Monsters and Ecology

[Yair]

I'm thinking of homebrewing a new setting, and I am starting with trying to come up with an ecology that almost makes sense while keeping to the core's "Environment" monster entires.
Can anyone help me out with how many predators to how much prey, how much herbivores can an area type (Temperate Forests, Cold Sarmp, or so on) support, and - critically - a decent Excel or similar database that can give me the monsters per environment entry?

 

[jgbrowning]

I tried to do this with A Magical Society: Ecology and Culture but quicky abandoned it. There's simply too many variables to make a chart that has any real worth. For example, a tribe of lions could live on as little as 10 sq miles or as much as 80-100. Such a large range depends on the carrying capacity of the soil.

So, I'd just guess and do a bit of self-education to try and make an educated guess.

joe b.

 

[Yair]

So, I'd just guess and do a bit of self-education to try and make an educated guess.

See, it's the self-education I'm trying to get around by asking for other people's education's fruits

I am trying to come up with a ration of predators/prey, taking Size Categories into account as mutliples of x4, and assign arbitrary but reasonable numbers for the soil's productivity of base herbivores for each Environment area. (Or land's productivity for magicvores, same thing.)

I'll be taking another look at Ecology and Culture when I get home.

Yair

 

[jgbrowning]

See, it's the self-education I'm trying to get around by asking for other people's education's fruits

Doh! I failed my Sense Motive check.

I am trying to come up with a ration of predators/prey, taking Size Categories into account as mutliples of x4, and assign arbitrary but reasonable numbers for the soil's productivity of base herbivores for each Environment area. (Or land's productivity for magicvores, same thing.)

I'm fuzzy here but... I think mammals to reptile predation is something like for every one mammal predator there can be 10-20 reptilian predators. Reptiles are much, much, more efficient than mammals in this regard.

I'll be taking another look at Ecology and Culture when I get home.

Cool. I really wanted to include something like you're asking for, but I simply couldn't make anything of value work.

joe b.

 

[Yair]

In an attempt to come to terms with some sensible ecology, I took advantage of my university’s library and skimmed Ecology, 83(11), 2002, pp. 3003-3013 (The Effect of Prey and Predator Densities on Wolf Populations, John A. Vucetich, Rolf O. Peterson, and Carrie L. Schaefer). The paper contrasts various ecological models with hard data, obtained in from 1971 to 2001 at the Isle Royale National Park (Michigan, USA), a 544 sq.km. island in Lake Superior.

The data clearly shows that a constant kill rate (linear prey dependent model) is totally off, but even more complex models don’t really capture the process. Nevertheless, some useful values can be determined.
In the 1980’s the population dropped rapidly from 50 wolves to around 12. Focusing on the healthier more populated period, it is clear that kill rate remained mostly at around 0.5 kills per wolf per month (although it varied significantly). More importantly, we have around 40 to 50 wolves (74 to 92 per 1000 sq. km.), and about 1000 moose (1.8 per sq. km), a ratio of about 4.5%. The article notes that the minimum amount of moose required to support a minimal (4-strong) pack is somehwere between 82 and 163, a ratio of 2.5 to 4.9%; of the same scale, speaking as a physicist.

In D&D terms, a wolf is a Medium animal, and while the SRD does not contain moose but Silver Marches lists Elk as Large. I am sure that the numbers can vary wildly, but under typical conditions it would seem a ratio of about 4.5% Medium predators to Large prey (in biomass) is appropriate, which implies a ratio of 18% predators to prey of equal Size. A ratio of less than 10% should be explained by magic, and one in excess of 40% would mean a very inefficient predator (which could be made much mor plausible with an active scavenger population).
It would also make sense to have something like 2 Large sized herbivores per square kilometer, multiplied a few times surely for more esoteric herbivores in the area.
This is in Michigan, and the island seems to be a fairly typical Temperate Forest environemt. I haven’t considered how to modify all these numbers for other environments. Yet.

 

[JamesDJarvis]

I'm fuzzy here but... I think mammals to reptile predation is something like for every one mammal predator there can be 10-20 reptilian predators. Reptiles are much, much, more efficient than mammals in this regard.

Actually reptiles need to eat much less then a mammal of the same relative size. A reptile needs to eat about a 1/10 th of what a mammal does because reptiles are cold blooded ( the 1/10 is very loose and not exact). A mamal however is more capable of surviving missed chances despite the higher nutritional requirements because mammals are capable of maintaining states of high activity longer then reptiles.

 

[Tonguez]

In D&D terms, a wolf is a Medium animal, and while the SRD does not contain moose but Silver Marches lists Elk as Large. I am sure that the numbers can vary wildly, but under typical conditions it would seem a ratio of about 4.5% Medium predators to Large prey (in biomass) is appropriate, which implies a ratio of 18% predators to prey of equal Size. A ratio of less than 10% should be explained by magic, and one in excess of 40% would mean a very inefficient predator (which could be made much mor plausible with an active scavenger population).
It would also make sense to have something like 2 Large sized herbivores per square kilometer, multiplied a few times surely for more esoteric herbivores in the area.
This is in Michigan, and the island seems to be a fairly typical Temperate Forest environemt. I haven’t considered how to modify all these numbers for other environments. Yet.

This Site looks at prey/predator ratio's amongst Dinosaurs and suggests that large dinosaur predators (and Sabre tooth cats) had a ratio of 3.5% to 5%, going to less than 1% in sub-optimal conditions (like Creatceous Mongolia). So we can probably presume that this range is standard across most mammals with a 3.5% ratio being average.

So taking a 3.5% ratio for Temperate environments I'd then assign different values to the other environments (eg Desert, Arctic 1%, Tropic 5%). I'd also presume that unusually large predators (like Griffons, Wyverns and of course Dragons have Reptile-like metabolisms and thus low ratios ie 1/10 of mammals)

 

[jgbrowning]

Actually reptiles need to eat much less then a mammal of the same relative size. A reptile needs to eat about a 1/10 th of what a mammal does because reptiles are cold blooded ( the 1/10 is very loose and not exact). A mamal however is more capable of surviving missed chances despite the higher nutritional requirements because mammals are capable of maintaining states of high activity longer then reptiles.

Wouldn't that then mean that roughly 10-20 reptile predators could exist on the same amount of food as 1 mamal predator?

joe b.

 

[Enamel_32]

jgbrowning: Yes, but being cold-blooded means they're less likely to adapt to sudden environmental changes.

 

[Algolei]

...while the SRD does not contain moose but Silver Marches lists Elk as Large.

Trust me, moose are Large.

--MyMotherWasAMoose Spikey.

P.S. Ooh! I think I found that website she posed for!

http://www.smouse.force9.co.uk/facts.htm

Moose Statistics and Appearance

Life span: 15-25 years
Weight: 550-700 kg (1200-1500 lbs)
Body length: 2.5-2.7 metres [edit: I think that's about 98.4-106.272 inches]

Alas, they seem to have replaced her photo with a hand-drawn image.