Vulnerability of native and exotic urban birds to housing densification and changing gardening and landscaping trends

Variation in densities of native and exotic urban-adapted birds across suburban habitats may reveal levels of adaptation that predict vulnerability to habitat modification over time. We investigated the densities of the eight most common exotic and native birds in a southern hemisphere city across suburbs of varying housing density, vegetative cover and garden vegetation characteristics to determine whether avian population densities are maintained across variations in habitat structure that mirror current temporal trends in land use and landscaping/gardening preferences. We also tested whether densities of birds in residential areas bordering undeveloped patches were higher due to spill-over from patches. Densities of all four native species were highest in undeveloped patches and these species were largely absent from high density housing areas. While densities of two exotic species (blackbird, song thrush) were slightly higher in suburbs with greater vegetation complexity, they were still present in higher density housing suburbs in significant numbers. Common starlings were similarly abundant across all residential habitats and house sparrow density increased in higher density housing areas. Blackbirds appeared least vulnerable to housing densification and landscaping and gardening trends because they were abundant across all habitats, including undeveloped patches. Densities of native birds were higher in residential areas bordering undeveloped patches indicating spill-over was occurring and emphasising the role that patches could play in boosting city-wide native bird abundances. Native birds are vulnerable to reductions in garden size and vegetation complexity that should have little or positive effects on populations of common exotic species.     

Vegetation structure along urban ephemeral streams in southeastern Arizona

Riparian areas in Arizona are being encroached upon by urban developments. This study investigated the impacts of different urban housing densities on riparian vegetation structure along ephemeral streams. Nine sites representing three levels of housing density were selected within the town of Marana, located in southeast Arizona. The housing densities were categorized as high (7–8 houses ha^?1), moderate (2.5–4.5 houses ha^?1), and low (< 1.5 houses ha^?1). Each treatment had three replications. The urban developments were relatively young (less than 15 years). No significant differences were found among the treatments for the tree variables (density, height, mean canopy volume and total canopy volume) or the herbaceous vegetation variables (species richness, percentage of introduced species and percentage of ground cover). However, the shrub variables (mean density, mean height, mean canopy volume, total canopy volume and species richness) showed some significant differences. Shrub density and species richness was significantly greater adjacent to ephemeral channels than just three meters upland. In addition, whitethorn acacia shrubs were significantly taller and larger adjacent to the stream channels in the high and moderate housing density sites than in the low housing density sites. Creosote shrubs showed the opposite trend. Increased runoff in the more heavily urbanized streams may have promoted the growth of the facultative riparian species (whitethorn acacia) but not the non-riparian species (creosote). Overall, in these young developments, vegetation was resilient across the levels of urbanization explored in the study.     

Relative importance of habitat quantity, structure, and spatial pattern to birds in urbanizing environments

Urbanization reduces the quantity of native vegetation and alters its local structure and regional spatial pattern. These changes cause local extirpations of bird species associated with native vegetation and increases in the abundance and number of bird species associated with human activity. We used 54–1 km² landscapes in the Seattle, Washington, USA metropolitan area to determine (1) the relative importance of habitat quantity, structure, and pattern to bird diversity and abundance and (2) whether housing developments can be managed to mitigate the negative impacts of urbanization on forest bird diversity. In general, bird species richness was high and many native forest species were retained where urban landcover comprised less than 52% of the landscape, tree density (especially that of evergreens) remained at least 9.8 trees/ha in developments, and forest was at least 64% aggregated across the landscape. These results suggest that the quantity, structure, and pattern of forested habitat affected breeding bird diversity in urbanizing landscapes. However, habitat pattern appeared less influential than other habitat attributes when results from all community- and population-level analyses were considered. Conservation of native birds in reserves can be supplemented by managing the amount, composition, structural complexity, and—to a lesser extent—arrangement of vegetation in neighborhoods.     

