Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Specialization

Ecology and Ecosystem Science (EES)

Department

Biological Sciences

First Advisor

Jason Kolbe

Abstract

Urbanization dramatically alters the abiotic and biotic environment of cities, favoring species that are pre-adapted or are able to adapt (e.g. through phenotypic plasticity or evolutionary mechanisms) to these novel conditions. While urbanization is widely considered one of the greatest threats to biodiversity worldwide, we still lack a general understanding of the mechanisms underlying organismal responses to human-induced rapid environmental change. The focus of this thesis was to examine how urbanization-induced changes in the abiotic and biotic environment influence the foraging behavior and body size of the brown anole, Anolis sagrei. In Chapter 1, we presented anoles from habitats differing in their levels of urbanization with an experimental feeding opportunity to assess differences in foraging decisions. We also manipulated perch availability and the presence of predators to determine whether and how these factors influence foraging decisions. While our manipulations had little effect, we found that anoles from natural forest habitats responded faster and more often to the experimental feeding opportunity compared to conspecifics from urban and suburban populations. In Chapter 2, we utilized gut-content and stable isotope analysis in the same populations as in Chapter 1 (i.e. natural forest, suburban and urban) to identify whether and how changes in the composition of prey and primary producers influence the trophic structure of urban food webs. We found that the differential consumption of prey by anoles among habitat types was only partially consistent with the differences observed among their isotopic niches. Specifically, greater proportional consumption of carnivorous arthropods in natural forest anoles compared to urban and suburban conspecifics was consistent with variation along the N15 axis of their isotopic niche. However, isotopic niches were primarily differentiated along the C13 axis, which was not consistent with differences in the proportional consumption of other taxa among habitat types. Furthermore, urban and suburban anoles incorporated substantial amounts of grass-based carbon into their tissues, suggesting that changes in trophic structure were primarily driven by the presence of C4 grasses in urban areas. In Chapter 3, we assessed the role of abiotic and biotic factors in determining the body size of brown anoles across an urbanized landscape. Given our finding of a positive relationship between anole body size and predator abundance, we then performed a manipulative field experiment and laboratory study to identify the mechanisms behind this pattern. In the field, we presented tethered male brown anoles that varied in body size to predatory curly-tailed lizards (Leiocephalis carinatus). Curly-tailed lizards attacked smaller anoles more frequently and at shorter latencies compared to larger anoles. Finally, we conducted a common garden experiment to determine whether body size differences between habitats with and without predators are genetically determined. Male brown anoles from habitats with predators had faster growth rates compared to males from non-predator habitats. However, we found no differences in female growth rate between habitats.

Urban habitats are currently considered hotspots of rapid environmental change and consequently, rapid adaptation of urban taxa. This dissertation demonstrates potential mechanisms through which urbanization can cause organismal change and identifies specific traits that may favor the persistence of taxa in these novel habitats. Yet, our results also emphasize that conditions both within and among urban areas are quite variable over relatively small spatial scales. Urban populations separated by even short distances are thus likely to experience markedly different selective pressures. This may be why organismal responses to urbanization-induced environmental change have been found to be mostly species and location specific. Indeed, the field of urban ecology has yet to produce conclusions that can be generalized beyond a few taxa, but this should not discourage future work on urban systems. In fact, our ability to predict and mitigate the negative impacts of urbanization is completely dependent on our knowledge of these specific effects. Therefore, we hope this work will encourage others to examine how urbanization influences other taxa and most importantly to determine the specific mechanisms underlying organismal change.

Available for download on Monday, July 27, 2020

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