Brightly colored corals are supplanted by dark, undulating seaweeds. Familiar fish species vanish — to be substituted by unknown strangers, or not replaced at all.

Pushed to the brink by warming oceans and human threats, “the places that we used to recognize as coral reefs no longer look the same,” said Gabby Ahmadia, director of ocean science at the World Wildlife Fund.

It’s a metamorphosis unfolding around the globe. A sweeping survey looked at tens of thousands of species counts from the past few decades and found that the world’s ecosystems are rapidly reorganizing. On average, more than a quarter of all plant and animal species within an ecosystem are being replaced every decade — probably the result of local extinctions, the introduction of invasive species and migrations motivated by climate change.

In the midst of a global environmental crisis, when an estimated 1 million species are said to be at risk of extinction, the study offers a look at biodiversity on the level of individual ecosystems. It suggests that for now, at least, human activities have resulted not so much in outright losses as in large-scale reorganization.

But the function of ecosystems — their capacity to filter water and clean air, to sustain the plants and animals we rely on — depends on the activities and health of their inhabitants. And those qualities are at risk.

“It’s a little bit like we’re playing musical chairs at the moment,” said macroecologist Maria Azeredo de Dornelas. “You have a lot of things moving around, and chances are that some things are going to end up without a chair.”

The study, published in the journal Science, draws on a massive, newly built database called BioTime. The database, which Dornelas helped build, contains more than 8 million measures of abundance on more than 40,000 species in roughly half a million locations.

This richness, said Sarah Supp, a Denison University data scientist, allowed the researchers to sift through global biodiversity trends. “This wasn’t really possible before,” she said. “But it’s important because the scale at which our actions take place are often much more pointed toward specific locations, or political boundaries that are not at the scale of the globe.”