Between Explanans and Explanandum: Biodiversity and the Unity of Theoretical Ecology

Abstract

Biodiversity is arguably a major topic in ecology. Some of the key questions of the discipline are: why are species distributed the way they are, in a given area, or across areas? Or: why are there so many animals (as G. Evelyn Hutchinson asked in a famous paper)? It appears as what is supposed to be explained, namely an explanandum of ecology. Various families of theories have been proposed, which are nowadays mostly distinguished according to the role they confer to competition and the competitive exclusion principle. Niche theories, where the difference between “fundamental” and “realised” niches (Hutchinson GE, Am Nat 93:145–159, 1959) through competitive exclusion explains species distributions, contrast with neutral theories, where an assumption of fitness equivalence, species abundance distributions are explained by stochastic models, inspired by (Hubbell SP, The unified neutral theory of biodiversity and biogeography. Princeton University Press, Princeton, 2001). Yet, while an important part of community ecology and biogeography understands biodiversity as an explanandum, in other areas of ecology the concept of biodiversity rather plays the role of the explanans. This is manifest in the long lasting stability-diversity debate, where the key question has been: how does diversity beget stability? Thus explanatory reversibility of the biodiversity concept in ecology may prevent biodiversity from being a unifying object for ecology. In this chapter, I will describe such reversible explanatory status of biodiversity in various ecological fields (biogeography, functional ecology, community ecology). After having considered diversity as an explanandum, and then as an explanans, I will show that the concepts of biodiversity that are used in each of these symmetrical explanatory projects are not identical nor even equivalent. Using an approach to the concept of biodiversity in terms of “conceptual space”, I will finally argue that the lack of unity of a biodiversity concept able to function identically as explanans and explanandum underlies the structural disunity of ecology that has been pointed out by some historians and philosophers.