Island biodiversity rides on the wings of birds

You can know a lot of things about birds just by the shape of their wings. A seafaring albatross, stretching out its sail-like airfoils, lives a very different life from a ground-dwelling antpitta with its long legs and short, stubby wings that it uses in rare, short bursts of flight.

But can bird wing shape tell scientists something useful about how nature is organized?

Research from Washington University in St. Louis says that bird wing shape — a proxy for long-distance flying ability — is a trait that influences biodiversity patterns on islands around the world.

“Our results reveal how a key dispersal trait — bird wing shape — can shape fundamental diversity patterns and island biogeography across scales,” said Justin Baldwin, first author of the new study in Ecology Letters. Baldwin is recent PhD graduate of the ecology and evolutionary biology program at WashU.

“To date, most studies have focused on how geographic factors such as area and isolation influence island species-area relationships,” he said. “Our results highlight the importance of trait differences among species, specifically traits related to dispersal ability.”

No bird is an island

The new study has its roots in one of the earliest recognized patterns of ecology.

The influential biologists Robert H. MacArthur and Edward O. Wilson coined the theory of island biogeography 50 years ago to help to explain patterns of species diversity on islands.

The theory relies on two main observations: first, more isolated islands have a lower number of species, because it is harder for species to reach them; and second, larger islands have a higher number of species, because they experience fewer extinctions. That second part gives rise to an ecological law known as the island species-area relationship.

“In larger patches of habitat, you will find larger numbers of species,” Baldwin said. But the area of an island or protected area is not the only important consideration for how many species are found in one place.

The theory of island biogeography, appealing in its simplicity, has its limitations. Notably, modern scientists have taken issue with how classical island biogeography theory assumes that species are ecologically equivalent in terms of their dispersal ability. “Ecologists have spent a lot of time trying to understand what other factors might drive biological diversity and species-area relationships,” Baldwin said.

A recently published study of bird traits offered Baldwin and his PhD co-advisor Jonathan Myers, a professor of biology in Arts & Sciences, an opportunity to examine one potentially important and species-specific factor: bird wing shape.

The scientists tapped into big data on bird hand-wing index values. “It’s a measure of how pointy bird wings are,” Baldwin said. He synthesized data for 6,706 bird species on 3,894 islands for this new study.

“Birds like chickens, wrens and cassowaries have low values and tend not to fly very far,” Baldwin said in explaining the index values. “But swifts, swallows and hummingbirds have high values and can move far.”

Dispersal ability matters for biodiversity

Baldwin and Myers used the hand-wing index data and two other independent datasets to calculate new species-area relationships for all 11 recognized biogeographical realms of the world. They also calculated 248 new species-area relationships representing unique combinations of bird families in biogeographical regions.

“We redid our analysis at multiple spatial and taxonomic scales,” Baldwin said. “So, not just for all birds at all different biogeographical realms, but we repeated the analyses within families across realms and across major orders.”

They found that the visually intuitive metric of bird wing shape does have a strong effect on biodiversity patterns on islands.

“In parts of the world where birds tend to have low dispersal abilities, the species-area relationships were steep,” Baldwin said — meaning that increasing island size caused a large gain in species numbers. “But in areas where birds could move around more, species-area relationships were flatter.”

They also found evidence of a kind of diversity “saturation” on certain types of islands. This saturation appears to be important when islands have a lot of bird species with wings that equip them for long-distance flights.

Findings from this study may be useful to inform conservation decision-making.

“The theory of island biogeography and species-area relationships have important applications for biodiversity conservation, especially on islands, which often harbor large numbers of endemic species found only within a single island or group of islands,” Myers said.

“The ecological and evolutionary processes that determine the steepness of the species-area relationship can be used to predict how and why habitat loss — due to factors such as deforestation, human land-use change and sea-level rise — causes extinctions and biodiversity loss on islands,” he said.

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