Homogenizing an urban habitat mosaic: arthropod diversity declines in New York City parks after Super Storm Sandy.

The frequency and intensity of hurricanes are increasing globally, and anthropogenic modifications in cities have created systems that may be particularly vulnerable to their negative effects. Organisms living in cities are exposed to variable levels of chronic environmental stress. However, whether chronic stress ameliorates or exacerbates the negative effects of hurricanes remains an open question. Here, we consider two hypotheses about the simultaneous consequences of acute disturbances from hurricanes and chronic stress from urbanization for the structure of urban arthropod communities. The tipping point hypothesis posits that organisms living in high stress habitats are less resilient than those in low stress habitats because they are living near the limits of their environmental tolerances; while the disturbance tolerance hypothesis posits that high stress habitats host organisms pre-adapted for coping with disturbance, making them more resilient to the effects of storms. We used a before-after-control-impact design in the street medians and city parks of Manhattan (New York City, New York, USA) to compare arthropod communities before and after Super Storm Sandy in sites that were flooded and unflooded during the storm. Our evidence supported the disturbance tolerance hypothesis. Significant compositional differences between street medians and city parks before the storm disappeared after the storm; similarly, unflooded city parks had significantly different arthropod composition while flooded sites were indistinguishable. These differences were driven by reduced occurrences and abundances of arthropods in city parks. Finally, those arthropod groups that were most tolerant to urban stress were also the most tolerant to flooding. Our results suggest that the species that survive in high stress environments are likely to be the ones that thrive in response to acute disturbance. As storms become increasingly common and extreme, this juxtaposition in responses to storm-associated disturbance may lead to diversity loss in cities, potentially leading entire urban landscapes to mirror the reduced diversity of street medians.

Responses of arthropod populations to warming depend on latitude: evidence from urban heat islands.

Biological effects of climate change are expected to vary geographically, with a strong signature of latitude. For ectothermic animals, there is systematic latitudinal variation in the relationship between climate and thermal performance curves, which describe the relationship between temperature and an organism's fitness. Here, we ask whether these documented latitudinal patterns can be generalized to predict arthropod responses to warming across mid- and high temperate latitudes, for taxa whose thermal physiology has not been measured. To address this question, we used a novel natural experiment consisting of a series of urban warming gradients at different latitudes. Specifically, we sampled arthropods from a single common street tree species across temperature gradients in four US cities, located from 35.8 to 42.4° latitude. We captured 6746 arthropods in 34 families from 111 sites that varied in summer average temperature by 1.7-3.4 °C within each city. Arthropod responses to warming within each city were characterized as Poisson regression coefficients describing change in abundance per °C for each family. Family responses in the two midlatitude cities were heterogeneous, including significantly negative and positive effects, while those in high-latitude cities varied no more than expected by chance within each city. We expected high-latitude taxa to increase in abundance with warming, and they did so in one of the two high-latitude cities; in the other, Queens (New York City), most taxa declined with warming, perhaps due to habitat loss that was correlated with warming in this city. With the exception of Queens, patterns of family responses to warming were consistent with predictions based on known latitudinal patterns in arthropod physiology relative to regional climate. Heterogeneous responses in midlatitudes may be ecologically disruptive if interacting taxa respond oppositely to warming.

Habitat and species identity, not diversity, predict the extent of refuse consumption by urban arthropods.

Urban green spaces provide ecosystem services to city residents, but their management is hindered by a poor understanding of their ecology. We examined a novel ecosystem service relevant to urban public health and esthetics: the consumption of littered food waste by arthropods. Theory and data from natural systems suggest that the magnitude and resilience of this service should increase with biological diversity. We measured food removal by presenting known quantities of cookies, potato chips, and hot dogs in street medians (24 sites) and parks (21 sites) in New York City, USA. At the same sites, we assessed ground-arthropod diversity and abiotic conditions, including history of flooding during Hurricane Sandy 7 months prior to the study. Arthropod diversity was greater in parks (on average 11 hexapod families and 4.7 ant species per site), than in medians (nine hexapod families and 2.7 ant species per site). However, counter to our diversity-based prediction, arthropods in medians removed 2-3 times more food per day than did those in parks. We detected no effect of flooding (at 19 sites) on this service. Instead, greater food removal was associated with the presence of the introduced pavement ant (Tetramorium sp. E) and with hotter, drier conditions that may have increased arthropod metabolism. When vertebrates also had access to food, more was removed, indicating that arthropods and vertebrates compete for littered food. We estimate that arthropods alone could remove 4-6.5 kg of food per year in a single street median, reducing its availability to less desirable fauna such as rats. Our results suggest that species identity and habitat may be more relevant than diversity for predicting urban ecosystem services. Even small green spaces such as street medians provide ecosystem services that may complement those of larger habitat patches across the urban landscape.