Urban areas may serve as habitat and corridors for dry-adapted, heat tolerant species; an example from ants

Urban abiotic stressors drive changes in the foraging activity and colony growth of the black garden ant Lasius niger

Changes in habitat characteristics are known to have profound effects on biotic communities and their functional traits. In the context of an urban-rural gradient, urbanisation drastically alters abiotic characteristics, e.g., by increasing environmental temperatures and through light pollution. These abiotic changes significantly impact the functional traits of organisms, particularly insects. Furthermore, changes in habitat characteristics also drive changes in the behavioural traits of animals, allowing them to adapt and thrive in new environments. In our study, we focused on the synanthropic ant species Lasius niger as a model organism. We conducted nocturnal field observations and complemented them with laboratory experiments to investigate the influence of night warming (NW) associated with Urban Heat Islands (UHI), light pollution (ALAN), and habitat type on ant foraging behaviour. In addition, we investigated the influence of elevated temperatures on brood development and worker mortality. Our findings revealed that urban populations of L. niger were generally more active during the night compared to their rural counterparts, although the magnitude of this difference varied with specific city characteristics. In laboratory settings, higher temperatures and continuous illumination were associated with increased activity level in ants, again differing between urban and rural populations. Rural ants exhibited more locomotion compared to their urban counterparts when maintained under identical conditions, which might enable them to forage more effectively in a potentially more challenging environment. High temperatures decreased the developmental time of brood from both habitat types and increased worker mortality, although rural colonies were more strongly affected. Overall, our study provides novel insights into the influence of urban environmental stressors on the foraging activity pattern and colony development of ants. Such stressors can be important for the establishment and spread of synanthropic ant species, including invasive ones, and the biotic homogenization of anthropogenic ecosystems.

Effect of ants on herbivory levels of Inga laurina: the interplay between space and time in an urban area

Extrafloral nectary plants not only occur in natural areas but also in urban parks. These areas are prone to edge effects, and plants face different microenvironmental conditions. We investigated the spatial variation of ant–plant interactions in an urban park, and we examined if plants with ants would show lower herbivory levels and if it depended on habitat type (interior or edges). Seedlings of Inga laurina were set in 200-m long transects (which covered both the west and east edges, and the interior) in an urban park and then experimentally assigned to be either ant-present or ant-excluded plants. Leaf herbivory was investigated throughout the wet season and was influenced by the interaction effect between ants and habitat type. Ants decreased the herbivory on the west edge, but on the east edge results were the opposite. The east edge had higher temperature and sunlight exposure in comparison to the other sites and was assumed to disrupt the stability of ant–plant interactions. In the interior of the fragment, herbivory depended on ant presence/absence and on the location of plants along the transect. Our study highlights how the outcomes of ant–plant interactions are spatially conditioned and affected by different types of habitats.

Anthropogenic Induced Beta Diversity in Plant–Pollinator Networks: Dissimilarity, Turnover, and Predictive Power

Biogeography has traditionally focused on the distribution of species, while community ecology has sought to explain the patterns of community composition. Species interactions networks have rarely been subjected to such analyses, as modeling tools have only recently been developed for interaction networks. Here, we examine beta diversity of ecological networks using pollination networks sampled along an urbanization and agricultural intensification gradient in east Leinster, Ireland. We show, for the first time, that anthropogenic gradients structure interaction networks, and exert greater structuring force than geographical proximity. We further showed that species turnover, especially of plants, is the major driver of interaction turnover, and that this contribution increased with anthropogenic induced environmental dissimilarity, but not spatial distance. Finally, to explore the extent to which it is possible to predict each of the components of interaction turnover, we compared the predictive performance of models that included site characteristics and interaction properties to models that contained species level effects. We show that if we are to accurately predict interaction turnover, data are required on the species-specific responses to environmental gradients. This study highlights the importance of anthropogenic disturbances when considering the biogeography of interaction networks, especially in human dominated landscapes where geographical effects can be secondary sources of variation. Yet, to build a predictive science of the biogeography of interaction networks, further species-specific responses need to be incorporated into interaction distribution modeling approaches.

