere in-between, around 45 or 48 per square mile (from memory).

- If there’s a village of 120 people, how much empty space (0 per square mile) is needed per village to achieve these overall densities? Zero to (120-D)/(D-1), or (in this case), (120-30)/(30-1) = 90/29 = 3.1 square miles.
- If there’s a town of 1200 people, how much empty space surrounds each town to achieve these densities? (1200-D)/(D-1), or, in this case (1200-120)/(119) = 9 (at 120/sqr mile) to (1200-30)/(29) = 40.3 square miles.
- If there’s a small city of 12,000 people, we get 99.8 (at 120) to 412.8 (at 30) square miles.
- If there’s a national capital of 120,000 people, we get 1007.4 to 4136.9 square miles.

Let’s put these another way. Think of each population centre as a spot surrounded by a circle of empty land. Assuming that each of these population centers is roughly a square mile (2 for the small city, 4 for the national capital):

- Village of 120 people = 0.56 – 1.14 miles to the surrounds of the next population centre, ie the edge of the next circle.
- Town of 1200 people = 1.8 – 3.6 miles to the surrounds of the next population centre.
- Small City of 12,000 people = 5.7 – 11.5 miles to the surrounds of the next population centre.
- National Capital of 120,000 people = 18 – 36.3 miles to the surrounds of the next population centre.

These were simply calculated by determining the radius of a circle of the required area.

Even reducing the population densities to 1/10th of those targets – from 120 to 12, from 30 to 3 – doesn’t change things as much as you might expect.

- Village of 120 people: 9.8 – 58.5 square miles, or 1.9 – 4.3 miles to the surrounds of the next population centre.
- Town of 1200 people: 108 – 598.5 square miles, or 5.9 – 13.8 miles to the surrounds of the next population centre.
- Small city of 12,000 people: 1089.8 – 5998.5 square miles, or 18.6 – 43.7 miles to the surrounds of the next population centre.
- National capital of 120,000 people: 10908 – 59998.5 square miles, or 58.9 – 138.2 miles to the surrounds of the next population centre.

From what I’ve seen of most D&D campaigns, most people use something close to these numbers – population densities more appropriate to stone age Britain than a medieval setting.

Either there are lots of people (humans, orcs, whatever) or there are virtually none at all. Let’s put this into context: 3 people per square mile is the current population density of the Western Sahara (i.e. the less habitable part).

This page should prove enlightening. Sure, the numbers are modern. NB: page will attempt to load an unnecessary popup. Don’t take chances, block it if you can.

When estimating population densities for non-humans, bear in mind how much food per person they need and adjust these density targets appropriately. In my Fumanor campaign, for example, Goblins occupy some of the best farmlands. Normally, that would suggest a population density of 120 per square mile, but each needs only about half as much food as an adult human – so double that to get a density of 240 per square mile. They keep domesticated food animals but don’t farm, so I would divide this number by about 5, to get 48 per square mile. In a 30×40 mile region, there may be as many as 57,600 Goblins. They occupy about 20 such areas, for a total Goblin population of 1,152,000 Goblins. Orcs need about 3/4 what a human does, but they are in a less fertile region. They will eat a lot of food that a human could not, so a reduction to 1/2 human requirements is not unreasonable – but they do not keep domesticated animals and do not cultivate farms, so that drops the population density to about 1/10th what it might otherwise have been. Net effect: 50 x 2 / 10 = 10 per square mile. Balancing that, they have about 400×150 miles of territory – so there are about 600,000 of them, all told.

As soon as the population density goes above about 50 per square mile, the chance of an non-plant encounter that is not drawn from that population pool drops to 10% or less.

Balancing all of this to some extent, and playing into the nature of the encounter, is the fact that the population in any medieval era is a LOT less mobile than you might expect. Unless going to a war, people rarely go more than a mile from their homes. Maybe 1 in 1000 people would have some reason (other than war) to travel, so the chances of an encounter with someone on the road is going to be less than that (there’s a 50% chance they are going in the same direction you are, for example). Seeing people working in the fields in the distance is far more likely.

### Significance – discarding the irrelevant

Having worked out the number of potential encounters per mile of travel, the next step is to discard the irrelevant. I like to build some of these into the narrative description, especially as cues to terrain and ecological change; that’s up to you.

