Testing ideas about herbivory and plant defence on islands versus mainlands
Island plants are thought to suffer lower rates of herbivory than plants on mainlands. The reasoning behind this idea is that islands host fewer herbivorous animals. As a consequence of lower herbivore pressure, plants on islands are predicted to be poorly defended in comparison to mainland plants. These ideas are well-accepted, but studies that investigate herbivory and plant defence in paired island and mainland sites are limited, find mixed results, and are narrow in geographical and taxonomic breadth. Our study is the first to include multiple plant and herbivore taxa at a broad geographic scale.
We asked if there was a difference between plants on islands in mainlands in 1) leaf area lost to herbivores; and 2) plant defence traits. We conducted both a worldwide data compilation including over 1358 species, and fieldwork including 42 unique species in paired island-mainland sites across 20 degrees of latitude. Our field study is unique its consideration of multiple vegetation types and plant species abundance.
Our data compilation study shows island plants suffer more damage when herbivory was measured as a one-off snapshot, but less damage when herbivory was quantified through a fixed number of days. This apparent discrepancy is explained by our trait data. Island plants have higher leaf toughness, a trait associated with longer-lived leaves. That is, island plants accrue damage more slowly than mainland plants, but long-lived island leaves appear more damaged in one-off assessments.
Our field study shows no consistent differences in rates of herbivory and plant defence traits between island and mainland pairs. These results align with each other but are counter to both the predictions of traditional theory and to our data compilation results. The difference in results between our field study and the literature compilation could be due to both the coastal mainland species and island species used in our field study tending to have tough, long-lived leaves, while the broader comparison in the literature compilation includes mainland species that are from less exposed, non-coastal areas.
Our results show the importance of testing venerable ideas, particularly those without substantial grounding in data appropriate to the scale of their predictions.