Pronghorn (Antilocapra americana) Offspring Recruitment on the Carrizo Plain National Monument: Evaluating the Effects of Low Population Density and Marginal Habitat Quality.
Creators: name: Johnson, Diego Ramirez
Publisher: name: University of Nevada, Las Vegas
Year: 2014

description: Demographic fluctuation among ungulate populations is strongly linked to variability in recruitment. Rates of recruitment are subject to various forms of density-dependent and density-independent regulation. For species which benefit from the presence of conspecifics, reduced population density can decrease rates of recruitment and trigger a decline in per capita growth. Termed the Allee effect, this scenario can cause demographic collapse and population extinction. For many ungulate species, predation on juveniles is reduced when the timing and distribution of births is synchronized within a local population. Because birth synchrony is density-dependent, it may act as a mechanism for the Allee effect if offspring production in small populations is not sufficient to limit predation. In addition to risks associated with the Allee effect, marginal quality habitat and stochastic environmental fluctuation can limit recruitment and contribute to declines in reduced populations. An understanding of how small populations are affected by density-dependence and habitat quality is critical for the ecology and conservation of ungulates. This study examines recruitment in a highly reduced population of pronghorn occupying sub-marginal habitat on the Carrizo Plain National Monument (CPNM) in California. 
In Chapter 2, I address the potential for birth synchrony to act as a mechanism for a component Allee effect by comparing survival of individuals born during "peak" and "non-peak" periods within annual birth distributions. Twenty of forty-five pronghorn fawns born on the CPNM from 2009-2011 were equipped with lightweight, detachable GPS/VHF collars. The status of uncollared fawns was monitored via the VHF tracking of a collared sibling, general location of fawning site and/or the pelage of individual does. All 12 surviving fawns in the study (26.7%) were born during peak periods of productivity. I then tested for the presence of a demographic Allee effect by investigating the density dependent feedback of population size on logarithmic per capita population growth rate using ten years of regional flight count information from 2000-2011. Using multi-model inference and Akaike's information criterion (AIC) I developed and selected a set of candidate models to best describe the pattern in the dataset. Results indicated that, at small population size, per capita growth rate tended to decline providing evidence for the presence of an Allee effect. 
In Chapter 3, I evaluate the effect of macro- and micro-environmental conditions on recruitment. I modeled fawn habitat selection and survival, as well as examined diet composition and forage quality/availability for adults. Fawn locations, in comparison to random locations, were closer to drinking water and large shrub communities and contained lower percent slope of terrain. Survival of fawns appeared to increase with closer proximity to water sources, greater distance from main roads, and with increased slope of terrain. Low seasonal precipitation and high summer temperatures appeared to negatively affect survival. Forage abundance and quality were adequate during spring, but low during summer and fall. Low overall shrub cover appeared to provide inadequate concealment opportunities for fawns and likely limited nutrient availability during summer and fall for adults. Collectively, this study demonstrates that low density pronghorn populations are likely at risk for Allee effects and that Allee effects may be manifested through mechanisms associated with birth synchrony. Additionally, this study provides information on specific environmental conditions which affect survival of pronghorn fawns associated with micro- and macro-habitat availability and selection by does and fawns.