Fragmentation metric proxies provide insights into historical biodiversity loss in critically endangered grassland

Abstract

Landscape transformation causes habitat loss and fragmentation, which poses the greatest threat to biodiversity globally. In a fragmented landscape the persistence of species is affected by the amount of habitat and the spatial attributes of individual habitat patches. As baseline biodiversity information is often unavailable to assess the effects of transformation on biodiversity, quantifying changes in patch metrics over time could be a useful indicator of biodiversity changes in fragmented landscapes. We investigate historical land cover changes due to increased anthropogenic land uses in a highly fragmented, critically endangered grassland, the Woodbush Granite Grassland, South Africa. We test how the spatial attributes of habitat patches were affected by habitat loss and fragmentation over two extended time-periods, and use these results to infer likely threats to biodiversity over these periods. We used repeat aerial photography to analyse the drivers and extent of land cover loss and the changes in five fragmentation metrics over a 60year period. The overall grassland extent decreased significantly over the 60-year period, mainly due to increased timber plantations, with an estimated 6.1% of the original extent of the grassland remaining in 2008. Increases in patch isolation due to habitat loss were most pronounced between 1948 and 1977, while between 1977 and 2008 patch sizes decreased and edge effects increased. This extensive habitat loss and increase in fragmentation are likely to have had considerable impacts on the biodiversity of the region, which is supported by anecdotal evidence of species extinctions and local extinctions. Because so little of the vegetation type remains, it is imperative that the last vestiges of this vegetation type are protected from transformation and further habitat degradation. In summary, we show how, faced with a lack of biodiversity data, fragmentation patch metrics can be used to quantify threats to biodiversity.