A world of specialists

Specialists are people who always repeat the same mistakes.

Walter Gropius

Imagine that most typical of ecological entities, a community of coexisting species. The community may be characterized by variables, such as the number of species and the number or type of interactions. The next several chapters will examine how evolution affects these fundamental properties of communities. This chapter specifically examines the evolution of the breadth of use of resources that govern the size of an organism's ecological niche.

Some species are generalist, having in some sense a broad niche (large range of environmental tolerance, broad diet, etc.). Most species, however, are probably specialists in at least one sense. This is curious from an evolutionary perspective; we can imagine that a wide ecological niche would be extremely beneficial from a fitness perspective, allowing an organism to survive unexpected changes in its environment. The problem then is why so many species apparently forsake such benefits, leading to a great subdivision of resources and environments among species. The widespread evolution of ecological specialization is at least a partial answer to the question 'why are there so many different kinds of animals [and plants]' (Hutchinson 1959).

It is conceptually useful to distinguish two types of niche; the 'fundamental' niche was defined by Hutchinson as the set of environments in which a species could maintain a positive growth rate (i.e. where it could in theory persist). The smaller,'realized' niche is the subset of the fundamental niche actually occupied in nature. The fundamental niche is governed by an organisms' adaptation to the environment in terms of morphology and physiology. Some of the fundamental niche may then not be used, either due to constraints, such as dispersal ability, or because the organism displays plastic behavioural rejection of some habitats.

Understanding the evolution of the realized niche therefore demands that we (1) understand the extent of the fundamental niche through genetic variation for the preference and exploitation of different numbers of environments, and (2) understand adaptive decision-making (see Chapter 7) that narrows the range of environments actually utilized. We will first explore the very rich body of theory on these subjects before proceeding to the evidence.

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