When we think about the removal of predators from a natural ecosystem system quite often what we expect to see is an increase in the previously hunted prey. What we don’t expect to see are significant changes in prey behaviour, which not only have impacts on food intake but also influence changes in reproductive output, growth rates and life history traits. Dr Robert Warner, currently in Australia, spoke this month at Macquarie University and demonstrated how prey behaviour, in response to the eradication or restoration of predators, can rapidly alter population dynamics and modify entire ecosystems.
Humans often have an impact on the largest animals be it either in a marine or terrestrial environment, and until recently the scientific community focused on the impacts of this via trophic cascades (figure 1). Warner explained that we are starting to explore what else happens when we remove predators from an ecosystem and we are seeing that prey will change their behaviour based on the presence or absence of predators. In the presence of predators prey will exhibit anti-predator, or risk averse, behaviour and Wirsing and Ripple (2011) explain that this can be through strategies such as escape facilitation, encounter avoidance and increased vigilance. Essentially animals may alter habitat use, feeding patterns, movements and other traits under risk of predation. When the risk of predation is removed then these behaviours can change with implications for populations and entire ecosystems over relatively short timeframes.
One of the most well-known and beautiful examples which demonstrates the ecosystem wide response to changes in prey behaviour was in Yellowstone Park following eradication of the Grey Wolf in the 1900s. The absence of wolves meant that the Elk no longer needed to manage risk of predation and so valleys and river banks that were previously inaccessible became prime areas for foraging. The increased numbers of Elk and overgrazing of vast areas resulted in fundamental changes to not only the assemblages of animals but the physical geography of the land as well. Following the reintroduction of the wolves in 1995 and a return to the risk averse behaviour in the Elks, the entire ecosystem, both biological and physical, was restored in relatively short timeframes (Fortin et al. 2005). Watch a stunning video on how wolves change rivers here.
Warner explains that while there have been increasing numbers of behavioural studies undertaken in the terrestrial environment there have, until recently, been very limited numbers in the marine environment. Warner has been working with Liz and Josh Madin, who are currently researching out of Macquarie University, to understand how prey feeding behaviours in reef ecosystems change when predation is altered. The Line Islands in the Central Pacific provide an excellent opportunity to undertake such research with the archipelago reflecting a continuum of environmental conditions from degraded (high human impact) to pristine (no human impact). Madin et al. (2010) demonstrated that large-scale human removal of predators from a natural ecosystem indirectly alters prey behaviour and this study showed that once predation risk is removed that prey excursions (or the distance prey travel for food) are increasing. Madin et al. (2010) explain that this change in feeding may drive unexpected effects across reef food webs.
The decreased vigilance and increase in excursion lengths can be seen beautifully on the halos surrounding coral reefs (Madin et al. 2011), a phenomenon which can even be seen from space. On intact reef systems with high predation rates fish exhibit risk averse behaviour and undertake very minimal travel from the safety of coral reef in which case we see relatively small halos around reefs. On those reefs that experience more disturbance and have lower predation rates fish will travel further from the safety of the reef and as a result we can see larger halos around the reef (figure 2). These studies are important for the development of fisheries management and shifting the focus from the direct consequences of removing top-level fish to the indirect effects on non-target prey.
We have also seen that predation can have positive effects on reproductive activity and while there is not enough room to provide extensive details here I do encourage readers to explore this further. The strange, and rather humerous mating behaviour of bumphead parrotfish has only been witnessed in marine park reserves, the places where fishing is banned and top-level predators are in tact (Aswani & Hamilton 2004). Further testament to the importance of protecting our ecosystems where possible. Watch a brief video of this entertaining behaviour here
Research in the marine environment has so far been limited to coral reefs and so we are unsure if this is happening in other marine systems. Whether these same processes are taking place in the open ocean is currently unknown but future research in this area could provide some key insights to the importance of conservation of large predatory pelagic fish such as shark and blue fin tuna. We already have seen the benefits of no-take reserves and marine parks which are focused on the return of fish to pre-fishing levels. What we have not been focussed on is behaviour of prey, these studies provide insight into why this is important and key for us to manage.
Aswani S. & Hamilton R.J. 2004. Integrating indigenous ecological knowledge and customary sea tenure with marine and social science for conservation of bumphead parrotfish (Bolbometopon muricatum) in the Roviana Lagoon, Solomon Islands. Environmental Conservation, 31 (1): 68-83.
Fortin D., Beyer H. L., Boyce M. S., Smith D. W., Duchesne T., & Mao J. S. 2005. Wolves influence elk movements: behavior shapes a trophic cascade in Yellowstone National Park. Ecology, 86(5): 1320-1330.
Madin E. M., Gaines S. D., & Warner R. R. 2010. Field evidence for pervasive indirect effects of fishing on prey foraging behavior. Ecology, 91(12): 3563-3571.
Madin E. M., Madin J. S., & Booth D. J. 2011. Landscape of fear visible from space. Scientific reports, 1: 14.
Wirsing A.J. & Ripple W.J. 2011. A comparison of shark and wolf research reveals similar behavioural responses by prey. Fronteir Ecology Environment, 9 (6): 335-341.