Toward increasing avian diversity: urban wildscapes programs

Residential development in urban and suburban settings is a major cause of habitat change affecting avian communities. The effects from land-clearing prior to initiation of construction followed by landscaping focused on traditional aesthetics has reduced habitat diversity at multiple levels. These residential developments generally emphasize the use of ornamental, nonindigenous vegetation with little to no concern for native species and natural vegetation parameters. As a result, avian community composition and diversity is changed. We compared avian communities at three sites in Bexar County, Texas; two residential neighborhoods in the city of San Antonio and an unincorporated undeveloped site, Government Canyon State Natural Area. The residential neighborhoods were a Texas Parks and Wildlife Department certified wildscapes development and a traditionally developed neighborhood. Bird abundance and species richness in the neighborhoods were greater than at Government Canyon State Natural Area (P < 0.01), but no differences existed between the neighborhoods. Bird diversity was greatest at the wildscapes neighborhood (P < 0.01). Further, bird diversity did not differ between the traditional neighborhood and the natural area. Evenness differed among sites (P = 0.006) with the natural area having the greatest value. Bird diversity measures correlated with woody plant density and vertical cover (r = 0.358 to 0.673, P < 0.05) at residential sites. Residential areas incorporating natural landscapes into their design attracted a greater variety of birds than traditionally landscaped residential areas.     

Using housing growth to estimate habitat change: detecting Ovenbird response in a rapidly growing New England State

Numerous measures of human influence on the environment exist, but one that is of particular importance is houses as they can impact the environment from species through the landscape level. Furthermore, because the addition of houses represents an important component of landscape change, housing information could be used to assess ecological responses (e.g., decline in wildlife habitat) to that change. Recently developed housing density data represents a potential source of information to assess landscape and habitat change over long periods of time and at broad spatial extents, which is critically needed for conservation and management. Considering the potential value of housing data, our goal was to demonstrate how changes in the number of houses leads to changes in the amount of habitat across the landscape, and in-turn, how these habitat changes are likely to influence the distribution and abundance for a species of conservation concern, the Ovenbird (Seiurus aurocapillus). Using a relationship between Ovenbird abundance and housing density, we predict suitable habitat in the forests of Massachusetts (USA) from 1970 to 2030. Over this 60-year period, the number of houses was projected to increase from 1.84 to 3.32 million. This magnitude of housing growth translates into a 57 % decline in Ovenbird habitat (6,002 km² to 2,616 km²), a minimum decline of ~850,000 (48 %) Ovenbirds, and an increase in the number of subpopulations across the landscape. Overall, housing data provide important information to robustly measure landscape and habitat change, and hence predict population change of a species. We suggest that time series of housing data linked to ecological responses (e.g., Ovenbird abundance) offers a novel and underutilized approach to estimating long-term and spatially broad predictions of ecosystem response to landscape change, which in turn can inform conservation and management.     

Bird responses to housing development in intensively managed agricultural landscapes

Housing a growing human population is a global issue and designing environmentally friendly developments requires identifying the species likely to be negatively impacted and finding mitigation solutions. Existing studies that consider fragmentation of natural habitats have limited application in countries such as Britain where a prime target for development is agricultural land where decades of intensive management have already diminished biodiversity. Here I used citizen science data on the abundance of 146 breeding and wintering birds to develop models linking abundance to human population density and habitat features. I used these as a proxy for the urbanisation process, finding that impacts of urbanisation were species-specific and context dependent. Low-density developments benefited a high proportion of birds, with wetland birds benefitting most and farmland birds least, but as human densities increased further, up to 75% of species were negatively impacted. Almost half of species currently occurring at 14 flagship residential development sites were predicted to decline based on projected human population density increases, with a third predicted to increase. Presence of wetlands, canopy cover and patches of trees all benefited certain species but efforts to identify more detailed habitat associations were hampered by collinearity among variables. I conclude that even in heavily degraded agricultural landscapes, a high proportion of species will be negatively impacted by residential development and that some will require spared land to persist in the wider landscape. As no single habitat benefited the entire bird community, urban planners wishing to design bird-friendly developments will need to make difficult decisions over which aspects of the bird community to prioritise.

