The island rule, also known as Foster’s Rule, is an evolutionary phenomenon observed in isolated animal populations, particularly those found on islands. This rule describes how the body size of animals tends to change significantly when they colonize islands. Large-bodied species often evolve to become much smaller, a process known as insular dwarfism, while small-bodied species tend to evolve into larger forms, a phenomenon called insular gigantism.
Core Principles of the Island Rule
Insular dwarfism occurs when large-bodied mainland animals, such as elephants or deer, evolve to be significantly smaller over many generations after becoming isolated on islands. This reduction in size is a gradual evolutionary adaptation driven by specific selective pressures present in island environments. For example, the extinct dwarf elephants on Mediterranean islands were much smaller than their mainland ancestors.
Conversely, insular gigantism describes the evolutionary tendency for small-bodied mainland animals, including rodents, insects, or certain birds, to grow much larger when isolated on islands. These size increases unfold over generations as species adapt to new ecological opportunities and pressures. Both dwarfism and gigantism represent evolutionary adjustments to island life.
Ecological Drivers
The primary ecological pressures driving the island rule involve resource availability, predator-prey dynamics, and competition. Limited resources on islands often lead to dwarfism in large animals, as a smaller body size requires less food to sustain itself. For small animals, abundant and unexploited resources can drive gigantism, allowing them to grow larger in an environment with fewer constraints.
The dynamics of predators and prey also play a significant role. The absence of large predators on islands can reduce the need for a large body size as a defense mechanism, contributing to dwarfism. For small animals, the lack of predators can remove the pressure to remain small for concealment, allowing them to evolve larger sizes. Reduced inter-species competition on islands can also allow species to expand their ecological niches, which can contribute to these size changes. Intraspecific competition among individuals for limited island resources can also specifically drive dwarfism.
Notable Examples
Many examples illustrate both insular dwarfism and gigantism across various animal groups. For insular dwarfism:
- The dwarf elephants of Sicily and Malta (Palaeoloxodon falconeri) were remarkably small, standing about one meter at the shoulder and weighing around 250 kilograms.
- The dwarf mammoths (Mammuthus exilis) found on California’s Channel Islands were significantly smaller than their mainland counterparts.
- The dwarf hippos (Hippopotamus creutzburgi) inhabited Crete.
- The Key deer of the Florida Keys, a subspecies of white-tailed deer, exhibit dwarfism.
For insular gigantism:
- The Dodo bird (Raphus cucullatus) from Mauritius evolved into a large, flightless bird from smaller pigeon ancestors.
- The Komodo dragon (Varanus komodoensis) of Indonesia represents gigantism from smaller lizard ancestors.
- Giant rats have evolved on various islands, such as the Flores Giant Rat (Papagomys armandvillei) in Indonesia, which is significantly larger than typical mainland rats.
- The Lord Howe Island stick insect (Dryococelus australis) grew much larger than its mainland relatives in the absence of predators.