Determining Minimum Habitat Requirements in Theory and Practice

A central problem in conservation biology is how to determine the amount of suitable habitat required for a population to persist. The critical minimum is what Lande (1987) terms the 'extinction threshold'. Lawton et al. (1994) and Nee (1994) have pointed out the fundamental equivalence of this quantity to the 'eradication threshold' used in epidemiology (Anderson and May 1991). The eradication threshold defines the maximum fraction of a susceptible population that an immunisation programme can afford to miss, while still succeeding in eradicating the disease. This has been shown to equal in magnitude the uninfected fraction of the susceptible population; in other words, it takes the same value as the unused amount of the disease's limiting resource (Anderson and May 1991). For a conservation biologist, the eradication threshold would define the smallest amount of suitable habitat or limiting resource that can sustain a population of animals. Lawton et al. (1994) give the example of a metapopulation consisting of local populations distributed among discrete patches. The metapopulation is at equilibrium when a local population colonises just one other patch before going extinct. Not all patches are occupied at equilibrium. This scenario parallels the epidemiological case of a human population of which some fraction are not immune to the disease and are either carrying it or susceptible to it. Some fraction, h, of the patches are habitable, and a fraction x* among these are unoccupied at equilibrium though susceptible to colonisation. By the simple device of presenting the equilibrium fraction of occupied patches as h x*, it follows that this reaches zero (extinction of the metapopulation) when h equals x*. In other words, the minimum fraction of patches required to avoid extinction, h, is simply x*, the unoccupied but susceptible fraction at equilibrium. Nee (1994) likewise demonstrates how the eradication threshold of a predator is equal in magnitude to the uneaten fraction of limiting prey. A population well buffered against extinction is thus one that occupies the majority of habitable patches; or it is one that maintains the equilibrium density of limiting prey well below what it would be in the absence of predation. Conversely, a population that is at risk of extinction in the event of a small reduction in habitable patches or prey is one whose members occupy few of the habitable patches at equilibrium, or make little impact on the density of their prey. Such situations can arise if the habitat is highly fragmented and dispersal incurs a substantial mortality risk. Equally a predator might make little impact on prey density if the prey have effective defences against predation (they are present in numbers, but difficult to catch). It is interesting to note that highly disturbed environments are likely to be inhabited by species that use the majority of suitable habitat, whereas historically undisturbed environments will support species that are less efficient in this respect. Knowledge of the unused fraction thus provides a way of indexing environmental disturbance. Most of the species-specific details that are commonly sought in conservation studies, such as migration rates between patches, foraging efficiency for limiting prey, birth rates, death rates, etc. are superfluous to this estimate of minimum habitat requirements (Nee 1994). This conclusion has obvious consequences for field research on rare species: the essential priority in conservation studies is to determine accurately what constitutes the limiting resource or the suitable habitat. Unfortunately, this is often the most difficult objective to realise in practice (see Caughley and Sinclair 1994 for examples). Several factors may combine to regulate population size (Sinclair 1989). Even if their combined influence points towards certain types of habitat as being more suitable than others, defining exactly what