The thermal ecology of prairie stream fishes

Abstract

Earth’s atmosphere has warmed by approximately 1°C over the past century and continues to warm at an increasing rate. The effects of atmospheric warming are already visible in most major ecosystems and are evident across all levels of biological organization. Understanding how organisms respond to spatial and temporal variation in temperature, as well as linking their functional responses to temperature, are critical steps toward predicting the responses of populations and communities to global climate change. The southern redbelly dace (Chrosomus erythrogaster) and the central stoneroller (Campostoma anomalum) are two common minnows (Cyprinidae) that occur in the Flint Hills region of the United States. These species fill similar ecological roles in streams where they co-occur but differ in their overall pattern of occurrence, with dace largely occupying cooler headwater reaches and stonerollers persisting in both headwaters and warmer intermediate-sized streams. Differences in the fundamental thermal niche of these species could underlie the observed differences in their realized thermal niches along a stream-size gradient of temperature. To better understand how temperature drives patterns of occurrence in functionally similar species of fish, I evaluated the thermal ecology of these two minnow species. First, I tested for interspecific differences in physiological functional traits along an ecologically realistic temperature gradient. The critical thermal maximum of the stoneroller was higher than dace at warm acclimation temperatures, indicating a greater capacity to buffer thermal stress. Additionally, temperature drove differences in activity levels between species; dace were more active when temperatures were warm, though behavioral differences between the benthic stoneroller and column-dwelling dace could also influence activity. Second, I tested whether acclimation to a diurnal temperature cycle affected the energy metabolism of dace and stoneroller compared to constant acclimation conditions. Dace acclimated to a diurnal thermal regime exhibited higher maximum metabolic rates, and subsequently higher aerobic scope, when exposed to temperatures above mean conditions. This indicates that diurnal variation in temperature is an important contributor to this species’ ability to maintain energy metabolism when exposed to above-average temperature. Third, I leveraged long-term fish community and environmental data to examine responses in body size, abundance, and growth to inter-annual variation in temperature and flow in two cohorts of dace and stoneroller. I found that the average body size of dace in November decreased during years when stream flows were reduced during the spring and summer, while the average body size of stonerollers increased during years with lower spring flows and stable flow persisting through the summer. The abundances of both species in November was not influenced by inter-annual variation in flow or temperature. Finally, while growth of dace between August and November was not influenced by inter-annual variation in flow or temperature, stonerollers grew less during years were flows were reduced during the spring and summer. Collectively, these studies demonstrate both the importance of using long-term data to infer patterns along environmental gradients and highlight how functional responses to temperature can inform patterns of occurrence along thermal gradients.