Endangered Species Act: Quantifying Threats Impacting Listing

With species increasingly becoming imperiled due to anthropogenic activities, conservation practitioners are tasked with determining conservation priorities in order to make the best use of limited resources. The United States’ Endangered Species Act (ESA) has two listing statuses into which imperiled species are placed to receive protections: Threatened or Endangered. In the first chapter, our objective was to identify differences between Threatened and Endangered species beyond what is outlined in their ESA definitions. To our knowledge, this is the first study to compare listing status for species protected by the ESA on the basis of types and number of threats they face. For six broad-scale threats (habitat modification, overutilization, pollution, species-species interactions, environmental stochasticity, and demographic stochasticity), we investigated whether there is a difference in the number and types of threats which impact Threatened and Endangered species at the time of their listing. We found that Threatened (X̄ = 2.9, SD = 1.4) and Endangered (X̄ = 3.0, SD = 1.1) species were faced by a similar number of threats at their time of their listing. The only broad-scale threat that disproportionately impacted Endangered species more than Threatened species was demographic stochasticity; Endangered species were 1.9 times (95% CI = 1.4 -2.7) more likely to have this threat than Threatened species. We found four finer-scale demographic stochasticity threats (few individuals in one population, few individuals in multiple populations, lack of reproduction, and genetic loss) to be strong predictors of Endangered status. The similarities in the number and types of broad-scale threats faced by Threatened and Endangered species suggest that changes recently made to the ESA may be detrimental to the recovery efforts of future Threatened species. In the second chapter, our objective was to identify temporal trends in threats facing the four major phyla protected by the United States Endangered Species Act: angiosperms, arthropods, mollusks, and vertebrates. For 24 threat types, we created models to determine whether there was a linear, quadratic, or pseudo-threshold association between year and the probability that a phylum was listed with a given threat. We were able to identify temporal trends for 79% of the 96 possible threat-phylum combinations. We found that angiosperms had the highest peak probability of being listed with the greatest number of threats (N = 10), followed by mollusks (N = 8) and more distantly by arthropods (N = 4) and vertebrates (N = 2). We found that vertebrates had the greatest number of threats (N = 16) for which the year with their greatest probability of their being listed with a threat was their most recent year of listing. The other three phyla were similar to each other for this metric, mollusks having 12 and angiosperms and arthropods each having 11 threats peak in their most recent year of listing. We only identified one threat/phylum combination for which the maximum probability of their being listed with a threat was in their first year of listing (i.e., the threat has been consistently decreasing): authorized take impacting angiosperms. Overall, we believe our findings can be used to assist conservation efforts by identifying which threats have been decreasing or stable over time and which have been increasing that are in need of more attention