Local and landscape‐scale environmental filters drive the functional diversity and taxonomic composition of spiders across urban greenspaces

Acute toxicity and bioaccumulation of common urban metals in Bombus impatiens life stages

Metal contamination is ubiquitous in urban areas and represents a risk to arthropod species. Bees are exposed to metals while foraging within contaminated landscapes from multiple sources. Eliminating the risk of bee exposure to metals is complex, and requires an understanding of how bees become contaminated, how metals accumulate within bee bodies, and how this exposure influences their health. We selected Bombus impatiens, the common eastern bumble bee, as our focal species because it is the most frequently encountered bumble bee species in the eastern United States and common within urban greenspaces. The aims of this study were to quantify the lethal concentration exposure limit (LC50) for B. impatiens foragers, assess the bioaccumulation ability of environmentally relevant concentrations of common urban metals in adults, larvae, and pupae, and compare the LC50 values against field relevant concentrations collected by foraging bumble bees within a legacy city. Bumble bees were orally exposed to arsenic oxide, cadmium chloride, or chromium oxide in sucrose solution to encourage consumption. The LC50 for arsenic (As2O3 36.4 mg/L), cadmium (CdCl2 10.3 mg/L), and chromium (CrO3 189.6 mg/L) are 202×, 79×, and 1459× greater than concentrations found within urban bumble bee collected provisions, respectively. Adult bumble bees fed field realistic concentrations of metals accumulate significant amounts of cadmium and lead within their bodies, but do not accumulate chromium and arsenic. Additionally, adults accumulate significantly higher concentrations of metals than brood. While bumble bee foragers are unlikely to encounter lethal metal concentrations while foraging in contaminated landscapes, it is crucial to consider and understand how sublethal concentrations impact overall colony functioning. The results from this study highlight the need to identify hazards and bioaccumulation ability of common metals as bees respond differently to each metal species, as well as the impacts of metal mixtures on bioaccumulation and toxicity.

Local Habitat Complexity and Its Effects on Herbivores and Predators in Urban Agroecosystems

In urban community gardens, cultivated vegetation provides variable levels of habitat complexity, which can suppress pests by promoting predator diversity and improving pest control. In this study, we examine three components of the structural complexity of garden vegetation (cover, diversity, and connectivity) to investigate whether higher garden vegetation complexity leads to fewer herbivores, more predators, and higher predation. We worked in eight community gardens where we quantified vegetation complexity, sampled the arthropod community, and measured predation on corn earworm eggs. We found that plots with high vegetation cover supported higher species richness and greater abundance of predatory insects. High vegetation cover also supported a greater abundance and species richness of spiders. In contrast, high vegetation diversity was negatively associated with predator abundance. While high predator abundance was positively associated with egg predation, greater predator species richness had a negative impact on egg predation, suggesting that antagonism between predators may limit biological control. Community gardeners may thus manipulate vegetation cover and diversity to promote higher predator abundance and diversity in their plots. However, the species composition of predators and the prevalence of interspecific antagonism may ultimately determine subsequent impacts on biological pest control.

Land use intensification alters the relative contributions of plant functional diversity and soil properties on grassland productivity

Understanding the mechanisms of grassland productivity variation is critical for global carbon cycling and climate change mitigation. Heretofore, it is unknown how different environmental factors drive small-scale spatial variation in productivity, and whether land use intensification, one of the most important global changes, can regulate the processes that drive productivity change. Here we performed an 18-year exclosure experiment across six sites with high-intensity mowing/grazing history in northern China to examine the effects of land use intensification on plant functional diversity, soil properties, and their relative contributions to above-ground net primary productivity (ANPP). We found that plant functional diversity and soil properties contributed to the variation in ANPP both independently and equally in enclosed grasslands (plant diversity: 20.6%; soil properties: 19.5%). Intensive land use significantly decreased the Rao's quadratic entropy (RaoQ) and community-weighted mean value (CWM) of plant height, and further suppressed the contributions of plant functional diversity to ANPP. In contrast, intensive land use increased soil available N, P, pH, electrical conductivity, and homogeneity of soil available P, and strengthened their contributions to ANPP (31.5%). Our results indicate that high-intensity land use practices in grasslands decrease the role of plant functional diversity, but strengthen the effects of soil properties on productivity. We, therefore, suggest that plant functional diversity can be used effectively to boost productivity in undisturbed grasslands, while soil properties might be a more critical consideration for grassland management in an areas with increased land use.

