Island tameness is a biological phenomenon observed in species inhabiting isolated island ecosystems. This behavior is characterized by a lack of fear toward potential predators, which contradicts basic survival instincts. This docility develops because the animals evolve in environments largely free of the threats found on mainland continents. The resulting fearless behavior makes these island species highly vulnerable when their environments are exposed to external threats, posing a serious conservation challenge.
Defining Island Tameness
Island tameness, sometimes called ecological naiveté, refers to the reduced or absent anti-predator responses in island-dwelling animals compared to their mainland relatives. A key measurable aspect of this behavior is a short flight initiation distance, which is the close proximity an animal allows a perceived threat, such as a human, to approach before fleeing. These animals often show little or no defensive or aggressive reaction to novel stimuli, making them generally approachable.
The famous, now-extinct Dodo bird from Mauritius is the most iconic example of island tameness, as it showed no fear of the sailors who first encountered it, leading to its easy capture. Many species across the Galápagos Islands, such as the marine iguanas and certain finches, also exhibit this docility, a fact first noted by Charles Darwin in the 1830s. This diminished wariness is a consistent trait seen in diverse island fauna, including lizards, birds, and some marsupials.
The Evolutionary Basis
The development of island tameness is a direct result of evolutionary selective pressure, or rather, the lack of it. Remote islands are often isolated from the mainland for thousands or even millions of years, which prevents the natural colonization of many large, fast-moving mammalian predators. In the absence of these significant threats, the necessity for maintaining anti-predator defenses diminishes over generations.
Remaining vigilant and fleeing at the slightest disturbance expends significant energy. Natural selection favors individuals who conserve this energy by not reacting to non-existent threats, allowing them to redirect resources toward foraging, growth, or reproduction. The longer an island remains isolated and predator-free, the more pronounced this behavioral shift toward tameness becomes. Studies on Aegean wall lizards, for example, demonstrate that flight initiation distance decreases as an island’s distance from the mainland increases.
Extreme Vulnerability to External Threats
Island tameness transforms from a beneficial energy-saving adaptation into a severe liability when external threats are introduced to the ecosystem. This extreme vulnerability means endemic species are unable to recognize new predators as a threat. These novel invaders, often introduced by humans, include animals like feral cats, rats, pigs, and stoats, all of which have evolved with well-developed hunting instincts.
The lack of an ingrained fear response means native animals fail to flee or defend themselves, leading to rapid population collapses. For example, the Laysan rail and Lyall’s wren are two species whose extinctions were hastened by this combination of tameness and introduced predators. The Falkland Islands wolf, or warrah, was so tame that early explorers could easily kill it by simply holding out a piece of meat with one hand while striking with the other. This ecological naiveté, combined with small population sizes and slow reproductive rates, leaves little time for species to adapt to new predation pressure.
Protecting Endemic Species
Conservation efforts for island species address the threat posed by island tameness and introduced predators. One effective strategy is implementing strict biosecurity measures, such as rigorous inspections of cargo and boats, to prevent the arrival of new invasive species. Stopping the introduction of new threats is far less costly and more successful than trying to manage an established invasive population.
Another focus is the large-scale eradication of existing invasive species from islands, a process that has been pioneered in places like New Zealand. Successful removal of predators like rats and feral cats can allow native populations to recover their numbers. In some cases, to ensure a species’ immediate survival, scientists create predator-free havens or translocate animals to smaller, already predator-free offshore islands to serve as temporary refuges.