Urban effects on saprophagous macroarthropods are mainly driven by climate: A global meta-analysis

Macrodecomposers provide important ecosystem services even in human dominated habitats including urban ecosystems, but the effect of urban land conversion on their species diversity and abundance has not been explored at global scale. Here, we present the first meta-analysis to quantify the general response of two major arthropod taxa, terrestrial isopods and millipedes to urbanization and to reveal the underlying mechanisms. Climatic (temperature, precipitation, growing season length), edaphic (pH, organic carbon, CaCO₃ and clay content of surface soils), urban (population density, city age, vegetation cover and mean actual evapotranspiration) parameters and methods of study (duration, sampling technique, replications) were used as moderators. We used a hierarchical meta-analytic approach to consider the dependence of multiple effect sizes obtained from one study. Altogether 156 paired observations were extracted from 59 urban studies conducted between 1980 and 2020. Urbanization had a negative effect on species diversity (species richness and Shannon index) of both macroarthropod taxa. However, both the direction and strength of their abundance response varied to a greater extent, resulting in a neutral effect of urban disturbance on them. The key drivers influencing the urban effects on macroarthropods were mean annual temperature and precipitation, absolute minimum temperature and length of growing season. The study also highlighted the importance of sampling methods: direct sampling (hand collecting) resulted in stronger urban effects presumably due to several sources of sampling bias. Our global synthesis highlighted that urbanization is a threat to soil arthropods, particularly to litter-dwelling detritivores, which potentially alters plant residue processing and ultimately soil biogeochemical cycles.

Water-seeking behavior among terrestrial arthropods and mollusks in a cool mesic region: Spatial and temporal patterns

Dehydration can have negative effects on animal physiological performance, growth, reproduction, and survival, and most animals seek to minimize these effects by reducing water losses or seeking water sources. Much-but not all-of the research on animal water balance comes from dryland ecosystems. However, animals inhabiting mesic regions may also experience desiccating conditions, for example within urban heat islands or during heatwaves and droughts. Here we examined how spatial variation in impervious surface and spatial and temporal variation in microclimate impact water demand behavior of terrestrial arthropods and mollusks in three areas of mesic Northwest Ohio, with analysis of taxa that exhibited the greatest water demand behavior. Water demand behavior was measured as the frequency that individuals were observed at an artificial water source (a moistened pouch), relative to the frequency at a control (a dry pouch). Overall, terrestrial arthropods and mollusks were found about twice as often at the water source than at the control (equivalent to 86 more observations on the wet pouch than on dry at each site, on average), with ants accounting for over 50% of the overall response in urban areas. Daily fluctuations in vapor pressure deficit (VPD) best predicted daily variation in water demand behavior, with increased demand at higher VPD. Mean VPD was generally highest near urbanized areas, but effects of VPD on water demand behavior were generally lower in urbanized areas (possibly related to reductions in overall abundance reducing the potential response). On certain days, VPD was high in natural areas and greenspaces, and this coincided with the highest arthropod water demand behavior observed. Our results suggest that terrestrial arthropod communities do experience periods of water demand within mesic regions, including in greenspaces outside cities, where they appear to respond strongly to short periods of dry conditions-an observation with potential relevance for understanding the effects of climate change.

Species and functional diversity - A better understanding of the impact of urbanization on bee communities

We examined site patterns in bee species for diversity and functional diversity in urban, suburban and rural areas. We sampled bees from all three habitat types and compiled a database of functional traits for each species. While species diversity decreased with urbanization, as expected, components of functional diversity showed differences between urban and suburban habitats. Functional dispersion (FDis) increased significantly in suburban areas as compared to urban sites, while functional divergence (FDiv) and functional redundancy (Fred) were higher in urban areas. Functional richness (FRic) and evenness (FEven) were not affected by urbanization. Moreover, assemblages in highly urbanized environments have a substantially different functional composition. Solitary species, cleptoparasites, soil nesters, bees with trophic specialization, and those with a short flight period turned out to be more sensitive to urbanization changes. This study highlights the importance of examining functional diversity in assessing human-induced biodiversity loss and its impacts on ecosystem functioning in urbanized areas. These results have significant implications for improving our understanding of the mechanisms of suburban community ecology and conserving bees in urban habitats.

