The behavioral and energetic responses of montane grasshoppers to climate change and the effects of competition in introductory college STEM classrooms

Abstract

Understanding thermal physiology can help us make mechanistic predictions about the organismal impacts of climate change. In my two ecology chapters, I explore the consequences of thermal physiology and behavior for the energy gain of montane grasshoppers. My study species – Melanoplus boulderensis and Melanoplus sanguinipes – are found along an elevational gradient (1740-3515m) in Colorado, and I compare populations at different sites along this gradient. I measure the temperature sensitivity of digestion (Chapter 1) and diel patterns in field body temperature, activity, gut fullness, and preferred body temperature (Chapter 2). I interpret my findings using a simple energy budget, accounting for the energy gained through digestion of food and the energy expended through metabolism (both temperature-dependent processes). Past work suggests that these grasshoppers optimize their digestion at higher temperatures (~40°C) than their locomotion (~25°C) and most environmental temperatures they encounter. This led me to hypothesize that this difference in thermal optima could be adaptive if grasshoppers are using microhabitat selection and temporal partitioning of activities. Specifically, I expected that grasshoppers would feed in the morning, filling their guts by the afternoon. At these peak afternoon temperatures, they would bask in a warm microclimate to optimize their digestion. In Chapter 1, I find that grasshoppers do indeed optimize their digestion – and consequently energy gain – at high temperatures ~40°C, with minimal differences between populations in thermal optima but a of higher peak digestion for populations at higher elevations. Consequently, both climate and physiology affect population energy balances, with both species exhibiting counter-gradient variation. I find that, since metabolic costs have a minimal effect on net energy gain assuming abundant food, climate change has led to an increase in net energy gain of grasshoppers. In Chapter 2, I find that field body temperatures are elevated in cool conditions. M. sanguinipes individuals select a higher body temperature in a thermal gradient in the afternoon than they do in the morning, but feeding state (unfed or fed) does not significantly affect thermal preference. I do not find evidence for diel patterns in gut fullness and the best model for predicting activity includes only the operative temperature and not time of day. Unlike I predicted, grasshoppers are most active at high temperatures, which generally correspond to midday, and guts are not fullest at midday. However, the evidence I found for behavioral thermoregulation via microhabitat selection supports the hypothesis grasshoppers are taking advantage of thermal opportunity to gain more energy. My third chapter is in the field of discipline-based education research. Understanding the interplay between contrasting theories of motivation can help us make mechanistic predictions about the effects of classroom experiences on student outcomes. Building on past literature about classroom experience, I use structural equation models to explore the relationships between students’ perception of classroom competition, sense of control, sense of belonging, and exam performance. Two contrasting theories of motivation, Control-Value Theory and Self-Determination Theory, posit sense of control and sense of belonging as keys to motivation and ultimately performance. I leverage a large survey dataset of students in introductory college STEM classes in the United States. I test contrasting mechanisms for the impact of competition on exam z-score: an indirect effect via sense of control and an indirect effect via sense of belonging. I find that perceived competition lowers students’ sense of belonging, sense of control, and, via sense of control, exam performance. I also disaggregate these effects by race to investigate whether the impact of perceived competition varies for students with different racial identities. I refer to the Quantitative Critical Race Theory literature to guide our analysis and interpretation of these results. I find that the relationships between perceived competition, sense of control, and sense of belonging do not differ for students with different racial identities. I do, however, find differences in the mean levels of perceived competition, sense of belonging, and sense of control among students with different identities. These differences reflect systems of oppression in our society and our classrooms.