Using population models to guide management of the San Cemente Sage sparrow

Background/Question/Methods

The military manages many species threatened with extinction, each of which face multiple threats against their persistence.  The multitude of potential threats that need to be addressed to successfully conserve threatened species poses a serious management challenge.  Because ultimately all threats to species persistence are manifested in their population dynamics, population models are widely used to guide conservation efforts.  Unfortunately, most models examine how potential threats affect only one aspect of population dynamics: growth potential, maximum population size, and, rarely, population variability.  Using the case study of the San Clemente Sage Sparrow (Amphospiza belli clementae, SCSS) I demonstrate the importance of evaluating all three aspects of population dynamics.  SCSS are endemic to San Clemente Island, which is owned by the U.S. Navy.  I used data from an ongoing intensive monitoring program to parameterize a simulation model of SCSS population dynamics using program VORTEX.  The original goal of the model was to evaluate the impact on sage sparrow extinction risk of habitat loss associated with development. 

Results/Conclusions

Although habitat loss and degradation from introduced grazers, resulting in a low population ceiling, was the primary threat cited when the SCSS was listed under the Endangered Species Act in 1977, I found that changes in habitat availability has little impact on predicted SCSS extinction risk.  Instead, the primary threat currently appears the impact of low juvenile survival on population growth potential.  If juvenile survival can be increased, greater population variability imposed by global climate change, in the form of more frequent drought events, will become the greatest threat.  Both the counter-intuitive robustness of SCSS to habitat loss and the increasing influence of global climate change on SCSS extinction risk with greater juvenile survival are explained by the species disturbance-driven ecology.