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.