Paradox of the clumps

Sean Nee and Nick Colegrave comment on the Scheffer and van Ness PNAS paper on the formation and persistence of ecological lumps (see the earlier post by Buzz Holling, and the commentary Discontinuities in ecological data by Craig Allen in PNAS).

Their commentary, Paradox of the clumps (Nature May 25, 2006) suggest that ecological clumpiness of species may change how we think about species. They write:

…Scheffer and van Nes 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? … 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?

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 ecology, and such phenomena can be studied effectively only by simulation. There is also good experimental evidence for long-lasting coexistence between similar species.

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 speciation.

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 Nes have revitalized the fundamental question of how we should look at the ecological identity of species.

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