The "Island Rule," also known as Foster's Rule, is a principle in evolutionary biology stating that members of a species will get smaller or larger depending on the resources available in their environment. When a large mainland species colonizes an island, it often undergoes "insular dwarfism" because limited food and space make smaller bodies more efficient (e.g., the extinct dwarf elephants of Sicily). Conversely, small mainland species often undergo "insular gigantism" because the absence of mainland predators allows them to grow larger without being easily hunted (e.g., the Dodo or the Komodo dragon). By 2026, scientists have used this rule to predict how modern animals might adapt to "habitat islands" created by urban sprawl and climate change. The rule highlights that island ecosystems are unique evolutionary laboratories where the standard "rules" of mainland competition are rewritten, often leading to highly specialized and, unfortunately, highly vulnerable endemic species that exist nowhere else on Earth.