Comparative Meta-analysis Effects of Nonnative Ants (Hymenoptera: Formicidae), Ground Beetles (Coleoptera: Carabidae), and Bark and Ambrosia Beetles (Coleoptera: Curculionidae) on Native Confamilials

Nonnative species often transform local communities to the detriment of native species. Much of the existing invasion ecology research focuses on the effects of a few extremely impactful species, and it is less clear how nonnative species which are not causing economic or ecological impacts alter closely related natives at risk of being displaced. Filling these knowledge gaps is critical because consequences of nonnative species are likely to vary depending on taxonomic scale, functional trait, and spatial or temporal niche. We conducted a meta-analysis to evaluate how biodiversity of native Formicidae (ants), Carabidae (ground beetles), and Scolytinae (bark and ambrosia beetles) species changes across a gradient of pressure from nonnative confamilials. We calculated Hill numbers for each group from data presented in literature and correlated native diversity metrics to proportion of nonnative species. Species richness of native ants was significantly negatively correlated with proportions of nonnative ants, whereas bark and ambrosia beetle metrics showed a nonsignificant negative correlation. Nonnative ground beetles had neutral effects on diversity of native ground beetles. Resulting contrasting patterns of invasive species effects on natives suggest complex biotic and abiotic factors driving effects of nonnative species in these groups. Our results suggest that a few extreme examples (e.g., red imported fire ants) drive most of the changes seen in native arthropod communities. To accurately assess impacts of invaders on native arthropod diversity, baseline data are needed, and community analyses must consider diverse functional traits of native taxa and improve the depth and breadth of community sampling.

Influence of Urban Landscape on Ants and Spiders Richness and Composition in Forests

As large amounts of natural environments are lost due to urbanization, the role of remnant native vegetation in the preservation of biodiversity has become even more significant. Remnant native forest patches are essential refugia for flora and fauna and are crucial for the maintenance of ecosystem processes in urbanized landscapes. We evaluated the influence of landscape structure on ants and spiders associated with Atlantic Forest remnants in urban landscapes. We sampled 14 forest areas in the Metropolitan Region of Salvador and tested the effect of the landscapes' proportion of forest cover, mean landscape isolation, and mean landscape shape complexity on the taxonomic and functional richness and the community composition of both groups. The species collected were classified into functional groups based on behavioral attributes and environmental preferences. Overall, there were strong adverse effects of forest loss, decreased connectivity, and an increase in edge effects associated with the mean shape complexity of the forest remnants. However, the spiders responded to all three landscape structure characteristics whereas the ants only responded to the landscape mean shape complexity. Our findings indicate that the maintenance of urban forest habitats is essential for the conservation of biodiversity in the Metropolitan Region of Salvador and the preservation of ecological functions performed by species within the forest areas.

Dispersal ability predicts spatial genetic structure in native mammals persisting across an urbanization gradient

As the rate of urbanization continues to increase globally, a growing body of research is emerging that investigates how urbanization shapes the movement-and consequent gene flow-of species in cities. Of particular interest are native species that persist in cities, either as small relict populations or as larger populations of synanthropic species that thrive alongside humans in new urban environments. In this study, we used genomic sequence data (SNPs) and spatially explicit individual-based analyses to directly compare the genetic structure and patterns of gene flow in two small mammals with different dispersal abilities that occupy the same urbanized landscape to evaluate how mobility impacts genetic connectivity. We collected 215 white-footed mice (Peromyscus leucopus) and 380 big brown bats (Eptesicus fuscus) across an urban-to-rural gradient within the Providence, Rhode Island (U.S.A.) metropolitan area (population =1,600,000 people). We found that mice and bats exhibit clear differences in their spatial genetic structure that are consistent with their dispersal abilities, with urbanization having a stronger effect on Peromyscus mice. There were sharp breaks in the genetic structure of mice within the Providence urban core, as well as reduced rates of migration and an increase in inbreeding with more urbanization. In contrast, bats showed very weak genetic structuring across the entire study area, suggesting a near-panmictic gene pool likely due to the ability to disperse by flight. Genetic diversity remained stable for both species across the study region. Mice also exhibited a stronger reduction in gene flow between island and mainland populations than bats. This study represents one of the first to directly compare multiple species within the same urban-to-rural landscape gradient, an important gap to fill for urban ecology and evolution. Moreover, here we document the impacts of dispersal capacity on connectivity for native species that have persisted as the urban landscape matrix expands.

