Estimating Abundance and Simulating Fertility Control in a Feral Burro Population

• Published in: The Journal of wildlife management 2021-08, Vol.85 (6), p.1187-1199
• Main Authors: GEDIR, JAY V., CAIN, JAMES W., LUBOW, BRUCE C., KARISH, TALESHA, DELANEY, DAVID K., ROEMER, GARY W.
• Format: Article
• Language: English
• Subjects: Abundance; abundance estimation; Aerial surveys; Arid environments; Arid zones; Aridity; Barriers; California; Demographics; Equus asinus; Fecundity; Females; feral burros; feral equids; Feral populations; Fertility; Fieldwork; Group size; Growth rate; immunocontraception; Immunologic contraceptives; Juveniles; matrix models; population dynamics; Population growth; Rangelands; Special Section on Management of Feral Equids; Survival; Vaccination; Vaccines; Wildlife; Wildlife management; Zona pellucida
• ISSN: 0022-541X; 1937-2817
• DOI: 10.1002/jwmg.22058

Overabundant populations of feral equids are negatively affecting rangelands in the western United States. To better manage these populations, robust estimates of abundance and demographic rates and cost-effective methods of reducing abundance are necessary. From August 2015 to April 2017, we estimated the abundance of feral burros (Equus asinus) at the Fort Irwin National Training Center (NTC; California, USA) using a double-observer-sightability aerial survey method; captured, radio-collared, and inoculated female burros with porcine zona pellucida (PZP), an immunocontraceptive control agent; estimated female demographic rates; and used matrix population models to simulate how changes in demographic and PZP delivery rates would influence burro abundance. We estimated there were 690 (95% CI=618–752) feral burros within the surveyed area, but these are part of a much larger population that is not geographically isolated from those in the survey area. Sighting probabilities ranged from 0.19–0.98 and were most strongly influenced by distance from observer and group size. We estimated age-specific demographic rates at the NTC and compiled mean rates across burro populations in arid environments from the literature. Mean fecundity varied from 0.17 to 0.58 foals/adult female with younger females having lower fecundity. Mean survival was 0.90 for foals, 0.98 for yearlings, and 0.96 for adults. The PZP vaccine treatment strategies that suppressed fertility for up to 10 years predicted that burro abundance would be reduced by 67–88% after 15 years (compared with no treatment), but none of these strategies resulted in population extirpation. Population growth rates shifted from increasing to decreasing at adult survival rates below 0.84 and the population was predicted to become extirpated when adult survival declined to