In the 1940s, the brown tree snake reached Guam hidden in post-WWII cargo and set off a chain reaction that scientists are still measuring today; birds gone, tree seedlings down by up to 92%, and spiders multiplying up to 40-fold on an island that once had neither

When people think about the damage an invasive predator can do, they mostly think about the animals it directly kills. What they rarely consider is what becomes of the trees.

A study titled ‘Effects of an invasive predator cascade to plants via mutualism disruption’ by researcher Haldre S. Rogers and colleagues at Iowa State University and the University of Washington, published in Nature Communications, shows that one invasive snake on a tiny Pacific island has started a cascade so devastating that the forest itself may be losing its ability to regenerate. The brown treesnake is to blame. The victim, ultimately, is the island of Guam.

The snake that silenced a forest
The brown treesnake (Boiga irregularis) was accidentally introduced to Guam in the mid-1940s, probably as a stowaway in military cargo after World War II. The snake contributed to the extinction of ten of the twelve native forest bird species on the island, and the other two were also functionally extirpated, leading to what researchers call a 'silent forest,' according to Rogers and colleagues.


That loss of birds was a familiar disaster. The new research shows that the damage is much greater than anyone thought.

Why birds are important to trees
This is what most people miss: birds are not just residents of a forest; they help to build it. Around 70% of Guam's tree species have fleshy fruits, whose seeds depend on birds to disperse them from the parent tree. When a bird eats a fruit, two important things occur. First, the seed passes through the bird's digestive system, which helps the seed germinate: the process removes compounds that inhibit sprouting and can scarify the seed coat in ways that boost germination rates. Second, the bird deposits the seed in a new, safer location away from the dangers that congregate under adult trees.

Seeds eaten by birds were two to four times more likely to germinate than whole fruits left intact. But taking the fruit flesh off by hand didn’t have the same effect; there’s something about passing through the gut that matters.

Seeds going nowhere
To understand how much bird loss has affected seed dispersal, the researchers compared Guam to three nearby islands, Saipan, Tinian, and Rota, which retain intact bird communities. The contrast was stark.

The Nature Communications study notes that for islands with birds, the mean dispersal distance was 5.37 m for Psychotria mariana and 8.19 m for Premna serratifolia. On Guam, those distances dropped to 0.73 meters and 1.18 meters, respectively. Thus, 94% of Psychotria seeds and 96% of Premna seeds fell on Guam land directly beneath the canopy of their parent tree, compared to only 26% and 40% on islands with birds.

In other words, nearly every seed on Guam is in the worst possible position to land.

Why landing near the parent is a death sentence
This is important because of a well-documented ecological phenomenon: seedlings growing too close to their parent trees suffer concentrated attacks by fungal pathogens, insect pests, and other natural enemies that tend to build up around adult trees of the same species. Getting out of that zone can often be the difference between life and death.

But the seedlings planted far from conspecific adults survived significantly better than those planted near them across all four islands, Rogers and colleagues studied. On Guam, where no seeds are dispersed outside that danger zone by birds, few seedlings survive.

When researchers combined all these factors: reduced germination without gut passage, shortened dispersal distances, and increased seedling mortality near parent trees, the overall picture was bleak. The Nature Communications study found that the loss of birds may have caused a 61 to 92 percent decline in seedling recruitment for the two tree species studied.

A problem bigger than one island
Guam may be an extreme case, but the dynamics it exposes are playing out more widely. According to Doherty and colleagues, writing in the Proceedings of the National Academy of Sciences, 30 species of invasive predators are responsible for the extinction or endangerment of 738 vertebrate species worldwide, accounting for 58% of the extinctions of all birds, mammals, and reptiles. Removing an animal that disperses seeds from an ecosystem breaks a mutualism that plants may have relied on for thousands of years.

Traveset and Richardson, in their study, ‘Biological invasions as disruptors of plant reproductive mutualisms,’ published in Trends in Ecology and Evolution, mention invasive species disrupt seed-dispersal mutualisms more often than previously thought, and the consequences for plant communities are still greatly underestimated

What can be done
Currently, the brown treesnake cannot be eradicated from the island. But Rogers and team note that localized control via fencing, trapping, and toxicants is feasible and, coupled with the reintroduction of native bird species to targeted areas, could begin to restore ecological function where it has been lost.

The birds that once thrived on Guam still live on nearby islands that lack snakes. Re-introducing them, even in protected pockets, could re-activate the seed dispersal system the forest desperately needs.

Guam’s trees are still standing. The next generation of trees may struggle to replace them, and the snake is a major reason