Soilscapes of Mortality Risk Suggest a Goldilocks Effect for Overwintering Ectotherms

Abstract

AbstractChanging climates are driving population declines in diverse animals worldwide. Winter conditions may play an important role in these declines but are often overlooked. Animals must not only survive winter but also preserve body condition, a key determinant of growing season success. We hypothesized that ectotherms overwintering in soil face a trade-off between risks of cold damage (including freezing) near the surface and elevated energy use at deeper depths. To test this hypothesis, we developed landscapes of mortality risk across depth for overwintering bumble bee queens. These critical pollinators are in decline in part because of climate change, but little is known about how climate affects overwintering mortality. We developed a mechanistic modeling approach combining measurements of freezing points and the temperature dependence of metabolic rates with soil temperatures from across the United States to estimate mortality risk across depth under historic conditions and under several climate change scenarios. Under current conditions, overwintering queens face a Goldilocks effect: temperatures can be too cold at shallow depths because of substantial freezing risk but too hot at deep depths where they risk prematurely exhausting lipid stores. Models suggest that increases in mean temperatures and in seasonal and daily temperature variation will increase risk of overwinter mortality. Better predictions of effects of changing climate on dormant ectotherms require more measurements of physiological responses to temperature during dormancy across diverse taxa.

Keywords: Bombus; climate change; cold tolerance; dormancy; overwintering energetics; winter ecology.