Mycorrhizal-Assisted Phytoremediation and Intercropping Strategies Improved the Health of Contaminated Soil in a Peri-Urban Area

Soils of abandoned and vacant lands in the periphery of cities are frequently subjected to illegal dumping and can undergo degradation processes such as depletion of organic matter and nutrients, reduced biodiversity, and the presence of contaminants, which may exert an intense abiotic stress on biological communities. Mycorrhizal-assisted phytoremediation and intercropping strategies are highly suitable options for remediation of these sites. A two-year field experiment was conducted at a peri-urban site contaminated with petroleum hydrocarbons and polychlorinated biphenyls, to assess the effects of plant growth (spontaneous plant species, Medicago sativa, and Populus × canadensis, alone vs. intercropped) and inoculation of a commercial arbuscular mycorrhizal and ectomycorrhizal inoculum. Contaminant degradation, plant performance, and biodiversity, as well as a variety of microbial indicators of soil health (microbial biomass, activity, and diversity parameters) were determined. The rhizosphere bacterial and fungal microbiomes were assessed by measuring the structural diversity and composition via amplicon sequencing. Establishment of spontaneous vegetation led to greater plant and soil microbial diversity. Intercropping enhanced the activity of soil enzymes involved in nutrient cycling. The mycorrhizal treatment was a key contributor to the establishment of intercropping with poplar and alfalfa. Inoculated and poplar-alfalfa intercropped soils had a higher microbial abundance than soils colonized by spontaneous vegetation. Our study provided evidence of the potential of mycorrhizal-assisted phytoremediation and intercropping strategies to improve soil health in degraded peri-urban areas.

Rich and abundant spider communities result from enhanced web capture breadth and reduced overlap in urban greenspaces

Urbanization is a key contributor to biodiversity loss, but evidence is mounting that cities can support rich arthropod communities, including rare and threatened species. Furthermore, greenspace is growing within hundreds of "shrinking cities" that have lost population resulting in a need to demolish an overabundance of infrastructure creating vacant land. Efforts are underway to transform vacant lots, often viewed as blighted areas, into habitats that promote biodiversity and generate ecosystem services, such as urban agroecosystems. To understand how reconfiguring these greenspaces might influence species conservation, elucidation of the factors that drive the distribution of an urban species pool is needed. In particular, the importance of species interactions in structuring urban communities is poorly understood. We tested hypotheses that (1) greater breadth of prey captured by web-building spiders and reduced overlap of prey capture among individuals facilitates the conservation of genera richness and abundance and (2) heterogeneity within a greenspace patch facilitates enhanced dietary niche breadth and greater resource partitioning. In 2013 and 2014, the abundance, breadth and degree of overlap in prey capture of sheet web spiders (Linyphiidae) was measured using web mimic traps at 160 microsites (0.25 m² ) situated in four urban vacant lots and four urban farms in the city of Cleveland, Ohio, USA. Within a subset of 40 microsites, we used vacuum sampling and hand collection to measure the abundance and genera richness of Linyphiidae. Spider richness and abundance were significantly reduced within urban farms relative to vacant lots. The distribution of spiders and prey was explained by habitat structure, with microsites dominated by tall grasses and flowering plants, with a high bloom abundance and richness, supporting greater prey capture and a higher genera richness and abundance of spiders. In 2014, web capture overlap was significantly greater within microsites dominated by bare ground. These findings illustrate that urban greenspace conservation efforts that focus on reducing bare ground and incorporating a diversity of grasses and flowering plant species can promote linyphiid spiders, potentially by relaxing exploitative competition for shared prey.