Animals could be trapped in their environment thanks to their own adaptations
Diving birds such as penguins, loons and grebes may be more likely to become extinct than their non-diving relatives, a new study has found.
Many waterfowl have evolved highly specialized bodies and behaviors that facilitate diving. Now, an analysis of the evolutionary history of more than 700 species of waterfowl shows that once a group of birds acquires the ability to dive, the changes become irreversible. Such inflexibility may help explain why diving birds have higher extinction rates than non-diving birds, researchers report in materials of the Royal Society from December 21.
“There are significant morphological adaptations for diving,” says Catherine Sheard, an evolutionary biologist at the University of Bristol in England, who was not involved in the study. For example, birds that dive into water from the air, such as gannets and some pelicans, can have damage to their neck muscles and breast bones.
It is possible that some diving birds evolve under an evolutionary “ratchet mechanism”, where adaptations to use a particular food source or habitat open up some new opportunities, but also contribute to increasingly specialized evolutionary adaptations. These birds can become trapped on their way, increasing the risk of their extinction. This is especially true if their habitat is rapidly changing in some negative way, perhaps due to human-induced climate change.
Evolutionary biologists Josh Tyler and Jane Younger studied the evolution of diving in Aequorlitornithes, a collection of 727 species of waterfowl in 11 bird groups. The team divided the species into non-diving birds, or into one of three types of divers: leg-propelled chasers (such as loons and toads), wing-propelled chasers (such as penguins and birds), and divers.
Diving has evolved in waterfowl at least 14 times, but the researchers found that there was no instance in which diving birds reverted to a non-diving form.
Scientists have also investigated the relationship between diving and the development of new species or their disappearance in different lineages of birds. Among the 236 species of diving birds, 75, or 32 percent, were part of lineages that experience 0.02 more species extinctions per million years than generations of new species. This increased extinction rate was more common in wing- and leg-driven pursuit divers compared to pirnage divers. On the other hand, lineages of non-diving birds create 0.1 more new species per million years than the rate at which species die out.
“The more specialized you are, the more dependent you are on a particular diet, foraging strategy or environment,” says Tyler of the University of Bath in England. “The range of habitats available for foraging is much greater for non-diving birds than for specialist divers, and this may affect their ability to adapt and thrive.”
In groups of diving birds, the less specialization, the better. Take penguins, a group that has been the subject of a fair amount of conservation concern. Researchers note that Papuan penguins ( Pygoscelis papua ), which have a broad diet, have larger populations than the related chinstrap penguins ( P. antarcticus ), which eat mainly krill, and may actually be four species that have recently diverged.
The International Union for Conservation of Nature considers both penguin species to be of “least concern” in terms of risk of imminent extinction.
If some diving birds are trapped in their environment by their own adaptations, this does not bode well for their long-term survival, say Tyler and Younger, who work at the University of Tasmania in Hobart.
According to the IUCN, 156 species, or about one-fifth, of the 727 species of waterfowl are considered vulnerable, threatened, or endangered. The researchers estimate that of the 75 species of diving birds from lineages with high rates of extinction, 24 species, or nearly one-third, are already listed as endangered.
