Introduction
Many fundamental aspects of an organism’s biology are controlled by body
size, including metabolic rate, life history characteristics, diet
breadth, and trophic position (Brown et al. 2004; Woodwardet al. 2005; White et al. 2007). Communities, particularly
in aquatic environments, are often size structured and characterized by
a strong negative relationship between the abundance or biomass of
individuals and body size, known as the size spectrum (White et
al. 2007; Blanchard et al. 2009). The slope of this relationship
is related to trophic transfer efficiency (Trebilco et al. 2013;
Sprules & Barth 2015). Therefore, understanding the distribution of
biomass within communities connects individual- and population-level
traits to community structure, function, and ecosystem dynamics (Dossenaet al. 2012; O’Gorman et al. 2012; Yvon-Durocher & Allen
2012; Trebilco et al. 2013).
Size spectra are one of the few well documented organizing principles in
ecology. A large body of literature has demonstrated the consistency of
size-spectra relationships in diverse ecosystems (Jennings & Blanchard
2004; Trebilco et al. 2013; Blanchard et al. 2017;
Mazurkiewicz et al. 2019, 2020). A strongly negative relationship
between the abundance or biomass of individuals for a given body size
range has been consistently documented in aquatic communities, to the
point that size-spectra parameters have been recommended as a potential
“universal” indicator of ecological health (Petchey & Belgrano 2010).
However, while estimated slope coefficients are universally consistent
in sign (i.e. always negative), their specific values can vary in
response to both natural and anthropogenic environmental drivers
(Yvon-Durocher et al. 2011; Dossena et al. 2012; O’Gormanet al. 2012; McGarvey & Kirk 2018; Pomeranz et al. 2019).
For example, because metabolic rates increase with both body size and
temperature, environmental warming may have asymmetrical effects on
community structure and biomass distributions (Brown et al. 2004;
Brose et al. 2012). Bergmann’s and James’ rule predict that
warmer regions will tend to have smaller species, or smaller-sized
populations within a species, respectively (Bergmann 1847; James 1970).
Further, the temperature-size rule states that warmer temperatures cause
smaller individual body sizes in ectotherm species (Atkinson 1994).
Finally, small body size is a predicted response to global warming
(Daufresne et al. 2009; Gardner et al. 2011). Therefore,
temperature is hypothesized to be one of the main drivers of variation
in size spectra (O’Gorman et al. 2012, 2017). Other anthropogenic
impacts, including land use (Martínez et al. 2016), acid mine
drainage (Pomeranz et al. 2019), as well as natural variation,
including seasonal variation (McGarvey & Kirk 2018), and resource
subsidies (Perkins et al. 2018), have also caused slope estimates
to vary.
While these and other studies suggest that environmental conditions can
alter size spectra relationships, it is difficult to know how those
effects scale up to broader patterns in size-spectra at the continental
scale and across multiple years. A major limiting factor in large-scale
studies of size spectra is the logistical challenge of obtaining
consistently collected, processed, and analyzed data across a large
spatiotemporal scale. As a result, empirical studies of size spectra
share a fundamental limitation of small geographic scales and limited
sampling through time (typically a single sample). To overcome this
limitation, we estimated size spectra across stream sites in the
National Ecological Observatory Network (NEON). We had two primary
objectives in this study: 1) assess the broad geographical consistency
of size spectra across North American streams, 2) test the hypothesis
that size spectra vary as a function of temperature (measured as mean
annual temperature).