Seeds of change: How invasive animals are replanting islands

Islands run on movement. Plants drop fruit, animals carry seeds, and forests rebuild themselves across valleys and slopes.

When native animals vanish or newcomers take over, that conveyor belt stutters, and a quiet shift begins in what grows where.

This is the conclusion of research by Donald Drake, an expert on Pacific island ecology at the University of Hawaii at Manoa (UHM).

How seeds are spread on islands

On islands, many plants rely on seed dispersal, the natural spreading of seeds by wind, water, or animals, to move offspring away from parent trees. This basic process cuts crowding and disease while opening new sites for growth.

Animals that eat fruit are called frugivores, animals that feed mostly on fruit and spread seeds through their droppings or food handling.

By shaping how far different seeds travel, these animals act as gardeners for entire landscapes.

Global changes in seed dispersal

The UHM team compared 120 islands in 22 archipelagos and found that introductions outnumbered extinctions (44 percent versus 23 percent).

The study also revealed that introductions affected 92 percent of islands, compared with 76 percent for extinctions.

Non-native mammals, often omnivores like pigs and rats, now replace lost birds, bats, or reptiles in many places. This shift alters the very traits that drive seed movement.

The transition comes with tradeoffs. Mammals that chew more and fly less tend to disperse fewer intact seeds and over shorter distances than wide-ranging birds.

Seed size shifts across islands

A previous analysis revealed a strong link between bird gape and fruit size, so when large gaped frugivores disappear, big seeded plants lose their ride.

This mismatch leaves heavy, large-seeded species stranded close to parent trees. Over time, that can thin future forests toward small-seeded plants that fit the mouths of the animals that remain.

Hawaii’s shrinking seed movers

Hawaii’s native forests once leaned on birds to move seeds of lama, hoawa, and olopua – and that system has frayed as birds declined.

Research focused on Hawaii found that modern bird communities have smaller mean gape width and body mass than historic assemblages, a pattern that cuts the effective dispersal of large seeds.

Introduced rodents add a second pressure by eating seeds outright. In a Hawaiian dry forest, a study documented substantial seed predation by invasive rats, a loss that chokes regeneration where animal couriers already run short.

“Understanding how introduced species reshape ecological interactions is crucial for planning effective conservation strategies,” said Drake.

“Many studies focus on individual species or single islands, but this research shows that introductions can fundamentally alter ecological interactions on a global scale.”

Changing island seed networks

An archipelago is a group of islands, and each one can host a different set of plants and seed movers, so changes do not play out the same way everywhere.

Introduced animals can sometimes carry seeds, yet their routes, diets, and handling of fruit differ from the native species they replace.

Those differences matter for people as well as plants. Forests fed by steady seed rain help maintain slope stability and clean water, linking ecology, the study of how organisms interact with each other and their environment, directly to daily life.

Forests, water, and culture

In Hawaii, watershed forests, forested areas that catch and channel rain into groundwater, capture mist and rain, slow runoff, and recharge aquifers that supply communities.

A 2023 report, notes that protecting native forests and removing feral ungulates, hoofed mammals like pigs and goats, can improve groundwater recharge and reduce erosion, benefits that ride on forests keeping their structure.

When seed dispersal weakens, forests can age in place, gaps fill with aggressive weeds, and the sponge function slips. That slide threatens water supply, native species, and the cultural practices that depend on them.

Island changes differ

The researchers emphasize that patterns vary from island to island, and that is important for action.

Different histories, plant seed sizes, and mixes of birds, mammals, and reptiles can lead to different outcomes even under the same broad pressures.

Managers and communities should expect patchy maps of risk and opportunity. Some islands may still hold key native frugivores to protect, while others may need creative fixes.

How restoration can help

Control of invasive species, non-native animals or plants that spread rapidly and harm native ecosystems, can lift pressure on seeds and seedlings, especially where rodents or pigs take a heavy toll.

Targeted planting can bridge gaps for big seeded natives that no longer move on their own, while keeping corridors for remaining birds sustains the flow that still works.

Restoration should focus on traits, not just names. Matching fruit and seed sizes to the mouths of the animals in each forest gives managers a practical checklist identifying the plants that need extra help.

The study is published in the journal Proceedings of the National Academy of Sciences.

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