Summary: 1. Recent observations indicate that climatic change is altering biodiversity, and models suggest that the consequences of climate change will differ across latitude. However, long‐term experimental field manipulations that directly test the predictions about organisms’ responses to climate change across latitude are lacking. Such experiments could provide a more mechanistic understanding of the consequences of climate change on ecological communities and subsequent changes in ecosystem processes, facilitating better predictions of the effects of future climate change.2. This field experiment uses octagonal, 5‐m‐diameter (c. 22 m³) open‐top chambers to simulate warming at northern (Harvard Forest, Massachusetts) and southern (Duke Forest, North Carolina) hardwood forest sites to determine the effects of warming on ant and other arthropod populations and communities near the edges of their ranges. Each site has 12 plots containing open‐top chambers that manipulate air temperature incrementally from ambient to 6 °C above ambient. Because the focus of this study is on mobile, litter‐ and soil‐dwelling arthropods, standard methods for warming chambers (e.g. soil‐warming cables or infrared heaters applied to relatively small areas) were inappropriate and new technological approaches using hydronic heating and forced air movement were developed.3. We monitor population dynamics, species composition, phenology and behaviour of ants and other arthropods occupying these experimental chambers. Microclimatic measurements in each chamber include the following: air temperature (three), soil temperatures (two each in organic and mineral soil), photosynthetically active radiation (PAR), relative humidity and soil moisture (one each). In two chambers, we are also measuring soil heat flux, associated soil temperatures at 2 and 6 cm and volumetric water content. To assess the composition, phenology and abundance of arthropod communities within the experiment, we use monthly pitfall trapping and annual Winkler sampling. We also census artificial and natural ant nests to monitor changes in ant colony size and productivity across the temperature treatments.4. This experiment is a long‐term ecological study that provides opportunities for collaborations across a broad spectrum of ecologists, including those studying biogeochemical, microbial and plant responses to warming. Future studies also may include implementation of multifactorial climate manipulations, examination of interactions across trophic levels and quantification of changes in ecosystem processes.

Insect mutualisms can have disproportionately large impacts on local arthropod and plant communities and their responses to climatic change. The objective of this study was to determine if the presence of insect mutualisms affects host plant and herbivore responses to warming. Using open‐top warming chambers at Harvard Forest, Massachusetts, USA, we manipulated temperature and presence of ants andChaitophorus populicolaaphids onPopulus tremuloideshost plants and monitored ant attendance and persistence ofC. populicola, predator abundance, plant stress, and abundance ofMyzus persicae, a pest aphid that colonized plants during the experiment. We found that, regardless of warming,C. populicolapersistence was higher when tended by ants, and some ant species increased aphid persistence more than others. Warming had negligible direct but strong indirect effects on plant stress. Plant stress decreased with warming only when both ants andC. populicolaaphids were present and engaged in mutualism. Plant stress was increased by warming‐induced reductions in predator abundance and increases inM. persicaeaphid abundance. Altogether, these findings suggest that insect mutualisms could buffer the effects of warming on specialist herbivores and plants, but when mutualisms are not intact, the direct effects of warming on predators and generalist herbivores yield strong indirect effects of warming on plants.

Ecological communities are being reshaped by climatic change. Losses and gains of species will alter community composition and diversity but these effects are likely to vary geographically and may be hard to predict from uncontrolled “natural experiments”. In this study, we used open‐top warming chambers to simulate a range of warming scenarios for ground‐nesting ant communities at a northern (Harvard Forest, MA) and southern (Duke Forest, NC) study site in the eastern US. After 2.5 years of experimental warming, we found no significant effects of accumulated growing degree days or soil moisture on ant diversity or community composition at the northern site, but a decrease in asymptotic species richness and changes in community composition at the southern site. However, fewer than 10% of the species at either site responded significantly to the warming treatments. Our results contrast with those of a comparable natural experiment conducted along a nearby elevational gradient, in which species richness and composition responded strongly to changes in temperature and other correlated variables. Together, our findings provide some support for the prediction that warming will have a larger negative effect on ecological communities in warmer locales at lower latitudes and suggest that predicted responses to warming may differ between controlled field experiments and unmanipulated thermal gradients.

Population changes and shifts in geographic range boundaries induced by climate change have been documented for many insect species. On the basis of such studies, ecological forecasting models predict that, in the absence of dispersal and resource barriers, many species will exhibit large shifts in abundance and geographic range in response to warming. However, species are composed of individual populations, which may be subject to different selection pressures and therefore may be differentially responsive to environmental change. Asystematic responses across populations and species to warming will alter ecological communities differently across space. Common garden experiments can provide a more mechanistic understanding of the causes of compositional and spatial variation in responses to warming. Such experiments are useful for determining if geographically separated populations and co‐occurring species respond differently to warming, and they provide the opportunity to compare effects of warming on fitness (survivorship and reproduction). We exposed colonies of two common ant species in the eastern United States, Aphaenogaster rudis and Temnothorax curvispinosus, collected along a latitudinal gradient from Massachusetts to North Carolina, to growth chamber treatments that simulated current and projected temperatures in central Massachusetts and central North Carolina within the next century. Regardless of source location, colonies of A. rudis, a keystone seed disperser, experienced high mortality and low brood production in the warmest temperature treatment. Colonies of T. curvispinosus from cooler locations experienced increased mortality in the warmest rearing temperatures, but colonies from the warmest locales did not. Our results suggest that populations of some common species may exhibit uniform declines in response to warming across their geographic ranges, whereas other species will respond differently to warming in different parts of their geographic ranges. Our results suggest that differential responses of populations and species must be incorporated into projections of range shifts in a changing climate.