Population viability of an island endemic bird facing climate-related threats

Background/Question/Methods

Climate change is expected to shuffle the deck for many vulnerable species by altering habitat while exacerbating climate-related threats such as introduced diseases and catastrophic wildfires. Island endemics are likely to be particularly hard hit because they are often unable to shift their geographic ranges in response to changing conditions. The Island Scrub-Jay (Aphelocoma insularis), found only on Santa Cruz Island off the coast of southern California, is the sole insular endemic bird species in the continental United States. The jay’s future viability in a changing climate has come under recent scrutiny in the scientific and lay media. Warming conditions are expected to make Santa Cruz Island more hospitable to arthropod vectors of disease, and the risk of West Nile Virus (WNV), to which corvids are highly susceptible, is a particular concern. Here we evaluate the viability of the Island Scrub-Jay population on Santa Cruz Island and compare the extinction risks posed by climate-related threats. Our approach was to simulate the population dynamics of Island Scrub-Jays in the presence of WNV, periodic wildfire, or both, and to compare these to dynamics in the absence of threats. We parameterized our model using habitat-related drivers of demographic rates identified through field studies, and incorporated and explored the effects of parameter uncertainty.

Results/Conclusions

Our model predicts that the jay’s baseline risk of quasi-extinction on Santa Cruz Island over 50 years is low (P_ext < 0.01) if starting populations sizes are ≥ 400, or ~25% of assumed carrying capacity for breeding habitat. Introduction of WNV to the interior of the island, where temperatures are warmer, does not significantly increase quasi-extinction risk for populations at carrying capacity (P_ext <0.02). However, if the virus spreads to the remaining habitat on the island, extinction risks rise, with risks ≥40% if exposure probabilities exceed 60%. The unique challenges of global climate change will require creative approaches to managing vulnerable range-restricted species such as the Island Scrub-Jay.