Indexing the health of the environment for breeding seabirds in the Benguela ecosystem

Abstract

Abstract Time-series of the sizes of breeding populations of 10 species of seabird were used to develop indices of the health of the Western Cape seabird community of South Africa. For each species, a target range was defined running from some minimum value to infinity or to some maximum value for species that may cause harm to other species or be a nuisance to humans. If populations were within the target range, their individual health index was set at 1, whereas outside the range, this index decreased linearly with population size. These individual indices were integrated into one for the total community, also running from 0 to 1 and therefore allowing representation as a percentage of the overall management target (=1). Three indices were developed, weighting each species equally and using different weighting methods to account for the IUCN conservation status of the species. All indices increased between the 1950s and 1970s and then decreased again, the lowest values being observed in the late 1990s.