Individual size distributions across North American streams vary with local temperature

Abstract

Parameters describing the negative relationship between abundance and body size within ecological communities provide a summary of many important biological processes. While it is considered to be one of the few consistent patterns in ecology, spatiotemporal variation of this relationship across continental scale temperature gradients is unknown. Using a database of stream communities collected across North America (18-68°N latitude, −4 to 25°C mean annual air temperature) over 3 years, we constructed 160 individual size distribution (ISD) relationships (i.e. abundance size spectra). The exponent parameter describing ISD's decreased (became steeper) with increasing mean annual temperature, with median slopes varying by ~0.2 units across the 29°C temperature gradient. In addition, total community biomass increased with increasing temperatures, contrary with theoretical predictions. Our study suggests conservation of ISD relationships in streams across broad natural environmental gradients. This supports the emerging use of size-spectra deviations as indicators of fundamental changes to the structure and function of ecological communities.

Community size structure varies with predator–prey size relationships and temperature across Australian reefs

“…A recent study (Heather, Blanchard, et al, 2021 ) showed that abundance‐based size spectrum slopes around Australia varied considerably around the theoretical expected mean of −1 (although our slopes are shallower, as unlike Heather, Blanchard, et al, 2021 , we included the full range of sizes observed). A large body of literature has demonstrated that temperature is an important predictor of community size spectrum slope and that these slopes are usually steeper in higher temperatures (Heneghan et al, 2019 ; Pomeranz et al, 2022 ). Here, we show that cPPMR also explains a significant amount of variation in size spectrum slopes, but not in cool temperate areas.…”

“…In the absence of fishing, both empirical and theoretical studies have shown that abundance declines with body size with a slope close to −1, corresponding to roughly equal biomass in size class bins on a logarithmic scale (Blanchard et al, 2017 ; Sprules & Barth, 2016 ). However, despite its conservative nature, several factors can affect size spectra, most notably the selective removal of larger‐bodied individuals (e.g., via fishing), which results in fewer larger‐bodied individuals relative to smaller‐bodied individuals, thus steeper size spectrum slopes (Dulvy et al, 2004 ; Graham et al, 2005 ; Robinson et al, 2017 ); along with increasing temperature (Blanchard et al, 2005 ; Pomeranz et al, 2022 ) and pollution (Arranz et al, 2021 ). The slope of size spectra provides a useful indicator of reef ecosystem health, and an improved understanding of ecological size spectra baselines and responses to different pressures is needed (Nash & Graham, 2016 ).…”

Fish life histories in a warming climate: a mechanistic basis of change and a community context

“…While studies on natural fish populations often (but not always) support this prediction, these cross scale patterns are biased because of fishing on those populations (Baudron et al 2014;Ikpewe et al 2020). This obscures detailed analysis of warming driven mechanisms underlying size shifts (Pomeranz et al 2022). I identify that negative effects of warming on the energy budget of large individuals, amplified by resource competition, increase with allocation to growth.…”

“…These time series are however rare or may provide little mechanistic understanding. For fish communities, another issue with studies of such temporal patterns of climate change inferred from time series data are that they are heavily biased because of fishing (Pomeranz et al 2022). The effects of intensive harvest on individual growth trajectories, maturation, reproductive patterns and population structure may obscure potential effects of climate change and thus demands a more diverse methodological approach (Enberg et al 2012;Audzijonyte et al 2016).…”

“…However, maximum and mean body size generally decreases with temperature over latitudinal gradients, in communities (Blanck & Lamouroux 2006;Emmrich et al 2014) and within species (Heibo et al 2005;van Dorst et al 2019). Also maturation occurs at a smaller size, growth rate and mortality increases, which in turn skews the population towards smaller individuals (van Dorst et al 2019;Pomeranz et al 2022). The possible effects of other environmental factors such as productivity or light conditions may however bias the analysis of effects of temperature on sizeat-age.…”