Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.
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...
Biodiversity continues to decline under the effect of multiple human pressures. We give a brief overview of the main pressures on biodiversity, before focusing on the two that have a predominant effect: land-use and climate change. We discuss how interactions between land-use and climate change in terrestrial systems are likely to have greater impacts than expected when only considering these pressures in isolation. Understanding biodiversity changes is complicated by the fact that such changes are likely to be uneven among different geographic regions and species. We review the evidence for variation in terrestrial biodiversity changes, relating differences among species to key ecological characteristics, and explaining how disproportionate impacts on certain species are leading to a spatial homogenisation of ecological communities. Finally, we explain how the overall losses and homogenisation of biodiversity, and the larger impacts upon certain types of species, are likely to lead to strong negative consequences for the functioning of ecosystems, and consequently for human well-being.
Climate warming has been linked to an apparent general decrease in body sizes of ectotherms, both across and within taxa, especially in aquatic systems. Smaller body size in warmer geographical regions has also been widely observed. Since body size is a fundamental determinant of many biological attributes, climate-warming-related changes in size could ripple across multiple levels of ecological organization. Some recent studies have questioned the ubiquity of temperature–size rules, however, and certain widespread and abundant taxa, such as diatoms, may be important exceptions. We tested the hypothesis that diatoms are smaller at warmer temperatures using a system of geothermally heated streams. There was no consistent relationship between size and temperature at either the population or community level. These field data provide important counterexamples to both James’ and Bergmann's temperature–size rules, respectively, undermining the widely held assumption that warming favours the small. This study provides compelling new evidence that diatoms are an important exception to temperature–size rules for three reasons: (i) we use many more species than prior work; (ii) we examine both community and species levels of organization simultaneously; (iii) we work in a natural system with a wide temperature gradient but minimal variation in other factors, to achieve robust tests of hypotheses without relying on laboratory setups, which have limited realism. In addition, we show that interspecific effects were a bigger contributor to whole-community size differences, and are probably more ecologically important than more commonly studied intraspecific effects. These findings highlight the need for multispecies approaches in future studies of climate warming and body size.
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