Unexpected changes in community size structure in a natural warming experiment

Abstract

2Natural ecosystems typically consist of many small and few large organisms 1-4 . The 1 scaling of this negative relationship between body mass and abundance has important 2 implications for resource partitioning and energy usage [5][6][7] . Global warming over the 3 next century is predicted to favour smaller organisms [8][9][10][11][12] , producing steeper mass-4 abundance scaling 13 and a less efficient transfer of biomass through the food web 5 . 5Here, we show that the opposite effect occurs in a natural warming experiment 6 involving 13 whole-stream ecosystems within the same catchment, which span a 7 temperature gradient of 5-25 C. We introduce a mechanistic model that shows how the 8 temperature dependence of basal resource carrying capacity can account for these 9 previously unexpected results. If nutrient supply increases with temperature to offset 10 the rising metabolic demand of primary producers, there will be sufficient resources to 11 sustain larger consumers at higher trophic levels. These new data and the model that 12 explains them highlight important exceptions to some commonly assumed "rules" about 13 responses to warming in natural ecosystems. 14 Body mass (M) is a key determinant of many ecological phenomena 6,7,14 (e.g. growth, 15 metabolism, feeding) and its relationship with abundance (N) at either the individual or 16 species level is well described by a simple power law, b NM  (hereafter "MN-scaling"). 17The exponent b and its controlling factors have generated considerable interest in community 18 ecology for decades 4,6 , with widespread recognition that b is related to energy flow through 19 food webs [5][6][7] . Many studies have found that MN-scaling is conserved in the face of 20 biodiversity loss or species turnover and so may be a relatively stable property of 21 ecosystems 1-3 . Thus, a change in MN-scaling may highlight a fundamental disruption to the 22 processes that govern energy flow through an ecosystem by environmental or anthropogenic 23 stressors. For example, steepening of size-spectra (i.e. a more negative exponent b) following 24 fisheries exploitation is indicative of widespread losses at higher trophic levels 5,15 . warming favoured smaller phytoplankton and led to steeper size-spectra 13 . 36We tested the generality of this predicted temperature effect on MN-scaling across 13 37Icelandic streams that span a natural temperature gradient of 5-25 °C (Fig. 1a), but are 38 otherwise very similar in their physical and chemical properties [20][21][22][23][24] . Natural experiments and 39 space-for-time substitutions have some limitations (e.g. non-random allocation of 40 temperature "treatments", no observation of the warming process but rather its end point; see 41Supplementary Methods for discussion of these limitations), however, the streams occur in 42 the same catchment and thus are free of the usual confounding effects of biogeographical 43 differences or other environmental gradients 23,25 . The constituent species are a subset of those 44 commonly fou...

Size‐based ecological interactions drive food web responses to climate warming

“…; O'Gorman et al . ). Our novel finding is that predator declines with increasing temperatures are not always gradual, but can be sudden and collapses can occur at temperatures much lower than those that would cause starvation.…”

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

“…As global warming intensifies, there is increasing urgency to understand and predict how species and their ecological interactions will respond. Here, we show that long‐term exposure to different temperatures affected metabolic rate and movement of snails in ways that are likely to help them sustain larger populations at higher temperatures (Nelson et al., ; O'Gorman et al., ). The thermal dependence of snail metabolic rates was steeper than expected from theory (Brown et al., ; Gillooly et al., ), increasing the capacity for speed and thus for foraging, mate‐finding, migration and dispersal.…”

“…Chemical and physical properties vary slightly among streams but do not covary with temperature (Woodward et al., ). This system has been relatively stable over the past few centuries (Saemundsson, ), with the streams isolated from one another and exhibiting their characteristic temperatures for at least the past 16 years of research in the area (Woodward et al., ; O'Gorman et al., , ). Thus, it provides an ideal natural experiment – using space‐for‐time‐substitution – to explore how species respond to temperature change over multiple generations (O'Gorman et al., ; Saemundsson, ).…”

The fiddler crab, Minuca pugnax, follows Bergmann's rule

“…For instance, in the seagrass beds of the eastern United States, the marine isopod, Idotea balthica , follows Bergmann's rule (Manyak‐Davis et al, ), but not the temperature–size rule. Diatoms shrink or do not respond to higher temperatures (Adams et al, ; O'Gorman et al, ). Even fiddler crabs have different body size responses to latitude and temperature.…”