The unique isolation of island ecosystems has fostered an extraordinary degree of biological specialization, giving rise to species found nowhere else on Earth. These organisms are known as endemic species, and they represent a disproportionate share of global biodiversity. The arrival of invasive species—organisms introduced by human activity outside their native range—threatens this delicate evolutionary balance. Islands are recognized as global biodiversity hotspots, and non-native species are a leading driver of extinctions for island flora and fauna.
Why Island Species Are Especially Susceptible
The intrinsic biological characteristics of island life make endemic species particularly vulnerable to new threats. Evolution on islands often occurs in the absence of major predators or strong competitors from the mainland. This lack of long-term co-evolutionary pressure means native species have not developed the necessary defenses to survive the sudden arrival of an aggressive newcomer.
Island birds, for instance, frequently lose the ability to fly or nest exclusively on the ground. These behaviors are maladaptive when faced with terrestrial predators like rats or cats. This evolutionary naiveté extends to flora, which may lack chemical or physical defenses against generalist mammalian herbivores.
Island populations are often small and geographically restricted, meaning that a single successful invasion can threaten the entire species with extinction, as there is no refuge elsewhere. Island ecosystems are often simpler in structure, with fewer species filling specific roles compared to continental systems. This lack of functional redundancy means the ecosystem is less able to absorb the impact of a disturbance. When an invader disrupts a single component of the food web, the resulting cascade of effects can lead to widespread decline.
The Threat of Direct Predation and Resource Competition
The most immediate impact of invasive species comes from direct consumption and intense rivalry for finite resources. Introduced predators, particularly mammals, are damaging because island endemics did not evolve with them. The black rat (Rattus rattus), the Polynesian rat (Rattus exulans), and the Norway rat (Rattus norvegicus) are globally considered among the most damaging island invaders.
These invasive rodents decimate populations of ground-nesting birds, reptiles, and invertebrates by consuming eggs, nestlings, and young. Feral cats and small Indian mongooses, often introduced to control rats, frequently prey on endemic species instead. They have a disproportionate impact on animals that lack a flight response or effective camouflage. For example, the brown tree snake, accidentally introduced to Guam, wiped out nearly all native forest birds because they had no defense against the snake’s arboreal hunting strategy.
The competition for limited resources further stresses endemic populations. Invasive herbivores, such as goats, pigs, and deer, rapidly consume unique vegetation that forms the base of the island food web. Feral goats, for instance, overgraze entire hillsides, preventing the regeneration of native plants. This grazing also causes significant habitat degradation, leading to soil erosion and the collapse of plant communities.
Competition is also pronounced among birds for specific nesting sites or food sources. Invasive birds, which tend to be generalists with high reproductive rates, quickly monopolize scarce resources like tree cavities or fruit. They aggressively displace endemic birds from their preferred foraging areas or nesting territories, reducing reproductive success. This direct struggle for finite space puts endemic specialists at a severe disadvantage against highly adaptable invasive generalists.
Altering the Ecosystem: Habitat Modification and Disease Spread
Beyond direct consumption and competition, invasive species inflict systemic damage by fundamentally changing the physical and biological environment. Invasive plants, such as certain grasses or trees, can alter the island’s landscape by changing the natural fire regime. Some invasive grasses are highly flammable and promote frequent, intense wildfires to which native island flora are not adapted, leading to their widespread destruction and replacement by fire-tolerant invasives.
Other invasive plants disrupt soil chemistry and water cycles, making the habitat unsuitable for specialized endemic plants. For example, some invasive trees fix nitrogen in the soil at a much higher rate than native species, creating conditions that only benefit other non-native species. Feral pigs also contribute to habitat modification by rooting and wallowing, which disrupts soil structure and promotes erosion, while also creating standing water pools.
These environmental changes are compounded by the introduction of novel diseases and parasites to which endemic species have no resistance. Invasive species can act as vectors, carrying pathogens that jump to the immunologically naïve native fauna. A documented example is the introduction of avian malaria, caused by the parasite Plasmodium relictum, transmitted by invasive mosquito species.
In Hawaii, the spread of avian malaria has restricted many native honeycreepers to higher, cooler elevations where the mosquitoes cannot survive. The wallows created by invasive feral pigs can increase the habitat suitability for these mosquito vectors, indirectly amplifying the disease’s impact on native birds. This combination of systemic habitat change and disease accelerates the decline of unique island biodiversity.