Species and ecosystems

The importance of the identity of the species forming an ecosystem is far from a resolved issue. In certain respects the exact identities of the species matter little because they all perform the same functions: plants photosynthesise, whilst fungi and bacteria decompose organic matter in the soil. In other respects the identity of the species is crucial because only certain species can perform specific and important functions. Examples of such species, keystone species, are nitrogen fixers and nitrifiers. The stability, whatever is implied in that term, may also depend on the identity and diversity of species.

Concepts of stability

An ecosystem can be stable or unstable with respect to disturbances in several senses. We will discuss the most important ones. A convenient analogue for understanding ecological stability is a ball in a landscape with hills and valleys (Figure 8.1). Note that the terminology in this area is sometimes confusing and different schools may use different terms for the same property or the same term for different properties (Peterson et al. 1998). First of all, the question is whether an ecosystem returns to the same state as before a disturbance. The ability to recover after a disturbance is often referred to as resilience (this term is sometimes used for the rate of return and then called engineering resilience as opposed to ecological resilience as used here). A more technical way of expressing resilience is to look at the range over which the system can be disturbed and still return to its initial state, its basin of attraction.