A terrestrial island habitat is a landmass surrounded by a barrier, most often water, that impedes the movement of species. This isolation is the primary driver of their unique biological characteristics. This separation from mainland environments creates distinct communities of life, making islands natural laboratories for observing evolution and ecology.
Formation of Island Ecosystems
The origins of islands are a primary factor in the ecosystems they support, forming in two distinct ways. Oceanic islands are born from the sea, never having been connected to a continent. They result from underwater volcanic eruptions that build up land over millennia, like the Hawaiian Islands, or the accumulation of coral reefs. Every plant and animal must cross a water barrier to colonize these landmasses.
Continental islands, in contrast, are pieces of a mainland that have been separated. This can occur due to the movement of tectonic plates or from rising sea levels that flood land bridges. Great Britain, for example, was once part of mainland Europe before being cut off by rising seas. These islands begin their existence with species inherited from the parent continent, which then evolve in new directions.
Unique Biodiversity and Evolution
Isolation is a powerful force for evolutionary change, leading to a high degree of endemism, where species are found only in one specific location. The lack of gene flow with mainland populations allows colonizing species to adapt to their specific surroundings. Over generations, these populations can become so different from their ancestors that they are classified as new species endemic to that island or archipelago.
This process can lead to adaptive radiation, where a single ancestral species evolves into multiple descendants to fill different environmental roles. A well-known example is the finches of the Galápagos Islands, where one ancestral species gave rise to many others, each with a beak specialized for a different food source. The Hawaiian honeycreepers similarly evolved a diversity of forms and functions from one colonizing species.
Other evolutionary patterns also emerge on islands. Island gigantism occurs when small mainland animals evolve to larger sizes, often due to the absence of predators, such as the Komodo dragon. Conversely, island dwarfism is when large animals evolve to be smaller as an adaptation to limited food resources, like the extinct pygmy elephants of various Mediterranean islands.
Ecological Fragility
The same isolation that fosters unique life also makes island ecosystems fragile. Island species populations are often small with limited geographic ranges, making them highly susceptible to extinction from sudden environmental changes. A single event, like a severe storm or disease outbreak, can have a large impact on a species confined to one landmass.
Many island species also exhibit “evolutionary naivety.” Having evolved with few or no predators, they often lose their defensive behaviors and physical traits, such as flightlessness in birds like the dodo. When non-native species are introduced, these naive island natives have few defenses, making them particularly vulnerable.
Impacts of Invasive Species and Human Activity
The introduction of non-native species by humans is one of the most severe threats to island ecosystems. Invasive predators, competitors, and diseases can devastate native populations that lack evolved defenses. The accidental introduction of the brown tree snake to Guam is a clear example, where its population grew unchecked and caused the extinction of nine of the island’s eleven native forest bird species.
Direct human activities also place immense pressure on island habitats. Land development for tourism, agriculture, and housing leads to habitat destruction and fragmentation. Pollution from these activities can degrade the surrounding land and water, stressing the ecological balance. These combined pressures have made island species account for a disproportionate number of recorded extinctions.