     

Spatial distribution of structure infesting ant species inhabiting urban housing developments of different ages

The distribution of ant colonies within a community is often well defined, yet this distribution can change due to changing environmental conditions, resource availability, and colony growth. In this study, structure infesting ant communities were sampled for one year within three Puerto Rican housing developments of different ages. These developments represented environments in different stages of recovery after a disturbance (secondary succession). Spatial mapping was used to plot the distribution and abundance of ant communities within these developments. At the beginning of the study, the youngest housing development (1 year since construction), had the fewest number of species present (ave. 1.6 per house). The second housing development (4 years old) had a greater number of species (ave. 2.6 per house), and a greater sampling frequency (the same species collected at multiple houses) than Site 1. The oldest development (8 years old) had both the greatest number of species (ave. 2.7 per house), and sampling frequency of the three sites. In all developments, the number of species, sampling frequency, and ant biomass increased throughout the year as colonies grew and foraging ranges expanded. It was observed in all developments, that species coexistence also increased throughout the year. As colony populations increased, there was a greater occurrence of multiple species being collected at the same sample house. In the youngest development, the two most dominant species increased in numbers during the year and began to be collected from the same sample houses. Spatial diagrams documented that multiple species in the older housing developments (later stages of succession), also shared a common distribution. Even the most dominant species (S. invicta) in all three sites, did not exclude additional species from foraging within its established range.     

Relationship of bird richness,abundance and assemblage to the built environment in a small island tropical urban setting: a Suva,Fiji case study

There have been few studies investigating the relationship between the built environment and the status of bird distributions in small island tropical urban areas. We present a study investigating the relationship between bird species richness, abundance and assemblage to the built environment in Suva, Fiji. Field surveys were taken at 54 randomly selected sites throughout the city, stratified by three building density classes and the central business district (CBD). At each site bird counts were recorded, along with environmental data such as average building height, within a 150 m radius. Land-use information was obtained from screen digitized high-resolution satellite imagery within the same radius. Distance to undeveloped patches of land within the urban area was calculated using a GIS. Analysis of the effects of the built environment was carried out for all species, and for exotic and native species separately. Abundance of exotics was significantly higher in the central business district (CBD) than all other urban density classes, and significantly higher than natives in all other density classes. We found a negative relationship between native species richness and distance to undeveloped patches, but no relationship for exotics. Species assemblage was not related to urban density class. We conclude that the status of native and exotic bird species in Suva is similar to what has been found in urban areas in temperate climates, and conservation efforts should focus on minimizing the amount of heavily urbanized “core areas” and protecting undeveloped areas of forested vegetation to improve bird biodiversity in small tropical islands cities.

     

Distribution and activity of bats at local and landscape scales within a rural–urban gradient

We examined the relationship between bat species activity and composition and the extent of forest cover and urbanization in and adjacent to 11 U.S. National Park Service, National Capital Region Parks in Maryland, Virginia, West Virginia, and Washington, D.C., from 2003–2004, using mist nets, harp traps, acoustical detectors, and visual observations in a variety of habitats. Our efforts included 363 trap nights across 74 sites along with acoustical sampling at 362 sites. We captured 383 bats and identified 6,380 echolocation passes of 6 species. Both overall and species-specific activities were affected more by forest fragmentation within parks than by urbanization adjacent to parks. With an ability to exploit anthropogenic structures for day-roosts, big brown bats (Eptesicus fuscus) were the most ubiquitous and probably the most abundant species in NCR Parks, particularly in forested, urban parks. Northern myotis (Myotis septentrionalis), and to a lesser extent, little brown myotis (Myotis lucifugus) were more prevalent in forested, rural parks of the Ridge and Valley and Blue Ridge than in eastern, less forested urban parks of the Piedmont and Coastal Plain physiographic provinces. Retention of larger, residual forest tracts and day-roosting habitat (i.e., trees and snags) would be beneficial to most species, as urban expansion continues throughout the region.     

