Conservation of long-lived species requires that populations either persist or increase over time. Determining population trajectories is challenging, as a species’ vital rates can vary widely in response to spatiotemporal variability, and individual vital rates (e.g., recruitment, survival) do not always respond in sync to changing environmental conditions. At best, tradeoffs in vital rate performance among populations generate “demographic compensation” that maintains a species’ viability across a broader spatial and temporal range. However, examples of these dynamics to help inform conservation efforts are limited. Here we present the results of a multi-year experiment evaluating the effects of managed grazing on the population dynamics of the perennial Stipa pulchra at Vasco Caves Regional Preserve (Livermore, CA). We tagged Stipa across three demographic stages (seedling, juvenile and adult) within six paired sheep or cow-grazed and ungrazed plots across an environmental gradient. For 5 years, we monitored survival and reproductive effort of tagged individuals and used stochastic life table response experiments to disentangle the effect of stochastic environmental processes (e.g. precipitation) and deterministic abiotic and biotic processes on population growth and demographic rates.
Results/Conclusions
We found that population growth tracked rainfall such that it declined in dry years but increased in wet years. This pattern was exacerbated by resource environment and grazing. In dry years, grazing reduced the growth of individuals in low resource areas where overall Stipa populations were in decline, whereas grazing increased growth rates in high resource areas where overall Stipa population growth rates were increasing. Growth and survivorship in the juvenile and adult life stages were key demographic processes that contributed to these differential responses to grazing over time. Variability in these demographic transitions was reduced when herbivores were present. These results provide an in-depth view into how multiple ecological processes interact to influence demographic responses across a landscape over time. Furthermore, they indicate that ideal management interventions must account for demographic tradeoffs, enhancing growth under good conditions without exacerbating declines under bad