Effects of small mammal outbreaks in the Serengeti

After more than 50 years of research into the ecology of large herbivores and predators in the greater Serengeti ecosystem, you might think that we know almost everything there is to know about this tropical savanna ecosystem. But in our article, “Episodic outbreaks of small mammals influence predator community dynamics in an East African savanna ecosystem” (Andrea E. Byrom et al.), we show that relatively little attention has focused on the role of small mammals (rodents and shrews) in tropical African savannas such as the Serengeti. This presents a critical gap in our understanding of one of Africa’s best known ecosystems.

Tropical savanna woodland, Serengeti National Park

We do know that in agricultural areas throughout East Africa, rodent populations fluctuate (outbreak) with large peaks in abundance, triggered by increased food availability during the dry season in response to the amount of rain in the preceding wet season. When outbreaks occur, species such as the multimammate rat Mastomys natalensis and the African grass rat Arvicanthis niloticus cause substantial economic damage in crop-growing areas. Before our study, however, little was known about the population dynamics of small mammals in tropical savanna, or their trophic importance, including as prey for some threatened carnivore species. Small mammals are a known food source for predators in this system, including mammalian carnivores in the weight range 1–18 kg, and birds of prey.

Arvicanthus niloticus, the African grass rat

It’s not surprising that researchers travel from all over the world to study an ecosystem as diverse and well-known as the Serengeti. Our team comprised researchers from Tanzania, New Zealand, Australia, the USA, Canada, and the UK – all scientists who had lived or worked in East Africa. Some of us have never met, but we all contributed to collection of the data that were used in this article. We pieced together a 42-year time series (1968-2010) on the abundance of 37 species of small mammals, derived from intermittent measures collected in Serengeti National Park and adjacent agricultural areas. Data on abundance of black-shouldered kites (1968–2010), eight other species of rodent-eating birds (1997–2010), and 10 mammalian carnivore species (1993–2010) were also collated.

Black-chested snake eagle, Circaetus pectoralis

We used climatic fluctuations and differences between unmodified and agricultural systems as perturbations to examine both bottom-up and top-down drivers of small mammal abundance: key to understanding responses to climate change and increasing human pressures adjacent to Serengeti National Park. Outbreaks occurred every 3–5 years in Serengeti National Park, with low or zero abundance of small mammals between peaks. There was a positive relationship between rainfall in the wet season and (a) small mammal abundance and (b) the probability of an outbreak, both of which increased with negative Southern Oscillation Index values. Rodent-eating birds and carnivores peaked 6–12 months after small mammals. In agricultural areas, abundance remained higher than in natural habitats.

 

The serval, Leptailurus serval

We conclude that small mammal outbreaks have strong cascading effects on predators in African savanna ecosystems. Changes in climate and land use may alter their future dynamics, with consequences for higher trophic levels, including threatened carnivores. Although outbreaks cause substantial damage to crops in agricultural areas, small mammals also play a vital role in maintaining some of the diversity and complexity found in African savanna ecosystems. Our study provides vital baseline data from which to monitor the future resilience of tropical savanna ecosystems.