IMPACTS OF URBANIZATION AND INVASIVE SPECIES ON FORAGING BEHAVIOR AND BODY SIZE IN ANOLIS SAGREI

Zachary Chejanovski

doi:10.23860/diss-chejanovski-zachary-2018

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

Highlights

Animals must feed to survive, and theory states that organisms maximize fitness by matching their foraging decisions to environmental conditions (Stephens and Krebs 1986; Dall et al 2005)
Our results suggest that urbanization-induced variation in the trophic niche of brown anoles is primarily driven by differences in the composition of primary producers and by the presence of grasses in both urban and suburban habitats
Isotopic niches were primarily differentiated along the δ13C axis (Fig. 3) and almost half of the carbon in urban and suburban lizard tissues was derived from C4 grasses, as indicated by the IsoError mixing models

Summary

Introduction

Animals must feed to survive, and theory states that organisms maximize fitness by matching their foraging decisions to environmental conditions (Stephens and Krebs 1986; Dall et al 2005). Urbanization, for instance, produces rapid environmental change that dramatically transforms the biotic and abiotic characteristics of populated areas worldwide (Shochat et al 2006) While these changes are associated with many novel stressors (e.g., habitat fragmentation, human activity, and predators) that may alter foraging decisions in urban habitats, the precise nature of these anthropogenically-driven changes in foraging behavior is still unclear. Body size appears to play a role in determining consumer diets in that the majority of feeding interactions within a food web involve a larger predator consuming smaller prey (Cohen et al, 1993). This has resulted in a large research effort aimed at identifying the factors that may contribute to body size variation both within and between species (Peters, 1986; Dmitriew, 2011)

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Results

Conclusion