Christofer Bang

Urban biodiversity: patterns and mechanisms.

The patterns of biodiversity changes in cities are now fairly well established, although diversity changes in temperate cities are much better studied than cities in other climate zones. Generally, plant species richness often increases in cities due to importation of exotic species, whereas animal species richness declines. Abundances of some groups, especially birds and arthropods, often increase in urban areas despite declines in species richness. Although several models have been proposed for biodiversity change, the processes underlying the patterns of biodiversity in cities are poorly understood. We argue that humans directly control plants but relatively few animals and microbes—the remaining biological community is determined by this plant “template” upon which natural ecological and evolutionary processes act. As a result, conserving or reconstructing natural habitats defined by vegetation within urban areas is no guarantee that other components of the biological community will follow suit. Understanding the human‐controlled and natural processes that alter biodiversity is essential for conserving urban biodiversity. This urban biodiversity will comprise a growing fraction of the worlds repository of biodiversity in the future.

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.

Reduced Wind Speed Improves Plant Growth in a Desert City

Background The often dramatic effects of urbanization on community and ecosystem properties, such as primary productivity, abundances, and diversity are now well-established. In most cities local primary productivity increases and this extra energy flows upwards to alter diversity and relative abundances in higher trophic levels. The abiotic mechanisms thought to be responsible for increases in urban productivity are altered temperatures and light regimes, and increased nutrient and water inputs. However, another abiotic factor, wind speed, is also influenced by urbanization and well known for altering primary productivity in agricultural systems. Wind effects on primary productivity have heretofore not been studied in the context of urbanization. Methodology/Principal Findings We designed a field experiment to test if increased plant growth often observed in cities is explained by the sheltering effects of built structures. Wind speed was reduced by protecting Encelia farinosa (brittlebush) plants in urban, desert remnant and outlying desert localities via windbreaks while controlling for water availability and nutrient content. In all three habitats, we compared E. farinosa growth when protected by experimental windbreaks and in the open. E. farinosa plants protected against ambient wind in the desert and remnant areas grew faster in terms of biomass and height than exposed plants. As predicted, sheltered plants did not differ from unprotected plants in urban areas where wind speed is already reduced. Conclusion/Significance Our results indicate that reductions in wind speed due to built structures in cities contribute to increased plant productivity and thus also to changes in abundances and diversity of higher trophic levels. Our study emphasizes the need to incorporate wind speed in future urban ecological studies, as well as in planning for green space and sustainable cities.

Control of Arthropod Abundance, Richness, and Composition in a Heterogeneous Desert City

There is a demand for mechanistic studies to explore underlying drivers behind observed patterns of biodiversity in urban areas. We describe a two-year field experiment in which we manipulated bottom-up (resource availability) and top-down (bird predation) forces on arthropod communities associated with a native plant, Encelia farinosa, across three land-use types—urban, desert remnant, and outlying natural desert—in the Phoenix metropolitan area, Arizona, USA. We monitored the trophic structure, richness, and similarity of the arthropod communities on these manipulated plants over a two-year period. We predicted that (1) increased water resources increase plant productivity, (2) increased productivity increases arthropod abundances, and (3) in the urban habitat, top-down forces are greater than in other habitats and limit arthropod abundances. We also predicted that urban remnant habitats are more similar to urban habitats in terms of arthropod richness and composition. Strong interannual differences due to an unusual cold and dry winter in the first year suppressed plant growth in all but urban habitats, and arthropod abundances in all habitats were severely reduced. In the following year, arthropod abundances in desert and remnant habitats were higher than in urban habitats. Water had positive effects on plant growth and arthropod abundance, but these water effects emerged through complex interactions with habitat type and the presence/absence of cages used to reduce bird predation. Plants grew larger in urban habitats, and phenology also differed between urban and desert habitats. The results from caging suggest that bird predation may not be as important in cities as previously thought, and that arthropods may retard plant growth. As expected, desert communities are strongly bottom-up regulated, but contrary to predictions, we did not find evidence for strong top-down control in the city. Remnant habitats were intermediate between desert and urban habitats in terms of diversity, richness, evenness, arthropod composition and phenology, with urban habitats generally lowest in terms of diversity, richness, and evenness. Our study shows that control of biodiversity is strongly altered in urban areas, influenced by subtle shifts in top-down and bottom-up controls that are often superseded by climatic variations and habitat type.

Urbanization affects plant flowering phenology and pollinator community: effects of water availability and land cover

IntroductionClimate change and urbanization have been shown to alter plant phenology. However, a mechanistic understanding of these changes in flowering phenology and associated pollinator communities is lacking. Thus, this study was designed to examine finer scale flowering phenological patterns and driving processes in an arid urban ecosystem. Specifically, we tested the effect of water availability and land cover type on the flowering phenology of brittlebush (Encelia farinosa) and investigated the arthropod pollinator community associated with brittlebush.MethodsThe fieldwork was carried out as part of a larger community ecology experiment following a factorial nested design. We chose three land cover types, each of which had three replicates, resulting in a total of nine sites. For water availability manipulations, 60 genetically different 5-gallon potted plants were placed on the ground within each site. Pan-trapping was used to collect potential pollinators.ResultsOur results showed that water availability did not produce significant differences in flowering phenology. However, brittlebush planted in mesiscaped urban sites bloomed later, longer, and at a higher percentage than those planted in desert remnant sites and desert fringe sites. Furthermore, desert remnant sites were significantly lower in pollinator abundance than desert fringe sites. Pollinator richness varied over time in all land cover types.ConclusionsThis study provides empirical evidence that land cover type, which is strongly correlated to temperature, is the primary cause for altered flowering phenology of brittlebush in the Phoenix area, although water availability may also be important. Moreover, land cover affects total abundance of bee pollinators.

Constructed wetlands: high-quality habitats for Odonata in cultivated landscapes

Abstract In Norway and throughout the rest of Europe, a continuous decline in the number of small lakes and ponds has taken place. As a consequence, many pond-dwelling organisms have become rare or extinct. Constructed wetlands (CWs) have since the 1990s been used as a remedial action against agricultural runoff. This study has investigated the potential these wetlands have as habitat for freshwater organisms, exemplified by Odonata. Four different CWs in southern Norway were investigated, and larval Odonata species composition was related to a wide range of environmental variables. The material was ordinated using Detrended Correspondence analysis (DCA) and Canonical Correspondence analysis (CCA). All the CWs had high nutrient values and high diversities of aquatic plants. Of the 11 Odonata species found, the richest CW contained 10 species. During the study, one of the CWs was exposed to diazinon (an insecticide). Sun exposure and nutrient content were the most important variables determining species composition. The species that dominated the wetlands were typically euryoecious species, indicating harsh living conditions. Despite the high nutrient content, the results clearly indicate that CWs have an obvious role in pond habitat creation, especially in areas managed according to pesticide-free management.