Trophic Interactions Drive Tree Establishment in the Serengeti Ecosystem

Photo 1. Tree seedlings in savanna ecosystems, such as this Vachellia tortillis seedling in Tanzania's Serengeti National Park, face competition from grasses and are regularly damaged by fire and browsers. These filters to survival result in demographic bottlenecks in savanna tree populations. We found that grass competition accounted for the majority of early seedling mortality. Photo credit: Jason Donaldson. Photo 2. Regular fires, fueled by a buildup in grass biomass, kill high numbers of tree seedlings with mortality rates highest in areas where grass competition is historically high. Photo credit: Jason Donaldson. Photo 3. Impala (Aepyceros melampus) and other browsing ungulate herbivores remove vegetative growth and kill seedlings, exacerbating the negative effects of fire and grass competition on seedling establishment. There was no evidence that browsers in the Serengeti were responding to lion kill risk in a way that changed their spatial impacts on tree establishment. Photo credit: Jason Donaldson. Photo 4. Expansive herds of wildebeest (Connochaetes taurinus), zebra (Equus quagga), and other ungulate grazers in the Serengeti remove grass biomass and can reduce the detrimental effects of grass competition and fire, resulting in increased tree seedling establishment. Despite this, lion kill risk does not meaningfully impact grazer effects on tree seedling survival. Photo credit: Jason Donaldson. Photo 5. Tree seedling establishment will substantially increase in systems containing high densities of grazing livestock, where wild browsing ungulates have been removed, and tree seedlings are released from grass competition and fire. It is likely that this scenario is occurring over broad areas of savanna globally. Photo credit: Jason Donaldson. These photographs illustrate the article “Fire, grazers and browsers interact with grass competition to determine tree establishment in an African savanna” by Jason E. Donaldson, Ricardo Holdo, Jeremia Sarakikya, T. Michael Anderson published in Ecology. https://doi.org/10.1002/ecy.3715