Data from: Extreme precipitation variability, forage quality and large herbivore diet selection in arid environments

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 [nitrogen, N],
moisture) within the constraints imposed by temporal and spatial
variability in forage conditions. We investigated the influence of
precipitation-induced shifts in forage nutritional quality and subsequent
large herbivore responses across widely varying precipitation conditions
in an arid environment. Specifically, we assessed seasonal changes in diet
breadth and forage selection of adult female desert bighorn sheep Ovis
canadensis mexicana in relation to potential nutritional limitations in
forage N, moisture and energy content (as proxied by dry matter
digestibility, DMD). Succulents were consistently high in moisture but low
in N and grasses were low in N and moisture until the wet period. Nitrogen
and moisture content of shrubs and forbs varied among seasons and climatic
periods, whereas trees had consistently high N and moderate moisture
levels. Shrubs, trees and succulents composed most of the seasonal sheep
diets but had little variation in DMD. Across all seasons during drought
and during summer with average precipitation, forages selected by sheep
were higher in N and moisture than that of available forage. Differences
in DMD between sheep diets and available forage were minor. Diet breadth
was lowest during drought and increased with precipitation, reflecting a
reliance on few key forage species during drought. Overall, forage
selection was more strongly associated with N and moisture content than
energy content. Our study demonstrates that unlike north-temperate
ungulates which are generally reported to be energy-limited, N and
moisture may be more nutritionally limiting for desert ungulates than
digestible energy.