An outstanding question in urban evolutionary biology is what factors influence a species’ success in urban environments. This is a topic we’ve talked about before (check out Urban Adaptation in Mammals) and several other recent papers have tackled this topic. A variety of factors may influence whether or not a species occupies an urban environment and if it will thrive once it is there, and these factors are probably variable across taxa. Physiology and morphology may limit the species’ ability to tolerate urban climate or deal with the urban structural habitat. Patterns and modes of dispersal and reproduction may limit a species’ ability to establish in urban environments. Behavior may act as a filter stopping some species from occupying the busy and open urban environment while others may have an innate predisposition to explore new environments and deal with complex new challenges (check out No City for Shy Dog). And as Paul Martin and Frances Bonier demonstrate in their recent paper “Species interactions limit the occurrence of urban-adapted birds in cities” – competitive interactions can be important as well.
What role does competitive behavior play?
Martin and Bonier set out to understand the roles of competition and behavioral dominance in determining urban success across 296 species of birds from 260 cities. They focused on species with well-documented dominance relationships between congeners and scored each based on their breeding occurrence in cities (absent, localized breeding, local to widespread breeding, and widespread breeding). Their rationale was simple: direct competition and aggressive dominance behaviors may directly influence urban success by restricting access to resources for subordinate species, or indirectly because these traits are correlated with other traits that confer an advantage in urban habitats. Specifically, they tested three main hypotheses:
Hypothesis 1: Subordinate Tolerance
Or what I’m going to call the underdog advantage (no one else calls it that to my knowledge, but that’s what it sounds like to me!). Species that have it hard in their native environments may find that they are well-suited for the challenges of urban life while dominant species may not find those preferential resources that they are used to in the city. Subordinate species are used to being excluded, fighting for resources, and dealing with scarcity. Whether they opportunistically take advantage of the urban environment or are forced there by dominant species that perceive the habitat as inferior, these species may come out on top in this novel environment.
Hypothesis 2: Competitive Interference
Since I’m giving nicknames here, let’s call this one the bully wins (again, no one calls it this but me). In this scenario, species that thrive in cities (which the authors term “urban adapted”) recognize and are able to capitalize on anthropogenic resources that give them advantages such as increased anthropogenic food waste, abundant fruits from ornamental plants, or decreased predators in urban spaces. The species that are best able to competitively exclude other species from those same resources are the ones that thrive. Importantly, this effect would occur only where two species that exploit the same resources co-occur in sympatry, although not necessarily in urban environments. Historical competitive interference in ancestral habitat may also lead to the more dominant species winning out in urban areas while the subordinate is excluded.
Hypothesis 3: Dominant Advantage
To carry out my poor analogies, let’s call this one the all-around rockstar. This last hypothesis posits that species that are dominant have associated traits that are advantageous in the urban environment. For example, dominance and aggression may be related to boldness. If these correlated traits give the dominant species an advantage in acquiring resources or tolerating urban disturbances then they will do better in cities than the subordinate species that lack these traits.
Dominant Passer domesticus (left) may limit the urban success of Passer montanus (right) by aggressively preventing access to nesting sites.
The bully wins… most of the time
Martin and Bonier found support for the first two hypotheses. Some patterns of urban bird breeding abundance were consistent with the bully winning and others were consistent with the underdog coming out on top. Interestingly, where species ranges did not overlap, both dominant and subordinate species were equally likely to thrive in cities! So dominance alone is not correlated with urban success in the absence of species interactions. But where competing species were sympatric, dominant species were more likely to thrive in urban environments… but only if they commonly breed in urban areas (i.e., are urban adapters). In species pairs where the dominant species less commonly breeds in urban environments, the subordinate species had the advantage (go underdog!).
Adding another layer to this, the authors investigated geographic variation in these patterns and found that these relationships varied by continent! They dove deep into this in their supplementary materials, testing 12 different hypotheses that could explain this variation related to latitude, climate, primary productivity, human population, economic development, evolutionary relatedness, and sampling design (e.g., number of people surveyed about breeding birds). Of all these, the one factor that best explained these geographic patterns was economic development. In more developed countries, dominant species were more widespread than subordinate species in sympatry while no pattern was observed in developing countries and an intermediate pattern in countries of intermediate economic development. This result is intriguing. Why would economic development be associated with patterns of competitive interference in birds? The authors put forward three possibilities: clustered and augmented anthropogenic resources, novel niche space in urban environments that dominant species avoid, and/or elevated mortality in developing countries.
Patterns of urban tolerance are influenced by species interactions
This research is significant as it highlights the role of ecological mechanisms in influencing patterns of urban colonization and success. For species which competition plays a significant role in natural environments, it is important that urban researchers consider how these species interactions influence presence and habitat use in urban environments, which in turn influence urban evolution. Additionally, the finding that economic development is correlated with patterns of urban tolerance emphasizes the importance of considering the complex ways in which urban environments impact species and the need move past simple metrics of defining and studying urbanization.
Want to read more? Check out the full manuscript:
Editor Notes:
This post is dedicated to the memory of my grandmother, Jean Winchell, who passed away in February and who loved birds, especially the cardinal, but not the blue jay, which she considered a bully.
The featured header image is a Northern Cardinal (Cardinalis cardinalis) by Wikimedia Commons user Tattooeddreamer (CC BY-SA 3.0).
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