Do arid-zone ants undergo boom-bust population fluctuations in response to the El Niño Southern Oscillation?

A project undertaken at the Department of Zoology, La Trobe University, and supervised by Heloise Gibb

Arid ecosystems in Australia undergo repeated cycles of boom and bust in response to quasiperiodic swings in climate patterns resulting from the El Niño Southern Oscillation (ENSO).  Dramatic fluctuations in populations of many animal species result from rainfall-driven changes in food availability.  These boom-bust cycles may make species particularly vulnerable to anthropogenic extinction drivers, including altered fire regimes, predation by feral animals and habitat disturbance through grazing.

Long-term data sets examining responses to climate are rare and most published studies focus on birds, mammals or vegetation.  Our poor understanding of arid-zone invertebrate assemblages is a significant knowledge gap.  Ants are of particular interest because they occupy a broad range of functional roles, acting as seed dispersers, predators, scavengers and mutualists, and show strong responses to disturbance.  Along with other social insects, they can make up over 30% of insect biomass.  They are thus an ideal model group to test the generality of responses to both spatial and temporal variation in water availability.

This will be the first long-term study to address responses of insect assemblages to ENSO-driven rainfall fluctuations in the Australian arid zone.  We will also test the role of spatial heterogeneity in moisture levels in buffering climatic effects by comparing productive 'refuges' with unproductive 'non-refuge' sites.  We will focus on changes in the abundance of ant species and the rate at which they perform ecosystem functions, including seed dispersal, predation and scavenging.  In particular, we will use a long-term data set from the Simpson Desert covering several major rainfall events and habitats differing in productivity (due to moisture) to address the following questions: 1) Do ant assemblages respond predictably to ENSO-driven fluctuations in rainfall; and 2) Are ant population and functional responses buffered in more productive 'refuge' areas?

This project will considerably improve our understanding of the how arid-zone invertebrates respond to fluctuations in climate due to ENSO.  This work will complement the twenty-one years of data collected on boom-bust responses of vegetation and a range of vertebrate species to rainfall.  It thus fills a missing gap in the food chain that is particularly important in understanding the responses of insectivorous vertebrates to ENSO-driven rainfall fluctuations.  Finally, a good knowledge of contemporary responses to fluctuating climates is necessary if we are to predict future responses to changes in climate.


Figure 1. Arid zone ants of the genus Calomyrmex (photo by H Gibb)

Figure 2. Simpson Desert dune (photo by Aaron Greenville)

Figure 3. Contrasting low and high productivity areas from above (Image from GoogleEarth)