UCLA

A central objective of ecology is to understand the factors, biotic and abiotic, that shape the survival and persistence of species. Developing such an understanding is particularly chal- lenging for organisms with complex life cycles, whose behavioral and physiological responses to the environment during one life stage can carry over to affect future life stages and scale up to affect population dynamics. Building on this understanding to then make predictions about how populations might respond to ongoing and future climate change is an additional defining challenge for contemporary ecological research.

The goal of my dissertation is to uncover behavioral and physiological mechanisms that shape survival, and evaluate the consequences of these processes for populations. Working across diverse taxa, I use statistical approaches to identify the behavioral mechanisms that affect survival and growth rate at the level of individuals, and develop a novel theoreti- cal framework of mathematical models that link behavior and physiology to demographic rates in order to quantify impacts at the population level. Chapter 1 introduces this novel theoretical framework, which incorporates temperature responses of the ectothermic (e.g. eggs, hatchlings) and endothermic (e.g. juveniles, adults) life stages that comprise the avian life cycle into a multi-season, stage-structured population model. I use this framework to compare the effects of different warming regimes, including deterministic increases to mean annual temperatures, seasonal differences in warming severity, and stochastic hot extremes. Using non-migratory arid-zone passerine birds as a case study, I find that stochastic hot extremes represent an immediate threat to population persistence and that realistic levels of warming over the next century may cause considerable – and in some cases, catastrophic – declines in abundance.

In Chapter 2, I expand this framework to account for the empirical observation that extreme temperatures can have persistent effects on a bird’s condition beyond the period of extreme temperature exposure. Focusing on temperature-induced condition changes for only the adult stage, I find that low levels of warming that are not enough to trigger acute increases in mortality rates can nevertheless cause adult condition to deteriorate and lead to declines in abundance. The severity of impacts from such condition changes on bird populations depends on two factors: first, on the negative consequences of being in poor condition for vital rates, and second, on how quickly adults in poor condition can recover to good condition under favorable environments. This work shows that failing to account for temperature-induced condition changes could lead to underestimation of warming costs to birds.

For Chapter 3, I turn my attention to the underlying factors that affect demographic rates. I focus on the juvenile stage of a territorial reef fish, which exhibits a trade-off between body size at the time of settlement on the reef and growth rate after settling, such that juvenile fish are more likely to survive if they are large at settlement but grow slowly. I assess possible behavioral mechanisms that give rise to this trade-off, and find that larger juveniles undertake foraging strategies that increase survival, but also experience higher costs from conspecific chasing, which reduce growth rate. Since water temperature during the larval (pre-settlement) period has been previously shown to determine size-at-settlement in this system, my work reveals how temperature effects during early life stages can impact survival and growth in a future life stage through condition and behavior.

Together, the chapters of my dissertation reveal an important role of temperature and be- havior interacting to affect vital rates that have downstream impacts on later life stages. My work underscores the value of integrating detailed empirical work on species with complex life histories with mechanistic models that scale individual-level mechanisms up to population dynamics.