In contrast to New England salt marshes, many African savannas have experienced significant losses of large herbivores. In Gorongosa National Park, Mozambique, these losses occurred during a civil war that spanned a two-decade period beginning in the late 1970s. Here, we used pre-war data, combined with experimental herbivore exclosures and DNA metabarcoding, to show that the loss of herbivores in this system allowed the encroachment of Mimosa pigra, and that the return of certain species following intensive management has begun to reverse this trend. At Mpala Research Centre, in contrast, we use large experimental herbivore exclosures to simulate the loss of herbivores; the site currently supports more than twenty wild herbivores >5kg. Using a combination of long-term exclosure data, herbivore diet analysis, and simple theoretical models, we have shown plant responses to simulated herbivore extinction that span levels of biological organization, from genes to entire plant communities.
African elephant numbers have declined precipitously over the last century, and the continent's remaining elephants are increasingly concentrated in fenced protected areas, where they occur at abnormally high densities. In these settings, elephants cause widespread damage to savanna trees while browsing, and elephant damage has been frequently cited as a major concern by those tasked with the conservation and management of savanna plant communities. Contrary to long-held expectations, we found that elephant damage actually increases understory plant biomass and species richness by indirectly inhibiting non-elephant herbivores and creating canopy-scale refuges for understory grasses and forbs. In this way, elephants enhance an existing associational resistance relationship between spiny trees and their understory neighbors. This work joins a growing body of evidence that the loss of this iconic megaherbivore may lead to a cascade of unintended, negative consequences for savanna ecosystems.
Declines of wild megafauna are expected to transform ecosystems and are known to influence tree-grass balance in African savannas, but the effects of large herbivores on lianas are unknown. Using diet analysis, long-term exclosure experiments, and smaller-scale manipulations, we show that liana infestation occurred rapidly after the loss of large herbivores and suppressed tree growth and reproduction. We show theoretically that extended absence of herbivores could potentially generate a liana-dominated alternative state—but both experimental and theoretical results indicate that herbivore reintroduction can reverse endemic infestation, even after decades. We found low functional redundancy among wild herbivore species, and between wildlife and livestock. Collectively, our results suggest that diverse large-herbivore assemblages promote resilience and robustness of savannas to liana encroachment.
Relevant publications:
Relevant publications:
- Coverdale, TC, TR Kartzinel, K Grabowski, RK Shriver, AA Hassan, JR Goheen, TM Palmer and RM Pringle. Elephants in the understory: opposing direct and indirect effects of consumption and ecosystem engineering. 2016. Ecology 97: 3219-3230.
- Guyton, JA, J Pansu, MC Hutchinson, TR Kartzinel, AB Potter, TC Coverdale, JH Daskin, AG da Conceição, MJS Peel, ME Stalmans, and RM Pringle. 2020. Trophic rewilding revives biotic resistance to shrub invasion. Nature Ecology and Evolution 4:712-724.
- Coverdale, TC, RD O'Connell, MC Hutchinson, A Savagian, TR Kartzinel, TM Palmer, JR Goheen, DJ Augustine, M Sankaran, CE Tarnita, and RM Pringle. Megaherbivores prevent harmful liana infestation in an African savanna. 2021. PNAS, 118: e2101676118.