Population Regulation of Frogs and Lizards in a Lowland Wet Neotropical Forest: Integrating Alternative Hypotheses
Total Page：16
File Type：pdf, Size：1020Kb
Population Regulation of Frogs and Lizards in a Lowland Wet Neotropical Forest: Integrating Alternative Hypotheses by Brian Patrick Folt A dissertation submitted to the Graduate Faculty of Auburn University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy Auburn, Alabama August 5, 2017 Keywords: Conspecific Attraction, Leaf Abscission, Patch Dynamics, Predators, Resources Copyright 2017 Brian P. Folt Approved by Craig Guyer, Chair, Emeritus Professor of Biological Sciences John W. Feminella, Professor of Biological Sciences Sharon M. Hermann, Assistant Professor of Biological Sciences Maureen A. Donnelly, Professor, Florida International University Conor P. McGowan, Assistant Research Professor, School of Forestry and Wildlife Sciences Abstract Populations can be regulated in space and time from the “bottom-up” by resources (e.g., food), from the “top-down” by predation, or from parallel factors, such as interactions with conspecifics or heterospecifics. Amphibians and reptiles are species rich and abundant in lowland wet forests of Central America, and are important components of trophic communities as low or mid-level consumers of arthropods, and as prey to a diverse assemblage of vertebrate and invertebrate species. Theory describing population and community structure of Neotropical amphibians and reptiles has been generated largely from studies of a species-rich assemblage of frogs (Craugastoridae, Dendrobatidae) and anole lizards (Dactyloidae) that occur terrestrially on the forest floor and have generally supported hypotheses of bottom-up regulation mediated by resources, such as leaf-litter and prey abundance. Because trees regulate the input of leaf litter into terrestrial systems, a conceptual model has been suggested where leaf-abscission phenology of trees regulates patch dynamics of terrestrial frogs and lizards through forests in ways consistent with bottom-up regulation (the litter-mosaic hypothesis). However, a comparative review of anole life history in mainland Central American and Caribbean island habitats has described an alternative, top-down hypothesis of population regulation, where anoles in Central America are more greatly influenced by predation than anoles in Caribbean islands (the mainland-island model). Additionally, studies of the island anole Dactyloa aenea have demonstrated that juveniles are attracted to conspecifics when settling habitat, and conspecific attraction may be an additional parallel factor influencing forest dynamics of anoles and frogs in Central America. ii In an effort to generate a more integrative conceptual model describing population regulation and forest dynamics of frogs and lizards assemblage in Central America, I tested three hypotheses describing bottom-up, top-down, and parallel population regulation of a model assemblage at La Selva Biological Station, Costa Rica. First, I tested the litter-mosaic hypothesis that leaf-drop phenology of trees regulates population cycles and patch dynamics of terrestrial arthropods, frogs (Craugastor bransfordii, Oophaga pumilio), and lizards (Norops humilis). I used a comparative experimental approach, where I (1) compared seasonal abundance cycles of terrestrial litter and fauna beneath two tree species differing in phenology of leaf abscission (Castilla elastica, Dipteryx panamensis), and (2) experimentally manipulated leaf abscission beneath C. elastica with litter supplementation. I observed seasonal patterns of leaf litter, arthropods, and vertebrate abundance, but these patterns did not vary between tree species. Experimental supplementation caused elevated abundance of N. humilis in manipulated C. elastica plots relative to controls, as a result of demographic shifts in either apparent survival, immigration, or recruitment. My results suggest the litter-mosaic hypothesis can be refined and restricted to describing spatiotemporal variance of terrestrial anoles, while excluding frogs. Second, to better understand how predators influence prey frog and lizard ecology, I tested the mainland-island model prediction that predation exerts stronger effects than food resources for anoles and, by extension, frogs in Central America. I modeled the relative contribution of food, microhabitat, and predatory spiders (Ctenidae) to frog and anole occupancy. Frog occupancy was most strongly influenced by predators, an effect which increased at reduced biomass of leaf litter. Anoles occupied sites independent of predators, an observation inconsistent with the mainland- island model. All species were positively associated with leaf-litter depth and had elevated detection when predators were present. Third and last, I used eighteen months of