Small mammal dynamics in Serengeti, East Africa: implications of climate and land use change for a savannah ecosystem

Zebra and wildebeest grazing on the Serengeti plains. Image - Andrea Byrom.

In Issue 10 of Kararehe Kino (June 2007), Andrea Byrom and Wendy Ruscoe reported on a visit to the Serengeti Biodiversity Programme in Tanzania. The programme has been running at a variety of sites in Serengeti National Park for almost 45 years, and is run jointly by the Tanzanian Wildlife Research Institute and Professor Tony Sinclair of the University of British Columbia. It specifically seeks to understand (1) the factors affecting all species in the park and (2) links between the protected area and human populations in areas surrounding the park. Andrea and Wendy, with colleague Guy Forrester, have reviewed the long-term Serengeti data specifically for rodent populations as indicators of the impacts of climate change.

In African savannah ecosystems, changes in climate and land use are two key drivers of environmental change. Serengeti National Park, a savannah woodland, is a good system in which to investigate the effects of these drivers of global change on ecosystem processes. There, many processes are driven by rainfall, which falls in two wet seasons: the short season rains (November–December) and long rains (March–May). The wet/dry seasonal cycle is critical to the functional dynamics between large predators and their prey within the park, but relatively little is known about how climate drivers such as rainfall affect other components of the ecosystem. For example, not much is known about the distribution and dynamics of small mammals (primarily rodents) in the greater Serengeti ecosystem.

Andrea, Wendy and Guy combined the scattered data on rodent abundance over 1968–2008 with more intensive trapping data collected from 1999–2010. Rodent populations showed interannual fluctuations, remaining at low levels for several years and occasionally ‘outbreaking’ to spectacularly high (plague) numbers (Fig. 1). The data revealed a positive relationship between short-season rainfall and rodent outbreaks, presumably because higher rainfall increased ecosystem productivity.

Andrea, Wendy and Guy also looked at time series data on the abundances of small mammalian carnivores recorded between 1991 and 2010 and of black-shouldered kites between 1968 and 2010, and both groups showed peaks in abundance coincident with peaks in rodent abundance (Fig. 2). These observations suggest that the dynamics of rodents in the Serengeti savannah have important consequences for carnivores, particularly lesser-known ones such as the smaller cats, jackals, viverrids (members of the mongoose family), and birds of prey. Indirectly, rodent outbreaks in the natural ecosystem may help with the conservation of small mammalian carnivores and birds of prey because they provide pulses of food (prey) at critical times.

To the west and north of Serengeti National Park are agricultural areas and villages, where the primary crops are maize and millet. These areas are undergoing unprecedented development, with intensification of land use and human population growth of 2–5% per year. The dynamics of rodents in agricultural areas are also characterised by outbreaks, but unlike in the natural savannah, some rodents were always detected even during ‘low’ phases between outbreaks. Outbreaks cause significant economic losses because the rodents damage crops and stored grain. In outbreak years, rodents can also transmit diseases directly to humans (e.g. bubonic plague) and to domestic dogs and cats, which are then thought to transmit diseases back to wild carnivores (e.g. distemper, parvovirus, and rabies). In agricultural areas, therefore, rodents are regarded as pests.Climate change predictions for East Africa suggest that both the amount and variability of rainfall will decrease over the next two decades, which might mean fewerodent outbreaks both in agricultural areas and the natural savannah. This is good news for managing rodents in agricultural areas, and may help reduce disease outbreaks in wildlife and humans. On the other hand, as Andrea and Wendy have discovered, rodents are a vital part of the natural savannah ecosystem because they provide food for many threatened carnivore species. This sets up a complex conflict between predicting and managing for desirable outcomes for natural verses human-dominated ecosystems in East Africa.

The research on small mammals in the Serengeti has increased the known number of rodent genera from 28 to 32, and extended the geographic range of three native and one introduced rodent genus. Riverine floodplains and woodlands had the greatest diversity of small mammals, with lower diversity on short grass plains and rocky outcrops (kopjes), and in villages and farms outside the park.

Travel for this work was funded by the Serengeti Biodiversity Program. The results have been written up as a chapter in Serengeti IV (due to be published in 2012) under funding from the Ministry of Science and Innovation (Programmes C09X0505 and C09X0909).

Andrea Byrom, Wendy Ruscoe & Guy Forrester

A team of international colleagues has contributed to research on rodents, carnivores, and birds of prey in the Serengeti. The team comprises: Anthony Sinclair and Kristine Metzger (Beatty Biodiversity Centre, University of British Columbia); Stephen Makacha, John Bukombeand Joseph Nkwabi (Serengeti Biodiversity Program, Tanzania Wildlife Research Institute); Meggan Craft and Katie Hampson (Division of Ecology and Evolutionary Biology, University of Glasgow); Sarah Durant (Institute of Zoology, Zoological Society of London); Simon Mduma (Tanzania Wildlife Research Institute, Arusha); and Denne Reed (Department of Anthropology, University of Texas)