A fresh look at an established model in ecology has generated insights into how species coexist with each other. But it has also raised a vexed question: what constitutes the ecological identity of species?
Classical ecology discovered the principle of competitive exclusion — or, more pithily, ‘one species, one niche’. In order to coexist, species must have their own individual way to make a living, otherwise the superior competitors would exclude the inferior. Niches might pre-exist: for example, if there are two types of seed in the environment, this provides two niches for specialist seed-eating birds. Or niches might be created by a species evolving into openings in the ‘marketplace’ of their ecology.
As they report in Proceedings of the National Academy of Sciences, Scheffer and van Nes1 have revisited a well-studied classical model of competing species and discovered something new. Even in the absence of any environmental discontinuities, they find that assemblages of species will self-organize into clumps of species with very similar niches within a clump and a large difference between clumps. So, paradoxically, species both do, and do not, organize themselves into discrete niches.
In the Origin of Species, Darwin asked: “Why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?” This evolutionary question has a closely related ecological counterpart: how similar can species be to one another and still coexist? A well-studied model for this problem is outlined in Figure 1. The question becomes this. With a single, continuous niche axis, how many species can you pack along it? Or, is there a limit to how close the species can be along this axis?
Although some confusion persists about the answer, the canonical result2,3 is that there are, indeed, limits to similarity, and coexistence is possible only for species spaced out along the axis. In other words, we will observe species occupying distinct, spaced niches even in the absence of environmental discontinuities. This has been analytically proven for an even more general class of models than those studied in the classical period4. So this aspect of the results of Scheffer and van Nes1 is not new. Rather, the novelty of their results is as follows.
Previous analytical results produced single species widely spaced along the niche axis. But Scheffer and van Nes find widely spaced clumps of species occupying very similar niches. Why the difference? Analytical work looks at the long-term equilibria of models, whereas a simulation study allows the system to be observed as it moves towards these equilibria. Scheffer and van Nes take the simulation approach, which starts out with a large number of species along the axis and then evolves the system according to standard equations that govern competition between species. The clumps they observe are transient, and each will ultimately be thinned out to a single species. But ‘ultimately’ can be a very long time indeed: we now know that transient phenomena can be very long-lasting and, hence, important in ecology5, and such phenomena can be studied effectively only by simulation. There is also good experimental evidence for long-lasting coexistence between similar species3.
Why clumps, as opposed, for example, to a slowly thinning, uniform cloud of species along the niche axis? Consider Figure 1. In a community consisting of species A and C, where would a third species be most likely to persist successfully? Species B, positioned halfway between A and C, would be competing strongly against two species, whereas if it was in the same location as A or C it would be competing with only one. In the second case, one species would ultimately exclude the other, but — as pointed out above — this will be on a very long timescale. Of course, this argument depends on the width of the utilization distribution curves shown in Figure 1, and the distance between A and C. But this is an aspect of the self-organization of the species: they move into positions such that the void between them is an inhospitable competitive environment.
The emergence of clumps of highly similar species resonates with a proposed solution to another possible problem: the coexistence of large numbers of species in environments that do not seem to allow for much niche differentiation. Plankton and tropical forest plants are the usual examples. These organisms have a simple set of requirements: light, carbon dioxide and a few nutrients. How is it possible to carve out thousands of distinct niches from so few requirements? It has been proposed that such high numbers of species can coexist precisely because their niches are so similar that exclusion takes a very long time, perhaps on the same timescale as speciation6,7,8.
So much for the theory: what about the data? Scheffer and van Nes present frequency histograms of species' body sizes for three data sets, which they claim show discrete clumps of species along a niche axis. For these data, body size is the niche axis — body size being the single most important variable determining a species' life history. But whereas two of their examples (aquatic beetles and phytoplankton) seem to occur in discrete clumps of species with similar body sizes, a third (American prairie birds) does not: to our eyes, these bird species look smeared out along the axis with little profound clumping. This impression illustrates the difficulty inherent in making objective judgements about clumping based simply on visual inspection of frequency histograms9. Statistical techniques have been developed that do, in fact, identify significant clumping among prairie birds as well as other species9. This leaves the extent of overlap between statistical and ecological significance as an interesting and open question.
We can go further: on what basis did Darwin make his assertion about the discreteness of species? This question is distinct from debates about the definition of species in nature. Blackberries reproduce asexually, and it is impossible to agree on how many ‘species’ there are; but, nonetheless, we all know a ‘blackberry’ when we see one and do not wonder if it is actually a raspberry. Great tits, blue tits and coal tits are all quite distinct when considered as a set, but are surely just more-or-less continuous variants on a tit theme when compared with flamingos. Bacteria that are vastly different genetically are all called Legionella because they clump along the single niche axis that matters to us: they all cause Legionnaire's disease.
So what is the correct or meaningful frame of reference when thinking about the ecological nature of species? As well as providing stimulating theoretical results, Scheffer and van Nes1 have revitalized the fundamental question of how we should look at the ecological identity of species.
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Theoretical Ecology (2009)