Mammalian herbivores exert strong selective pressures on plants and favor physical and chemical defensive traits that minimize leaf tissue loss. These defenses may be costly in the absence of herbivores, and previous studies have shown that oceanic islands whose plants have evolved in the absence of mammalian herbivores may lose many of their defenses. However, these studies generally compare single island and mainland sites and do not account for potentially important environmental covariates that may affect expression of putative leaf defense traits. In this study, I use chaparral shrubs from three oceanic islands (ungulate herbivores historically absent) and three mainland populations (ungulate herbivores present) to investigate whether island taxa do indeed show reduced expression of physical and chemical leaf defensive traits against herbivores. Five commonly occurring chaparral shrub conspecific/congener pairs were compared between island and mainland locations. In total, 293 individual plants were sampled, and nearly 5,000 individual leaves were phenotyped. Leaf traits of primary interest include marginal spinescence, specific leaf area, total leaf area, and concentrations of cyanogenic glycosides.
Results/Conclusions
Island taxa were significantly less spiny than their mainland relatives (p < 0.001) and had leaves that were, on average, significantly larger (p = 0.032). Interestingly, the degree of spinescence heteroblasty (a measure of ontogenetic changes in spinescence) was substantially reduced in island taxa (p < 0.001), consistent with a recent analysis of broad patterns associated with defense against large herbivores. Both island and mainland taxa in the Rosaceae produced toxic cyanogenic glycosides; forthcoming analyses will quantify the magnitude of these differences. Past experiments have shown that traits measured in this study are relevant for mediating feeding behavior in mammalian herbivores. Thus, this study provides support for the widespread observation that island plant taxa may be especially susceptible to the impacts of introduced herbivores.