Islands hold a disproportionate share of both unique species and recorded extinctions. The same isolation that produced the distinctiveness also removed the pressures those species would have needed defences against.
What isolation does to evolution
Reaching a remote island requires crossing ocean, so the species that arrive are a filtered subset, typically birds, bats, insects and plants with dispersing seeds.
Ground-dwelling mammalian predators rarely make the crossing, so island communities frequently develop with no such predator present at all.
Over long periods this produces distinctive forms found nowhere else, which is why island endemism rates are so high relative to the land area involved.
Defences that were not needed disappear
Traits that cost energy and confer no benefit are lost over time, and predator avoidance is expensive to maintain.
Island birds have repeatedly become flightless or nearly so, nested on the ground, and lost the alarm responses their mainland relatives retain.
The same applies to plants, many of which have reduced spines, toxins and other defences against browsing where no browsers existed.
Why introduced predators are so destructive
Rats, cats, pigs, goats and mongooses arrived with shipping, and they encountered prey with no evolved response to them.
Ground-nesting birds continue to sit on nests as a rat approaches, because the behaviour that would save them was never selected for.
Rats reach nearly every island they are carried to, which is why they account for a large share of island extinctions despite being small animals.
Small ranges leave no margin
An island endemic may occupy a few square kilometres in total, so a single fire, storm or disease outbreak can affect the entire global population at once.
There is no source population elsewhere to recolonise from, and no refuge to retreat to, which is the structural difference from a mainland species with a patchy range.
Population sizes are correspondingly small, which brings the genetic and chance-driven problems that afflict any small population regardless of habitat quality.
Why islands are also where recovery works
The same boundedness that creates vulnerability makes intervention tractable, because a predator can be removed from an island completely in a way that is impossible on a continent.
Eradication programmes have cleared rats and cats from a growing number of islands, followed by rapid recovery of seabird colonies and ground-nesting species.
Success depends on removing every individual and preventing reinvasion, so biosecurity on vessels and cargo continues indefinitely after the eradication itself is complete.