Background/Question/Methods While the species-area relationship (SAR) is often recognized as one of the few general patterns present in ecology, we suggest that usage of these data to predict species extinctions in light of habitat loss may be inappropriate. Using both historical and extant distributions of species within three major taxonomic groups (plants, mammals, and fishes) we examined the relationship between observed and predicted extinctions in response to agricultural expansion and urbanization. To assess this relationship, we compared observed extinctions in each of the lower 48 states in the US to expected values calculated from SARs and estimates of contemporary land loss.

Results/Conclusions Our findings indicate that estimates of extinctions calculated from the observed SARs tend to overestimate the actual number of species lost within all three taxonomic groups. However the amount of overestimation varied greatly between plants and our two vertebrate study groups. Further, we noted that for plants, the species-area relationship is a bad predictor for both individual states and on average across the United States, where it is off by almost an order of magnitude. For mammals and fishes, however, we saw a different trend. On average we noted that for these two groups, overestimates of extinctions were off by less than one individual species. This finding is in sharp contrast to estimations for individual states where only 2% of the total variance is captured in our comparison of actual versus expected extinctions. Ultimately, we believe that these findings will have important implications for both conservation and land management in the future. Further analyses however, will be needed to address why SARs are limited in their ability to estimate plant extinctions and at higher resolution spatial scales.