Grazing by large savanna herbivores indirectly alters ant diversity and promotes resource monopolisation.

Jean Purdon,Catherine L Parr,Michael J Somers

doi:10.7717/peerj.6226

Abstract

In savannas, grazing is an important disturbance that modifies the grass layer structure and composition. Habitat structural complexity influences species diversity and assemblage functioning. By using a combination of natural sites and manipulated experiments, we explored how habitat structure (grazing lawns and adjacent bunch grass) affects ant diversity and foraging behaviour, specifically the efficiency of resource acquisition, resource monopolisation and ant body size. We found that in the natural sites there was no difference in the amount of time ants took to locate resources, but in the manipulated experiments, ants were faster at locating resources and were more abundant in the simple treatments than in the more complex treatments. Ant body size was only affected by the manipulated experiments, with smaller ants found in the more complex treatments. In both the grazing lawn and bunch grass habitats there were differences in assemblage patterns of ants discovering resources and those dominating them. Seasonality, which was predicted to affect the speed at which ants discovered resources and the intensity of resource monopolisation, also played a role. We show that ants in winter monopolised more baits and discovered resources at a slower rate, but only at certain times within the experiment. Grazing in conjunction with season thus had a significant effect on ant diversity and foraging behaviour, with dominant ants promoted where habitat complexity was simplified when temperatures were low. Our results indicate that structural complexity plays a major role in determining ant assemblage structure and function in African savannas.

Highlights

Determining the factors that govern species diversity, interactions and patterns is essential to understanding ecological processes and functioning (Oliver et al, 2015)
Our results suggest a more complex environment in the bunch grass habitat compared with the grazing lawn habitat (Table 1)
Our study shows that the creation of grazing lawns by large mammals plays an important role in promoting local heterogeneity as ant assemblage composition differed significantly between the two habitats

Summary

Introduction

Determining the factors that govern species diversity, interactions and patterns is essential to understanding ecological processes and functioning (Oliver et al, 2015). These interstitial sizes will decrease, excluding certain species of a particular size class, restricting their foraging area and altering competitive interactions (Kaspari & Weiser, 1999; Parr, Parr & Chown, 2003; Farji-Brener, 2004; Sarty, Abbott & Lester, 2006). As the structural complexity of a habitat decreases, the size and number of the interstices decrease, enabling species to access and locate resources more efficiently This will increase the number of encounters among species, resulting in competition for resources (Gibb & Parr, 2010). In the savannas of northern Australia, ants were found to disperse seeds significantly farther after a fire that consumed much of the ground layer (Parr et al, 2007)

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