Island gigantism is a biological phenomenon in which the size of animals isolated on an island increases dramatically in comparison to their mainland relatives. Large mammalian carnivores are often absent on islands, due to their large range requirements and/or difficulties in over-water dispersal.
In their absence, the ecological niches for large predators may be occupied by birds or reptiles, which can then grow to larger-than-normal size. Since small size usually makes it easier for herbivores to escape or hide from predators, the decreased predation pressure on islands can allow them to grow larger.
The Daily Omnivore 
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