According to a new study published in the journal Science Advances, human-caused bird extinctions through habitat destruction or climate change are driving losses of functional diversity on islands all around the globe. Unfortunately, these gaps cannot be filled by introduced (alien) species, even on islands where non-native birds actually outnumber the extinct species.

“Humans have drastically changed bird communities, not only by driving animals to extinction but also by introducing species into new habitats across the globe,” said study lead author Ferran Sayol, a research fellow in Genetics and Evolution at University College London (UCL). “There has been some debate as to whether introduced species might replace the roles of the extinct species, thus maintaining functional diversity within the ecosystem; here, we found that is unfortunately not the case.”

By analyzing 1,302 bird species populating nine different archipelagos, including 265 extinct species and 355 new introductions from 143 separate species, Sayol and his colleagues have found that, before human arrival, island bird communities were more morphologically diverse, and that human-driven extinctions have disproportionately affected some types of birds, leading to the loss of certain ecological roles.

“Some of the extinct species had a role in their ecosystem that has not been replaced by other birds,” explained Sayol. “For example, some giant flightless species, like the moas of New Zealand and the elephant birds of Madagascar, were probably acting like large terrestrial herbivores as grazers, similar to ungulates like cattle and sheep on the continents, before being driven extinct by humans.”

“Other valuable functions that may be lost with bird extinctions can include pollination and seed dispersal, which can have cascading harmful effects on other species.”

The scientists also found that different archipelagos are becoming increasingly similar in terms of trait diversity as native birds go extinct and the same species of alien birds are introduced in various places. “Some groups of birds have been particularly successful at establishing outside their natural areas – for example, many species of parrot and starling,” said study co-author Tim Blackburn, a professor of zoology at UCL. “Because of this, islands are becoming more homogeneous as the same kind of birds are established everywhere.”

“Our findings add to evidence that conservation efforts should be focused on preserving functionally distinct threatened species, to stem the tide of harmful losses to biodiversity that are driven by human actions,” concluded co-author Alex Pigot, a senior research fellow at UCL.