Absence of evidence and evidence of absence for dingo control-induced trophic cascades

Abstract

The direct and indirect effects that human actions have on top-predators are of major importance to ecosystems and the people that study them. Top-predators may shape terrestrial ecosystems through trophic cascade effects, and because humans can influence top-predators, predator management actions (such as restoration or extirpation) may have unexpected and sometimes negative effects on ecosystems. This hypothesis had led to a common view that top-predator conservation is essential for ecosystem resilience, and that the lethal control of top-predators produces cascading detrimental outcomes for biodiversity. However, evidence for the ecological roles of top-predators is disputed and highly context-specific, much of what is known about top-predator function is derived from a few unique observational case studies, and the effects of temporary suppression of common top-predators has received little attention. In this thesis, I present a logical progression of publications that interrogate and provide evidence about the effects of contemporary lethal control programs on the ecological roles of Australia’s iconic top-predator, the dingo (Canis lupus dingo and hybrids). Lethal dingo control is commonly practiced across Australia to protect livestock and some threatened fauna from dingo predation and is typically characterised by the broad-scale distribution of poisoned meat or manufactured baits. According to prevailing ecological theory, dingoes are expected to suppress mesopredators, and mesopredators are expected to suppress smaller prey species, which are often rare and/or threatened. Thus, dingo control, which is expected to suppress dingoes, is also expected to produce indirect negative consequences for smaller prey species through trophic cascade effects, including mesopredator release. Firstly, I examine the reliability of available data on dingoes’ ecological roles and the effects of lethal control on those roles in a comprehensive and systematic series of quantitative and qualitative critical reviews. Secondly, I report a series of large-scale manipulative experiments undertaken to investigate the functional effects of baiting on dingo populations and the numerical effects of baiting on dingoes and other fauna across Australia’s beef-cattle rangelands. I demonstrate that the available literature is characterised by speculative reviews built on snap-shot or correlative studies of low inferential value, which are often confounded by a range of factors that render their results unreliable. For example, 38 of the 40 field studies assessed in Chapter 6 contain various methodological flaws (e.g. invalid seasonal comparisons), sampling bias (e.g. very small spatial and/or temporal scales) and experimental design constraints (e.g. correlative data only). Such studies have very restricted inferential power and provide few data suitable for assessing lethal dingo control as a cause of trophic cascades. Moreover, the results of my experiments provide compelling, demonstrable evidence that contemporary dingo control practices do not release mesopredators or initiate trophic cascades (Chapter 11 and Chapter 12), which is consistent with the results of all other manipulative experiments addressing this issue. For example, prey populations were almost always in similar or greater abundances in dingo-baited areas, short-term prey responses to baiting were seldom apparent, and longer-term prey population trends fluctuated independently of baiting for every prey species at all nine sites assessed (Chapter 12). In essence, this thesis provides evidence of absence for dingo control-induced trophic cascades and demonstrates an absence of reliable evidence to the contrary. Future studies should seek to (1) further quantify the effects of baiting on calf production, (2) identify thresholds of dingo suppression, beyond which, dingoes are no longer able to perform their ecological roles, (3) better understand the net effects of contemporary dingo control on both calf production and threatened fauna populations, and (4) define and reach consensus on dingo conservation and management goals. Making progress on these issues will be incredibly challenging in the current eco-political and economic climate, but the future of livestock production, dingoes and many threatened fauna may depend on it.