American Society of Naturalists

Paul R. Martin, Kevin W. Burke, and Frances Bonier (Jan 2021)

A global study of urban birds suggests both plasticity and evolutionary divergence underlie habitat partitioning

Read the Article (Just Accepted)

What causes habitat partitioning in urban-adapted birds?

Ecologically similar species often minimize competition and other costs of living together by occupying distinct habitats. The mechanisms that cause this habitat partitioning include plasticity, where subordinate species occupy different habitats to avoid or minimize competitive interactions with dominant species, and evolutionary divergence, where subordinate species evolve ecological differences from dominant species that lead them to occupy distinct habitats.

How important is plasticity versus evolutionary divergence for habitat partitioning in nature? In this study, researchers from Queen’s University in Ontario, Canada use a global dataset on urban birds to provide one of the few tests of the relative importance of plasticity versus evolutionary divergence underlying habitat partitioning. They find evidence for both. Greater habitat partitioning was associated with increased range overlap among dominant and subordinate species – a factor that is expected to increase the intensity of selection favoring evolutionary divergence. For birds that thrive in cities, however, the greatest impact on habitat partitioning appears to result from subordinates actively shifting out of cities when dominant species occur there, consistent with plasticity in response to aggressive, dominant species.

The study results suggest distinct ways to mitigate loss of biodiversity caused by urbanization. When dominant species thrive in cities, providing resources for subordinates that cannot be monopolized by the dominant (e.g., nest boxes with entrance holes too small for the dominant species to use) would help subordinates to persist. In the case of evolutionary divergence, adding distinct habitat refuges suited to subordinate species could help them colonize or persist in cities.

Overall, this global study provides new insight into the importance of two distinct processes that shape patterns of diversity in an urbanizing landscape.

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Abstract

Habitat partitioning can facilitate the coexistence of closely related species, and often results from competitive interference inducing plastic shifts of subordinate species in response to aggressive, dominant species (plasticity), or the evolution of ecological differences in subordinate species that reduce their ability to occupy habitats where the dominant species occurs (evolutionary divergence). Evidence consistent with both plasticity and evolutionary divergence exist, but the relative contributions of each to habitat partitioning have been difficult to discern. Here we use a global dataset on the breeding occurrence of birds in cities to test predictions of these alternative hypotheses to explain previously described habitat partitioning associated with competitive interference. Consistent with plasticity, the presence of behaviorally dominant congeners in a city was associated with a 65% reduction in occurrence of subordinate species, but only when the dominant was a widespread breeder in urban habitats. Consistent with evolutionary divergence, increased range-wide overlap with dominant congeners was associated with a 56% reduction in occurrence of subordinates in cities, even when the dominant was absent from the city. Overall, our results suggest that both plasticity and evolutionary divergence play important, concurrent roles in habitat partitioning among closely related species in urban environments.