Public Engagement Provides First Insights on Po Plain Ant Communities and Reveals the Ubiquity of the Cryptic Species Tetramorium immigrans (Hymenoptera, Formicidae)

Simple Summary

Public involvement in biodiversity research in the form of Citizen Science is a powerful tool to improve our understanding of the natural world, and it is especially suitable for the study of heavily populated environments. Ants’ ubiquity and diversity, their role as ecological bioindicators, and the fact that most species can easily be sampled makes them ideal candidates for this kind of studies. In the framework of the international School of Ants citizen science project, we joined the “BioBlitz Lombardia” in which citizens are invited to collect biodiversity data on several parks from Lombardy (Po Plain, Italy). As a result, we recorded 30 ant species and obtained a first characterization of the region’s ant assemblages. We studied their patterns of variation in relation with the ecological difference between the studies sites, which ranged from urban to subalpine areas. In addition, we detected the presence of a cryptic species (Tetramorium immigrans) whose distribution and identity were only recently clarified. It likely represents an under-recorded introduced species in the region. Advantages and critical aspects of using CS methodology for the study of biodiversity are discussed in light of our experience.

Abstract

Ants are considered a useful model for biodiversity monitoring and several of their characteristics make them promising for citizen science (CS) projects. Involving a wide range of public figures into collecting valuable data on the effect of human impact on ant biodiversity, the School of Ants (SoA) project represents one of the very few attempts to explore the potential of these insects in CS. Through the collaboration with the “BioBlitz Lombardia” project, we tested the SoA protocol on 12 Northern Italy parks, ranging from urban green to subalpine protected sites. As a result, we obtained some of the very first quantitative data characterizing the ants of this region, recording 30 species and highlighting some interesting ecological patterns. These data revealed the ubiquitous presence of the recently taxonomically defined cryptic species Tetramorium immigrans, which appears to be probably introduced in the region. We also discuss advantages and criticisms encountered applying the SoA protocol, originally intended for schools, to new categories of volunteers, from BioBlitz participants to park operators, suggesting best practices based on our experience.

Landscape configuration and habitat complexity shape arthropod assemblage in urban parks

The urbanization process systematically leads to the loss of biodiversity. Only certain arthropods are resilient to the urbanization process and can thrive in the novel conditions of urbanized landscapes. However, the degree to which arthropod communities survive in urban habitats depends on landscape and local effects and biological interactions (e.g., trophic interactions). In the present study, we examined the relative importance of various factors at landscape (isolation, edge density and area of surrounding greenery) and local (size of park, canopy cover, understory vegetation cover, defoliation depth, weight of dried leaves, soil temperature, soil moisture, and soil pH) spatial scales on the diversity of ants, beetles and spiders in urban parks. Our results indicated that park edge density was negatively correlated with diversity metrics in ants, beetles, and spiders in urban parks relative to the degree of proximity with the peri-urban forest. In other words, parks that located adjacent to the peri-urban forest may not necessarily have high biodiversity. The results suggested that man-made structures have been effective dispersal barriers that limit the spillover effects of ants and spiders but not the spillover of comparatively strong fliers, such as beetles. However, the area of surrounding greenery may have facilitated the colonization of forest-dependent taxa in distant parks. Large parks with reduced edge density supported a higher arthropod diversity because of the minimal edge effect and increased habitat heterogeneity. Vegetation structure consistently explained the variability of ants, beetles, and spiders, indicating that understory plant litter is crucial for providing shelters and hibernation, oviposition, and foraging sites for the major taxa in urban parks. Therefore, efforts should focus on the local management of ground features to maximize the conservation of biological control in urban landscapes.

City limits: Heat tolerance is influenced by body size and hydration state in an urban ant community

Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Therefore, urban animals may be particularly susceptible to warming associated with ongoing climate change. We used a comparative and manipulative approach to test three related hypotheses about the determinants of heat tolerance or critical thermal maximum (CT max) in urban ants-specifically, that (a) body size, (b) hydration status, and (c) chosen microenvironments influence CT max. We further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. We manipulated water access and determined CT max for 11 species common to cities in California's Central Valley that exhibit nearly 300-fold variation in body size. There was a moderate phylogenetic signal influencing CT max, and inter (but not intra) specific variation in body size influenced CT max where larger species had higher CT max. The sensitivity of ants' CT max to water availability exhibited species-specific thresholds where short-term water limitation (8 hr) reduced CT max and body water content in some species while longer-term water limitation (32 hr) was required to reduce these traits in other species. However, CT max was not related to the temperatures chosen by ants during activity. Further, we found support for our fourth hypothesis because CT max and estimates of thermal safety margin in native species were more sensitive to water availability relative to non-native species. In sum, we provide evidence of links between heat tolerance and water availability, which will become critically important in an increasingly warm, dry, and urbanized world that others have shown may be selecting for smaller (not larger) body size.

