Extreme drought and adaptive resource selection by a desert mammal

Abstract

When animals select areas to occupy, decisions involve trade‐offs between the fitness benefits of obtaining critical resources and minimizing costs of biotic and abiotic factors that constrain their use. These processes can be more dynamic and complex for species inhabiting desert environments, where highly variable spatial and temporal distribution of precipitation can create high intra‐ and inter‐annual variability in forage conditions and water availability, and thermal constraints can differ significantly among seasons and diel periods. We examined resource selection in desert bighorn sheep (Ovis canadensis mexicana) in Cabeza Prieta National Wildlife Refuge, Arizona, USA, at multiple spatial and temporal scales to gain insight into how a desert mammal responds to variations in climatic conditions. We used resource selection functions to test topographic, forage, and environmental features among seasons and diel periods, and between non‐drought and drought conditions at the population and home‐range scale. When precipitation was average, sheep selected for topographic features that were beneficial for predator avoidance (i.e., escape terrain—steep, rugged areas with high visibility) and locations near perennial water. When drought occurred, they ranged further from preferred escape terrain and perennial water, perhaps seeking forage conditions suitable to meet their nutritional requirements. On early (April–June) and late (July–September) summer days, sheep selected for more northerly aspects and locations with lower solar radiation, and in some periods, selection for these cooler areas coincided with periods when forage covariates, proximity to perennial water, and several topographic features were uninformative in resource selection models. These choices may be necessary trade‐offs, foregoing good escape terrain and foraging areas, and access to water, for improved thermoregulation. This study highlights the importance of identifying resource selection at variable spatial and temporal scales when investigating the interrelationship between species and their environment. It provides insight into the dynamics of resource selection in desert mammals, and how they respond to constraints imposed on them by their environment. This work can serve to inform strategies for managing and conserving species living in arid environments when faced with climate change.

Factors affecting bighorn sheep activity at water developments in southwestern Arizona

“…This result supports concerns that poorly placed water developments may have unintended negative consequences regarding predation (Rosenstock et al 1999(Rosenstock et al , 2004DeStefano et al 2000). To be clear, we did find that sheep avoided developments with intermediate percentages of bare ground, but interpretation is convoluted by collinear terms pertaining to elevation and shrub density (Gedir et al 2020). Further investigation is needed.…”

“…We found little evidence of association between NDVI and the frequency of visits by bighorn sheep to water developments. Investigations of space use intensity in bighorn sheep have similarly failed to find associations with NDVI during summer seasons when visits during our study were most concentrated (Hoglander et al 2015, Gedir et al 2020. Key forage species of bighorn sheep are capable of maintaining relatively high water content throughout the year (Alderman et al 1989;Cain et al 2008bCain et al , 2017Gedir et al 2016), and bighorn sheep will restrict their diets to forage with the greatest moisture during water-stressed periods (Cain et al 2017) may not be able to meet hydration needs through vegetation alone during the hottest and driest periods of summer (Turner 1973, Gedir et al 2016.…”

“…Under such conditions, it is intuitive that sheep may frequent water developments as convenient sources of free water that provide hydration and thermoregulatory relief. Sheep in our study visited water developments more often when temperatures were high and precipitation low and increasingly avoided areas with higher SRI (Campbell and Remington 1979, Foster et al 2005, Waddell et al 2007, Gedir et al 2020). Despite their many adaptations to xeric environments (Cain et al 2006), periods of extreme heat and aridity exert thermoregulatory tolls and diminish water availability, which influence survivorship (Porter and Gates 1969, Douglas 2001, McKinney et al 2001).…”

“…Normalized difference vegetation index is a commonly used remotely sensed metric of vegetative greenness with broad availability of spatial and temporal coverage. Because it is an accurate predictor of primary productivity, it is often used as an indicator of forage in ungulates in arid regions (Pettorelli et al 2005, Marshal et al 2006 b , Hoglander et al 2015, Villepique et al 2015, Gedir et al 2020). In addition, NDVI is a determinant of water availability in vegetation and soils with lower values of NDVI indicating greater moisture stress (Tucker and Choudhary 1987, Gessesse and Melesse 2019), despite some inherent limitations as a chlorophyll‐based metric (Ceccato et al 2002).…”

Predicted distribution of a rare and understudied forest carnivore: Humboldt martens (Martes caurina humboldtensis)

“…For populations that have declined or are currently declining, modeling species distributions may be complicated by additional factors – for example, contemporary locations may not include all areas where a species could physically colonize in the future, if the species is constrained by mobility or time (Barry et al In review) or barriers that were not in place prior to the species’ decline (e.g., human development; Brown et al 1996). Species may also require resources at multiple spatial scales, during different seasons, or for different durations of time – a female desert bighorn sheep ( Ovis canadensis nelsoni ) requires high nutritional-value vegetation during spring when providing for young (fine spatial extent, short duration), a water source within her range during the heat of the summer (large spatial extent, short duration), and immediate cover from predation year-round (fine spatial extent, long duration; e.g., Gedir et al 2020; Hoglander et al 2015). As such, limited observations in space or time may miss the conditions necessary for population persistence.…”