In this paper, we focus on macropodid marsupials-kangaroos, wallabies and their relatives-and ask whether insularity affects anti-predator behaviour, and if so, whether this effect is specifically due to the loss of predators.
Thus, we might expect different types of behaviour to vary in the degree to which different anti-predator adaptations respond to isolation ( Blumstein & Daniel 2002). 2003), and some anti-predator traits may be more phenotypically plastic than others ( Blumstein 2002). Species have a variety of anti-predator behaviours ( Van Buskirk 2001 Stoks et al. Thus, it is important to isolate the effects of insularity from isolation from predators. 2003) and many islands may retain a subset of predators.
Species often have more than one predator ( Lima 2002 Stoks et al. 2001 Gittleman & Gomper 2001).Ĭurrently, there is only one systematic, quantitative study demonstrating that isolation on islands with negligible predation risk is responsible for the loss of any anti-predator behaviour ( Beauchamp 2004). For instance, insular birds and mammals are reported to be more vulnerable to human exploitation as well as the introduction of exotic predators than mainland species ( Berger et al. If generally true, and regardless of the mechanism, this evolutionary effect has important consequences for conservation biology. Alternatively, loss of anti-predator behaviour in insular populations could be due to founder effects. Because maintaining anti-predator behaviour in the absence of predators is assumed to be costly ( Magurran 1999), we would expect that it would be eliminated by selection if there were no benefit. The mechanism underlying this loss is hypothesized to result from a reduction of predation risk on islands either because islands are able to support fewer top predators than adjacent mainland sites, or because islands lost predators following Pleistocene sea-level changes ( Blumstein 2002). Indeed, species on isolated islands, such as the Galápagos, are often described as being predator-naive ( Darwin 1839 Lack 1947 Curio 1966). When isolated from predators, costly and no longer functional anti-predator behaviour should be eliminated by selection ( Kavaliers 1990 Magurran et al. Together, these results demonstrate that anti-predator behaviour may indeed be lost or modified when animals are isolated on islands, but it is premature to assume that all such behaviour is affected. Remarkably, we also found no evidence that isolation from all predators per se is responsible for these effects. However, we found no evidence that animals on the mainland are ‘flightier’ than those on islands. Moreover, insular animals forage more, and are less vigilant, than mainland ones. We report the first comparative evidence that, in macropodid marsupials, isolation on islands may lead to a systematic loss of ‘group size effects’-a behaviour whereby individuals reduce anti-predator vigilance and allocate more time to foraging as group size increases. However, isolation on islands involves other processes, such as founder effects, that might be responsible for naiveté or reduced anti-predator behaviour. Predator naiveté is often pronounced on islands, where species are found with few or no predators. When isolated from predators, costly and no longer functional anti-predator behaviour should be selected against.
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