Ecological and evolutionary systems–populations, communities, ecosystems, species, food webs, whatever–only occupy certain regions of parameter space. That is, the systems we observe are only a small subset of those which could possibly exist. Individuals organisms may live for hours or years or even a century or more, but they don’t live for millenia. There are tens of millions of species, not dozens or trillions, and those species typically persist for millions of years, not a century or a million centuries. Food webs have connectances ranging from about 0.03-0.3, not 0.001 or 0.99. Most populations exhibit negative density dependence, but less than 30% of populations exhibit regular cycles. And so on.

So why do ecological and evolutionary systems occupy the regions of parameter space they do? This isn’t a question that often gets asked (but see Joe Felsenstein’s classic article on why there are so few kinds of species, compared to what we might have expected). For instance, in my own field of community ecology, analysis of theoretical models is typically concerned with identifying the regions of parameter space that allow stable coexistence. But why should nature occupy any particular region of parameter space? After all, nature may abhor a vacuum, but why should it abhor lack of stable coexistence, or a food web with connectance >0.3, or whatever?

I can think of several possible answers:

No reason. It just so happens that nature occupies certain regions of parameter space. If there are any reasons why particular systems occupy particular regions of parameter space, those reasons are idiosyncratic and system-specific. Fundamental physical constraints. We only observe certain kinds of systems because those are the only ones consistent with the constraints imposed by the laws of fundamental physics. Historical constraints. We only observe certain kinds of systems because those are the only ones consistent with the constraints imposed by past historical events. Natural selection. We only observe certain kinds of systems because natural selection works; others would only exist if species were highly maladapted. The anthropic principle. Systems that occupied other regions of parameter space wouldn’t exist long enough to be observed, or else wouldn’t be consistent with human life (so that we wouldn’t be around to observe them).

#1 surely has something to it. After all, the regions of parameter space occupied by ecological systems clearly can change (e.g., due to things like climate change, anthropogenic impacts, and mass extinction events), so perhaps there’s nothing special about the regions of parameter space nature happens to occupy at the moment. But I doubt that’s the whole story. There’s never been, and never will be, a time when species lifetimes were measured in centuries, or when food webs had connectances of 0.99.

At the risk of offending any biomechanicists reading this, I suspect #2 is largely irrelevant. I don’t think very many features of ecological and evolutionary systems reflect constraints imposed by the laws of fundamental physics. What organism would move faster, except for the constraint imposed by the speed of light? What does fundamental physics have to do with the strength of density dependence? Etc. And less-fundamental physical constraints, while they absolutely exist, have a way of turning out not to be constraints at all over sufficiently long time scales. For instance, the evolution of multicellularity freed multicellular organisms from many of the physical constraints (e.g., diffusion-imposed constraints on body size) to which single cells are subject.

#3 is sort of a special case of #1. For instance, all organisms use nucleic acids as their genetic material. That’s a ‘frozen accident’ of history which probably isn’t going to change in future, and which may impose some sort of constraint on what’s evolutionarily possible. Or maybe not. As Francis Crick once said, “Evolution is cleverer than you are.” In general, whenever we claim that X is some kind of absolute constraint on evolution, we have be very careful that we’re not merely expressing the limits of our own imaginations.

I think #4 is a big one, not just in evolution (obviously!), but also in ecology. In the context of community ecology, I’ve lately been wondering if there might not be fairly general reasons to expect natural selection to promote ecological coexistence. That is, if you incorporate adaptive evolution into models of ecological coexistence, so that the model parameters can (co)evolve, are there reasons why you are likely to end up in a region of parameter space permitting coexistence? These reasons, if they exist, would of course have exceptions. After all, natural selection doesn’t actually ‘care’ about coexistence, it only ‘cares’ about relative fitness. Any coexistence you get due to selection is just a side effect from an evolutionary perspective. But perhaps it’s a particularly likely side effect. With the recent explosion of interest in ‘eco-evolutionary dynamics’, #4 is likely to be thoroughly explored in the near future.

#5 is the inspiration for the title of this post. In physics, the anthropic principle is the idea that observations of the universe must be compatible with the conscious life that observes it. No less than Alfred Russel Wallace prefigured a (strong) version of the anthropic principle in 1904 when he wrote,

“Such a vast and complex universe as that which we know exists around us, may have been absolutely required … in order to produce a world that should be precisely adapted in every detail for the orderly development of life culminating in man.”

By the ecological anthropic principle, I don’t just have in mind the idea that we wouldn’t be here to observe nature unless nature were a certain way. I’m also (actually, mostly) thinking of the fact that we can only observe those bits of nature that are sufficiently stable and persistent (so maybe a broader term like ‘selection bias’ would actually be better). It’s like the old joke in economics, about why you never find $10 bills on the ground (if they were there, someone would’ve picked them up by now). For instance, many food web theoreticians have asked whether observed topological features of natural food webs are such as to promote their stability, implicitly arguing that unstable webs should not persist long enough to be observed. Some population ecologists have made a similar argument to explain why most populations exhibit at least weak density dependence (if they didn’t, they’d be expected to go extinct much faster than they do). But I don’t think the ‘ecological anthropic principle’ has been systematically explored as an explanation for why nature is the way it is, rather than some other way. How strong a constraint does it impose on states of nature? What happens if it opposes natural selection (for instance, Laurence Mueller‘s studies of Drosophila find no tendency for selection to favor stable population dynamics)?

The take-home message here is that ecologists shouldn’t just worry about explaining how the world is. If we want an ultimate explanation for the way the world is, we should probably pay more attention to the way it isn’t.