Current species distributions have a strong historical signal. Darwin noted that the extinct fossilized species from a given locality were more closely related to contemporary species from the same locality rather than to, say, contemporary species living far away in a similar environment. The geographical locations of ancient adaptive radiations often can be inferred from the geographical distribution of the current descendants of those radiations, because those descendants often are most diverse and abundant where they or their ancestors originated. This sort of ‘inertia’ in the distributions of contemporary species and their ancestors is what I mean by a ‘historical signal’.
This historical signal has sometimes (often?) been taken to imply that community ecology is of minor importance, that the effects of local species interactions are too weak, or else too spatially and temporally localized, to generate a signal over large spatial and temporal extents.
I think this argument is both a mistake, and a missed opportunity. A mistake because it’s hard to see how the geographical distribution of species past and present could represent anything but the effects of community ecology ‘writ large’ (plus speciation, of course, and remember that speciation is often driven by community ecology). There are only four processes that can affect distribution and abundance of existing species: birth, death, immigration, and emigration. Individuals are born, die, and move around. That’s it. They don’t arise by spontaneous generation, or vanish into the ether. And the fate of a species is just the sum of the fates of all the individuals comprising it. A species can’t, say, go extinct while the individuals comprising it are giving birth faster than they’re dying, or shift its geographic range north while all the individuals comprising it move south. And what is community (and population) ecology but the study of the determinants and consequences of rates of birth, death, immigration, and emigration? So I literally don’t know what it means to say, for any reason, that community ecology could be ‘unimportant’, on any spatial or temporal scale.
The missed opportunity is to ask a question that, as far as I know, hasn’t been asked (please correct me in the comments if I’m wrong on this!) The question is the title of the post: Why doesn’t community ecology erase the signal of historical biogeography? Because it certainly could. Under the right conditions, species can expand, contract, or shift their geographical distributions very far, very fast, thereby obscuring or erasing historical signal. Think for instance of the rapid northward expansion of many species at the end of the last glaciation, or the rapid and vast spread of certain exotic species. But those kinds of history-erasing changes are fairly exceptional. Why? Why is community ecology (mostly) history preserving?
I think this is a really interesting question (is that just me?). And I don’t think the answer is obvious (is that just me?). I highly doubt the answer is as simple as ‘community ecology is history preserving when species interactions are weak’. So here’s a challenging but cool modeling project: Set up some kind of metacommunity model, say some habitat patches connected by dispersal, and seed it with some initial distribution of interacting species (say, a bunch of competitors). You probably want to be able to vary the spatial (and maybe temporal) heterogeneity of the environment. Maybe you have a speciation component too, or microevolution, or periodic invasions of exotic species. What features does the model have to have to have so that it is history preserving, so that species and their descendants tend to persist for lengthy periods of time (tens or hundreds of thousands of generations) only in the neighborhood of the patch where they originated?
The take-home message here is simple. It’s not that, because history matters, contemporary community ecology doesn’t. ‘History’, here, there, and everywhere in between, is nothing but community ecology, over and over and over again. The interesting question is why community ecology now, or recently, doesn’t routinely erase what was produced by community ecology way back when.
UPDATE: An amusing but probably apocryphal anecdote summarizes the previous paragraph. A prominent academic once gave a public lecture on astronomy, describing how the earth is a globe which revolves around the sun. After the lecture a little old lady approached him and said “This is all rubbish. The world is really a flat plate supported by the back of a giant turtle.” The academic smiled indulgently at the old lady and asked “But what is the turtle standing on?” The old lady replied “You are very, very clever, young man–but it’s turtles all the way down!” Similarly, my claim is that it’s community ecology all the way down.
p.s. Before any evolutionarily-oriented readers get upset with me, I’ve ignored microevolution (selection, drift, mutation, migration) just because the post was easier to write that way. But really, the question is why community ecology and microevolution now don’t routinely erase what was produced by community ecology and microevolution way back when.