The potential utility of rodents and other small mammals as indicators of ecosystem 'integrity' of South African grasslands

Abstract

Context The expansive grassland biome is one of the most extensively transformed in South Africa, yet no strategy for monitoring its integrity is in place. A grassland health program, incorporating different ecosystem levels, was recently initiated. The suitability of three taxonomic groups as indicators has been tested so far: vegetation (by calculating an ecological index value, El), insects (using the South African grassland scoring system, SAGraSS) and small mammals (this study). All of these methods aim to be rapid and easy to perform. Whereas SAGraSS still needs further refinement, several factors already indicate the importance of including small mammal community parameters in integrity assessments. Aims This contribution reports on more than 12 years of results from various studies on small mammals in the Free State Grasslands, with the aim of exploring the utility of small mammal survey for assessment of ecosystem integrity. Methods The hypothesis was based on the outcomes of several short-term studies conducted in the grassland biome. Combining all previous results, this paper re-evaluates the parameters of trap success, species richness, diversity, evenness and individual species as bio-indicators. Key results By combining data from many sites and years, the effect of seasonal and inter-annual variations in habitat and population parameters was diminished, and a more general picture of small mammal community structure revealed. New insights were gleaned into the status of several indicator species. By providing a summary of small mammal community parameter scores and indices, the study establishes a benchmark for future small mammal assessments and monitoring. To be effective, small mammal surveys in the grassland biome of southern Africa should be carried out in autumn and early winter. Conclusions This study suggests that small mammal species richness and diversity decline with habitat degradation; that the generalist rodent Mastomys coucha dominates community numbers at low ecological integrity; that the number of specialist species increases towards ecological climax; and that specific species act as indicators during the successional process. Implications This study should benefit the monitoring, conservation and management of grassland ecosystems, make environmental impact assessments more effective, and identify new topics for ecological research.