Abstract
The presence of introduced species is one of the most problematic sources of environmental degradation worldwide. Introduced species often lack natural controls in foreign environments (Cox, 1999) and establish new populations that may alter the natural functions of an ecosystem. The Central American black spiny-tailed iguana, Ctenosaura similis, has disjunct populations throughout south Florida (Krysko et al. 2003; Townsend et al. 2003b), one residing on Keewaydin Island (KI) in Collier County. Currently, little is known about how this population has adjusted to the environment on KI. In this study, I investigated the diet of C. similis to determine if life history or temporal factors contribute to diet variation. I also consider potential negative impacts of the iguanas on the local flora and fauna. I obtained specimens by noose pole capture, opportunistic hand captures, and by donation from Rookery Bay. Immediately post-mortem I measured morphometries. I performed necropsies on each individual to verify age class and sex, and reveal mouth, esophageal and stomach contents. Parametric analyses, including analysis of covariance and linear regression determined how each independent factor (age class, sex and sampling period) influenced the dependent variables (volume of food item types). Logistic regression analyses predicted how each independent factor influenced the dependent variables. I also used niche breadth and Pianka's diet overlap equation (Pianka, 1986) calculated for each age class, sex, and sampling period of capture. Data from 54 iguanas revealed 197 unique prey (56 different prey categories) within the stomachs of iguanas. Results revealed that age class, sex, and sample period all influence diet variation in C. similis. An ontogenetic diet shift was observed near 100- mm snout-vent length (SVL). At 100-mm SVL, juveniles appear to switch from a diet composed of predominantly animal prey to a vegetative diet as adults. Sex also appears to have influence on diet when considering the size of the iguana, but only in regards to volume of animal prey consumed. Diet varied among sample periods as well, with iguanas captured in August 2010 consuming significantly more plant prey volume than in any other sampling period. There was no significant difference in the amount of native versus non-native prey items consumed (QR=-354.378, df=3). Dietary niche breadth analysis found juvenile iguanas (8=3.24) had the highest niche breadth between age classes, and iguanas captured in August 2010 (B=3.26) had the highest niche breadths among sampling periods. The diet of C. similis on KI is influenced by life history traits (e.g. age class and sex) and environmental factors (e.g. time of capture), though this list is not inclusive. Iguanas on KI consumed a diversity of native and non-native plant and animal prey. Some prey items on KI belong to families with protected species in Florida, including tree snails (Bulimulidae), and the plants Galactia (Fabaceae), Eugenia (Myrtaceae), Forestiera (Oleaceae), Chamaesyce (Euphorbiaceae), Opuntia (Cactaceae), Asclepias (Apocynaceae), and Salicaceae. The combination of its dietary breadth and plastic feeding capabilities warrants concern for C. similis as a potentially successful invasive species in south Florida and wherever it has established introduced populations. The diet of C. similis on KI suggests that this population has potential to be a successful invader in a non-native environment. They are feeding as opportunistic omnivores, depredating native wildlife, and consuming potentially protected prey. They are also capable of consuming noxious prey with no immediately visible adverse effects to their health. Natural resource managers may be able to use these data to prioritize control efforts on properties where C. similis has become established. Not only do these data show exact GPS locations of the population, but they also provide information regarding which groups consume certain prey and when these prey items may be at the greatest risk of depredation. As Cooper and Yitt (2002) proposed, this population of iguanas is probably foraging to best meet their daily energetic requirements. Future studies may consider additional sampling periods and years to expand our understanding of temporal variation, or investigating population demographics of C. similis on KI, prey availability on KI, prey choice by C. similis, gut retention times in C. similis, seed dispersal, and germination rates of seeds passed through the gut of C. similis. Each of these additional research components would continue to build upon the knowledge base established in this study, and aid resource managers in their control efforts.