Island Gigantism - Aman Lalwani

Island gigantism is a biological phenomenon in which animals living on an island grow much larger in size than their mainland relatives in the absence of predators. The phenomenon is  corroborated by Foster’s rule, which explains how an organism’s size differs in relation to its surrounding environment and resources. Miscellaneous islands shelter organisms which have been subject to this phenomenon and they are continually studied to provide concrete evidence for evolution. New Zealand is a renowned example of island gigantism and has intrigued scientists for a long time. Although the phenomenon has existed for millennia, widespread expansion of humanity and introduction of invasive species (predators) onto fragile islands have led to the extinction of various giant organisms. 

In 1964, J.Bristol Foster introduced the concept of island gigantism in the published paper ‘Evolution of Mammals on Island.’ This was done based on a survey which involved the comparison of 116 island-living species and their mainland counterparts. It was concluded that larger species reduce in size because of limited food and smaller species grow larger due to lower predation pressures. These concepts provided the foundations for the study of island gigantism and they have been developed over time by biogeographers. 

There are various reasons for island gigantism which have been deduced over time and te

• The first one is credited to the absence of large mammalian predators due to a sparse range and problems in over-water dispersal (sea-crossing). Taking advantage of their absence, birds/reptiles/smaller carnivores occupy the ecological niches of larger carnivores and can grow to larger-than-normal sizes. 

• Animals with smaller sizes are more efficient when escaping or hiding from predators, therefore lower predation pressures on islands allow them to grow larger.

• Animals may grow to larger sizes to minimize vulnerability to food or water resources. Being larger allows them to survive longer without food or water and travel longer distances in search of these resources.

• Territorialism also plays a role in island gigantism. Islandic territorial species are usually larger as this enables them to better defend their territories. 

A well-known example of island gigantism involves a reptile called the Komodo dragon. They inhabit a few islands in Indonesia and are the largest lizard species in the world. Growing much larger than the mainland monitor lizard, Komodo dragons exhibit characteristics of island gigantism and have evolved to be the apex predator of their ecosystem.