mark-recapture iii observations of O. pumilio in a homogenous, fallow cacao plantation to test predictions of the conspecific attraction hypothesis. I found that juveniles settled habitat and used space in significant association with adults with previous history in plots, in ways consistent with conspecific attraction of juveniles and strong male territoriality. Together, my results contribute to our understanding of patch dynamics for terrestrial frogs and lizards in Neotropical wet forests. My experimental results suggest that Norops humilis populations respond to pulses of terrestrial litter in ways consistent with the litter-mosaic hypothesis, but the O. pumilio and C. bransfordii do not. Dispersal of O. pumilio and other frogs through the landscape may be driven in part by conspecific attraction. Because predatory spiders exert stronger effects on the occupancy of terrestrial frogs at lower abundance of terrestrial litter, seasonal declines in frog abundance during the wet season may be driven in part by elevated predation pressure when leaf litter is scarce. Predator-prey models reject a large literature invoking predation as the dominant force shaping ecology of diverse mainland anoles, and suggest similar mechanisms may regulate the ecology and evolution of mainland and island anoles. iv Acknowledgments I experienced a rich learning environment during graduate school at Auburn University, and my productivity there was enabled and enriched by a diverse group of colleagues, friends, and family members. First, thanks are extended to my graduate adviser, Craig Guyer. My development as a scientist benefited tremendously from his expertise and patience, while my development as a person similarly profited from his exemplar generosity and humility. I am grateful for the excellent experiences fostered by Craig during my time at Auburn, and I extend him my deepest gratitude for these fine years. Other scientists and peers contributed greatly to my experiences at Auburn University, by sharing expertise, example, and/or generosity. Maureen Donnelly, Jack Feminella, and Sharon Hermann kindly participated as helpful members of my committee, and Conor McGowan participated as an outside reader. I am very appreciative of Jason Bond, Brian Helms, David Laurencio, and Jon Armbruster, who supported projects in the Auburn University Museum of Natural History. A notable undergraduate, Sean Kelly, provided great help working in the museum. Thanks are deserved to Jim Godwin, who taught me practical skills and was generous with his time, thoughts, and resources. A graduate program secretary, Sandra Abate, was a rare example of terrific administrative efficiency. Thanks are also due to all those who develop and contribute to open-source statistical software in the Program R. Without those efficient and effective tools, my capacity as a scientist would be greatly reduced, and this endeavor would have been nearly impossible. v Numerous individuals deserve thanks for enabling and enriching my experiences in Costa Rica. I am grateful for staff members at La Selva Biological Station, who were welcoming and vastly resourceful: Danilo Brenes, Bernal Matarita, and Ronald Vargas. I overlapped with many researchers while at La Selva, who made my time there so enjoyable: Diego Dierick, John Smiley, Kim Smiley, Meaghan Gade, Linus Günther, and Ben Taylor. My life was enriched by friends during my time in Auburn, and I thank the following for friendship and memories: I.E.P. Turner, D. Steen, D. Laurencio, B. Mosholder, D. Mosholder, J. Ferguson, J. Goessling, J. Roberts, S. Riddle, G. Johnson, and A. J. Jenkins. Cheers! Last, I am forever grateful for my immediate family members who supported this chapter of my life: my parents, Robert and Claudia Folt, my brothers, Jason and David Folt, and my sister-in-law, Molly Folt. Matthew Painter, Cliff Painter, and Rosemary Warmenhoven welcomed me in the Southeast. Thanks to family, near and far, for great support and memories. The following document contains my doctoral dissertation. Additional acknowledgments, including appreciation for funding sources, are associated within relevant chapters hereafter. I take responsibility for any errors or oversights that exist herein. vi Table of Contents Abstract ........................................................................................................................................... ii Acknowledgments........................................................................................................................... v Chapter 1 General Introduction ...................................................................................................... 1 Population Regulation ......................................................................................................... 1 The Focal Assemblage