The Loss (and Gain) of Defensive Adaptations in Island Plants and Animals: A Comparative Review

Compositional differences between islands and continents had marked consequences for the evolution of island life. Regardless of their taxonomic rank, many island species lost defences that evolved in mammal-dominated ecosystems and gained new defences against bird-dominated islands. However, the study of island evolution has been taxonomically compartmentalised, with research on island animals and plants being conducted independently of one another. Thus, whether a common pattern in the evolution of island defences exists remains unclear. To help bridge this gap in our understanding, we conducted a comparative review of defensive adaptations in island animals and plants to (1) better understand differences in research effort between plants and animals, and (2) establish unified principles in the loss (and gain) of defences in island life. To do that, we manually screened 1600 studies extracted from Google Scholar. Of them, 127 were included in our review. The majority of studies focused on island animals. Most studies on the gain of defences focused on plants, while loss of defence was explored more thoroughly in animals. Differences in terminology and idiosyncrasies between animals and plants hindered our ability to compare research findings. Nonetheless, some commonalities have emerged. In particular, a general pattern of loss and gain of defences can be delineated. Insularity appears to promote the loss of non-bird-specific defences and the gain of bird-specific defences. This pattern though is clearer in plants than in animals and more research is needed to unify these two bodies of work.