Urbanisation alters ecological interactions: Ant mutualists increase and specialist insect predators decrease on an urban gradient

The modification of habitats in urban areas is thought to alter patterns of species interactions, by filtering specialist species and those at higher trophic levels. However, empirical studies addressing these hypotheses remain limited in scope and number. This work investigates (1) how main urban land uses affect predator-prey and mutualistic interactions, and (2) how specialist and generalist predators respond to size and availability of urban green spaces. In a large town in the UK, experimental colonies of ant-attended Black bean aphid Aphis fabae and non-ant-attended Pea aphid Acyrthosiphon pisum were monitored over two years. Ants were more frequently found in highly urbanised sites; however mutualistic ants were also more often encountered when the habitat was more plant diverse. Aphids were not affected by urban land uses, but A. fabae numbers were positively related to the presence of mutualists, and so indirectly affected by urbanisation. Predators were the only group negatively affected by increased urbanisation, and specialist species were positively related to increased proportion of urban green areas within the habitats. While this work supports the hypothesis that specialist predators are negatively affected by urbanisation, we also show that a fundamental ecological interaction, mutualism, is affected by urbanisation.

Urbanization drives cross-taxon declines in abundance and diversity at multiple spatial scales

The increasing urbanization process is hypothesized to drastically alter (semi-)natural environments with a concomitant major decline in species abundance and diversity. Yet, studies on this effect of urbanization, and the spatial scale at which it acts, are at present inconclusive due to the large heterogeneity in taxonomic groups and spatial scales at which this relationship has been investigated among studies. Comprehensive studies analysing this relationship across multiple animal groups and at multiple spatial scales are rare, hampering the assessment of how biodiversity generally responds to urbanization. We studied aquatic (cladocerans), limno-terrestrial (bdelloid rotifers) and terrestrial (butterflies, ground beetles, ground- and web spiders, macro-moths, orthopterans and snails) invertebrate groups using a hierarchical spatial design, wherein three local-scale (200 m × 200 m) urbanization levels were repeatedly sampled across three landscape-scale (3 km × 3 km) urbanization levels. We tested for local and landscape urbanization effects on abundance and species richness of each group, whereby total richness was partitioned into the average richness of local communities and the richness due to variation among local communities. Abundances of the terrestrial active dispersers declined in response to local urbanization, with reductions up to 85% for butterflies, while passive dispersers did not show any clear trend. Species richness also declined with increasing levels of urbanization, but responses were highly heterogeneous among the different groups with respect to the richness component and the spatial scale at which urbanization impacts richness. Depending on the group, species richness declined due to biotic homogenization and/or local species loss. This resulted in an overall decrease in total richness across groups in urban areas. These results provide strong support to the general negative impact of urbanization on abundance and species richness within habitat patches and highlight the importance of considering multiple spatial scales and taxa to assess the impacts of urbanization on biodiversity.

Gridlock and beltways: the genetic context of urban invasions

The rapid expansion of urban land across the globe presents new and numerous opportunities for invasive species to spread and flourish. Ecologists historically rejected urban ecosystems as important environments for ecology and evolution research but are beginning to recognize the importance of these systems in shaping the biology of invasion. Urbanization can aid the introduction, establishment, and spread of invaders, and these processes have substantial consequences on native species and ecosystems. Therefore, it is valuable to understand how urban areas influence populations at all stages in the invasion process. Population genetic tools are essential to explore the driving forces of invasive species dispersal, connectivity, and adaptation within cities. In this review, we synthesize current research about the influence of urban landscapes on invasion genetics dynamics. We conclude that urban areas are not only points of entry for many invasive species, they also facilitate population establishment, are pools for genetic diversity, and provide corridors for further spread both within and out of cities. We recommend the continued use of genetic studies to inform invasive species management and to understand the underlying ecological and evolutionary processes governing successful invasion.

Urbanization without isolation: the absence of genetic structure among cities and forests in the tiny acorn ant Temnothorax nylanderi

Urban alteration of neutral and adaptive evolutionary processes is still underexplored. Using a genome-wide SNP dataset, we investigated (i) urban-induced modifications of population demography, genetic diversity and population structure and (ii) signature of divergent selection between urban and forest populations in the ant species, Temnothorax nylanderi. Our results did not reveal an impact of urbanization on neutral processes since we observed: (i) analogous genetic diversity among paired urban/forest sites and two control populations; (ii) weak population genetic structure explained neither by habitat (urban versus forest) nor by geography; (iii) a remarkably similar demographic history across populations with an ancestral growth followed by a recent decline, regardless of their current habitat or geographical location. The micro-geographical home range of ants may explain their resilience to urbanization. Finally, we detected 19 candidate loci discriminating urban/forest populations and associated with core cellular components, molecular function or biological process. Two of these loci were associated with a gene ontology term that was previously found to belong to a module of co-expressed genes related to caste phenotype. These results call for transcriptomics analyses to identify genes associated with ant social traits and to infer their potential role in urban adaptation.

