Discussion
We analyzed the largest collection of size spectra relationships in
stream communities that we are aware of, and we found that slope
estimates varied in response to a broad temperature gradient.
Size-spectra slopes represent the efficiency of energy transfer from
small, abundant, individuals to fewer large predators (Trebilco et
al. 2013) with clear implications for ecosystem functioning (O’Gormanet al. 2012; Cross et al. 2015). Shallow slopes indicate
efficient transfer of energy by supporting a relatively higher
proportion of larger individuals, while steeper slopes indicate
inefficient energy transfer with relatively fewer large individuals. The
steeper slopes reported here with increasing mean annual temperatures
implies that warmer sites have fewer large individuals relative to the
number of small individuals within a site compared to colder sites.
These results help to resolve previous uncertainty in how size spectra
slopes scale with temperature. Variation in size spectra slopes is
driven by variation in body size distributions and body size is in turn
altered by temperature, either through reductions in taxon-specific body
size, species turnover, or through changes in community structure
(Bergmann 1847; Atkinson 1994; Daufresne et al. 2009; Winderet al. 2009; O’Gorman et al. 2012). Thus, it is widely
expected size-spectra slopes should vary across temperature gradients,
though the direction of change is uncertain (Daufresne et al.2009; Dossena et al. 2012; O’Gorman et al. 2012). Theory
predicts that warm environments should favor smaller individuals, and
this is supported by empirical (James 1970; Atkinson 1994; Daufresneet al. 2009) and experimental observations (Yvon-Durocheret al. 2011; Dossena et al. 2012). However, O’Gormanet al. (2017) found that warmed Icelandic streams had shallower
slopes, perhaps due to increased nutrient availability and changes in
trophic transfer efficiency, leading to increased top-down effects of
consumers on diatoms. In contrast, Dossena et al. (2012) found
that slopes declined with temperature, but the effect varied over
seasons. Mazurkiewicz et al. (2020) found no relationship between
marine benthic size spectra and temperatures in arctic systems. These
contrasting outcomes, derived from different experimental approaches,
generate uncertainty in how size spectra slopes should scale with
temperature across large spatial gradients. The results presented here
support the hypothesis that slopes become steeper in response to
increasing temperature.
However, while size spectra slopes scaled with temperature, the overall
change was relatively small, with median slopes declining by only
~0.1 units across the temperature gradient. Direct
comparisons of this effect size with previous studies of size spectra
responses to temperature (e.g., Yvon-Durocher et al. 2012, O’Gorman et
al. 2017) are hampered by the different approaches to estimating size
spectra exponents (Edwards et al. 2017). However, in a 30-year dataset
from the International Bottom Trawl Survey (ICES 2015), Edwards et al.
(2020) found that size spectra slopes calculated using maximum
likelihood varied ~0.4 units among years. In addition,
among sample variation at NEON sites in this study was
~0.3 to 0.8 units, with measured slopes at one site
varying from -1 to -1.8 among sample dates. Placing our results into
this context, the influence of temperature appears small relative to
variation due to other factors, including natural variation over time.
In contrast to the negative relationship of temperature with
size-spectra slopes, community biomass was positively related to mean
annual temperature. This is also in agreement with predicted effects of
increasing environmental temperature supporting more small-bodied
individuals. For example, an increase in community biomass could be
driven by small-bodied individuals alone, if their increase was larger
than the relative decrease in larger-bodied individuals. It is important
to account for community biomass, as increased biomass at lower trophic
levels may be able to support biomass at higher trophic levels (O’Gormanet al. 2012), even if trophic transfer efficiency is affected by
temperatures (Trebilco et al. 2013). However, the magnitude of
the random effects of site were large, and the effect of
temperature was relatively small. Including additional predictor
variables thought to affect community biomass, such as productivity or
nutrient availability at the base of the food web (Morin et al.2001; Daan et al. 2005), may help explain additional variation.
Given the relatively small influence of temperature and the overlap in
site-specific averages of size spectra (Figure 3A), our results provide
an opportunity to use our range of size-spectra as a baseline indicator
in studies of disturbance. Size-spectra relationships have been proposed
as a universal indicator of ecological health, with deviations from
“natural” size spectra representing disturbed systems (Jennings &
Blanchard 2004; Petchey & Belgrano 2010; Trebilco et al. 2013).
Defining “natural” is difficult without accounting for variation among
broad spatial and temporal scales. By accounting for the effect of
temperature on size spectra slopes in relatively undisturbed systems
across 50 degrees of latitude over three years, our results reveal
bounds that could help to gauge the severity of size spectra change in
response to disturbance. For example, one approach would be to compare
size spectra from disturbed sites to the posterior predictive
distribution of size-spectra at a similar site in our study, with
deviations outside of the expected range of natural variation indicating
the level of disturbance. This may represent a powerful tool for
assessing ecological condition. Indeed, as NEON data continues to be
collected, it will be possible to compare our predictions to
size-spectra collected after intense disturbances, such as extremely
high or low flow events, temperature anomalies due to climate change,
wildfires, flow debris, etc. This represents an exciting opportunity to
test responses to disturbances at higher levels of organization, which
has typically been difficult or impossible due to the large logistical
efforts needed to collect community-wide data across broad spatial
scales. Furthermore, data on post-disturbance size spectra within the
NEON sites will provide valuable information on community recovery, and
the magnitude, direction, and expected duration of altered size spectra.