Susannah B. Lerman

Invasion, Competition, and Biodiversity Loss in Urban Ecosystems

The global decline in biodiversity as a result of urbanization remains poorly understood. Whereas habitat destruction accounts for losses at the species level, it may not explain diversity loss at the community level, because urban centers also attract synanthropic species that do not necessarily exist in wildlands. Here we suggest an alternative framework for understanding this phenomenon: the competitive exclusion of native, non-synanthropic species by invasive species. We use data from two urban centers (Phoenix and Baltimore) and two taxa (birds and spiders) to link diversity loss with reduced community evenness among species in urban communities. This reduction in evenness may be caused by a minority of invasive species dominating the majority of the resources, consequently excluding nonsynanthropic species that could otherwise adapt to urban conditions. We use foraging efficiency as a mechanism to explain the loss of diversity. Thus, to understand the effects of habitat conversion on biodiversity, and to sustain species-rich communities, future research should give more attention to interspecific interactions in urban settings.

Linking Optimal Foraging Behavior to Bird Community Structure in an Urban‐Desert Landscape: Field Experiments with Artificial Food Patches

Urban bird communities exhibit high population densities and low species diversity, yet mechanisms behind these patterns remain largely untested. We present results from experimental studies of behavioral mechanisms underlying these patterns and provide a test of foraging theory applied to urban bird communities. We measured foraging decisions at artificial food patches to assess how urban habitats differ from wildlands in predation risk, missed‐opportunity cost, competition, and metabolic cost. By manipulating seed trays, we compared leftover seed (giving‐up density) in urban and desert habitats in Arizona. Deserts exhibited higher predation risk than urban habitats. Only desert birds quit patches earlier when increasing the missed‐opportunity cost. House finches and house sparrows coexist by trading off travel cost against foraging efficiency. In exclusion experiments, urban doves were more efficient foragers than passerines. Providing water decreased digestive costs only in the desert. At the population level, reduced predation and higher resource abundance drive the increased densities in cities. At the community level, the decline in diversity may involve exclusion of native species by highly efficient urban specialists. Competitive interactions play significant roles in structuring urban bird communities. Our results indicate the importance and potential of mechanistic approaches for future urban bird community studies.

Homeowner Associations as a Vehicle for Promoting Native Urban Biodiversity

The loss of habitat due to suburban and urban development represents one of the greatest threats to biodiversity. Conservation developments have emerged as a key player for reconciling new ex-urban residential development with ecosystem services. However, as more than half of the world population lives in urban and suburban developments, identifying conservation partners to facilitate retrofitting existing residential neighborhoods becomes paramount. Homeowner associations (HOA) manage a significant proportion of residential developments in the United States, which includes the landscape design for yards and gardens. These areas have the potential to mitigate the loss of urban biodiversity when they provide habitat for native wildlife. Therefore, the conditions and restrictions imposed upon the homeowner by the HOA could have profound effects on the local wildlife habitat. We explored the potential of HOAs to promote conservation by synthesizing research from three monitoring programs from Phoenix, Arizona. We compared native bird diversity, arthropod diversity, and plant diversity between neighborhoods with and without a HOA. Neighborhoods belonging to HOAs had significantly greater bird and plant diversity, although insect diversity did not differ. The institutional framework structuring HOAs, including sanctions for enforcement coupled with a predictable maintenance regime that introduces regular disturbance, might explain why neighborhoods with a HOA had greater bird diversity. For neighborhoods with a HOA, we analyzed landscape form and management practices. We linked these features with ecological function and suggested how to modify management practices by adopting strategies from the Sustainable Sites Initiative, an international sustainable landscaping program, to help support biodiversity in current and future residential landscapes.

Linking Foraging Decisions to Residential Yard Bird Composition

Urban bird communities have higher densities but lower diversity compared with wildlands. However, recent studies show that residential urban yards with native plantings have higher native bird diversity compared with yards with exotic vegetation. Here we tested whether landscape designs also affect bird foraging behavior. We estimated foraging decisions by measuring the giving-up densities (GUD; amount of food resources remaining when the final forager quits foraging on an artificial food patch, i.e seed trays) in residential yards in Phoenix, AZ, USA. We assessed how two yard designs (mesic: lush, exotic vegetation; xeric: drought-tolerant and native vegetation) differed in foraging costs. Further, we developed a statistical model to calculate GUDs for every species visiting the seed tray. Birds foraging in mesic yards depleted seed trays to a lower level (i.e. had lower GUDs) compared to birds foraging in xeric yards. After accounting for bird densities, the lower GUDs in mesic yards appeared largely driven by invasive and synanthropic species. Furthermore, behavioral responses of individual species were affected by yard design. Species visiting trays in both yard designs had lower GUDs in mesic yards. Differences in resource abundance (i.e., alternative resources more abundant and of higher quality in xeric yards) contributed to our results, while predation costs associated with foraging did not. By enhancing the GUD, a common method for assessing the costs associated with foraging, our statistical model provided insights into how individual species and bird densities influenced the GUD. These differences we found in foraging behavior were indicative of differences in habitat quality, and thus our study lends additional support for native landscapes to help reverse the loss of urban bird diversity.

Biodiversity in the City: Fundamental Questions for Understanding the Ecology of Urban Green Spaces for Biodiversity Conservation

&NA; As urban areas expand, understanding how ecological processes function in cities has become increasingly important for conserving biodiversity. Urban green spaces are critical habitats to support biodiversity, but we still have a limited understanding of their ecology and how they function to conserve biodiversity at local and landscape scales across multiple taxa. Given this limited view, we discuss five key questions that need to be addressed to advance the ecology of urban green spaces for biodiversity conservation and restoration. Specifically, we discuss the need for research to understand how green space size, connectedness, and type influence the community, population, and life‐history dynamics of multiple taxa in cities. A research framework based in landscape and metapopulation ecology will allow for a greater understanding of the ecological function of green spaces and thus allow for planning and management of green spaces to conserve biodiversity and aid in restoration activities.

Ecological homogenization of residential macrosystems

Similarities in planning, development and culture within urban areas may lead to the convergence of ecological processes on continental scales. Transdisciplinary, multi-scale research is now needed to understand and predict the impact of human-dominated landscapes on ecosystem structure and function.