Pronghorn fawn survival, bed-site characteristics, and adult doe habitat selection in southeastern New Mexico

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2019-12

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Abstract

Pronghorn (Antilocapra americana) population growth can be limited by low fawn recruitment, which is often limited by high predation, poor habitat condition, population density, and doe condition. Quantifying fawn survival and evaluating relationships between the factors that affect it are critical for understanding pronghorn population dynamics.
An isolated herd of pronghorn in the northern Sacramento Mountains near Fort Stanton, NM, that was augmented with 150 translocated pronghorn from 2013-2015 still suffers from low fawn survival, though adult survival is high. The objectives for this study were to (1) quantify pronghorn fawn survival during the most vulnerable stage (first 45 days), (2) quantify cause-specific mortality of pronghorn fawns, and (3) assess habitat selection at 3 scales: a) fawn microsite (bed site) selection, and doe home range selection at the b) Design II and c) Design III levels. Fawn births occurred between 3 May and 1 June in both 2016 and 2017, with fawning peaks from 7-22 May and 9-20 May, respectively. A total 101 fawns were captured and equipped with VHF radio-tag ear transmitters and monitored to 45 days old. Of those, 24 (23.8%) survived, which was equal to or lower than other fawn survival estimates from across pronghorn range. Fawning season has shifted earlier and become more synchronous over time, and mortality has improved from 2014. Out of 75 investigated mortalities, fawns succumbed to predation (73.3%; n = 55), exposure/abandonment (2.7%; n = 2), and injury (1.3%, n = 1), with 17 (22.7%) unknown. The three dominant predator species (or groups) in both years were bobcats (24.0%; n = 18), coyotes (26.7%; n = 20), and raptors (9.3%; n = 7). Habitat data were collected at 98 capture locations (53 in 2016, 44 in 2017) and 170 random locations (94 in 2016, 75 in 2017), both manually and remotely via GIS. These were considered “Used” and “Available” locations, respectively, for subsequent habitat selection analyses. Analysis of Variance (ANOVA) tests were used to examine differences between fawn capture locations and random locations, as well as surviving fawn and non-surviving fawn bed sites. Mean vegetation height mean woody height (specifically), woody density, % woody cover, % forb cover, % succulent cover, total visual obstruction, trees per hectare, distance to nearest tree, distance to nearest tree line (edge), distance to nearest main/county road, and % slope varied (P < 0.05) between fawn bed sites and random locations in one or both years. Bed site habitat characteristics such as mean vegetation height, % woody cover, total visual obstruction, and distance to nearest road (P < 0.05) varied between surviving and non-surviving groups in one or both years. Mass (kg) and body length (cm) of fawns varied between surviving and non-surviving fawn groups in 2016, but not 2017. A multi-step process was used to convert NAIP 1-m resolution imagery into a landcover class map with 13 classifications. Landcover composition within 50-m buffers centered on capture locations (Used) and random sites (Available) were compared using Chi-square analyses to assess fawn bed site landcover class selection. Compositional analyses showed fawns selected for Grasslands with 0-9.9% shrub, and savannas (5-20 tpha) with majority grass understory, while avoiding savannas (20-40 tpha) with majority shrub understory, and woodlands with >40 tpha in both years. Kaplan-Meier estimate of seasonal fawn survival were 8% (0-22%) and 15% (0-31%) in 2016 and 2017, respectively. When limited to fawns surviving at least 3 weeks, estimated fawn survival improved to ~23% in both years. Daily (DSR) and seasonal survival rates (SSR) were estimated per year via Mayfield models (RMARK) were 8% and 96% in both years. Mayfield models incorporating biological and ecological parameters were explored, but were ultimately not very useful due to low sample sizes and few differences between surviving and non-surviving fawn habitat selection variables. Adult translocated female pronghorn were monitored throughout 2016 and 2017 fawning seasons, and relocation data were recorded. Kernel density (KDE) home range estimates were produced for all translocated does with >10 relocations in May and June within each year (16 does in 2016, 9 does in 2017). Mean home range (95% isopleth) was 3,715 ha (37 km2) and 4,133 ha (41 km2) in 2016 and 2017, respectively, and core area was 858 ha (9 km2) and 970 ha (10 km2), respectively, though these estimates likely suffer from small sample sizes (low number of does per year, and low number of relocations per doe). Translocated doe selection at the Design II scale compared vegetative composition within available (aggregated 95% isopleths) and used (50% isopleths of individual does) areas, and found that does were selecting for Grasslands with <50% shrub and savannas (5-20 tpha) with majority grass understory, and against woodlands with >60 tpha in 2016. In 2017, only savanna (5-20tpha) with majority grass understory was selected for, and woodlands with >60 tpha were selected against. Sample size issues may have contributed to reduced selection results in 2017. At the Design III scale, composition within translocated doe 70% isopleths (Available) and 50-m buffers of relocations (Used) were compared, and does selected for grasslands with 10-20% shrub cover and savannas (5-20 tpha) with majority shrub understory, while selecting against woodlands with >60 tpha in both years. Grasslands with 20-30% shrubs were also selected in 2016, while grasslands and shrublands with >30% shrub cover were selected in 2017. Pronghorn selected for slopes <10% (90% of relocations), and avoided areas with >10% slope. The original estimate of available pronghorn habitat produced by aggregating the 95% isopleths of all doe home range KDEs was adjusted using selection information gained during this study. Areas with >10% slope and/or >40 tpha were removed from the original area, reducing the estimated functional pronghorn habitat by ~60%.
The high early mortality and sources of mortality were similar to other studies throughout pronghorn range. Drought conditions from 2011 through 2017 may have contributed to do stress and poor condition, and thus reduced fawn survival. Though fawn survival has improved and is similar to survival estimates across pronghorn range, many of those estimates were from studies instigated by low fawn survival or local population declines. Available habitat for pronghorn in the Fort Stanton area is limited. We speculate that low fawn survival may be a symptom of either poor forage conditions during drought years, leading to poor doe condition, high local pronghorn densities, and/or woody encroachment leading to high success rates for predators.


This thesis won 2nd Place in the Texas Tech University Outstanding Thesis and Dissertation Award, Biological Life Sciences, 2021.

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Keywords

Antilocapra americana, Bed site characteristics, Cause-specific mortality, Fawn predation, Fawn survival, Habitat selection, Home range, Neonate, New Mexico, Pronghorn

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