Legacy effects of top-down disturbances on woody plant species composition in semi-arid systems.

Savanna vegetation is controlled by bottom-up (e.g. soil and rainfall) and top-down (e.g. fire and herbivory) factors, all of which have an effect on biodiversity. Little is known about the relative contribution of these factors to biodiversity, particularly the long-term effects of top-down disturbance on patterns of woody plant composition. The aim of this study was to identify if various degrees of disturbance regimes create distinct woody species community assemblages. Data were collected over 1820 plots across Kruger National Park, South Africa. Woody species were identified and categorized into one of three height classes: shrub (0.75-2.5 m), brush (2.5-5.5 m), and tree (>5.5 m). Species richness and composition were calculated for each site and height class. A combination of long-term fire and elephant density data were used to delineate areas with varying degrees of top-down disturbance (i.e. low, medium and high). Using these degrees of disturbance, species composition was identified and community assemblages constructed according to each disturbance regime. Our results suggest that areas with similar disturbance regimes have similar species composition. Shrub composition was mainly responsive to the number of fires between the years 1941-1990, while tree composition was more responsive to elephant disturbance. A few dominant species were found equally under all degrees of disturbance at all height classes, while others were more regularly found under specific disturbance regimes at particular height classes. This study highlights that while species richness does not appear to be influenced by long-term, top-down disturbance regimes, species community composition may be responsive to these disturbances. Most species and structural classes persisted across all disturbance regimes, but the long-term effects of top-down disturbances can influence compositional and structural biodiversity. This information provides context for management policies related to artificial water provision, elephants and fire. [ABSTRACT FROM AUTHOR]