##### Little Plants

I would estimate that no more than 1 in 10,000,000 small plant encounters would be significant or noteworthy in any way. At best. Since a typical small plant might weigh only a few dozen grams, at best (even assuming there are several specimens of the plant variety in one spot), 1 doubt we’re talking more than a couple of pounds, perhaps a kilogram, of significant plant. These will usually be wild herbs of some sort, or something similar. Since each mile brings 222,000 tonnes of small plants into view, we can estimate the significant encounters as 222,000,000/10,000,000 = 22.2 encounters per mile. But even these “significant” encounters are largely irrelevant. Relevant encounter count: zero.

##### Medium Plants

Medium plants are more likely to have edible berries (in season) or fruits. Maybe 1% of such potential encounters are significant in that way. Since a bush can easily grow to weigh 100kg, and there’s 1/4000th as much plant mass as for small encounters, that gives 222,000,000/4000 /100 x 1% = 5.55. Again, most of these encounters will be irrelevant (aside from making good flavor text) and can be ignored. Relevant encounter count: zero.

##### Big Plants

Some trees will have fruit (in season) or edible nuts. Depending on the exact terrain, either a substantial number of trees will fall into this category or virtually none will. There’s not much that both grows on trees and is edible to humans in a pine forest, for example. More interesting is that some of these plants are potentially sentient enough to pose a hazard – since this is the logical category in which to find Treants and Verdonne and any other special plants. I would estimate that 1 in 100,000 encounters with a tree might be significant and *not* irrelevant in terms of an encounter. In our particular example terrain, we ended up with stands of 4,000 trees, with 15,000 trees visible for each mile within a corridor that’s 9000′ across. Per mile, then, 15,000 / 100,000 = 0.15 significant encounters per mile.

##### Insects and little critters

There’s lots of insects and other tiny critters and most of them are utterly irrelevant. We determined, earlier, that per mile, there would be 521 potential encounters of such life forms. Of these, perhaps 1 in 10,000 will be significant – a venomous spider, or a wasp’s nest, or a centipede, or whatever. So that’s 521/10,000 = or 0.0521 significant encounters per mile of travel.

##### The Picnickers

All sorts of nasties fall into this category – everything from snakes to spiders. Since close examination is often required before these are determined to be insignificant, everything that comes to the PCs attention is relevant to an encounter table. We earlier determined that there would be 5.3 potential encounters per mile; if 1% of these are genuinely hazardous enough to mention, that’s 0.53 relevant encounters per mile of travel.

##### The Hunters

Even wild animals of the Hunter variety will usually leave something as big as a human alone, unless it is especially hungry; there is usually easier prey around. Perhaps 1 in 25 times, a potential encounter with a hunter will become relevant in an encounter-table sense; at other times, the PCs may or may not observe a Hunter watching them carefully, or hear one attacking somewhere within earshot. With a potential encounter rate of 6.36 per mile, discarding the irrelevant gives 0.25 significant encounters per mile.

##### The Vegetarians

These creatures are often paranoid and prone to overreaction. Those reactions might be to flee or to attack, depending on the size and nature of the creature in question. We earlier got a total of 2.45 potential encounters per mile – but most of those will take place at some distance from the PCs. If the potential encounter takes up 5° of visible arc, and the creatures aren’t likely to be a problem unless the characters close to within 5° of the creatures, that gives a wedge of 15° out of 360° that puts the herd in a location where they will have to react if the PCs continue to travel in the direction they are going. So that estimates that 1 in 72 will be significant. And that gives a significant encounters tally of 0.03 encounters per mile.

##### The Pack Hunters

Are pack hunters more or less likely to take on humans than a lone Hunter type? There are some lines of arguement that suggest yes and some that suggest no. So, overall, let’s suggest a similar significance rate of 1 in 25. With one pack encounter every 42 miles, the chance of a significant pack encounter is 1/42 /25, or approximately 0.001 per mile.

##### Scavengers

Scavenger encounters are less likely to be significant (unless they are attending a kill) than a Hunter encounter, but we established that the two have roughly equal numbers of potential encounters per mile. So let’s say that 1 in 50 scavenger encounters will be significant – half as many as for a hunter – which gives us 0.125 significant encounters per mile.

##### Masters of all they survey

One possible encounter per 157.5 miles sounds good – but when you’re talking T-rex or Dragon or Frost Giant or something of that order, every potential encounter is going to be significant. So that’s 0.0063 encounters per mile. But these creatures are so attention-getting that even the sign that they are/were active in the area can be considered a significant encounter – and nine times out of ten, that’s all you’re going to find, some trace that they have left behind. A footprint. A scorch mark (red dragon?). A lightning-blasted tree. The remains of a kill made within the last couple of weeks (after the scavengers have had their share of the spoils). Heck, even just a lot of blood soaked into the ground.

