Spinescence and the Island Plant Defense Syndrome

Island plants are predicted to have weak or absent defenses as part of the island plant syndrome. Evidence supporting the weak island defense prediction stems largely from observations of intense damage from invasive mammalian herbivores on islands. However, this evidence is misleading because most oceanic island plants have not evolved with native mammalian herbivores, and so should not have evolved defenses against them. In contrast, many islands have been home to other native vertebrate megafaunal herbivores, including flightless birds, tortoises, and turtles, many of which are now extinct or rare and therefore easy to overlook as agents of selection for island plant defenses. We review the evidence that island megaherbivores have selected for spinescence in island plants, supplementing published data with new estimates of spinescence for island floras varying in historical legacies of megafaunal herbivores. While the proportions of spinescent species are generally low, there are many spinescent island plants, likely functioning in defense against extant herbivores or persisting as defense anachronisms, no longer functioning due to the losses of native island megaherbivores. Future research exploring the evolvability of spinescence, including rates of losses or gains as herbivory selection pressure shifts, will be particularly enlightening for assessing island plant defenses in response to complex and variable historical legacies of megafaunal herbivory.