"What is the niche-space? The exact theory"Meszéna, GézaWhat is niche-space? The exact theory Géza Meszéna, György Barabás, Liz Pásztor The Hutchinsonian concept of niche-space, that is, that species partition an abstract space among themselves, had been widely regarded as the big picture of ecology. It lost this status through endless debates on interpretation and competitive exclusion. The Lotka-Volterra model, which was regarded as the mathematical underpinning, fell out of favor also, as something grossly oversimplifies reality. Since then, the prevailing view about ecology is that it is too complicated and heterogeneous to have a unifying theory. Accordingly, theoretical/mathematical ecology is mostly a huge collection of independent models. In contrast, the theory of natural selection is considered as the unified background for understanding evolution. Obviously, all the complications of ecology affect evolution, as well. Still, it is widely accepted that all these complications affect evolution through a single quantity, fitness, i.e., the Malthusian growth rate. Theory of evolution is unifying at this level, either at the genetic, or the phenotypic level. Specific cases are studied within this fixed framework. We suggest that the same type of abstraction unifies ecology also. For this, one has to complement Malthusian growth rates of the populations with the notion of regulating environmental variables that affect, and affected by, the populations. Perturbation analysis provides the mathematical condition for robust coexistence: species should be sufficiently different both in their effect on, and in their sensitivity towards these variables. Accordingly, niche space of Hutchinson must be identified with the index set of the regulating variables. Species partition this space for coexistence. As this analysis is based on linearization for small perturbations, it is fully general, without becoming intractable. There exists a clear connection between the top-level theory of niche-segregation and the theory of evolutionary branching in adaptive dynamics. This is in line with the concept of ecological speciation emerging from empirical studies. The top-level description can be concretized to the specific model description of any ecological situation. Specific cases for trophic networks and different kinds of spatiotemporal variabilities will be discussed. The main advantage of this approach is that it preserves the intuitive connection between the mathematical description and the biological intuition. It is hopeless to build a mathematical model of a rainforest of hundreds of tree species with sufficient quantitative fidelity to study coexistence and stability issues. Instead, if we understand qualitatively, what kinds of differentiation of the species is required for their coexistence (i.e., differentiation in their regulations), then we may be able to recognize the main “niche axes” in real ecosystems. Specific empirical examples will be discussed. |
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