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Biotic determinants of heterogeneity in a South African savanna

Biotic determinants of heterogeneity in a South African savanna
Cleo Mirianthe Gosling

2014

Department of Biological Sciences, University of Groningen, 9700 AB Groningen, THE NETHERLANDS.

ABSTRACT

Mesic savannas are defined by a heterogeneous mix of different vegetation structural types. The cause and maintenance of this heterogeneity has been attributed to a number of different, but interlinked, drivers across a range of scales. These drivers can broadly be grouped into either abiotic factors that influence plant abiotic stress and resource availability, versus biotic factors that create disturbances or influence plant competitive interactions. It has been widely assumed that abiotic factors create large scale templates within which biotic factors drive smaller scale pattern (thus a spatial and causal hierarchy from abiotic to biotic factors in driving vegetation structure), but this is rarely explicitly tested.

The aim of this thesis was to describe, and explain, vegetation structural heterogeneity across a range of scales, from 1 m2 to 0.25 km2 in Hluhluwe iMfolozi Park (HiP), South Africa. The particular focus was on the effect of 2 groups of biotic drivers of vegetation heterogeneity, termites and rhino, against variation in background abiotic drivers. Both termites and rhino have been described as ecosystem engineers. The effects of termite mounds have been well documented elsewhere, but rarely have direct comparisons been made of mounds built by multiple different genera in a single study. Unlike termites, the effects of rhino have often been referred to as important, especially as creators of heterogeneity, but little research has been done to support these claims. This thesis documents the density of rhino features (such as middens, wallows, paths, etc.) and their effects on the surrounding vegetation. An additional, and unique, feature of this thesis is the direct comparison between the rhino and termites; mega- versus micro- herbivores.

At the largest scales we found that biotic and abiotic factors were equally important in explaining the distribution of the different vegetation structural types across scales. Rainfall and rhino features did not feature as strong predictors of any of the vegetation structural types, contrary to expectation from the general literature, where both feature as key factors in structuring savanna ecosystems. In our study the presence of termite mounds, specifically Macrotermes mounds, was the strongest single preditor of vegetation structure, with additional important effects of fire frequency. At more local scales, both termite mounds and rhino created features had significant effects on the vegetation in close proximity to them. The spatial extent of effects varied from 2–5 m from the features. The spatial extent of the effects was least pronounced for the smallest and most predated feature, Trinervitermes mounds. The other features affected similar areas per feature. However, the comparatively low densities at which Odontotermes mounds and rhino middens occur, even in an area with one of the highest rhino densities in the world, means that Macrotermes mounds were by far the most important of these features in shaping the savanna vegetation landscape.

Since termite mounds have been found as important features in creating and maintaining vegetation heterogeneity we then looked at a possible mechanism through which they create such effects; erosion of soil from above ground mounds to the surrounding soil surface. This may alter the nutrient status of the surrounding soils and thereby influence vegetation heterogeneity through altered plant-plant, and plant-herbivore, interactions. Although Trinervitermes mound soils had the largest impact on vegetation in a greenhouse bioassay, we found that in the field, Macrotermes mounds had the largest impact on surrounding vegetation. By linking the different nutrient availability of the mounds, we conclude that the type of mound plays a crucial role in determining the nature of the effects.

In the final data chapter we examine the interactive effects of rainfall, vegetation structure, large herbivore presence, and soil physical factors on termite activity. We found that termite activity declined with increasing rainfall and grass height and in the presence of large herbivores. Our results suggest that increased rainfall acts on termite abundance across large spatial and temporal scales, while vegetation biomass influences termite activity on smaller temporal and spatial scales. We suggest that conditions that stimulate microbial decomposition have a negative effect on termite activity. This may be interpreted as increased competition for food resources between termites and free living microbes. Therefore, the impacts of termites on nutrient cycling seem most pronounced when abiotic and biotic conditions limit decomposition by free living microbes.

In this thesis we have shown that termites are as, if not more, important for generating vegetation heterogeneity as other, larger animals, and processes such as fire and rainfall. However, we suggest that their effects maybe mediated, at still smaller scales, by free living microbes. Re-wilding efforts for general conservation purposes, especially in areas of limited size would do well to encourage healthy termite populations and the development of termite mounds before introducing suites of large herbivores.