Understanding the importance of small patches of habitat for conservation.
Abstract
Conservation activities in fragmented landscapes have largely focused on keeping remaining large patches intact, often disregarding the increasingly important role of smaller patches in the conservation of remaining vegetation. As habitat loss proceeds in fragmented landscapes, there is an increasing need to measure the relative contribution of all patches (large and small) to overall ecosystem persistence, in a way that helps deliver effective conservation strategies aimed at preventing the death of ecosystems by a thousand cuts. Using Australian vegetation communities as a case study, we calculated the historical change in the contribution of patches below different sized thresholds to overall extent. We introduced a new patch assessment metric based on the Gini coefficient that indicates how unequal the distribution of patch sizes is relative to historical distributions. At least 22% of major vegetation communities in Australia have >50% of their remaining extent in patches <1000 ha. Loss does not always match fragmentation status: though some vegetation communities are exposed to the double jeopardy of high loss and high fragmentation, others are far more affected by fragmentation than loss of extent. For some communities, actions focused on protecting large patches are critical but for many others, protecting and managing small patches is crucial for community persistence. Synthesis and applications. Arbitrary patch size thresholds for permitting native vegetation clearing are dangerous for ecosystems whose distribution is now restricted to small patches. We recommend that clearing thresholds be scaled to reflect the fact that some ecosystems are more dominated by small patches than others. With a renewed focus on formally assessing the threat status of ecosystems as well as species, ecosystem accounts such as those demonstrated in this study are the first step to reliably assessing vulnerability. Measures of ecosystem vulnerability that only consider the extent of vegetation loss and not the size of remaining patches are likely to be ineffective for impact assessment, conservation planning and preventing ecosystem loss.