Invasive Species Compendium

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Abstract

Estimating abundance and simulating fertility control in a feral burro population.

Abstract

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 <0.60. In the absence of other methods to reduce burro numbers, our findings indicate that current formulations of PZP immunocontraception, which require multiple doses, would be inadequate for controlling population growth rates at the NTC. Our fieldwork also highlighted the difficulty of administering PZP vaccination to large, free-ranging animals. Development of longer-term fertility reduction agents or more efficient vaccine delivery techniques would likely improve the efficacy of fertility control for overabundant ungulate populations. Lack of geographic closure (physical barriers to migration) further complicated efforts to reduce burro numbers.