Seeing one in the distance has to be at least five times more likely than coming to the attention of one, probably ten times. Finding such a trace is also going to happen nine times as often as an actual encounter. Add those together and add 1 for an actual encounter with one, and that base chance should be increased 20-fold (10+9+1=20). Which gives 20 per 157.5 miles, or 0.127 significant encounters per mile.

##### Exotica

We arbitrarily equated this value with that of the apex carnivores, with extras if undead are especially common. Most of these don’t leave the telltale evidence that the Apex Critters do, though, and it’s not often that they are significantly larger or more visible than a humanoid figure. So we use the unmodified value: 0.0063 encounters per mile.

##### Sentient Citizens

This is by far the biggest variable we have. In a densely-settled wilderness area, nine out of ten significant encounters will be with a sentient citizen, simply because everything else has been driven away – so this would have a value of nine times the total of all the rest. In a sparsely-settled region, there might be as many encounters with sentient citizens as there are with Hunters – but they will all be significant. In the real wilds, the chance might be vanishingly small, on the order of encountering exotica. And all points in between.

Here’s a more reliable guide: 1 in 1,000 people have a reason to travel (wars excepted), as noted earlier. That means that the population density per mile /1000 = the chance of a significant encounter per mile travelled. Half of those (or less) will be going in the same direction you are, so all you might find is traces of them. That makes the number Density/2000, or maybe /2500 to allow for the occasional person travelling at right angles.

If following a known road or trail between two population centers, total the size of the two population centers, divide by the distance between them, and use that as the “local” population density instead of the general value.

People make campfires and the like. So they can be detected from a reasonable distance – a couple of miles at least, possibly more, especially at night.

Here’s a list of handy values to work from:

- Non-significant: General Density 120 / square mile: 4.8 encounters per mile
- Non-significant: General Density 30 / square mile: 1.2 encounters per mile
- Non-significant: General Density 12 / square mile: 0.48 encounters per mile
- Non-significant: General Density 3 / square mile: 0.12 encounters per mile

- General Density 120 / square mile: 0.048 encounters per mile (day), 0.48 encounters per mile (night)
- General Density 30 / square mile: 0.012 encounters per mile (day), 0.12 encounters per mile (night)
- General Density 12 / square mile: 0.0048 encounters per mile (day), 0.048 encounters per mile (night)
- General Density 3 / square mile: 0.0012 encounters per mile (day), 0.012 encounters per mile (night)
- Between two villages of aprox 120, General Pop Density 120: (120+120)/(0.56+0.56)/2000=0.107 encounters per mile
- Between two villages of aprox 120, General Pop Density 30: 0.51 encounters per mile
- Between two villages of aprox 120, General Pop Density 12: 0.032 encounters per mile
- Between two villages of aprox 120, General Pop Density 3: 0.014 encounters per mile
- Between a Village of aprox 120 and a Town of 1200, General Pop Density 120: 1.18 encounters per mile
- Between a Village of aprox 120 and a Town of 1200, General Pop Density 30: 0.14 encounters per mile
- Between a Village of aprox 120 and a Town of 1200, General Pop Density 12: 0.085 encounters per mile
- Between a Village of aprox 120 and a Town of 1200, General Pop Density 3: 0.036 encounters per mile
- Between a Village of aprox 120 and a Small City of 12,000, General Pop Density 120: 0.95 encounters per mile
- Between a Village of aprox 120 and a Small City of 12,000, General Pop Density 30: 0.48 encounters per mile
- Between a Village of aprox 120 and a Small City of 12,000, General Pop Density 12: 0.3 encounters per mile
- Between a Village of aprox 120 and a Small City of 12,000, General Pop Density 3: 0.126 encounters per mile
- Between a Village of aprox 120 and a National Capital of 120,000, General Pop Density 120: 3.24 encounters per mile
- Between a Village of aprox 120 and a National Capital of 120,000, General Pop Density 30: 1.6 encounters per mile
- Between a Village of aprox 120 and a National Capital of 120,000, General Pop Density 12: 1 encounter per mile
- Between a Village of aprox 120 and a National Capital of 120,000, General Pop Density 3: 0.42 encounters per mile
- Between two towns of aprox 1200, General Pop Density 120: 0.333 encounters per mile
- Between two towns of aprox 1200, General Pop Density 30: 0.167 encounters per mile
- Between two towns of aprox 1200, General Pop Density 12: 0.102 encounters per mile
- Between two towns of aprox 1200, General Pop Density 3: 0.043 encounters per mile
- Between a Town of aprox 1200 and a Small City of 12,000, General Pop Density 120: 0.88 encounters per mile
- Between a Town of aprox 1200 and a Small City of 12,000, General Pop Density 30: 0.437 encounters per mile
- Between a Town of aprox 1200 and a Small City of 12,000, General Pop Density 12: 0.27 encounters per mile
- Between a Town of aprox 1200 and a Small City of 12,000, General Pop Density 3: 0.115 encounters per mile
- Between a Town of aprox 1200 and a National Capital of 120,000, General Pop Density 120: 3.06 encounters per mile
- Between a Town of aprox 1200 and a National Capital of 120,000, General Pop Density 30: 1.519 encounters per mile
- Between a Town of aprox 1200 and a National Capital of 120,000, General Pop Density 12: 0.935 encounters per mile
- Between a Town of aprox 1200 and a National Capital of 120,000, General Pop Density 3: 0.4 encounters per mile

