Does spatial heterogeneity mitigate the effect of extreme temporal variability on large herbivores? A test at local spatial scales in a semi-arid grassland
Dryland ecosystems are characterized by dramatic temporal variability in precipitation, which presents a major challenge for herbivores attempting to track such variability. Herbivore movement and selection of foraging sites in response to spatial variability at multiple scales is recognized as an important strategy for coping with temporal variability, but empirical studies on the scale and type of heterogeneity that mitigates temporal extremes are rare. We examined the influence of spatial heterogeneity in soils and topography on the performance of cattle grazing semi-arid rangeland (shortgrass steppe of eastern Colorado) at the scale of individual pastures (~65 – 130 ha) over 3 years (2010 – 2012) that varied from above-average precipitation to extreme drought. Each year, we measured cattle weight gains in 27 different pastures stocked at the same rate with yearling steers (0.16 animals.ha-1 during May – October growing season), but that varied in terms of pasture-scale, mean forage productivity as well as spatial heterogeneity in soils and topography. We quantified heterogeneity in terms of the diversity of ecological sites within each pasture, and in terms of the variance and skew of a topographic wetness index derived from a 1-m resolution digital elevation model.
Cattle weight gains were strongly correlated with pasture-scale estimates of mean forage production derived from soil maps and their associated ecological site descriptions for years with above- and near-average precipitation (r2 = 0.41 – 0.62). However, during a severe drought year, cattle weight gains were uncoupled from pasture-scale mean forage production (r2 = 0.04), and instead were best predicted by a model based on the topographic wetness index (reflecting the amount of the pasture consisting of swales/drainages) and ecological site diversity (r2 = 0.52). This result suggests that access to multiple, distinct plant communities mitigated the impacts of drought on cattle performance, possibly because cattle could shift grazing distribution away from upland, shortgrass communities in those pastures with a diverse mix of ecological sites. We discuss the influence of local heterogeneity on grazer performance during drought in relation to the role of heterogeneity at larger spatial scales (e.g. spatial variation in storm events, landscape-scale topoedaphic heterogeneity), and other strategies for coping with drought, such as conservative and flexible stocking, drought forecasting, and grassbanking.