Xoaquín Moreira, Misión Biológica de Galicia (CSIC, Spain), discusses his article: Global insular leaf size shifts follow the island rule, independently of insect herbivory and macroclimate
This work came out of a collaboration between 17 institutions across five continents, all brought together by a shared interest in island biogeography. By combining data, expertise, and fieldwork from many parts of the world, we were able to tackle a simple but surprisingly overlooked question: do plants follow the same “island rule” that has been so influential in animal ecology?
The island rule was first proposed to explain how animals change in size after colonising islands. In broad terms, it predicts that small animals on the mainland often become larger on islands—a phenomenon known as gigantism—while very large animals tend to become smaller, or dwarf. As a result, if you compare mainland and island populations across many species, you expect a curved relationship: small species get bigger, large species get smaller, and medium-sized species change relatively little. This idea has been tested many times in mammals, reptiles, and birds, and it has become a cornerstone of how we think about evolution on islands. Yet plants have largely been left out of this conversation. That gap matters, because plants form the foundation of island ecosystems and face many of the same ecological pressures as animals, such as limited space, altered climates, and changes in herbivores.
One reason plants have been overlooked is that they do not have a single, obvious measure of “body size” like animals do. Instead, they are modular organisms made up of repeated parts. In this study, we focused on leaf size as a meaningful and comparable trait. Leaves are central to how plants capture light, manage water, and defend themselves from herbivores, and they vary enormously across species. In this sense, leaf size can be thought of as an organ-level equivalent of body size.
To test whether plant leaves follow the island rule, we examined leaf size variation in 48 plant species from six oceanic island systems around the world, pairing each island species with a mainland counterpart. We did this in two complementary ways. First, we compared the same species growing on islands and on the mainland, allowing us to see how leaf size changes within species after island colonisation. Second, we compared island endemic species—those found only on islands—with their closest mainland relatives, which captures patterns over longer evolutionary timescales. Together, these approaches let us ask whether any island rule-like pattern is consistent across different depths of evolutionary history. Because islands differ from mainlands in many ecological factors, we also measured insect damage on leaves and gathered climate data such as temperature and rainfall. This allowed us to test whether differences in herbivory or climate could explain any island–mainland shifts in leaf size.
At first glance, the results seemed to suggest that nothing much was going on. On average, island plants did not have significantly larger or smaller leaves than mainland plants, regardless of whether we looked at the same species or closely related species. However, averages can hide important patterns. When we looked more carefully at how mainland leaf size related to island leaf size across all species, a clear trend emerged. We found a curved, non-linear relationship that closely matches the island rule. Species with small leaves on the mainland tended to have larger leaves on islands. As mainland leaf size increased, this island effect weakened, and for species with already large leaves, island populations showed little change. This pattern appeared consistently in both types of comparisons, suggesting that it is robust across evolutionary scales. Interestingly, neither insect herbivory nor climate explained this relationship, indicating that other factors—such as changes in competition, resource use, or growth strategies—may be involved.
Taken together, our findings show that plants do follow the island rule, at least for the size of their leaves. Importantly, increases in leaf size among small-leaved species and stability or slight reductions among large-leaved species balance each other out, which explains why there is no overall difference in average leaf size between island and mainland plants. This work furthers our understanding of plants in the broader framework of island biogeography and highlights how simple evolutionary rules can apply across very different forms of life.