NB: There should never be a direct connection between two small cities or two national capitals without towns and villages in between, so I haven’t bothered showing them.

### Forging An Encounter Table

We’re now ready to actually create the encounter table.

##### Populating the entries

I start by populating each entry in the table with an appropriate number of possible encounters. How many is an appropriate number? Divide the each Significant Encounter by the smallest of the values (ignoring any with a value of zero). In the case of our example, the smallest value appears to be the Pack Hunter chance of 0.001 per mile. Start at the bottom of the list and move backwards so that you get the encounters you most want to be on the table and then build an ecology around them.

I make notes about all the encounter types, even if the chance of a significant encounter is zero. The day the PCs realize that Minotaurs prefer regions with stone fruit is the day your campaign takes a massive step forward in verisimilitude, followed by another when the PCs start trying to guess their likely sources of danger based on the ecology that you have described.

With so many categories unrepresented in the Monster Manual, there is plenty of scope for using an appropriate reference book – or for getting a little creative. Plants, insects, and herbivores are your primary venues for creativity, though scavengers are underrepresented in the MM as well. I like to always include at least one “ringer” – a completely new creation – in each table. Sometimes these will be whimsical, and at other times, dangerous. Contemplate, for example, a leech which is 1/100th the size of the typical specimen – small enough to be a completely different species – that likes to crawl under fingernails to gorge itself. The larger it grows, the more tightly it gets wedged in under the nail (causing acute pain which can only be relieved by having a friend pull out the affected fingernail)…

At times, I will split a particular encounter category. For example, I might want to have two groups of pack-hunters out there, so I will assign each half the chance of the standard pack encounter.

Another tip: older editions of D&D list encounters by terrain. Use as a reference, double-checking as necessary with your edition.

##### Totaling the encounter chances

The next step is to add up all the individual encounter chances. In the case of our example, and selecting the general sentient population value as 0.48 (i.e. 12 per square mile) gives a total of 1.7514.

##### Divide 100 by the total

This is as straightforward as it sounds. In the case of our example, it gives 57.1, near enough.

##### First Draft

Multiply each individual chance by the result, rounding off as necessary. If the individual result is less than 0.75 after multiplying by the results of the previous step (57.1 in our example), set it aside with an asterisk – it has been selected to appear in a subtable. Note that there will usually be no need for a subtable, UNLESS the alternative approach presented below (which does away with rounding) is employed.

##### IF there is a subtable, Total the subtable entry values

Add up the (adjusted) encounter chance values for everything that came to less than 0.75 the first time around. Create a new entry in the main table that reads “roll on the subtable” and assign it the total that you have calculated from all these leftovers. Round the total up or down as necessary UNLESS the alternative given below is employed.

##### Recalculate the main table

Using the rounded values, and including any “lump sum” pointing at a subtable, get a new total, divide 100 by the result, multiply each value by *that* result, and round off as necessary. Then tweak until the total comes to 100% – or, if the value is a little under 100%, include the gap as a “roll again” entry at the bottom of the table.

