The shortgrass steppe ecosystem has a long evolutionary history of large animal grazing by bison, which were replaced by domesticated livestock in the mid 1800s. In addition, this ecosystem is characterized by a semi-arid environment with low annual precipitation amounts, but high inter- and intra-annual variability. Variability in forage production in this ecosystem has been shown to be explained mostly by cool-season precipitation, and average daily gains of livestock decrease as the ratio of animal demand to forage increases. The relative influence of intra- versus inter-annual variation in precipitation on the ecosystem services of livestock productivity, and how these may vary with grazing intensity, has not been quantitatively evaluated. Here, we utilize long-term (1940-2008) precipitation and livestock gain data from pastures grazed at light, moderate and heavy intensities to determine environmental variability in precipitation for 1) annual, 2) early-season (April to June), 3) late-season (July to September), and 4) total growing season (April to September) components, as well as for variability in ecosystem services of 5) livestock grazing season gains (kg/head), 6) average daily gains (kg/head/day), and 7) beef production (kg/hectare). Relationships between environmental variability and variability in the provision of ecosystem services (livestock gains) were evaluated.
Results/Conclusions Environmental variability in precipitation was evident across the 69 years with ten-fold differences between lowest and highest values in the early (39mm vs. 348 mm) and late (28 vs. 292) season, with the entire growing season (92 vs. 515) and annual precipitation (110 vs. 581) exhibiting five-fold differences. Coefficient of variations were higher for early (42%) and late (43%) season precipitation compared to the entire grown season (33%)and annual (29%) values. Daily and seasonal livestock weight gains per animal were similar under light and moderate grazing, but declined 15-21% under heavy grazing. Conversely weight gains per unit area were 29-60% greater with heavy compared to light and moderate grazing, and coefficients of variation for all three ecosystem services were higher with heavy grazing. Relationships between environmental variability and provision of these ecosystem services were not observed. Findings suggest that there is a disconnect between forage production responses to cool-season precipitation and livestock production responses to the ratio of animal demand to forage availability as these relationships function at different temporal scales. Therefore, temporal patterns and variation in ecosystem services pertaining to livestock productivity in shortgrass steppe are more likely driven by forage quantity/quality tradeoffs.
