Moving to suburbia: ontogenetic and evolutionary consequences of life on predator‐free islands

Abstract

Aim Many species find themselves isolated from the predators with which they evolved. This situation commonly occurs with island biota, and is similar to moving from the dangerous inner‐city to the suburbs. Economic thinking tells us that we should expect costly antipredator behaviour to be lost if it is no longer beneficial. The loss of antipredator behaviour has important consequences for those seeking to translocate or reintroduce individuals from predator‐free islands back to the predator‐rich mainland, but we have neither a detailed understanding of the mechanisms of loss nor information on the time course of relaxed selection. Some antipredator behaviours are experience‐dependent: experience with predators is required for their proper performance. In these cases, antipredator behaviour is lost after only a single generation of isolation, but it should be able to be regained following exposure to predators. Other behaviours may be more `hard‐wired'. The evolutionary loss of antipredator behaviour may occur over as few as several generations, but behaviours may also persist for many thousands of years of predator‐free living. Location Australia and New Zealand. Methods I discuss the results of a series of studies designed to document the mechanisms and time course of relaxed selection for antipredator behaviour in macropodid marsupials. Controlled studies of visual, acoustic and olfactory predator recognition, as well as field studies of antipredator vigilance focused on several species of kangaroos and wallabies. Results Visual predator recognition may be retained following 9500 years of relaxed selection, but olfactory and acoustic predator recognition may have to be learned. Insular populations allow humans to approach closer before fleeing than mainland animals. Insular species may retain `group size effects' – the ability to seek safety in numbers – when they are exposed to any predators. Main conclusions I suggest that the presence of any predators may be an important factor in maintaining functional antipredator behaviour. Managers should pay particular attention as to the source and evolutionary history of their population when planning translocations or reintroductions.

Island tameness: living on islands reduces flight initiation distance

“…Similar findings have been reported for iguanian lizards [2,6,17]. Antipredatory responses including FID and vigilance are weak where predation is relaxed in macropodid marsupials [9][10][11][12] and birds [8]. Our evidence strongly supports the existence of island tameness, but does not provide any direct evidence that it is a consequence of reduced predation.…”

“…He believed that escape behaviour had diminished where predators were rare or absent on remote islands, which resulted in loss of costly escape responses [2][3][4][5][6] in the absence of strong natural selection to maintain them. Island tameness has been reported anecdotally in birds, lizards and other taxa [6][7][8][9]. If Darwin's island tameness hypothesis is correct, predation intensity and escape responses should be diminished on islands compared with the mainland and should also vary with distance from the mainland [10,11].…”

“…In the lizard Sceloporus jarrovii, FID when approached by a person is longer in larger individuals, presumably because larger lizards are more likely to be detected and attacked by (or less able to escape from) a large predator [21]. We predicted that FID increases as body size (snout -vent length; SVL) increases, as it does in birds [22] and macropod marsupials [9]. Because occupation of islands affects body size of lizards [23], we used multiple regression to examine the independent statistical influences of distance from the nearest mainland and body size on FID.…”

Relaxation of risk-sensitive behaviour of prey following disease-induced decline of an apex predator, the Tasmanian devil

“…Anti-predator behaviours can be energetically demanding and therefore costly to maintain for an organism [7,30]; however, in the absence of predators, some traits can be retained for hundreds of generations [15]. If a predator is functionally absent from ecosystems for a long time, the presence of other predators can help to maintain general anti-predator behaviours even if risk-sensitive behaviours specific to a predator species that has declined are lost [3].…”

“…If predators are lost or absent from the environment for sufficient periods of time, selective pressure to maintain expensive risk-sensitive behaviours is reduced and some specific learned and heritable anti-predator behaviours and traits may be lost [3,14]. Others are retained in species isolated from their predators for hundreds of generations [15], possibly because they are not energetically costly to maintain or because they are relevant to other types of predators.…”