Process-based insights into insect responses to climate change: linking microclimate, ecophysiology, and demography in a butterfly model system

Abstract

Insects are in the warp and weft of the fabric of life on which we rely. Constituting 10% of animal biomass, insects play vital roles in ecological functions within terrestrial and freshwater ecosystems worldwide. However, many insect populations are experiencing declines, jeopardizing their significant contributions to both nature and human societies. These declines are primarily attributed to shifting land uses, habitat loss, agricultural intensification, and, as an increasingly influential factor, climate change._x000D_ Decades of research on the impact of climate change on insects have revealed that their responses are multifaceted, influenced by a complex interplay of processes across various scales. However, existing predictive models often overlook the underlying climatic and ecological processes that shape insect exposure and sensitivity. These models frequently aggregate climatic data on broad spatiotemporal scales that do not accurately reflect insects' climatic experiences, which are more tightly linked to the conditions measured in their microhabitats and host plants._x000D_ The objective of this thesis is to build more realistic predictive models that depict the current and future impacts of climate change on a well-known insect species -the green-veined white butterfly, Pieris napi- in the Mediterranean region. These models will be grounded in fundamental physiological and demographic processes._x000D_ The initial chapters characterise P. napi's microclimatic exposure and the dynamics of their host plants across diverse populations. Additionally, I examine various traits of the butterfly species that determine their susceptibility to climate variations. Findings indicate that P. napi populations are predominantly influenced by summer conditions which shapes butterfly microclimatic exposure and the availability and quality of their host plants. _x000D_ In the subsequent chapters, I employ the empirically collected data to parameterise two process-based models. The first model employs the species' experimentally estimated thermal tolerance to compute heat-induced mortality in response to fluctuating microclimatic temperatures recorded in the field. Mortality rates caused by extreme thermal stresses are relatively low in declining lowland populations of P. napi, despite their limited access to microclimatic buffering. These outcomes underscore the pivotal role microclimatic mosaics and microhabitat choices play in mitigating the impacts of climate change on insects._x000D_ The data from previous empirical work are then utilized to build a matrix population model. This second model integrates P. napi's vital rates across its life cycle, considering the effects of extreme microclimatic heat exposure and drought-induced scarcity of host plants on larval mortality and pupation. The model initially simulates present-day regimes of microclimatic heat extremes, drought, and their concurrent action (i.e. extreme hot-dry compound events). Results indicate that the existing declines in certain P. napi populations are primarily driven by drought-induced effects on host plants, corroborating prior field observations and highlighting the critical role indirect processes play in mediating insect responses to climate change. Subsequently, the model forecasts future regimes of extreme events under increasing global warming levels. These future scenarios anticipate a rise in the frequency of currently low-likelihood high-impact events relative to more moderate and recurrent extremes. Due to the nonlinear relationship between temperature and heat-induced mortality, simulations predict that under a global warming trajectory not aligned with the Paris Agreement, these low-probability, high-impact heat events will trigger more extensive and severe declines in P. napi populations. This effect could potentially surpass the impact of drought-induced plant scarcity._x000D_ In conclusion, this thesis underscores the value of employing process-based models that leverage field and experimental data to predict insect responses to climate change. The predictive models developed herein unravel the core underlying processes identified in empirical studies and quantify their individual and combined influences. Furthermore, these process-based models unveil an overlooked threat to insect populations: the disproportional escalation of low-likelihood high-impact extreme heat events.

Date of Award	11 Dec 2023
Original language	English
Supervisor	Jofre Carnicer Cols (Director) & Josep Peñuelas Reixach (Director)