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“The problem that is presented by the phytoplankton is essentially how it is possible for a number of species to coexist in a relatively isotropic or unstructured environment all competing for the same sorts of materials.” –G.E. Hutchinson, 1961

In this one sentence, which he calls the paradox of the plankton, G.E. Hutchinson has captured the crux of the problem that has perplexed community ecologists for nearly a century: why does diversity exist in nature? Why is everything not just replaced by one or two species that are really good at what they do?

The above statement was inspired when Hutchinson noticed that nearby temperate lakes harbored a large number of phytoplankton species, but was having trouble reconciling this observation with the emerging concept of competitive exclusion. Competition theory—at least at that time—predicted that when a group of species all compete for the same resource, eventually the most effective competitor dominates the resource and drives all the other species in the system to extinction. In relatively stable systems with few resources (such as Hutchinson’s lakes), competitive exclusion was expected to be especially swift, yet here were dozens of coexisting phytoplankton species.

Hutchinson came up with a few potential solutions to this paradox. He rejected the idea that different species capitalized on different microhabitats created by variation in physical conditions—depth, for instance–on the basis that these microhabitats were likely not varied enough to support as many species as he observed. He went on to suggest that perhaps some plankton existed in a symbiotic relationship, with one inferior competitor providing essential vitamins for the superior competitor, and then briefly touched on the idea that predators can disrupt competition between two species enough to promote coexistence (hinting at correspondence that would form the basis for Robert Paine’s influential paper on keystone predation five years later, highlighted in this blog post).

But Hutchinson’s greatest and most enduring insight is the idea that different species are favored under different sets of environmental conditions, and if the environment changes sufficiently through time, no single competitor could remain superior long enough to exclude other species. He goes on to provide a few different scenarios under which competitive exclusion would or would not be expected to occur, based on the time it takes for one species to exclude another, and the time it takes for the environment to shift from one set of conditions to another. The idea that diversity is prevented from reaching an equilibrium by environmental fluctuations can be found throughout many concepts in community ecology, particularly the intermediate disturbance hypothesis initially proposed by Joseph Connell in the late 1970s, which suggests that diversity is maximal under medium levels of environmental fluctuation (later extended to other kinds of disturbance, such as herbivory and predation).

Since Hutchinson’s paper, experimental evidence and theoretical models have generally supported disturbance-mediated coexistence, as long as species differ in their susceptibility to different environmental conditions (a tacit but key assumption of Hutchinson’s original argument). In particular, some excellent work by Descamps-Julien and Gonzalez demonstrated experimentally what Hutchinson postulated some 50 years prior. They utilized two species of freshwater diatoms, one of which demonstrated superiority under low temperatures, the other under high temperatures. When they put both together in experimental microcosms under constant temperature, one or the other came to dominate (depending on the temperature). However, when they put both together and oscillated the temperature between high and low, both species were able to persist at moderate abundances. But additional experimental evidence also suggests that this isn’t the whole story. Some interesting experiments have asked what happens under the opposite scenario: when the environment is fixed in a constant state for long periods of time. Heerkloss and Klinkenberg stocked experimental microcosms with 20 different species of phytoplankton and kept the environmental conditions constant for 10 years. They found large fluctuations in dominance and relative abundances during this time, even in the absence of all external disturbances. While the mechanisms underlying these trends have not yet been identified, it does raise the idea that biological, in addition to environmental, interactions among large assemblages of species somehow lead to non-equilibrium community states.

For me, this paper presents an approachable example that makes you consider the role of environment in structuring communities, and the open-endedness of the questions makes you consider your own systems: after all, this paper presents testable questions over concrete conclusions. And by presenting testable questions, Hutchinson helped ignite the transition from observing nature to explaining it, and has fueled a large body of both theoretical and experimental work attempting to explain why we see so much diversity in nature.

References:

Descamps-Julien, B. and A. Gonzalez. 2005. Stable coexistence in a fluctuating environment: an experimental demonstration. Ecology 86(10): 2815-2824.

Heerkloss, R. and G. Klinkenberg. 1998. A long-term series of a planktonic foodweb: a case of chaotic dynamics. Verg. Int. Verg. Limnology. 26: 1952-1956.

February 2, 2013