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A simple, but spatially explicit, model of competition between two hypothetical species is presented. The two species exist as metapopulations, where habitat patches are characterised by local extinctions and colonizations. One species is a superior competitor and the other a superior disperser. The model determines the proportion of habitat patches that can be destroyed before the species become extinct, confirming the results of deterministic models. It further shows that the pattern of habitat destruction affects the minimum amount of habitat required to avoid extinction. If intensity of habitat loss occurs along a gradient, rather than at random, species persist until a much greater overall habitat loss has occurred. This is also the result if habitat is lost in blocks or in lines. The implication of this result for conservation policy is discussed.

The extension of the Levins model (1969) of a metapopulation to two competing species by Nee & May (1992) predicted that habitat destruction may actually result in an increase in abundance of 'weedy' species. The Nee & May model was not spatially explicit but their main observation was supported by the spatially explicit models of Dytham (1994) and Moilanen & Hanski (this issue). A further, spatially explicit analogue of the Nee & May model is presented here (Fig. 1) again showing that the inferior competitor will increase in absolute abundance as habitat is lost. This

In a recent issue of Journal of Animal Ecology, Nee & May (1992) presented a two-species model which described how a competitively inferior species would increase in both frequency and absolute numbers as habitat patches were destroyed. Their model was very simple: the inferior competitor, B, could only colonize patches which were empty, but not destroyed, while the superior competitor, A, could additionally colonize patches occupied by B, extinguishing B in the process. The species were assigned extinction rates, eA and eB, and colonization rates CA and CB. Three equations described the rate of change in number of patches which are either empty or occupied by A or B. Nee & May stated that an inferior competitor would persist if cB/eB > cA/eA and that neither species could persist when the proportion of habitable patches is below e/c. The Nee-May model outlined above describes a situation where there are a very large number of patches which are all equally accessible to colonizers and therefore takes no account of any spatial variation in metapopulations across the patchy landscape. Here I describe the results of a cellular analogue of the NeeMay model which does take into account the spatial variation in species distribution and shows both similarities and difference in results.

Keystone species and community effects of biological introductions planned introductions in biological control surface transport of micro-organisms soil and groundwater transport of micro-organisms aerial transport of bacteria physiochemical and biological factors affect the transfer of genetic information among micro-organisms genetic exchange and genetic stability in bacterial populations models for the population dynamics of transposable elements in bacteria fitness and gene stability quantitative approaches to questions about the spread of recombinant genes or recombinant organisms.