Earth is losing biodiversity at an unprecedented rate – and these losses are due largely to human actions. While habitat destruction and fragmentation have been a large focus of many experimental and theoretical studies, there are many other widespread threats to species resulting from climate change, direct hunting and harvesting, pollution, and spread of invasive species. Many species may face 5 or more such threats concurrently. Individually, such threats may not pose as large of an extinction risk as habitat destruction or habitat fragmentation, but the impact of multiple simultaneous smaller threats may be as large or larger. To understand the consequences for species extinction, we developed a theoretical framework to determine the probability of extinction through time when a species is impacted by multiple simultaneous threats.
Results/Conclusions
Overall, we show that multiple threats to a species greatly increase its extinction probability, especially when looking at the cumulative impact through time. Specifically, when considering multiple independent threats, there is an additive increase in extinction probability when each threat alone has a small extinction probability (less than ~1%). Further, the cumulative extinction probability increases linearly through time, even when extinction probability is held constant between time steps. However, species living in multiple, independent sites have a higher chance of survival at any one time step because it is unlikely that it will go locally extinct from all sites at once. Small threats, which may be difficult to detect currently, may actually have a relatively large impact on species’ extinction risk. Thus, we may lose more species than expected when we only focus on the largest drivers of extinction.