The tails of two geckos tell the story of dispersal in a fragmented landscape.

The fragmentation of habitat is a major cause of biodiversity loss. However, while numerous studies have suggested that reducing the size of populations and isolating them on fragments leads ultimately to the extinction of a species (small isolated populations are extinction prone), the evidence has been rather conjectural. This is because dispersal is so difficult to measure and isolation difficult to confirm. In past studies, evidence that populations become small and isolated on fragments, leading to declines, has relied on spatial patterns of distribution and abundance. Thus, a species not trapped in the matrix in which fragments are embedded might be assumed isolated on fragments, and if low in abundance on fragments compared to continuous habitat is assumed to have declined on fragments due to this isolation. However, without accurately measuring the degree of isolation, it is difficult to distinguish the role of isolation from other important causes of population decline that are correlated with fragment and population size, such as habitat degradation. Developments in molecular techniques and statistical methods now make it possible to measure isolation. Refreshingly, in this issue Hoehn et al . analyse microsatellite DNA with a suite of statistical methods to show convincingly that a declining species of gecko suffers from greater isolation on habitat fragments than a contrasting gecko that is able to disperse between fragments and hence persist in the severely fragmented wheatbelt of Western Australia. The Western Australian wheatbelt is a region outstanding both for its endemic biodiversity, and for its degree of habitat fragmentation (7% of the original habitat remains) and history of extinctions (the region has already lost five other gecko species). It is the perfect setting to illustrate the power of modern genetic techniques in getting to the bottom of why one species is at risk of extinction in a fragmented landscape while another persists. Using DNA samples taken from the tails of two gecko species (Fig. 1), the study compares the reticulated velvet gecko ( Oedura reticulata ) whose populations are declining in the fragmented wheatbelt, to a more widespread gecko ( Gehra variegata ) that maintains persistent populations in the fragmented landscape. While the declining species is endemic to the southwest corner of Australia and is a habitat specialist, preferring the arboreal haven of smooth-barked Eucalyptus woodland, the persistent species is widespread in southern Australia, and is a habitat generalist, although neither species can live in the hostile wheat fields that comprise the matrix in which habitat fragments are embedded (Fig. 2). An elegant feature of the study by Hoehn et al . (2007) is the crossed comparison of unfragmented to fragmented habitat and declining to persistent species. They show that the genetic structure and diversity of both species have been affected by habitat fragmentation by comparing the fragmented populations with populations in the continuous habitat of nature reserves. In the fragmented landscape, genetic diversity is lower and the genetic structure of both geckos reflects the subdivision of the population into habitat fragments, whereas individuals in continuous woodland appear to belong to one well-mixed population. While indices of genetic structure are almost the same for both species in the continuous habitat, the declining species is more strongly structured among fragments than the persistent species, consistent with the hypothesis that the declining species is suffering from greater isolation on fragments.