The Effects of Urbanization on Performance, Habitat Selection, and Persistence of Anolis Lizards
Urbanization is a global change phenomenon that is increasing in frequency and magnitude worldwide. As a greater proportion of the human population resides in urban areas, cities must grow, therefore exposing an increasing number of species to human-modified habitat. While some species become extirpated when their habitat is urbanized, others persist and even spread throughout cities. Furthermore, human activity increases the rates of species invasions around the world, and many introductions occur in urban areas. The objective of this dissertation is to evaluate the consequences of some major environmental changes caused by urbanization for non-human urban dwellers. Specifically, I measure the effects of changes in the structural and thermal properties of urban habitats on two species of Anolis lizards introduced to Miami, FL: the Cuban brown anole ( Anolis sagrei) and the Puerto Rican crested anole (Anolis cristatellus). These species, and anoles in general, are arboreal and ectothermic, therefore likely sensitive to the habitat changes caused by urban development. Separate from the following manuscripts, Appendix 2 documents standing phenotypic variation in morphology, thermal traits, and behavior in an urban and natural population of A. sagrei. ^ One of the most dramatic effects of urbanization is the change in the structural habitat, the matrix of physical objects that comprise a habitat. In order to develop a landscape for human use, vegetation is removed and modified and artificial structures are added. In the first chapter, I evaluate the qualities and magnitude of differences in the structural habitat between natural and urban habitats, and how lizards express their habitat preferences, given the changes in the urban environment. First, in the lab at URI, I assessed lizard preference for perch diameter using individuals from natural populations of both species. I allowed lizards to choose between vertical perches of three different diameters and recorded the proportion of time spent on each perch. Second, in four urban and four natural sites in the field in Miami, FL, I measured the diameter, height, and type (e.g., “tree trunk”, “branch”, “metal pole”) of available vegetation and artificial structures upon which lizards might perch. I could therefore asses habitat selection by also recording these values for perches that lizards used. From the preference trials, I discovered that lizards of both species prefer the largest available perches. In the field, I found that both vegetation and artificial structures in urban areas were broader compared to vegetation in natural areas. Lizards expressed their preference for broad diameters by selecting broader perches than were randomly available in both habitat types, including artificial structures. Therefore, in urban habitats, lizards used broader perches than they did in natural habitats, demonstrating an expansion of the structural niche axis in urban areas.^ Using the broadest perches in urban areas means that lizards often use artificial structures (the broadest available), exposing them to substrate properties they rarely encounter in natural habitats. Artificial structures, such as metal poles and painted walls, can be extremely smooth, challenging locomotion for species that primarily utilize vegetation (and man-made structures in cities) for daily activity. In the second chapter, I assess how lizard sprint performance is impacted by substrate smoothness and whether urban populations, more often exposed to smooth, artificial substrates, perform better. I measured the velocity and two-dimensional hindlimb kinematics of lizards running on three substrates of increasing smoothness (rough bark, concrete, and smooth wood) for two inclinations (inclined: 37°; vertical: 90°). I filmed lizards from urban and natural populations of both species using a high-speed video camera to capture limb positions at footrise and footfall during a sprint. I found that on vertical tracks, lizards ran slower, took shorter strides, moved their bodies shorter distances with a single step, kept their foot in contact with the substrate for longer (duty factor), and exhibited more contracted limb postures upon finishing a step than when running on the inclined track. I also observed these kinematic effects on the smooth wood substrate compared to the rough bark, though this effect was not as strong as with incline. I did not find an overall effect of habitat type, such that urban lizards did not run faster or use different gait characteristics or hindlimb positions compared to natural lizards.^ An effect of the structural changes caused by urbanization evaluated in the first chapter, along with increased impervious coverage (e.g., rooftops, parking lots, roads), is increased ambient and surface temperatures in cities. Known as the urban heat island effect, warmer urban temperatures could have great impact on ectothermic organisms, such as the anoles studied in this dissertation. To regulate their internal body temperature (Tb), ectotherms depend on not only ambient conditions, but also the presence of sunny basking sites and cool, shady sites, between which they can shuttle to raise or lower their temperature. This shuttling, or active thermoregulation, incurs costs because time spent shuttling is time not available for performing other tasks, such as foraging or territory defense. In the third chapter, I asses how the structural changes of urbanization affect thermal conditions and in turn the body temperatures of lizards. I first measured operative temperatures (Te), the body temperatures lizards would have if they did not actively thermoregulate, using copper models distributed randomly throughout each of the four urban and four natural sites used in the first chapter. Then, while the operative temperature models were recording data, I captured lizards and took internal body temperature measurements. Next, in the lab, thermal preferences were established for lizards of both species from urban and natural areas. Finally, I evaluated temperature-dependent sprint performance by measuring lizard sprint speed at six temperatures for A. sagrei and five temperatures for A. cristatellus. I found that urban areas had more open canopies compared to natural areas, which led to higher Te in urban sites than in natural habitats. I also found that lizards actively thermoregulated, maintaining Tb higher than Te in all sites. While thermal preference ranges and thermal performance did not differ between urban and natural populations for either species, overall, A. sagrei preferred warmer temperatures and sprinted faster at higher temperatures than did A. cristatellus. Urban sites may lower thermoregulatory costs for both species, but I found only A. sagrei Tb more often within their preferred temperature range in urban compared to natural habitat. Furthermore, based on available T e within each species’ preferred temperature range, urban sites with only A. sagrei appear less-suitable to A. cristatellus , while natural areas, even those that contain A. sagrei , are more suitable to A. cristatellus. While A. sagrei may find opportunities for dispersal in many urban locations, A. cristatellus is likely constrained to relatively cooler microclimates provided by forested locations and areas with higher canopy coverage.^ My dissertation evaluates several important ways that urbanization alters habitats for arboreal ectotherms. Together, the first two chapters constitute a complete evaluation of one mechanism behind persistence in urban habitats. From how the urban structural habitat differs from that of the ecologically and evolutionary historical natural habitat, to how lizards express preferences for habitat elements and the resultant performance consequences. The third chapter explores another mechanism influencing persistence that is critically important for ectotherms. As urban areas grow and more species are introduced to areas outside of their native ranges, studies such as these are important to understand and predict persistence and invasion dynamics. Furthermore, a deeper understanding of the mechanisms behind persistence in urban habitats may be critical for designing cities that maintain the biodiversity of a region and preserve ecosystem function.^
Andrew Clark Battles,
"The Effects of Urbanization on Performance, Habitat Selection, and Persistence of Anolis Lizards"
Dissertations and Master's Theses (Campus Access).