A Species Vulnerability to Population Decline Essay

Policy- and decision-makers claim information about the causes of population declines which driving species on the brink of extinction (). However, it is pointed out that the causes of vulnerability remain unclear to explain the variation to the risk of extinction (). According to Reynolds (2003), vulnerability is determinate by both the decline of species population and the reduction of their geographical range. These two forms of vulnerability are affected by both species’ mortality and habitat loss, which are the causes of intrinsic biological traits, extrinsic human induced effects and stochastic factors that likely determine the population trends (). It is suggested that extinction risk is not distributed by chance between taxa,†¦show more content†¦Strong sexual selection is also considerate important in predispose species to extinction risk, because species with exaggerated traits or sexually dimorphic have high selection reducing their genetic variability (Mor row Pitcher 2003). In addition, densities of migrants and residents species are likely different because is predicted that migratory species could affront more risky in breeding and stopover sites than residents ones, then migratory behaviour should be related with extinction risk (). However, it is clear that intrinsic factors by themselves are not explained the entire extinction phenomenon (). On the other hand, species vulnerability might not be explained only by their internal biological traits because they are interacting with the external environment. Therefore, extrinsic hypotheses have been proposed to elucidate their populations range size and abundance. For instance, human impact is one of the main current causes of habitat loss and mortality of natural populations due to a massive human-population growth (). It is also thought that the biogeographical region occupied by a species has an effect in their risk of becoming extinct such as species living in islands (). Nevert heless, the interactions between intrinsic and extrinsic factors may give more details about the vulnerability to extinction in some species, because some intrinsic