##### An Alternative

A more precise alternative is to include any fractions from the first set of entries in the subtable. In other words, if you got a value of 5.4, instead of rounding off to 5, you would cut the 5.4 into two parts: a 5% entry in the main table and a 0.4% entry in the subtable. If you have a lot of entries in your encounter table, this can actually be less work, so consider the possibility.

##### Optional Step

I like to list the entries in order of largest to smallest. It makes the resulting table easier to read.

##### Optional Step 2

Any encounter with a chance of more than 20% should be subdivided. In theory, any encounter entry with a chance of more than 10% should probably be divided, but 20% is good enough.

##### Convert into a d% table

If you have total chances of 18,15,14,11,9,7,7,5,5,9 (subtable):

the first entry has a chance of 01 to 18%. The second entry has a value of 18+1=19 to 18+15=33. The third entry has a chance of 33+1=34 to 33+14=47. And so on.

(I realize that this may be stating the obvious, but I’ve met at least one GM who didn’t know how to do it. For everyone, there is a first time.)

##### Generate the subtable

Using exactly the same steps but only those encounter entries that have been allocated to the subtable. Continue until all encounters have an entry in the table or in a subtable.

##### Playing with the numbers

Splitting an entry in an encounter table presents all sorts of opportunities. You can adjust relative chances for one species over another, for example, to reflect differences in population levels, or attitude. A pack of Veloceraptors would probably get a bigger share of the encounter chance than a pack of wolves of equal size. You can subdivide sentient encounters to reflect social values. You can increase the number of creatures of any given type encountered by a proportionate reduction in chance – so twice as many in an encounter = half the chance. Using this same principle, you can include entries for “double the normal number” or “eight times the normal number” – useful when talking about Orcs (a lone hunter, a scouting party, a hunting party, a war unit, a tribal raiding force – each double the number and half the chance of the previous one). You can do a separate table for night encounters, or just a different set of numbers on the one table – or simply a notation that at night, an encounter of “X” should be read as an encounter of “Y”.

Get creative. It’s *your* table, after all.

### Chance Of An Encounter

There’s pronounced light at the end of the tunnel. Only one step remains: working out the chance of their being ANY encounter. The table created in the previous step tells you *what* is encountered, *when* an encounter takes place. I used to use the second almost exclusively, now I use the first for all daylight hours and the second for party rest stops and night encampments.

##### By distance

The total we got when first adding up the chances of individual encounters is the overall % chance per mile of an encounter. So to determine the chance by distance, simply pick your distance and multiply by that number of miles. distance can be the separation between two points on the map; it can be the range limit of this particular table (because there is a terrain chance); it can be the distance between two settlements or the distance between two landmarks. Using rivers as boundaries can be especially useful.

##### By time

Convert the time into a distance describing how far the party can walk in a given period. One hour, Two hours, Four hours, six hours, eight hours, twelve hours – these are all valid choices. This can be especially useful if you employ the concept of “virtual movement” to cover time spent camping somewhere. For increased sophistication, use daylight minus 1, 1.5, or 2 hours – so that the value is how much daylight is left after breaking camp and setting up camp at the end of the day. A minute or two’s difference per day might not be all that noticeable, but the knowledge that you are tracking things that accurately can give you a lot of confidence.

### The Reset Button

Every time there is an encounter, reset the chance of an encounter at zero. You don’t have to recalculate the chance if you’re using the “by time” method, but will have to do so if using the “by distance” approach because you will want to know what the chances are for an encounter within the remaining distance to the end-point you have chosen.

### One More Thintg…

I almost forgot to mention one of the coolest things about this approach: Once you have set it up for a given patch of terrain, you can reuse the basic calculations for all matching terrain while still customising the individual entries in your encounter table! That’s because this is, fundamentally, an ecology-based system for deriving encounters, and while different occupants may be found in different ecological niches, those general niches will remain the same. What’s more, the approach is deliberately universal in design – one pack totalling X kilogrammes can replace another without a problem.

Whew! 11,900+ words, but I got there in the end. Next time, in Encounters With Meaning, I’ll go into encounter tables for Urban Settings, Dungeon settings, and talk about ways of integrating Wandering Monster encounters into your plotlines, infusing them with meaning. There

isa reason I labeled the encounters in the table as “significant” encounters…

- Creating ecology-based random encounters: The Philosophy of meanderings
- Creating ecology-based random encounters: This Eats That
- Creating ecology-based random encounters: Encounters with meaning

April 13th, 2013 at 4:43 pm

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