Shifts in community size structure drive temperature invariance of secondary production in a stream‐warming experiment

Abstract

A central question at the interface of food-web and climate change research is how secondary production, or the formation of heterotroph biomass over time, will respond to rising temperatures. The metabolic theory of ecology (MTE) hypothesizes the temperature-invariance of secondary production, driven by matched and opposed forces that reduce biomass of heterotrophs while increasing their biomass turnover rate (production : biomass, or P:B) with warming. To test this prediction at the whole community level, we used a geothermal heat exchanger to experimentally warm a stream in southwest Iceland by 3.8°C for two years. We quantified invertebrate community biomass, production, and P : B in the experimental stream and a reference stream for one year prior to warming and two years during warming. As predicted, warming had a neutral effect on community production, but this result was not driven by opposing effects on community biomass and P:B. Instead, warming had a positive effect on both the biomass and production of larger-bodied, slower-growing taxa (e.g., larval black flies, dipteran predators, snails) and a negative effect on small-bodied taxa with relatively high growth rates (e.g., ostracods, larval chironomids). We attribute these divergent responses to differences in thermal preference between small- vs. large-bodied taxa. Although metabolic demand vs. resource supply must ultimately constrain community production, our results highlight the potential for idiosyncratic community responses to warming, driven by variation in thermal preference and body size within regional species pools.

Extreme drought pushes stream invertebrate communities over functional thresholds

“…Certain chironomid subfamilies found in our study, such as Orthocladiinae, primarily comprise cold-adapted stenotherms (Friberg et al, 2009;Worthington, Shaw, Daffern, & Langford, 2015) and wide temperature fluctuations would have constrained their presence in severely dewatered channels. The results reported here therefore accord with those of Nelson et al (2017), who reported unexpected body size responses to stream warming attributable to variability in thermal preference, and suggest that r-selection is a necessary but not sufficient condition for success during extreme drought.…”

“…However, body size also dictates drought resistance, as small size entails lower metabolic demand and facilitates easier access to suitable refugia(Griswold, Berzinis, Crisman, & Golladay, 2008;Ledger, Brown, Edwards, Milner, & Woodward, 2013;Woodward et al, 2016). The results reported here therefore accord with those ofNelson et al (2017), who reported unexpected body size responses to stream warming attributable to variability in thermal preference, and suggest that r-selection is a necessary but not sufficient condition for success during extreme drought.Biotic adaptation to disturbance depends greatly on the predictability of the event(Lytle, Bogan, & Finn, 2008). Certain chironomid subfamilies found in our study, such as Orthocladiinae, primarily comprise cold-adapted stenotherms(Friberg et al, 2009;Worthington, Shaw, Daffern, & Langford, 2015) and wide temperature fluctuations would have constrained their presence in severely dewatered channels.…”

Energy‐based top‐down and bottom‐up relationships between fish community energy demand or production and phytoplankton across lakes at a continental scale

“…Furthermore, we estimated the community production of omnivorous fish (CP om , non‐normalized values) by summing the product of the mean annual biomass per each size class ( B , predicted biomass per size class from the calculated biomass spectrum) and the biomass turnover rate (production to biomass ratio, P : B ), calculated within the MTE framework and according to Nelson et al (): where M (g) is the body mass of a size class, E (0.65 eV) is the activation energy of cellular respiration, k (8.6173 10 −5 eV K −1 ) is the Boltzmann constant, and T is the absolute air temperature in K (constrained to a temperature range of 0–40°C).…”

“…Here, we apply the MTE framework to determine the strength and directionality of top‐down and bottom‐up trophic cascades between fish and phytoplankton in lakes at a continental scale in Europe. Specifically, we calculated the size‐ and temperature‐corrected energy demand of the omnivorous fish community (i.e., community energy demand, CED om ) (Nelson et al ) and the size‐ and temperature‐corrected production of the omnivorous fish community (i.e., community production, CP om ) (Nelson et al ) to evaluate whether phytoplankton biomass measured as Chl a concentration is a predictor of omnivorous fish CP om (bottom‐up) or a response to omnivorous fish CED om (top‐down) across 227 European lakes. The CED of piscivorous fish (CED pi ) was added to both top‐down and bottom‐up models as explanatory variable to account for potential control of omnivorous fish communities by fish predation.…”

Long‐term exposure to higher temperature increases the thermal sensitivity of grazer metabolism and movement

“…Radix balthica is a freshwater snail commonly found throughout European ponds and streams (Pfenninger, Salinger, Haun, & Feldmeyer, 2011) that can exert strong top-down control on primary production (O'Gorman et al, 2012;Wullschleger & Ward, 1998). Thus, temperature-dependent changes in the metabolism and movement of R. balthica are likely to alter the structure and function of stream ecosystems (Nelson et al, 2017a;Woodward et al, 2010). Furthermore, little genetic mixing of R. balthica populations occurs among Hengill streams (Johansson, Quintela, & Laurila, 2016) and so populations from each stream are probably exposed to these thermal habitats over many generations.…”