Annual turnover in bat occupancy patterns: predictions from life history theory with implications for conservation and monitoring

Background/Question/Methods

Bats exhibit “slow” life history strategies, with inherently low rates of extrinsic mortality and fecundity, and high longevity. These traits should result in low annual turnover in the regional occurrence patterns of bats, particularly in temperate regions where species exhibit the “slowest” life histories. We expect that the high site fidelity observed in many species would further enhance this. Variation in turnover rates among species may also follow predictions from life history theory, with hibernating species exhibiting lower rates than migrating species, for example. We tested these predictions across 13 bat species in northwestern USA by modeling turnover and associated occupancy rate parameters over a 4-year period with Bayesian occupancy models using data obtained from a regional monitoring program across Oregon and Washington during 2006-2009. Our models accounted for imperfect detection and also for the influence of forest cover on the sample unit (site) survival and recolonization probabilities.

Results/Conclusions

Turnover, the probability that an occupied sample unit is a newly occupied one, was low (< 0.2) for most species, with “flat” trends in occupancy over the study period (λ≈1). Rare species exhibited the highest rates of turnover. Contrary to our predictions, the two long-distance migrating species Lasiurus cinereus and Lasionycteris noctivagans exhibited low rates of turnover (e.g., 95% posterior interval for L. noctivagans in 2009=0.0-0.1). Turnover did not vary consistently among species with tree versus rock crevice or cave roosting habits, or among forest- and desert-dwelling species. Predictably, forest cover was negatively associated with survival and recolonization rates for desert-dwelling species and positively associated for forest-dwelling species, reflecting the importance of the forest habitat gradient to our understanding of bat distribution dynamics in the region. Our results provide insight into patterns of rarity and commonness in the regional bat fauna. Slow life histories of temperate-zone bats result from the evolutionarily successful strategies of flight and hibernation, now on a collision course with accelerated wind energy development and White-nose Syndrome. Our results suggest that regional monitoring programs will benefit from low intrinsic turnover with higher statistical power to detect the kinds of imminent declines anticipated for many bat species.