Idiosyncrasies in cities: evaluating patterns and drivers of ant biodiversity along urbanization gradients

Urbanization is expected to reduce biodiversity. However, an increasing number of studies report urban biodiversity comparable to that of surrounding nonurban areas, leaving open the question: what maintains biodiversity in cities? We characterized patterns of ant biodiversity across urbanization gradients of three major cities in the Midwestern United States and evaluated the support for two mechanisms underlying the maintenance of biodiversity in cities, specifically via introduced non-native species and differential phenology of communities along each urbanization gradient. We observed idiosyncrasies in ant species diversity such that each city displayed either increased, decreased or no change in biodiversity across the urbanization gradient. We found partial support (one of the three cities) for the hypothesis that non-native species can contribute positively to overall species diversity in cities, though even with introduced species removed from consideration, native ant biodiversity was maintained along the urbanization gradient. We found no support for systematic differential phenology across urbanization gradients, although species diversity did vary over time across all sites. Our results further challenge the assumption of biodiversity loss in cities, as two of our three cities exhibited maintained species diversity along the urbanization gradient. Most importantly, our study demonstrates that urban biodiversity can be maintained entirely by native communities.

Effects of Urbanization on the Diversity, Abundance, and Composition of Ant Assemblages in an Arid City

Cities within arid regions make up a significant but understudied subset of the urban ecosystems of the world. To assess the effects of urbanization, fragmentation, and land-use change in an arid city, we sampled the ant assemblages in three habitat types in Tucson, Arizona: irrigated neighborhood parks, urban desert remnants, and preserved desert. We analyzed the abundance, species richness, evenness, as well as the species and functional group composition of ant assemblages. We found no significant differences in species richness or evenness. However, irrigated parks had significantly greater ant abundances. Although some exotic species were present in the urban habitats, they did not have significant effects on ant diversity. Ant assemblages from all three habitat types were distinct from each other in their composition. Irrigated parks included a significantly higher proportion of species typically found in cooler and wetter climates. The differences in abundance and species composition between irrigated parks and the other habitats are likely the effect of irrigation removing water as a limiting factor for colony growth and increasing resource availability, as well as producing a localized cooling effect. Our results show that arid urban ecosystems may include considerable biodiversity, in part thanks to increased landscape heterogeneity resulting from the irrigation of green areas.

Miami heat: Urban heat islands influence the thermal suitability of habitats for ectotherms

The urban heat island effect, where urban areas exhibit higher temperatures than less-developed suburban and natural habitats, occurs in cities across the globe and is well understood from a physical perspective and at broad spatial scales. However, very little is known about how thermal variation caused by urbanization influences the ability of organisms to live in cities. Ectotherms are sensitive to environmental changes that affect thermal conditions, and therefore, increased urban temperatures may pose significant challenges to thermoregulation and alter temperature-dependent activity. To evaluate whether these changes to the thermal environment affect the persistence and dispersal of ectothermic species in urban areas, we studied two species of Anolis lizards (Anolis cristatellus and Anolis sagrei) introduced to Miami-Dade County, FL, USA, where they occur in both urban and natural habitats. We calculated canopy openness and measured operative temperature (T_e ), which estimates the distribution of body temperatures in a non-thermoregulating population, in four urban and four natural sites. We also captured lizards throughout the day and recorded their internal body temperature (T_b ). We found that urban areas had more open canopies and higher T_e compared to natural habitats. Laboratory trials showed that A. cristatellus preferred lower temperatures than A. sagrei. Urban sites currently occupied by each species appear to lower thermoregulatory costs for both species, but only A. sagreihad field T_b that were more often within their preferred temperature range in urban habitats compared to natural areas. Furthermore, based on available T_e within each species' preferred temperature range, urban sites with only A. sagrei appear less suitable for A. cristatellus, whereas natural sites with only A. cristatellus are less suitable for A. sagrei. These results highlight how the thermal properties of urban areas contribute to patterns of persistence and dispersal, particularly relevant for studying species invasions worldwide.

