Herbivory and drought reduce the temporal stability of herbaceous cover by increasing synchrony in a semi-arid savanna

Authors: EBEL, CARMEN - University Of Oregon; CASE, MADELON - Mpala Research Centre And Wildlife Foundation; KIMUYU, DUNCAN - Karatina University; LANGENDORF, RYAN - University Of Colorado; Porensky, Lauren; VEBLEN, KARI - Utah State University; WELLS, HARRY - University Of Leeds; WERNER, CHHAYA - University Of Wyoming; YOUNG, TRUMAN - University Of California, Davis; HALLETT, LAUREN - University Of Oregon

Submitted to: Frontiers in Ecology and the Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2022
Publication Date: 6/27/2022
Citation: Ebel, C.R., Case, M.F., Kimuyu, D.M., Langendorf, R.E., Porensky, L.M., Veblen, K.E., Wells, H.B., Werner, C.M., Young, T.P., Hallett, L.M. 2022. Herbivory and drought reduce the temporal stability of herbaceous cover by increasing synchrony in a semi-arid savanna. Frontiers in Ecology and the Environment. 10. Article e867051. https://doi.org/10.3389/fevo.2022.867051.
DOI: https://doi.org/10.3389/fevo.2022.867051

Interpretive Summary: Both environmental (bottom-up) processes like drought and consumptive (top-down) pressures like herbivory can influence the abundance and temporal stability of vegetation biomass and cover in semi-arid ecosystems. Moreover, stability can change due to multiple different plant community mechanisms, including 1) species asynchrony, in which a decline in one species is compensated for by a rise in another, 2) stable dominant species driving overall stability, and 3) the portfolio effect, in which multiple species contribute to enhancing stability. Understanding how bottom-up and top-down drivers alter vegetation stability, as well as its underlying mechanisms, will enhance managers' ability to predict and adapt to changing conditions. This study explored drivers of vegetation stability in a semi-arid Kenyan savanna. The authors found that both drought and herbivory tended to reduce stability. Drought and herbivory reduced palatable dominant species, creating space for subordinate species to fluctuate synchronously in response to rainfall variability. This increased synchrony was associated with lower stability. Herbivory but not drought also reduced dominant species population stability, which led to lower overall community stability. The portfolio effect (driven by species diversity) was not important for stability in this system. Results demonstrate that this system is naturally dynamic, and instabilities are associated with weather variability and herbivory. A future of increasing drought may further reduce stability, and drought effects could be either amplified or buffered by concurrent changes in herbivory, such as increased stocking rates. How “stability” itself aligns with management goals will depend on the values applied to a particular system; the constancy of total herbaceous cover, for example, may be a desirable quality for forage availability and erosion control. Alternatively, temporal variability and heterogeneity support other key ecosystem functions and biodiversity. Overall, our ability to understand the sources of variation and stability in dynamic ecosystems will be essential for predicting future patterns of stability and supporting manager decision-making in a changing world.

Technical Abstract: Ecological stability in plant communities is shaped by bottom-up processes like environmental resource fluctuations and top-down controls such as herbivory, each of which have demonstrated direct effects but may also act indirectly by altering plant community dynamics. These indirect effects, called biotic stability mechanisms, have been studied across environmental gradients, but few studies have assessed the importance of top-down controls on biotic stability mechanisms in conjunction with bottom-up processes. Here we use a long-term herbivore exclusion experiment in central Kenya to explore the joint effects of drought and herbivory on three biotic stability mechanisms: 1) species asynchrony, in which a decline in one species is compensated for by a rise in another, 2) stable dominant species driving overall stability, and 3) the portfolio effect, in which a community property is distributed among multiple species. We calculated temporal stability and biotic stability mechanisms over a 22-year time series and with a moving window to examine changes through time. Both drought and herbivory additively reduced asynchronous dynamics, leading to lower stability during droughts and at high herbivore pressure. This effect is likely attributed to a reduction in palatable dominant species under higher herbivory, which creates space for subordinate species to fluctuate synchronously in response to rainfall variability. Dominant species population stability promoted community stability, an effect that did not vary with precipitation but depended on herbivory. The portfolio effect was not important for stability in this system. Our results demonstrate that this system is naturally dynamic, and a future of increasing drought may reduce stability. However, these effects will in turn be amplified or buffered depending on changes in herbivore communities and their direct and indirect impacts on plant community dynamics.