Islands are quirky. They don’t always play by the same rules that continents do. And even though continents are just really big islands, their large size means that evolution managed to fit many forms of life onto them, all eating, parasitizing, and competing with each other. Not so on islands. Only a “lucky” few species cross the waters to reach those isolated bits of land. And when they get there, there are fewer predators, fewer competitors, fewer parasites, and ironically, more ecological space available.
This extra space and safe-haven from enemies has led biogeographers to hypothesize several patterns for how animals and plants on islands differ from their close relatives on the mainland. In a recent issue of Global Ecology and Biogeography, three scientists in New Zealand test one such pattern, somewhat arrogantly named, ‘The Island Rule’. The island rule posits that when small mammals colonize islands, they evolve to become larger because they don’t have to hide from predators, while large colonists evolve to become smaller because they don’t have as much food available. The generality of this “rule” has been debunked for mammals- mice and rats get larger (of course they do), artiodactyls (such as deer) get smaller, but this has more to do with what type of animal they are than with how large they began.
However, the researchers wondered whether the island rule might occur in plants, since other scientists had previously discovered that some weeds colonizing islands evolved into larger woody plants, whereas other plants get smaller. Using plants growing in a botanical garden at the southern tip of New Zealand, they compared the sizes of leafs, stems, and seeds of plant species that were native to small islands that surround New Zealand with the sizes of leaves, stems, and seeds of the closest relatives of these species that were native to the large main islands of New Zealand. Presumably, plants on the smaller islands evolved from ancestors that had come from the mainland of New Zealand, so by comparing the size of pairs of relatives they could see whether species from the smaller islands had changed in size.
Their findings? The island rule really isn’t much of a rule- it’s more like an option in a Choose Your Own Adventure novel. Most of the plants just got larger, regardless of how big they started out. But, even though both leaves and stems evolved to be larger on the islands, the scientists believe that it is pressures on the leaves that drive these changes- the stems only get bigger because they have to support larger leaves. What might make leaves larger on islands? Fewer animals eating them, for one. Small leaves might be one way that plants avoid rapacious salad-eaters.
Based on this, we might be inclined to propose the “half-island rule” for plants in which plants evolve to become larger on islands. Insert big yellow triangular caution sign here. First- this is just New Zealand, and everyone knows New Zealand is weird and populated by a tribe of small folk with big feet. Second- as the island rule (among many others) proves, “rules” in biogeography are great for generating controversy, but don’t have such a great track record for holding up to careful scrutiny. The patterns from which such rules are proposed are easy to demonstrate or refute. The causes for these patterns are conspicuously more elusive, but ultimately more important.
You can find this article at:
Kevin C. Burns, Nadine Herold, & Ben Wallace (2012). Evolutionary size changes in plants of the south-west Pacific Global Ecology and Biogeography (21), 819-828 DOI: 10.1111/j.1466-8238.2011.00730.x