Changes in community composition are an important, but hard to predict, effect of climate change. Here, we use a wild-bee study system to test the ability of critical thermal maxima (CT, a measure of heat tolerance) to predict community responses to urban heat-island effects in Raleigh, NC, USA. Among 15 focal species, CT ranged from 44.6 to 51.3°C, and was strongly predictive of population responses to urban warming across 18 study sites ( = 0.44). Species with low CT declined the most. After phylogenetic correction, solitary species and cavity-nesting species (bumblebees) had the lowest CT, suggesting that these groups may be most sensitive to climate change. Community responses to urban and global warming will likely retain strong physiological signal, even after decades of warming during which time lags and interspecific interactions could modulate direct effects of temperature.

Cities experience elevated temperature, CO2 , and nitrogen deposition decades ahead of the global average, such that biological response to urbanization may predict response to future climate change. This hypothesis remains untested due to a lack of complementary urban and long-term observations. Here, we examine the response of an herbivore, the scale insect Melanaspis tenebricosa, to temperature in the context of an urban heat island, a series of historical temperature fluctuations, and recent climate warming. We survey M. tenebricosa on 55 urban street trees in Raleigh, NC, 342 herbarium specimens collected in the rural southeastern United States from 1895 to 2011, and at 20 rural forest sites represented by both modern (2013) and historical samples. We relate scale insect abundance to August temperatures and find that M. tenebricosa is most common in the hottest parts of the city, on historical specimens collected during warm time periods, and in present-day rural forests compared to the same sites when they were cooler. Scale insects reached their highest densities in the city, but abundance peaked at similar temperatures in urban and historical datasets and tracked temperature on a decadal scale. Although urban habitats are highly modified, species response to a key abiotic factor, temperature, was consistent across urban and rural-forest ecosystems. Cities may be an appropriate but underused system for developing and testing hypotheses about biological effects of climate change. Future work should test the applicability of this model to other groups of organisms.

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The biological diversity and composition of microorganisms influences both human health outcomes and ecological processes; therefore, understanding the factors that influence microbial biodiversity is key to creating healthy, functional landscapes in which to live. In general, biological diversity is predicted to be limited by habitat size, which for green areas is often reduced in cities, and by chronic disturbance (stress). These hypotheses have not previously been tested in microbial systems in direct comparison to macroorganisms. Here we analyzed bacterial, fungal and ant communities in small road medians (average area 0.0008 km 2 ) and larger parks (average area 0.64 km 2 ) across Manhattan (NYC). Bacterial species richness was not significantly different between medians and parks, but community composition was significantly distinct. In contrast, ant communities differed both in composition and richness with fewer ant species in medians than parks. Fungi showed no significant variation in composition or richness but had few shared taxa between habitats or sites. The diversity and composition of microbes appears less sensitive to habitat patchiness or urban stress than those of macroorganisms. Microbes and their associated ecosystem services and functions may be more resilient to the negative effects of urbanization than has been previously appreciated.

1. Global urbanisation is rapidly expanding and most of the world's humans now live in cities. Most ecological studies have, however, focused on protected areas.2. To address this issue, we tested predictions from studies of protected areas in urban ecosystems.3. Because most cities are heterogeneous habitat mosaics which include habitats with varying levels of chronic environmental stress, we focused on predictions from studies of less modified ecosystems about community-wide responses to variation in chronic stress. 4. We sampled ants across Manhattan's urban habitat mosaic, at sites with varying levels of chronic environmental stress.5. Many predictions derived from less modified ecosystems were supported by our findings: despite being the most intensively sampled habitat, high stress urban medians had less variability in ant composition -both within and among sitesthan either urban parks or urban forests, the lowest stress habitaturban forests-had significantly more accumulated species and a higher number of unique species than higher stress habitats, and urban parks, which have intermediate levels of chronic environmental stress, also had intermediate levels of variation in among-site species composition, accumulated species richness, and the incidence of unique species. The most common species also differed across Manhattan's urban habitat mosaic.6. Nevertheless, the prediction that exotic species would occur more frequently in higher stress habitats was not supported; exotic species were equally common across all habitats.7. These findings suggest that fine-scale heterogeneity in the chronic stress of urban habitats may be an underappreciated, but important structuring force for urban animal communities.