Seabird Islands Are Undone By A Single Mammal

Seabird colonies concentrate enormous numbers of birds on small islands. The reason those islands were chosen is the same reason they are so easily ruined.

The islands were chosen for the absence of predators

Seabirds feed at sea and come ashore only to breed. They select islands and stacks that ground predators cannot reach, which is why colonies are so densely packed.

Safety from mammals removes the pressure that shapes nesting behaviour elsewhere. Many species nest on open ground, in shallow burrows or on bare ledges.

They also lay a single egg and invest heavily in it, a strategy that works while adult survival is high and nest losses are rare.

Introduced mammals arrive with shipping

Rats reach islands as stowaways, cats and goats as deliberate introductions, and mice on supply vessels. Most arrivals trace to trade, settlement or wrecks.

The birds have no behavioural response. Adults do not defend nests against ground predators and chicks do not hide, because neither behaviour was ever needed.

Burrow-nesting species suffer most, since a burrow that concealed a chick from gulls presents no obstacle at all to a mammal.

Losses compound through the population

Predation on eggs and chicks cuts recruitment first, and because seabirds are long-lived and breed slowly, the colony can look healthy for years while producing almost nothing.

Where adults are also taken the decline is much faster, because losing breeding adults removes decades of future output at once.

Smaller species are frequently eliminated entirely, and colonies retreat to stacks and ledges that the predator cannot reach.

The island itself changes

Seabirds import nutrients from the sea and deposit them on land, and island soils and plant communities are built around that input.

Remove the birds and the enrichment stops. Vegetation shifts, soils change, and the effect extends into the shallow water around the island where the runoff once fed algae.

Which means predator removal is not only about the birds. It restores a nutrient pathway running from open ocean to land and back to the shore.

Eradication works but has to be total

Removing rodents from an island is one of the more reliable conservation interventions, and hundreds of islands have been cleared successfully.

The requirement is completeness. A single surviving pregnant female rebuilds the population, so the work is planned around the last individuals rather than the first.

Preventing reinvasion is the ongoing cost, which means biosecurity on every vessel and cargo landing thereafter, indefinitely.