Part 1: Summary
Currently, human activities have increased the rate of species’ extinction. It is, however, difficult to predict the actual rate. Skeptics point out that some predictions in the past have failed to produce the magnitude of the forecast extinctions. Various ecologists use the species-area relationship (SAR) to predict the extinction rates associated with habitat loss (Rybicki and Hanski). The SAR works on the principle that the number of species in a given location increases as the size of the habitat area enlarges. In backward SAR, the reduction in habitat area decreases the species’ number. Though successful in some estimates, the assumption often overrates the species’ number that will become extinct. He and Hubbell proposed an explanation for the overestimation.
They used mathematical reasoning to show that the backward SAR usually overrates the species’ loss. They also showed that the endemics-area relationship (EAR) method is more appropriate than the SAR. The EAR uses the assumption that an endemic species will go extinct if it loses its habitat area. Using data collected from several surveys in North America, the researchers noted that the use of backward SAR overestimated the extinction rate in some cases by over 160%. Unlike the SAR, therefore, the EAR method provides a more accurate estimate of species’ extinction and helps in identifying measures to mitigate the extinction.
Part 2: Discussion
He and Hubbell observe that the current methods of estimating extinction rates fail to account for the full complexity of species’ number. For example, the overestimates obtained from the backwards SAR result from the false assumption that the reverse of the SAR principle gives the rate of species’ extinction. In many cases, the extinction of a species requires a larger loss of habitat area than the area required to increase the species’ number. In a biologically unrealistic and particular case, the independent and random distribution of species creates the possibility of deriving the EAR from SAR. For such a case, the EAR and SAR are mirror images because the total habitat area matches the sum of the areas of finding the first and the last individuals of the species. The mirror-image relationship, however, holds for randomly spread species, which are rare in nature. For aggregated species, a discrepancy exists between the forward EAR and the backward SAR methods.
As a result, the backward SAR method overrates all extinction rates. The backward SAR technique falsely assumes that any population loss resulting from habitat loss will lead a species to extinction. The discovery by He and Hubbell, however, may appear to imply that habitat loss is not a serious issue. There is, therefore, concern that the discovery threatens conservation efforts. The researchers, however, maintain that their work does not discourage the conservation of biodiversity but clarifies the extinction estimates. They also support the Millennium Ecosystem Assessment’s position that habitat loss is a major threat to the conservation of biodiversity. Their study, however, points out that the conservation measures should not use the backward SAR because it is inaccurate in determining the extent of the present extinction event. He and Hubbell argue that the EAR enhances forecasts of the rate of extermination and improves the conservation planning. Unlike the SAR, therefore, the EAR technique provides better geographical information on endemism and the distribution of species.
The increasing rate of species’ extinction due to habitat loss has become a major obstacle to conservation efforts. The determination of extinction rates and mitigation measures is ineffective due to the use of inappropriate methods. The widely used SAR method is unreliable because the backwards SAR overrates the extinction rate that follows habitat loss. The SAR aims at determining the number of species gained by an increase in habitat area while the EAR method determines the number of species lost through the loss of habitat. The use of the EAR technique, therefore, increases the accuracy of the extinction estimates. In addition, the EAR helps to formulate effective mitigation approaches.
He, F., and Stephen P. Hubbell. “Species-area relationships always overestimate extinction rates from habitat loss.” Nature 473 (2011): 368–371. Web. 28 July 2014.
Rybicki, Joel and Ilkka Hanski. “Species–area relationships and extinctions caused by habitat loss and fragmentation.” Ecology Letters 16 (2013):27-38. Web. 28 July 2014.