Evolution in island environments

Understanding the evolutionary shifts that wildlife populations undergo following isolation on islands, helps us not only shed light on the very nature of evolutionary processes, but is also critical for managing many island species which are becoming increasingly susceptible to human-caused disruptions. Research by lab members and collaborators has demonstrated how island wildlife evolve profound differences in anti-predator defenses, social behavior, body size, reproductive strategies and digestive capabilities.

Antipredator defenses

Because Mediterranean lizard populations are subjected to widely divergent predation levels, while living on islands of different sizes and ages, they have evolved striking differences in their biology. Perhaps the most telling example of such different strategies is the relationship between predator presence on an island and resulting extent of anti-predator defenses in that lizard population.

In a collaborative project spearheaded by Prof. P. Pafilis (Univ. of Athens, Greece), and the collaboration of Prof. S. Meiri, (Univ. of Tel Aviv) we investigated why body size changes in a Greek island endemic lizard, the Skyros Wall lizard (Podarcis gaigeae) which has evolved gigantic body sizes on some of the satellite islands surrounding the main island of Skyros. That research suggests that the evolution of very large body sizes may be the result of two interacting factors: intensified social interactions in island populations, and increased amounts of nutrients transfered to these islands by nesting seabird colonies in the form of food scraps, deceased offspring and guano. Thus, increased nutrients lead to an abundance of invertebrate food, and an increase in lizard densities, which in turn, lead to more intense social interactions.

On the study islands, increased lizard densities result in elevated competitive interactions between lizards which culminate in higher levels of aggression. Field work of Prof. Pafilis, has shown that on such islands, lizards are more likely to attack, remove and consume each others tails, toes and even limbs (Pafilis et al. 2009). On the most extreme situations, there appear to be cannibalistic attacks of the largest male lizards on young of the species. This form of intraspecific predation puts strong selective pressure on the young to grow fast so at to get as quickly outside the body size window in which they are susceptible to attacks. Thus, in the presence of relatively abundant resources, cannibalism leads to the evolution of larger body sizes as larger lizards both avoid being preyed on and have the advantage of being able to secure additional food by preying on other animals (see also Pafilis et al. 2011).

Research conducted together with Dr. P. Pafilis (a former postdoctoral researcher in the lab) has shown that on islands that have specialized lizard predators such as vipers (e.g. V. ammodytes), lizards have retained a well-developed ancestral tail-shedding defense. In contrast, on islands where these predators are absent, the expression of this defense is down-regulated, suggesting that there are important costs associated with a ‘hair-trigger’ tail-shedding strategy. Interestingly, only ease of tail autotomy appears to change in relationship to predation pressure. Other aspects, such as duration of thrashing of the shed tail (a characteristic that further diverts predator attention away from an escaping lizard) appear to be more phylogenetically conservative and are not apt to change in response to reduced predation pressure.

Loss of anti-predator defenses in island lizards explains why many of these populations are so susceptible to extinction following the introduction of exotic predators (see Pafilis et al. Evolution, 2009, Brock et al. 2014). This work now permits scientists to predict based on basic principles (e.g. island size, duration of isolation) which populations are susceptible to predation, without having to actually measure anti-predator defenses in the field. This in turn, allows populations to be ranked in terms of their susceptibility to extinction, hence helping set robust management priorities and prioritizing the use of scarce conservation funds.

Effects on digestion

This research has also shown that living in an island environment produces strong and consistent shifts in digestive efficiency among lacertid lizards occurring across the Aegean Sea region. Thus, these species that occur in island habitats have strikingly elevated digestive efficiencies in comparison to mainland taxa. This increased ability to extract nutrients from food, is particularly useful in areas where lizard densities are high and food is scarce. Such conditions are particularly prevalent on many Aegean islands that are both aridly unproductive and harbor dense lizard populations due to the lack the predators (see Pafilis et al. 2006).

Effects on body size

Body size may differ substantially between island vertebrates living on different islands. Body size changes among island species may either be adaptive, or may simply be the inadvertent side effects of other ecological or physiological processes. For example, it may be adaptive for island species to be bigger in order to better compete for resources or mates. On the other hand, island species may grow bigger because on average they simply survive to an older age. Thus, in a risky, predator-rich environment, continuously-growing species may achieve only small body sizes simply because they succumb to predation at a young age.

Two Aegean island Wall lizards from neighboring islands experience similar ecological conditions but look different because they have different evolutionary histories.