Quantifying the functional importance of ants in savanna ecosystems

Ants (Hymenoptera: Formicidae) are the dominant invertebrate group in the tropics and mediate a range of ecosystem processes and perform a wide variety of functions, but few studies have quantified their importance in ecosystems at large scales. In savannas, ants may constitute up to 70% of ground-active invertebrates, yet the role they play in structuring these ecosystems is largely unknown. It is thought that ants affect ecological processes on a number of levels. At basal levels, ants affect soil structure and nutrients through their nest-building and scavenging behaviours. At higher trophic levels, ants exert top-down control on invertebrates via predation, including regulating termites which may indirectly alter basal processes such as decomposition rates and soil bioturbation. Through their regulation of herbivorous invertebrates, ants may indirectly limit invertebrate herbivory which may have consequences for plant growth and survivorship. This project will utilise an ant-suppression treatment to exclude ants and measure the changes that occur in the ecosystem in their absence. There are four control plots and four ant-suppression plots. The suppression treatment targets ants without affecting the other invertebrates in the community. This study will determine the extent to which ants affect savanna ecosystem functioning through direct and indirect control of a range of ecosystem processes.
Aim: To quantify the roles of ants in mediating ecosystem functioning in savannas.
Objectives: To quantify the effect of ants on the following ecosystem processes:
1. Soil nutrients, carbon storage and heterogeneity
We will measure the impact of ants on soil properties by analysing soil pH, and concentrations of carbon and other organic soil nutrients associated with soil fertility (pH, P, Ca, Mg, K, Na, N). The structure of the soil will also be analysed.

2. Food resource distribution
We will quantify how much resource redistribution in the savanna can be attributed to ants, and whether vertebrates or non-ant invertebrates compensate in the removal of food resources when ants are excluded from the ecosystem. Bait removal by ants, vertebrates and non-ant invertebrates will be quantified through a combination of ant-suppression treatment and vertebrate-exclusion cages.

3. Termite activity, soil bioturbation and decomposition
We will quantify the impact of ants on termite activity by measuring termite consumption of cellulose baits on all plots every 4 weeks. At various time points over the course of a year, we will measure the amount of termite-mediated decomposition of wood blocks, and we will measure termite soil bioturbation under transects, in all experimental plots.

4. Invertebrate community regulation
We will assess the degree to which ants regulate and structure the invertebrate community by sampling invertebrates from different strata (soil, ground, grass, arboreal) and comparing community composition in the treatment and control plots,

5. Tree growth and herbivory
We will determine the impact of ants on rates invertebrate-mediated herbivory on seedlings of two native tree species, Terminalia sericea and Acacia exuvialis, and on rates of survivorship and growth in all experimental plots over the course of two years. Invertebrate-mediated herbivory on adult T. sericea trees will also be measured.
Through these experiments we aim to shed light on the many roles ants play in savanna ecosystems. We hope to highlight the importance of ants in structuring tropical systems and emphasise that ant diversity is key to healthy ecosystem functioning in the tropics.