Herbivory and drought generate short-term stochasticity and long-term stability in a savanna understory community

Authors: RIGINOS, CORINNA - University Of Wyoming; Porensky, Lauren; VEBLEN, KARI - Utah State University; YOUNG, TRUMAN - University Of California

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2017
Publication Date: 10/2/2017
Citation: Riginos, C., Porensky, L.M., Veblen, K.E., Young, T.P. 2017. Herbivory and drought generate short-term stochasticity and long-term stability in a savanna understory community. Ecological Applications. 28(2):323-335. https://doi.org/10.1002/eap.1649.
DOI: https://doi.org/10.1002/eap.1649

Interpretive Summary: Rainfall and herbivory are two fundamental drivers of grassland plant dynamics, yet few studies have examined the combined effects of these drivers on grassland plant communities. In many grasslands, a better understanding of the interactions between rainfall and herbivory is needed because livestock are replacing native wild herbivores and droughts are becoming more frequent. We examined herbaceous community composition through multiple drought periods that occurred over 15 years of the Kenya Long-term Exclosure Experiment (KLEE), which uses different types of fencing to allow six different combinations of cattle, native meso-herbivores (e.g. zebras, gazelles) and native mega-herbivores (giraffes and elephants) access to experimental plots. We found that plant community change was greatest in the ungrazed and lightly grazed treatments, and least in the more heavily grazed, cattle-present treatments. Community change was greatest during and immediately after drought in all herbivore treatments. Droughts contributed to consistent directional change in the less grazed treatments, but it had more variable effects on community composition in the more heavily grazed treatments, and generally helped to stabilize community composition over the long-term. Much of the community change in lightly grazed treatments was due to increases in the cover of a competitively dominant but grazing-sensitive grass, Brachiaria lachnantha, especially after droughts. These results illustrate how herbivory and rainfall act together to cause change in grassland communities not only at heavy grazing intensities, but also at exceptionally low grazing intensities. Further, they show that livestock grazing at moderate intensities need not destabilize — and in fact, may help to stabilize — long-term plant community composition in grassland systems with a long evolutionary history of grazing.

Technical Abstract: Rainfall and herbivory are two fundamental drivers of grassland plant dynamics, yet few studies have examined the long-term interactions between these factors in an experimental setting. An understanding of these interactions is important, as native wild herbivores are being replaced by livestock and droughts are becoming more frequent in grassland systems. The combination of livestock grazing and droughts are thought to interact, leading to much greater community change than either factor alone. We examined patterns of change and stability in herbaceous community composition through multiple drought periods that occurred over 15 years of the Kenya Long-term Exclosure Experiment (KLEE), which is comprised of six different herbivory treatments including no large herbivores, cattle only, native wild herbivores (e.g. zebras, gazelles) excluding and including mega-herbivores (giraffes and elephants), and different combinations of cattle and native herbivores. We used principal response curves (PRC) to analyse the trajectory of change in each herbivore treatment in relation to a common initial community and in response to four distinct drought periods, in order to ask whether herbivore treatment and drought interact to cause community change. Additionally, we quantified community “stability” using three different metrics: resistance, temporal variability, and turnover. Net community change was greatest (and resistance least) in the ungrazed and lightly grazed treatments (no large herbivores and wild herbivores alone), and least in the more heavily grazed cattle-present treatments. Community change was greatest during and immediately after episodic droughts in all herbivore treatments, but while droughts contributed to consistent directional change in the less grazed treatments, it had more variable effects on community composition in the more heavily grazed treatments. Much of the community change in lightly grazed treatments was due to substantial increases in the competitive dominant but grazing sensitive grass, Brachiaria lachnantha, especially after droughts. Synthesis: These results illustrate how herbivory and rainfall act together to cause change in grassland communities not only at the high end, but also at the low end of the grazing continuum. Further, they show that livestock grazing at moderate intensity need not destabilize — and in fact, may help to stabilize — long-term plant community composition in grassland systems with a long evolutionary history of grazing.