Final Report: The Effects of Drought on Desert Bighorn Sheep
James W Cain, Jay Gedir, Colleen A Caldwell, and Scott A Carleton, 2018, Final Report: The Effects of Drought on Desert Bighorn Sheep: .
The responses of individual species to environmental changes can be manifested at multiple levels that range from individual-level (i.e., behavioral responses) to population-level (i.e., demographic) impacts. Major environmental changes that ultimately result in population level impacts are often first detected as individual-level responses. For example, herbivores respond to limited forage availability during drought periods by increasing the duration of foraging periods and expanding home range areas to compensate for the reduction in forage. However, if the individual-level responses are not sufficient to compensate for reduced forage availability, reduced survival and reproductive rates may result. We studied the impacts of drought [...]
The responses of individual species to environmental changes can be manifested at multiple levels that range from individual-level (i.e., behavioral responses) to population-level (i.e., demographic) impacts. Major environmental changes that ultimately result in population level impacts are often first detected as individual-level responses. For example, herbivores respond to limited forage availability during drought periods by increasing the duration of foraging periods and expanding home range areas to compensate for the reduction in forage. However, if the individual-level responses are not sufficient to compensate for reduced forage availability, reduced survival and reproductive rates may result. We studied the impacts of drought on desert bighorn sheep (Ovis canadensis mexicana), American pronghorn (Antilocapra americana), Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis), and scaled quail (Callipepla squamata), including assessments of individual- (e.g., desert bighorn sheep) and population-level (e.g., pronghorn, cutthroat trout, scaled quail) responses to drought.
Nutritional ecology forms the interface between environmental variability and large herbivore behaviour, life history characteristics, and population dynamics. Forage conditions in arid and semi-arid regions are driven by unpredictable spatial and temporal patterns in rainfall. Diet selection by herbivores should be directed towards overcoming the most pressing nutritional limitation (i.e., energy, protein, moisture). We investigated the influence of precipitation-induced shifts in forage nutritional quality and subsequent responses of desert bighorn sheep (Ovis canadensis mexicana, DBS) across widely varying precipitation conditions. Succulents were consistently high in moisture but low in protein and grasses were low in protein and moisture until the wet period. Protein and moisture content of shrubs and forbs varied among seasons and climatic periods, whereas trees had consistently high nitrogen and moderate moisture levels. Shrubs, trees and succulents composed most of the seasonal sheep diets but had little variation in energy content. Across all seasons during drought and during summer with average precipitation, forages selected by sheep were higher in protein and moisture than that of available forage. During drought, desert bighorn relied on a few key forage species including palo verde (Parkinsonia microphylla, P.florida) and ironwood (Olneya tesota) trees. Overall, forage selection was more strongly associated with protein and moisture content than energy content.
Wildlife managers often provide supplemental water to help desert ungulates endure the hottest, driest periods. When surface water is unavailable, the only source of water for ungulates comes from the forage they consume, and they must make resourceful foraging decisions to meet their daily requirements. We calculated water and nutrient intake and metabolic water production from forage intake and forage moisture to determine whether desert bighorn sheep could meet their seasonal daily water requirements solely from forage. Under drought conditions without any surface water available desert bighorn would be unable to meet their daily water requirements in all seasons, except winter. We determined that DBS could achieve water and nutrient balances in all seasons by shifting their total diet proportions by 8–55% from lower to higher moisture and nitrogen forage species.
Climate often drives ungulate population dynamics, and as climates change, some areas may become unsuitable for species persistence. Therefore determining the relationships between climate and population dynamics can contribute to more informed management and conservation decisions. Growth rates of American pronghorn (Antilocapra americana) populations are particularly sensitive to climatic conditions. We analyzed long-term pronghorn population, precipitation, and temperature data from 18 populations in the southwestern United States and used these relationships to project population trends through 2090 under two climate change scenarios. Fifteen of the pronghorn populations declined in abundance since the 1990s. Sixteen populations demonstrated a significant relationship between precipitation and population growth. Models predicted that nine populations would be extirpated or approaching extirpation by 2090. In the southwestern United States, the climate underpinning pronghorn populations is shifting, making conditions increasingly inhospitable to pronghorn persistence.
Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis, RGCT), the southernmost subspecies of cutthroat trout, is endemic to the Rio Grande, Canadian, and Pecos River basins of Colorado and New Mexico. The subspecies is reduced to less than 11% of its historic range with most populations occupying isolated high elevation headwater streams. One of the greatest threats to its survival is the effect that low stream flow due to drought will have on this coldwater trout. We set out a specialized datalogger across 54 sites in 29 populations of RGCT to measure temperature and intermittency (no flow) over one year. While few RGCT populations experienced intermittency because of the serendipitous wet cycle from 2013 to 2014, the loggers provided water temperature data deemed comparable to more sophisticated and expensive temperature loggers. Thus, these intermittency loggers offer a low-cost and long-duration (battery can be replaced) solution that not only provides unambiguous and continuous water temperature, but also provides continuous intermittency information of stream flow. An important caveat of our research is that while RGCT populations may not experience intermittency during average to above average precipitation years, very little is known of their vulnerability during below average precipitation years.
Grassland birds are among the most imperiled bird guilds in North America. Scaled quail (Callipepla squamata) are a semi-arid grassland bird whose populations have declined over the past half century. We monitored scaled quail in New Mexico to study the effects of habitat, temperature and precipitation on survival of scaled quail adults, nests, and broods. Seasonal est survival (39.4%) had a positive relationships with increasing average weekly maximum temperature and grass density, and negative relationships with increasing average minimum temperature and percent bare ground. Seasonal brood survival (49.0%) had a negative relationship with increasing average weekly minimum and maximum temperature, and with increasing precipitation. These results illustrate the importance of managing ground cover for scaled quail to ensure adult survival and successful recruitment. Ground cover provides protection from thermal and precipitation related stress, as well as for visual obstruction from predators.