Ecology is often described as the web of life. Many complex interactions occur across this web, including between individuals of the same or different species (biotic) or between the environment and a species (abiotic). Understanding ecological interactions, such as the role of predators in an ecosystem or how climate influences the abundance of a species, are fundamental for describing how this web of life works.
After working for over 10 years in central Australia I had become hooked on the Simpson Desert. The sand dunes of the Simpson Desert that lie in south-western Queensland and towards the border of the Northern Territory are like a second home to me. The red parallel dunes roll across the landscape like waves across the ocean, carrying spinifex, a spiky grass that form odd-looking donuts floating the red surface. There is something about the red sand that gets under your skin and keeps drawing you back. This setting was where I would tackle the question—what are the relative roles of both biotic and abiotic interactions for driving an ecosystem? The Simpson Desert, like many areas of Australia, has another characteristic I could take advantage of: every trip to the Simpson is different. A small storm may have gently swept rain across the sand and marked out a patch of green for all to see against a canvas of red. Further down the road, the vegetation may lay twisted and grey, prostrate as if it collapsed from the effort of begging for rain. This high variability across time and the landscape must be important for explaining how species live in such a hostile environment. Furthermore, the earth’s climate is changing. How will climate change affect the ecology of central Australia, a region of Australia where the extinctions of native animals has already hit the hardest? What lessons can this remote and understudied part of the globe teach us about our world? It was from these questions pressing on my mind that my PhD was born.
Small storms can fall through-out the year in the Simpson Desert turning patches green, but other areas remain dry. This variability added an extra dimension to my PhD project. Photo by Aaron Greenville.
I found that the operation of both biotic (species‑species) and abiotic (species‑environment) interactions were important for the ecology of the Simpson Desert (see figure below). More interestingly, interactions are not fixed and can change across the landscape and through time. For example, the relative influence of predation compared to increases in food from large rainfall events varied over the years. How this affected the role of the dingo in supressing smaller predators, such as the red fox and feral cat, and the implications for feral predator management were highlighted in an earlier article.
Figure 8.1 from my thesis: Summary of thesis findings on biotic and abiotic interactions in an arid environment. Here we see how interactions are context-specific and can change over the landscape and time. Grey arrows are possible future interactions to explore. Diagrams by A. Foster.
Fierce debates raged in 1950s and 1960s the about the importance of biotic verses abiotic factors for populations after some Australian ecologists argued that the established dogma of the northern hemisphere ecologists may not be the whole story. The upstarts, H.G. Andrewartha and L.C. Birch, published a seminal text: The distribution and abundance of animals, which used empirical evidence, rather than the largely theoretical evidence from the northern hemisphere, to demonstrate that the environment (abiotic factors) could influence the population abundance of a species. Today, findings such as mine are showing that both biotic and abiotic factors are important. This kind of understanding is crucial if we are going to try and predict how species will be affected by climate change.
Over the past 100 years the climate of central Australia has changed. The deserts have warmed and the frequency of extreme rainfall events have increased. This may effect populations of native species. For example, extreme rainfall events (>95th quantile, or >400 mm, for the Simpson Desert) were required for rodent populations to increase, resulting in a boom in population numbers. One example was long-haired rat which I wrote about here . However, not all small mammals responded to the same drivers, as dasyurid marsupials (such as dunnarts) were not influenced by extreme rainfall events. These interactions are becoming increasing important to understand, as extreme weather events, such as tropical cyclones, heat waves and flooding rains, are increasing in magnitude and frequency. Increases in extreme weather events may change species’ populations through increases in mortality rates, decreases in reproduction, and by facilitation of invasive species or novel interactions. In addition, extreme weather events, such as flooding rains, are important not only for driving food pulses in arid systems, but also other abiotic events such as wildfire.
Not only do different species show a different response to rainfall over time, they also show different responses across the landscape. Native rodents, like the spinifex hopping mouse, exhibit a high level of population synchrony across the study region. Even though each population was separated by at least 20 km, thus preventing dispersal between populations from being a big factor (the rodents I was studying are only 10‑35 grams and the reptiles are even smaller), large rainfall events that occurred through-out the region could drive all the rodent populations up in the same way. The highs and lows of each population were in synchrony, dancing in uniform across an 8000 km2 study area. The dunnarts did what dunnarts do best: whatever they want. Each population I looked at was dancing to its own beat. Contrasting strategies in responses to rainfall in a water-limited environment. Such information is vital for knowing how best to conserve each species. A “one size fits all” approach to conservation management will not work for the small mammals and reptiles I studied.
Species do not only respond to biotic and abiotic factors differently over the landscape or across time. We also need to look at multiple factors, such as rainfall and wildfire, simultaneously when trying to predict how species will interact with each other and their environment. Projecting my findings that the frequency of extreme rainfall events had increased into the future (the next 100 years), populations of rodents did not increase as much as expected, as increases in wildfire frequency and current levels of introduced predators limited rodent populations. After removing introduced predators and keeping dingoes in the system, rodents could take advantage of increases in extreme rainfall events.
The final product after 3.5 years of work: a thesis and several scientific and popular science articles.
I am truly indebted to all the help and support I have had in this journey. Particularly to my supervisors Professors Glenda Wardle and Chris Dickman, my wife, friends and family. Funding was provided by the Australian Research Council, an Australian Postgraduate Award and the Australian Government’s Terrestrial Ecosystems Research Network. Perhaps it is best to end this post with the last paragraph of my thesis:
“This thesis has attempted to take advantage of over 20 years of ecological research focused on the ecology of central Australia. Of this history, I have been fortunate enough to have been part of the last 15 years of the endeavour, working in various capacities. Even though the hand of extinction has been brought down most strongly in Australia’s arid regions, studies such as this elucidate the complexity of the current ecology of central Australia and the sense of wonder that this environment conveys. This wonder feeds curiosity and surely curiosity, the most noble of human traits, needs to be conserved.”
Andrewartha, H. G. and L. C. Birch. 1954. The distribution and abundance of animals. University of Chicago Press, Chicago, USA.
Dickman , C., G. M. Wardle, J. Foulkes, and N. de Preu. 2014. Desert complex environments, Pages 379-438 in D. Lindenmayer, E. Burns, N. Y. Thurgate, and A. Lowe, editors. Biodiversity and environmental change: monitoring, challenges and direction. CSIRO Publishing, Vic.
Greenville, A. C. 2015. The role of ecological interactions: how intrinsic and extrinsic factors shape the spatio-temporal dynamics of populations. PhD Thesis. University of Sydney, Sydney.
Greenville AC, Wardle GM, Tamayo B, Dickman CR (2014). Bottom-up and top-down processes interact to modify intraguild interactions in resource-pulse environments. Oecologia, 1-10.
Greenville, A.C., Wardle, G.M. and Dickman, C.R. (2013). Extreme rainfall events predict irruptions of rat plagues in central Australia. Austral Ecology, 38: 754–764.
Greenville A. C., Wardle G. M., Dickman Christopher R. (2012). Extreme climatic events drive mammal irruptions: regression analysis of 100-year trends in desert rainfall and temperature. Ecology and Evolution, 2, 2645-2658.
Greenville A. C., Dickman C. R., Wardle G. M. & Letnic M. (2009). The fire history of an arid grassland: the influence of antecedent rainfall and ENSO. International Journal of Wildland Fire, 18, 631-639.
Top Dog: How Dingoes Save Native Animals. Australasian Science, November 2014.