Species pools and the fallacy of composition (UPDATED)

A species pool is the set of all species that could potentially colonize some local site. Many important ideas in community ecology and biogeography start from assumptions about the species pool, and about dispersal from the species pool to the local site, and from those assumptions derive the predicted consequences for properties of the ecological community occupying the local site. MacArthur and Wilson’s theory of island biogeography is the classic example. Other examples include studies of the relationship between local and regional species richness, Hubbell’s neutral theory and modifications thereof, and almost any study of local community structure which asks whether the species occupying a local site comprise a non-random subset of some larger pool of species (e.g., non-random with respect to their phylogenetic relatedness, or with respect to their phenotypic traits).

The notion of a species pool is valuable in calling attention to the fact that local communities aren’t closed systems, and that the species occupying any local site typically came from somewhere else. And if your main focus is on what happens locally, it’s natural to just take “somewhere else” as a given, and call it “the species pool”.

But just because this move is natural doesn’t make it right. Not that it’s necessarily wrong, either. But at best, any explanation of local community structure which appeals to the properties of the species pool is incomplete. Not because it leaves the properties of the species pool itself unexplained (every scientific explanation takes something for granted). But because, in any complete explanation of local community structure, the properties of the species pool won’t be exogenously determined. There’s nowhere on earth that’s external to community ecology; every population of every species is and always has been part of some local community somewhere. Species abundances, always and everywhere, are determined by birth, death, and dispersal: it’s “community ecology all the way down”. So a complete, ultimate explanation for local community structure wouldn’t involve an external species pool at all, or if it did the “species pool” would simply be the sum of the species present at all the localities. The species pool would be a dependent rather than independent variable, emerging as the aggregate outcome of local scale processes of demography and dispersal within and among many local sites.

Appeals to properties of the “species pool” as an ultimate explanation for local community structure suffer from the fallacy of composition. This is the fallacy of thinking that something which is true of some part of a whole must (or even can) be true of the whole. For instance, in economics, an individual who wants to accumulate more savings can do so by cutting back on his or her spending. But if everyone tries to do this at the same time, the result is a recession (the “paradox of thrift“): in the limit, if no one spends money on anything, the entire economy grinds to a halt. As a second economic example, an individual nation can attempt to grow its economy by devaluing its currency in order to make its exports cheaper. But it’s impossible for every nation to simultaneously devalue its currency relative to that of every other nation, or for every nation to be a net exporter.

The fallacy of composition is especially tempting when any given part of the whole is only one of many. Any given local community is a small part of the world, and might receive colonists from a very large area. And the collective dynamics of all the local communities in that large area are not much affected by what happens in any one locality within that area. But just because regional dynamics are independent of what happens in any one locality does not mean they are independent of what happens in all localities. To think otherwise is to commit the fallacy of composition.

The issue I’m raising may seem obvious, and I doubt any ecologist would actually deny what I’ve written above. But in practice, I think we’re so used to taking the notion of a “species pool” for granted that it steers our research efforts without us even realizing that the steering is happening. For instance, if you think of the “regional species pool” as exogenous to the local communities within the region, then it’s natural to do things like treat local species richness as a “dependent” variable which can be regressed on an “independent” variable like regional species richness. Which means you probably don’t even ask about how the resulting local-regional richness relationship would behave if, as is actually the case, the “region” was just a bunch of local communities linked by dispersal (a metacommunity). Which means you fail to ask questions about the joint determinants of local and regional richness, and the (epiphenomenological) local-regional richness relationships to which they give rise (e.g., Shurin and Allen 2001).

I’ve been surprised that the recent surge of interest in metacommunities hasn’t lead to a major shift in research effort, away from lines of research which depend on assuming exogenous species pools. I’m not sure why that shift hasn’t happened.

UPDATE: Ace macroecologist Brian McGill pops up in the comments with some sensible pushback. I was going to reply to him at length–when I realized that I’d already done so, months ago! See this old post, which I’d forgotten about–and which the present post basically just rephrases. I find this slightly embarrassing, to be accidentally repeating myself after so short a time. Especially since I like the old version better!