Date of Award
Program or Major
Natural Resources and Environmental Studies
Doctor of Philosophy
Rebecca J Rowe
As the emergence of novel diseases in wildlife becomes more common, a better understanding of the impact of disease on populations and their demographic rates will be critical for their conservation. Populations naturally fluctuate over time as a function of the rates of birth, death, immigration, and emigration, while at the individual level the chances of mortality and reproduction may be influenced by age and physiological condition. Diseases, particularly emerging infectious diseases, can cause immediate and severe changes in demographic rates. While some populations and species may go extinct due to high mortality rates, others may persist in smaller, but stable numbers by evolving, adapting, or demographically responding. Diseases can also have sublethal impacts that may differentially effect individuals, sexes, or ages and may threaten the viability of a population. White-nose syndrome (WNS) is a recently emerged fungal disease that infects hibernating bats in North America. Little brown bat (Myotis lucifugus) populations have declined in some areas by more than 90%, but have stabilized in remnant winter and summer colonies. In this study I investigated the long-term impacts of WNS and changes in demographic rates on little brown bat populations in New England.
Age-related demographic rates are often important to understand for conservation and management, but are hard to determine in long-lived species with no clear external indicators of age. In Chapter 1, I investigated whether relative telomere length could be used as a genetic marker of age for little brown and big brown bats (Eptesicus fuscus). I found that in big brown bats, there was a quadratic relationship with chronological age, where middle-aged individuals had the longest telomeres. For little brown bats, individuals with more wing damage due to WNS had shorter telomeres, suggesting an impact of disease on their physiological condition. Finding no relationship of telomere length to chronological age, little brown bats should, for now, continue to be grouped as either juveniles or adults, which may be appropriate as there is little evidence for senescence in Myotis bats.
In Chapter 2, I compared yearling (one-year-old) and adult reproductive rates using banding records from an extensive study at one summer maternity colony prior to WNS and from work at eight colonies since WNS emerged. I found that yearling reproductive rates have significantly increased and are now very similar to the observed adult reproductive rate of 0.95, suggesting a shift in life-history related to age of first reproduction. I also found that reproductive phenology has advanced by 6–10 days, most likely driven by warming spring temperatures, but also potentially as a response to WNS. Earlier reproduction could benefit offspring by giving them more time to accumulate fat stores before hibernation, thus increasing the chances of first-winter survival and reproduction as yearlings. Bats with more wing damage due to WNS had later parturition dates, suggesting that the energetic costs of infection delay reproduction for individuals, but are not delaying the overall timing of reproduction at the population level.
Long-term survival rates are also important to understand for population modeling. In Chapter 3, I used mark-recapture and colony count data to estimate survival in three different types of population models. I found that survival estimates were similar among models and showed that survival probabilities were lowest immediately after WNS invasion, but have since returned to or surpassed pre-WNS survival probabilities. Juvenile survival was lower than adult survival, but was generally higher than those reported from pre-WNS. I then used these survival estimates and the reproductive rates from Chapter 2 to conduct population viability analyses under different management strategies, which showed colony growth even without additional human intervention. These findings suggest that little brown bat populations in New England have responded to WNS and are beginning to rebound, but some management actions that boost survival could help ensure long-term recovery.
Ineson, Katherine, "DEMOGRAPHY OF A RECOVERY: TRACKING THE REBOUND OF LITTLE BROWN BAT (MYOTIS LUCIFUGUS) POPULATIONS" (2020). Doctoral Dissertations. 2529.