Estimates of Minimum Patch Size Depend on the Method of Estimation and the Condition of the Habitat

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density–area relationship, extinction threshold, Florida, USA, gopher tortoise, Gopherus polyphemus, habitat loss, habitat quality, minimum viable population size, patch context, patch size, scaling

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Minimum patch size for a viable population can be estimated in several ways. The density–area method estimates minimum patch size as the smallest area in which no new individuals are encountered as one extends the arbitrary boundaries of a study area outward. The density–area method eliminates the assumption of no variation in density with size of habitat area that accompanies other methods, but it is untested in situations in which habitat loss has confined populations to small areas. We used a variant of the density–area method to study the minimum patch size for the gopher tortoise (Gopherus polyphemus) in Florida, USA, where this keystone species is being confined to ever smaller habitat fragments. The variant was based on the premise that individuals within populations are likely to occur at unusually high densities when confined to small areas, and it estimated minimum patch size as the smallest area beyond which density plateaus. The data for our study came from detailed surveys of 38 populations of the tortoise. For all 38 populations, the areas occupied were determined empirically, and for 19 of them, duplicate surveys were undertaken about a decade apart. We found that a consistent inverse density–area relationship was present over smaller areas. The minimum patch size estimated from the density–area relationship was at least 100 ha, which is substantially larger than previous estimates. The relative abundance of juveniles was inversely related to population density for sites with relatively poor habitat quality, indicating that the estimated minimum patch size could represent an extinction threshold. We concluded that a negative density–area relationship may be an inevitable consequence of excessive habitat loss. We also concluded that any detrimental effects of an inverse density–area relationship may be exacerbated by the deterioration in habitat quality that often accompanies habitat loss. Finally, we concluded that the value of any estimate of minimum patch size as a conservation tool is compromised by excessive habitat loss.

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Ecology, v. 88, issue 6, p. 1401-1407