Urban plants and climate drive unique arthropod interactions with unpredictable consequences

Urban areas, a rapidly expanding land cover type, are composed of a mix of impervious surfaces, ornamental plants, and remnant habitat, which alters abiotic conditions and affects arthropod community assemblages and trophic interactions. Importantly, these effects often reduce arthropod diversity and may increase, reduce, or not change individual species or trophic interactions, which affects human and environmental health. Despite the pace of urbanization, drivers and consequences of change in urban arthropod communities remains poorly understood. Here, we review recent findings that shed light on the effects of urbanization on plants and abiotic conditions that drive arthropod community composition and trophic interactions, with discussion of how these effects conflict with human values and can be mitigated for future urbanization.

Getting ahead of the curve: cities as surrogates for global change

Urbanization represents an unintentional global experiment that can provide insights into how species will respond and interact under future global change scenarios. Cities produce many conditions that are predicted to occur widely in the future, such as warmer temperatures, higher carbon dioxide (CO2) concentrations and exacerbated droughts. In using cities as surrogates for global change, it is challenging to disentangle climate variables-such as temperature-from co-occurring or confounding urban variables-such as impervious surface-and then to understand the interactive effects of multiple climate variables on both individual species and species interactions. However, such interactions are also difficult to replicate experimentally, and thus the challenges of cities are also their unique advantage. Here, we review insights gained from cities, with a focus on plants and arthropods, and how urban findings agree or disagree with experimental predictions and historical data. We discuss the types of hypotheses that can be best tested in cities, caveats to urban research and how to further validate cities as surrogates for global change. Lastly, we summarize how to achieve the goal of using urban species responses to predict broader regional- and ecosystem-level patterns in the future.

Urbanization drives community shifts towards thermophilic and dispersive species at local and landscape scales

The increasing conversion of agricultural and natural areas to human-dominated urban landscapes is predicted to lead to a major decline in biodiversity worldwide. Two conditions that typically differ between urban environments and the surrounding landscape are increased temperature, and high patch isolation and habitat turnover rates. However, the extent and spatial scale at which these altered conditions shape biotic communities through selection and/or filtering on species traits are currently poorly understood. We sampled carabid beetles at 81 sites in Belgium using a hierarchically nested sampling design wherein three local-scale (200 × 200 m) urbanization levels were repeatedly sampled across three landscape-scale (3 × 3 km) urbanization levels. First, we showed that communities sampled in the most urbanized locations and landscapes displayed a distinct species composition at both local and landscape scale. Second, we related community means of species-specific thermal preferences and dispersal capacity (based on European distribution and wing morphology, respectively) to the urbanization gradients. We showed that urban communities consisted on average of species with a preference for higher temperatures and with better dispersal capacities compared to rural communities. These shifts were caused by an increased number of species tolerating higher temperatures, a decreased richness of species with low thermal preference, and an almost complete depletion of species with very low-dispersal capacity in the most urbanized localities. Effects of urbanization were most clearly detected at the local scale, although more subtle effects could also be found at the scale of entire landscapes. Our results demonstrate that urbanization may fundamentally and consistently alter species composition by exerting a strong filtering effect on species dispersal characteristics and favouring replacement by warm-dwelling species.

Stable isotopes reveal links between human food inputs and urban ant diets

The amount of energy consumed within an average city block is an order of magnitude higher than that consumed in any other ecosystem over a similar area. This is driven by human food inputs, but the consequence of these resources for urban animal populations is poorly understood. We investigated the role of human foods in ant diets across an urbanization gradient in Manhattan using carbon and nitrogen stable isotopes. We found that some-but not all-ant species living in Manhattan's most urbanized habitats had δ(13)C signatures associated with processed human foods. In particular, pavement ants (Tetramorium sp. E) had increased levels of δ(13)C similar to δ(13)C levels in human fast foods. The magnitude of this effect was positively correlated with urbanization. By contrast, we detected no differences in δ(15)N, suggesting Tetramorium feeds at the same trophic level despite shifting to human foods. This pattern persisted across the broader ant community; species in traffic islands used human resources more than park species. Our results demonstrate that the degree urban ants exploit human resources changes across the city and among species, and this variation could play a key role in community structure and ecosystem processes where human and animal food webs intersect.

