Coexistence and resource competition

Abstract

How large numbers of species coexist on a seemingly limited number of different resources is a classic problem in ecology¹, and attempts have been made to solve it experimentally. But we are not convinced that Huisman and Weissing's² proposal to add non-stationary dynamics in species abundance to the list of possible explanations offers any new insight into this biodiversity enigma.

Main

Early ideas were based on competitive exclusion, in which two (or more) species compete for shared resources but only the more proficient survives³, and the number of coexisting species, n, does not exceed the number of available resources, k. But these ideas based on competitive exclusion cannot explain how rich ensembles of species can coexist on a limited number of resources (that is, n>k), as in aquatic ecosystems.

A solution to this biodiversity problem has been suggested by Huisman and Weissing² in the form of a model demonstrating the possibility of coexistence of two or more competitors under conditions of fluctuating (periodic or chaotic) community dynamics, a finding central to contemporary ecological theory⁴. However, we believe that their solution to the n>k puzzle has long been known^{5,6,7,8,9,10,11,12,13}.

It was first inferred from a computer simulation over 25 years ago⁵ that two species can coexist on one biotic resource, with coexistence occurring along what appeared to be a periodic orbit, as was also shown by Huisman and Weissing². This was confirmed analytically and expanded to cover the more general case of n species coexisting on one biotic resource⁹. The fact that coexistence depends on the nonlinearity of the species-specific growth functions and on the lack of system equilibrium — the central issue discussed by Huisman and Weissing² — has also been demonstrated earlier⁸. The plankton paradox (whereby n>k) was thus resolved about two decades after it was first posed as a biodiversity problem.