Biodiversity as spatial insurance: the effects of habitat fragmentation and dispersal on ecosystem functioning

Anthropogenic habitat destruction (e.g. strip mining or clear cutting of forests), conversion to agriculture (e.g. conversion of grasslands to croplands or rangelands, or conversion of forests to plantations), and fragmentation (e.g. dividing ecosystems inhabited by native species into parcels that are separated by inhospitable terrain) are generally considered the dominant drivers of biodiversity loss. The loss of inhabitable area is the predominant cause of population (Hughes et al. 1997) and species extinctions (Pimm et al. 1995). Isolation of fragments of habitat and edge effects associated with such fragmentation can cause further declines in both the number of species, changes in their relative abundance, and other aspects of biodiversity within remnant habitat patches (e.g. Andrén 1994, Fahrig 2003, Ewers and Didham 2006). Although other anthropogenic drivers (e.g. climate change, overexploitation, and the spread of non-indigenous species that adversely affect indigenous species) are growing in importance, it is clear that their impacts will be felt within the context of ongoing habitat loss. Indeed, strong synergies between habitat fragmentation and climate change are expected (Holt 1990, Travis 2003) and will likely compound the loss of biodiversity at local and regional scales. The threat of widespread and rapid loss of biodiversity across most regions has prompted two decades of research on the impacts of biodiversity loss on ecosystem functioning and services. A number of controlled experiments have established that reduced levels of species diversity can impact community processes, such as biomass production and nutrient uptake (Cardinale et al. 2007), although data from unmanipulated plant communities suggest that these effects may be weaker or masked by other covarying factors in the environment (Grace et al. 2007, Hector et al. 2007). Overall, the beneficial effects of biodiversity in experimental conditions have been shown to saturate at relatively low to moderate levels, even when several functions are considered simultaneously (Hector and Bagchi 2007). The relevance of results from biodiversity and ecosystem function experiments, given the rapid saturation of biodiversity effects, has questioned their utility as a case for conservation biology and has led to calls for a broadening of empirical and theoretical perspectives within the field (Gonzalez and Chaneton 2002, Srivastava 2003, Srivastava and Velland 2005, Lawlor et al 2002). Biodiversity effects on ecosystem functioning (BEF), though small or sometimes negligible in small-scale studies, may nevertheless be more significant at larger spatial and temporal scales (Yachi and Loreau 1999, Loreau et al. 2003, Cardinale et al. 2004). Typically, experimental BEF studies have been performed over small spatial and temporal scales, relative to the size, mean habitat range, and generation times of the organisms involved. Although these limitations are most acutely associated with studies of terrestrial plant and tree communities, experiments with aquatic systems can also have similar limitations. The results of BEF experiments, whether terrestrial or aquatic, although clearly valuable for establishing the effect