Rain, resources and reproduction: Linking resource dynamics in ecological communities to life history trade-offs in lizards

Understanding how animals balance limited resources among the competing life history demands of reproduction, growth and survival in the face of climate and ecosystem variability is a central challenge in contemporary evolutionary ecology. In the following chapters I seek to integrate three projects broadly focused on the ecology, physiology and evolution of life history patterns in lizards. In my first project I used stable isotopes to link climate and pulsed resource dynamics at the ecosystem level to patterns of resource use in an arthropod and lizard consumer community. Second, I conducted an experimental study in which I examined physiological rates of carbon resource allocation and turnover in lizards. Last, I compiled a dataset from published literature to test theoretical evolutionary predictions of the importance of resources for the clutch-offspring size trade-off in lizards.;In chapter two I examine how the seasonal and inter-annual resource dynamics in a Chihuahuan desert food web are influenced by the interplay between pulsed precipitation and primary production patterns. Plants that use the C 4 photosynthetic pathway account for more than one-fifth of global annual primary production, largely as a result of highly productive grasses in semi-arid ecosystems. In semi-arid systems water is often the limiting factor, and precipitation events induce rapid, pulsed vegetative growth. In the Chihuahuan desert a bimodal precipitation pattern drives a spring pulse of C3 production and a summer pulse of C4 production. Although plants using C 4 and C3 photosynthesis differ in their seasonal availability, abundance and nutritional quality, we know little about their importance as sources of nutrients and energy for consumers at any level of a food web. Through carbon isotope analysis of a lizard community I showed that seasonally pulsed production by C4 and C3 plants resulted in a shifting resource landscape for arthropods and lizards in the Chihuahuan desert. Following summer rains, increasing amounts of energy and nutrients derived from C4 sources were incorporated into this food web. During a second year of study however, a failure in winter precipitation and an accompanying failure in spring C3 plant productivity resulted in a dramatic shift to increased C4 resource use across the arthropod and lizard food web. In the face of climate change data such as this is critical to understanding how communities will respond to predicted climatically driven changes in the nutrient quality, abundance and distribution of C3 and C4 plants.;In chapter three, I use a controlled diet switch experiment and carbon stable isotopes to conduct the first carbon - tissue turnover analysis of vertebrate ectotherms. My results showed that the small bodied lizard Sceloporus undulatus consubrinus had carbon turnover rates that were over four-times faster than a larger bodied Crotaphytus collaris. These ectothermic lizards also had carbon turnover rates that were seven-times slower than those reported for similar sized endotherms such as birds and mammals. These data, thus, provide new insight into carbon turnover rates in vertebrates of differing size as well as metabolic strategies and provide information critical for interpreting and understanding future studies using isotopic methodologies in vertebrate ectotherms.;In chapter four, I explore two fundamental life history trade-offs that result from the allocation of finite resources. The optimal size of an offspring and the optimal schedule of allocation to reproduction are the two central trade-offs of life-history evolution. Life history theory predicts an inverse proportionality between the number of offspring and the resources allocated per offspring. Though researchers have used lizards to test this theory empirically for decades, few have found the expected negative relationship between clutch size and offspring size. By correcting for size dependent resource allocation, however, we reveal this often concealed trade-off using an inter-specific comparison. This correction is robust to a full phylogenetic analysis using independent contrasts. Our dataset also shows a tight allometry for annual production that is consistent with life history models for indeterminate growers. Combined, these results demonstrate that the offspring-size/clutch-size tradeoff is not isolated to a single clutch but is shaped by resource investment over an entire year.