Riverine Landscape Patch Heterogeneity Drives Riparian Ant Assemblages in the Scioto River Basin, USA

Although the principles of landscape ecology are increasingly extended to include riverine landscapes, explicit applications are few. We investigated associations between patch heterogeneity and riparian ant assemblages at 12 riverine landscapes of the Scioto River, Ohio, USA, that represent urban/developed, agricultural, and mixed (primarily forested, but also wetland, grassland/fallow, and exurban) land-use settings. Using remotely-sensed and ground-collected data, we delineated riverine landscape patch types (crop, grass/herbaceous, gravel, lawn, mudflat, open water, shrub, swamp, and woody vegetation), computed patch metrics (area, density, edge, richness, and shape), and conducted coordinated sampling of surface-active Formicidae assemblages. Ant density and species richness was lower in agricultural riverine landscapes than at mixed or developed reaches (measured using S [total number of species], but not using Menhinick’s Index [D_M]), whereas ant diversity (using the Berger-Park Index [D_BP]) was highest in agricultural reaches. We found no differences in ant density, richness, or diversity among internal riverine landscape patches. However, certain characteristics of patches influenced ant communities. Patch shape and density were significant predictors of richness (S: R² = 0.72; D_M: R²=0.57). Patch area, edge, and shape emerged as important predictors of D_BP (R² = 0.62) whereas patch area, edge, and density were strongly related to ant density (R² = 0.65). Non-metric multidimensional scaling and analysis of similarities distinguished ant assemblage composition in grass and swamp patches from crop, gravel, lawn, and shrub as well as ant assemblages in woody vegetation patches from crop, lawn, and gravel (stress = 0.18, R² = 0.64). These findings lend insight into the utility of landscape ecology to river science by providing evidence that spatial habitat patterns within riverine landscapes can influence assemblage characteristics of riparian arthropods.

Odorous house ants (Tapinoma sessile) as back-seat drivers of localized ant decline in urban habitats

Invasive species and habitat disturbance threaten biodiversity worldwide by modifying ecosystem performance and displacing native organisms. Similar homogenization impacts manifest locally when urbanization forces native species to relocate or reinvade perpetually altered habitat. This study investigated correlations between ant richness and abundance in response to urbanization and the nearby presence of invasive ant species, odorous house ants (Tapinoma sessile), within its native region. Surveying localized ant composition within natural, semi-natural, and urban habitat supported efforts to determine whether T. sessile appear to be primary (drivers) threats as instigators or secondary (passengers) threats as inheritors of indigenous ant decline. Sampling 180 sites, evenly split between all habitats with and without T. sessile present, yielded 45 total species. Although urbanization and T. sessile presence factors were significantly linked to ant decline, their interaction correlated to the greatest reduction of total ant richness (74%) and abundance (81%). Total richness appeared to decrease from 27 species to 18 when natural habitat is urbanized and from 18 species to 7 with T. sessile present in urban plots. Odorous house ant presence minimally influenced ant communities within natural and semi-natural habitat, highlighting the importance of habitat alteration and T. sessile presence interactions. Results suggest urbanization releases T. sessile from unknown constraints by decreasing ant richness and competition. Within urban environment, T. sessile are pre-adapted to quickly exploit new resources and grow to supercolony strength wherein T. sessile drive adjacent biodiversity loss. Odorous house ants act as passengers and drivers of ecological change throughout different phases of urban ‘invasion’. This progression through surviving habitat alteration, exploiting new resources, thriving, and further reducing interspecific competition supports a “back-seat driver” role and affects pest management strategies. As demonstrated by T. sessile, this article concludes native species can become back-seat drivers of biodiversity loss and potentially thrive as “metro-invasive” species.

The southern megalopolis: using the past to predict the future of urban sprawl in the Southeast U.S

The future health of ecosystems is arguably as dependent on urban sprawl as it is on human-caused climatic warming. Urban sprawl strongly impacts the urban ecosystems it creates and the natural and agro-ecosystems that it displaces and fragments. Here, we project urban sprawl changes for the next 50 years for the fast-growing Southeast U.S. Previous studies have focused on modeling population density, but the urban extent is arguably as important as population density per se in terms of its ecological and conservation impacts. We develop simulations using the SLEUTH urban growth model that complement population-driven models but focus on spatial pattern and extent. To better capture the reach of low-density suburban development, we extend the capabilities of SLEUTH by incorporating street-network information. Our simulations point to a future in which the extent of urbanization in the Southeast is projected to increase by 101% to 192%. Our results highlight areas where ecosystem fragmentation is likely, and serve as a benchmark to explore the challenging tradeoffs between ecosystem health, economic growth and cultural desires.