SIZE ISN'T EVERYTHING: THE IMPORTANCE OF SMALL HABITAT PATCHES WHEN PLANNING THE CONSERVATION OF SPECIES IN FRAGMENTED LANDSCAPES

Abstract

Metapopulation theory and landscape ecology indicate that larger patches of habitat are more likely to support self-sustaining populations of more species. In spatial conservation planning, it makes sense that if all else is equal; one would prefer to conserve a large patch of habitat over a small patch. However, simplistic and selective application of these theories is having perverse impacts on the viability of rare and threatened species in fragmented landscapes. Preference toward conserving large patches and ambivalence toward protecting small patches is manifest in many land management policies and regulations. However, due to historical patterns of land-use and habitat loss, it is seldom the case that all else is equal in a choice between large and small patches. In many instances, small patches and large patches have different species composition. We hypothesized that the distribution of species most rare in the landscape will tend to be biased toward smaller patches. We analysed the relationships between species composition, rarity, irreplaceability and habitat fragment size in fragmented landscapes with contrasting histories of land-use and habitat change in Australia, Europe, Africa, and North America. We find that small habitat patches are inordinately important in the bulk of landscapes simply on the basis of their species compliment. We found a surprisingly clear and strong negative relationship between irreplaceability and most habitat fragmentation indices. This is driven by the fact that rare species distributions are biased toward small patches, while common species exist across all patch sizes. This result has strong implications for land-clearing regulations and offsetting policies. Selective application of ecological theory that downplays importance of small patches in fragmented landscapes will lead to catastrophic outcomes for rare and threatened species. Priority should be given to improving integrity of small patches through management of threats and restoration