Programme de bourses "Jeunes Chercheurs"

Herbivore-vegetation interactions in oligotrophic savanna: exploring the limits for grazers to drive landscape dynamics

Savannas form Africa’s largest biome and support the majority of the rural human population. Most of the national parks generating income from safari tourism are also located in savanna ecosystems as these can support high densities of large mammalian herbivores. Savannas may look like stable ecosystems at a first glance but in fact are inherently dynamic ecosystems. Fire and grazing or browsing by mammalian herbivores are the major agents driving dynamics; they influence each other and ultimately depend on soil moisture and nutrient availability. Whereas the well studied ‘grazing systems’ of drier nutrient-rich short grass savannas such as the Serengeti area are clearly herbivore-driven, dynamics of the many wetter and nutrient-poor tall grass savannas are poorly understood but are thought to be mainly fire-driven. Annual burning of savannas is widespread, not only among subsistence farmers or to ease hunting and poaching, but also in the official management of national parks. While the short-term effects are obvious – initiating fresh flush of nutrient-rich grasses – long-term effects may be less beneficial as they include loss of nutrients, especially nitrogen, from the ecosystem.

The Saadani National Park (gazetted in 2003) is one of Tanzania’s youngest national parks and protects an area of coastal savanna north of Dar-es-Salaam. Current vegetation is a mosaic of both tall- and shortgrass savanna and, as is not untypical for protected areas in Africa, reflects a varied land-use history which includes sisal plantations and cattle ranching. Saadani NP has a unique community of wild herbivores including some species not native to coastal tall grass, and larger herds concentrate in some short grass areas that largely coincide with the area formerly used for sisal production. These patches of short grass are highly dynamic in space and time as the larger herbivores such as wildebeest and waterbuck graze them down to so-called grazing lawns. However, burning is intense as well, as fires are created both by poachers and within the burning scheme operated by the park authorities. The main question, relevant for park management as well as for basic understanding of how savannas function, is whether larger grazers can maintain a short-grass environment by grazing alone, or whether periodic burning is necessary for providing the starting conditions for grazing lawn formation in a tall grass savanna. Moreover, there is the question of whether more selective herbivores are better adapted to tall grass environment and might thus form a community whose needs have to be addressed by park management.

In short, this study aims to improve our understanding of the mechanisms by which grazing herbivores drive vegetation dynamics in oligotrophic savannas, how this process is mediated by fire, and how ungulate species with contrasting feeding strategies thereby coexist.

The project combines four research approaches:

• Mapping spatial distributions and movements of two abundant mesoherbivores with contrasting feeding ecology, wildebeest (bulk grazer) and bohor reedbuck (selective grazer), by mapping and detailed observation of foraging individuals, and analyzing the results for habitat selection and foraging tactics on three spatial scales.
• Measuring and modelling grazing intensity across gradients taking in the variability of the open savanna, and by focusing on grazing lawns and their dynamics, in relation to grass species composition, soil nutrient availability, nutritional value, fire incidence, and other attributes.
• Establishing a field experiment to investigate the factors influencing the establishment and maintenance of grazing lawns.
• Determining population composition and reproductive parameters of the selected herbivores to assess the degrees of adaptation to tall grass savanna of native and non-native herbivores.

This research will contribute to an improved understanding of (a) how grazing ecosystems function under poor nutrient supply, (b) where the limits are for larger herbivores to drive ecosystem structure, and (c) how their influence is mediated by fire. As two of the grazers, wildebeest and zebra, are not native in the region, it may also reveal whether the Saadani ecosystem is capable of maintaining itself, or whether it will develop to become unsuitable for these animals. This type of information will be valuable for managing Saadani National Park, particularly with respect to fire management, as well as in attempts to recreate grazing ecosystems in other savanna areas that have lost their wildlife.

The project is part of an ongoing research program in which the Swiss Federal Institute of Technology Zurich (ETHZ), the Swiss Federal Research Institute WSL and Sokoine University of Agriculture, Morogoro (SUA) collaborate. Emerging from this, further cooperation between Ghent University, Belgium, WSL and SUA to address biodiversity conservation in coastal forest fragments of the Saadani region has just been established.

Fig. 1. Bohor Reedbuck (Redunca redunca) is a tall grass-adapted herbivore. An uncommon species over much of its distribution area, it is present in high densities in Saadani National Park.

Fig. 2. Strongly grazed patch surrounded by tall grass savanna, Saadani National Park.

Fig. 3. Researchers and game ranger in discussion over the species identity of antelope dung pellets which are used to assess grazing intensity of study plots.

Contacts

PhD student:
Annette Stähli
Swiss Federal Research Institute WSL
Zürcherstrasse 111
8903 Birmensdorf
Switzerland
Email: annette.staehli@wsl.ch

MSc student:
Cecilia Leweri
Sokoine University of Agriculture
PO Box 3073
Morogoro
Tanzania
Email: cleweri@yahoo.com

Project leader, supervisor, main contact:
Dr. Werner Suter
Swiss Federal Research Institute WSL
Zürcherstrasse 111
8903 Birmensdorf
Switzerland
Tel. ++41 (0)44 739 25 67
Email: werner.suter@wsl.ch

Supervisors:
Prof. Peter J. Edwards
Swiss Federal Institute of Technology (ETHZ)
8092 Zürich
Switzerland
Email: edwards@env.ethz.ch

Dr. Harry Olde Venterink
Swiss Federal Institute of Technology (ETHZ)
8092 Zürich
Switzerland
Email: harry.oldeventerink@env.ethz.ch

Research partner:
Prof. Alexander N. Songorwa
Head, Dept. of Wildlife Management
Sokoine University of Agriculture
PO Box 3073
Morogoro
Tanzania
E-mail: bhugoji@yahoo.com