In 1960, Spain planted an invasive tree thinking it was a great idea, and now it's hammering local birdlife because the ecosystem can't keep up

Imagine two patches of forest next to each other. Same rainfall, same hill, same part of the map. Walk through one and birdsong follows you all the way. Walk through the other, and it's noticeably quieter, even though the trees look just as thick. That’s no accident and, as a 2025 study published in the journal Forest Ecology and Management finds, that’s exactly what’s going on all over Galicia, in northwest Spain.

Researchers from the University of Santiago de Compostela and the Biological Mission of Galicia sampled 240 locations in and around the Fragas do Eume Natural Park, comparing native woodland to eucalyptus plantations. That research found that native forests had 1,806 birds across 35 species, while eucalyptus stands had only 754 birds across a similar 36 species. The species list hardly changed at all. The actual number of birds living there was reduced by nearly half.

Same number of species, way fewer birds
It is this detail that makes the study interesting rather than merely sad. Researchers found that a typical plot in native forest held around 9 species and 14 birds, compared with about 5 species and 7 birds in eucalyptus plantations. So the eucalyptus forests are not empty. They can only support a fraction of the population density of a native forest, almost like comparing a full neighborhood to one where half its people have moved out.


Why one fast-growing tree changes the math
Eucalyptus was brought to Spain in the 1800s and has expanded ever since paper and pulp companies discovered how fast it grows. The same study found that it now accounts for around 30 percent of the forested land in Galicia, and the percentage of eucalyptus in a given plot was the single best predictor of bird decline, more so than any other factor measured.

There were two reasons. First, eucalyptus is rarely felled with the cracks and hollows that birds need for nesting. Second, its leaves are less palatable to insects and offer less food for birds. A 2023 review in the journal Forests found that eucalyptus litter and root compounds also suppress the native shrubs and seedlings that would otherwise nourish that insect population in the first place.

The birds caught in the middle
The research showed some species, including the Eurasian nuthatch, short-toed treecreeper and crested tit, are dependent on mature native trees and do badly without them. The great spotted woodpecker is one of the few birds that can carve its own nest cavity, but it gets a bit of help from older eucalyptus trees, along with the common firecrest and coal tit. But that help is only for a small segment of the bird community. Indeed, the song thrush did worse around the older eucalypts, because the increased leaf litter there seems to suppress the ground insects on which it depends for food.

This may sound familiar to American readers. The US has its own fast-growing tree farms, mainly loblolly pine plantations in the South, raised on tight harvest cycles for the same reason eucalyptus took off in Spain: speed and profit. Europe isn’t the only place grappling with how much wildlife those plantations can actually support.

Not just the birds
The effects are felt in the water too. According to a study published in the journal Hydrobiologia, alder leaves decomposed much faster than eucalyptus leaves after they fell into nearby streams, and alder attracted far more of the small invertebrates that are the base of a stream’s food chain. Slower decomposition means less food at the bottom of that chain, which works its way up to fish, amphibians and the birds that consume them.

A fight that's been brewing for years
This is no new debate. According to reporting by Spain's environment ministry, a scientific advisory committee recommended in 2017 that several eucalyptus species be added to the country’s official catalog of invasive species. The government declined partly because eucalyptus accounts for about a third of the wood used by the country’s paper industry while covering only about 3 percent of its forested land. The regional government of Galicia has taken a smaller step on its own, extending a moratorium on new eucalyptus plantations to 2030.

What scientists are really asking for
No one involved in this research is arguing for the disappearance of the eucalyptus, since the regional economy depends so much on it. The study recommends leaving strips of native vegetation within the plantations themselves to allow shrubs, insects and birds to establish themselves alongside the eucalyptus, rather than instead of it. It's a tiny, modest fix for a forest that still looks perfectly healthy from a distance.