BackgroundAnts provide essential ecosystem functions such as biological pest control, seed dispersal, and soil modification. Consequently, ant communities are a powerful monitoring and assessment tool in land management due to their sensitivity to environmental conditions combined with their great functional importance and ease of sampling.
On the other end of the scale is the African elephant (Loxodonta africana), considered to be one of the most influential of the African megafauna as far as removing, or transforming, vegetation from the environment. Consequently, they have the ability to transform landscapes by reducing plant biomass leading to increased landscape patchiness at the local scale. However, it has been argued that although such local scale changes might increase habitat heterogeneity and consequently biodiversity, such benefits are short-term and could potentially have long-term implications for overall landscape heterogeneity, often reducing such heterogeneity in the long-term with potential implications for biodiversity conservation.
This study is using ants to investigate how herbivore pressure and elephant-induced changes to vegetation, as a function of distance from water, influences functional diversity and species richness patterns of ant communities at several spatial scales. This work complements the existing body of literature focusing on herbivore utilisation impacts as a function of distance from water at the local scale. Questions related to this issue have been well studied for plants and more well-known animals groups, e.g. mammals, but less so for invertebrates. Knowledge generated through this study is not only important for our general understanding of what factors shape the spatial dynamics of ecosystem functions, but will also provide empirical directions for biodiversity conservation in the face of environmental change.
- To quantify diversity changes along a herbivore pressure gradient away from a perennial water source within the mopane woodlands of northern Botswana.
- To quantify the following as a function of distance to water:
- Herbivore pressure gradient, using vegetation parameters such as changes in grass and shrub species diversity, percentage grass cover, and stem density of woody vegetation, as well as herbivore pellet counts.
- Elephant-induced changes to vegetation, by assessing utilization of woody plant species.
- Changes in species richness and abundance, and taxonomic and functional diversity in ant assemblages.
MethodologySix transects, stretching away from a perennial water source (the Savute Channel) and extending into mopane woodlands will be sampled. Several components of biodiversity will be investigated at five sampling points, spaced 10 metres apart, along six distance categories away from the water source.
To investigate the presence of herbivore species at each sampling point along a distance category we will perform dung pellet counts. The data will then be divided into presence/absence data as well as the number of herbivore species present based on dung identification.
We will identify and note all grass and shrub species present within a 5 x 5 m plot at each of the sampling points. A 0.5 x 0.5 m quadrat will be used to assess the percentage cover of grass species at each sampling point. Grass and shrub species will be identified using field guides.
The Point Quarter Distance method will be used to investigate stem density of woody vegetation at each sampling point. The area around each sampling point will be divided into 4 equal areas extending from the centre point. Within each area, the distance to the nearest woody vegetation stem will be measured using a range finder.
Within each plot, data reflecting the vegetation structure will be recorded on all woody vegetation using a 50 m tape that will be laid out in a northerly direction from the zero position of the GPS coordinate of each plot. Vegetation data will include the vegetation type, the species of woody plant, its height, and diameter at 50 cm above ground, and will be recorded at several tree height classes. All woody species will be assessed for new elephant-induced damage, i.e. debarking and breaking of stems and branches.
Finally, pitfall traps will be employed to capture ants in the study areas. Each sampling point along the distance categories will contain a triplet of pitfall traps within a 2 m radius of the sampling point. The pitfall traps will remain in the field for five nights and will be serviced on the third night. This is to ensure that as few pitfall traps as possible are lost due to interference by animals. Pitfall traps will contain a diluted industrial-grade preservative; ant specimens will be stored in evacuated blood collection tubes and will be identified and sorted according to morpho-species.
Data collection will take place during two time intervals, namely the end of the wet season (March/April), and the end of the dry season (September/October). This is to ensure that we encompass the extremes of water availability within the mopane woodlands. Full-time data analysis will begin in January 2013 and should be completed by the end of March 2013.