The perfect lawn: exploring neighborhood socio-cultural drivers for insect pollinator habitat

In the United States, residential yards are typically overlooked for biodiversity conservation, yet they account for a significant portion of urban green space. Yard vegetation can serve as valuable habitat patches for insect pollinator populations in cities, providing important foraging and nesting resources. Based on long-term native bee sampling data, we investigate the social and cultural drivers shaping front yard vegetation composition and configuration at two study sites with consistently low native bee species diversity and abundance. We employ quantitative remote sensing approaches with analysis of qualitative interview data to examine residential vegetation patterns and analyze the socio-cultural relationships between people and vegetation. Data analyses reveal both study sites have lower levels of vegetation composition and complexity, resulting in reduced habitat resources. We find neighborhood public-facing landscaping is shaped by various socio-cultural influences: aesthetics, norms, reference-group behavior, institutions, socioeconomics, and identity. Front yard land-use and decision-making practices are particularly meaning laden, as these spaces are often perceived as visible representations of longstanding neighborhood identity and contiguous common areas to be managed to a “perfect lawn” ideal. The quantitative and qualitative data are used to characterize the two study sites and inform future urban conservation and development efforts salient to citizen stakeholders.     

Squirrels in suburbia: influence of urbanisation on the occurrence and distribution of a common exotic mammal

Urbanisation is widely considered to promote the establishment of non-native species, but there is limited empirical evidence of the ecological factors driving their responses. The grey squirrel Sciurus carolinensis (Gmelin 1788) is native to North America, but is widespread in the UK and is starting to spread across Europe. It is regarded as one of the world’s worst invasive animals due to its adverse impacts on native biodiversity. We use the non-native grey squirrel population in Sheffield (UK) as a case study to assess which factors limit its distribution and abundance in urban environments. In 2010 the city-wide population of adult squirrels peaked at an estimated 6539 in autumn (0.46 squirrels/ha), with maximum local densities of 8.29/ha. These densities appear to be slightly lower than those recorded in urban environments in the species’ native range. Grey squirrels occurred more frequently at urban sites with larger amounts of green-space in the surrounding region. Local habitat characteristics were, however, more powerful predictors of urban grey squirrel occurrence and abundance than regional availability of green space. Canopy cover, seed bearing trees and supplementary feeders, provided for garden birds, positively influenced grey squirrels. The potential for grey squirrels to connect city dwellers with nature thus appears to be highest in urban locations that have considerable capacity to support native biodiversity. The beneficial impacts of supplementary feeding on grey squirrel populations is notable given concerns that squirrels can adversely influence bird populations. These habitat associations also imply that grey squirrels typically respond negatively to urbanisation, which challenges arguments that urbanisation favours exotic species.     

Insect species composition and diversity on intensive green roofs and adjacent level-ground habitats

While it is expected that green roofs support a wider variety of insects compared with conventional roof surfaces, few studies have quantified insect diversity on green roofs. Even fewer have attempted to determine whether green roofs can support insect communities comparable to level-ground urban habitats. In this study, insect richness, abundance and diversity indices were compared between five pairs of intensive green roofs and adjacent ground-level habitat patches in downtown Halifax, Nova Scotia. Pitfall traps were set at each site, collected bi-weekly between May-October 2009 and captured insects were identified to morphospecies (except where taxonomic expertise was available). No significant differences in richness, abundance or any of the indices (S, H’, E_var) were detected in analysis, which included plant species richness, site area and sampling effort as covariables. However, richness and abundance tended to be greater at ground level for all orders (except Heteroptera), and diversity appeared to increase away from the downtown core. Insect composition differed slightly between green roof and ground-level sites; only 17 species were collected from a single site type in numbers greater than five specimens. Nevertheless, a wide variety of insects, including many uncommon species were collected from green roofs, supporting the idea that these habitats can contribute to sustaining biodiversity in cities.     

Impacts of urbanisation on the native avifauna of Perth, Western Australia

Urban development either eliminates, or severely fragments, native vegetation, and therefore alters the distribution and abundance of species that depend on it for habitat. We assessed the impact of urban development on bird communities at 121 sites in and around Perth, Western Australia. Based on data from community surveys, at least 83 % of 65 landbirds were found to be dependent, in some way, on the presence of native vegetation. For three groups of species defined by specific patterns of habitat use (bushland birds), there were sufficient data to show that species occurrences declined as the landscape changed from variegated to fragmented to relictual, according to the percentage of vegetation cover remaining. For three other groups (urban birds) species occurrences were either unrelated to the amount of vegetation cover, or increased as vegetation cover declined. In order to maximise the chances of retaining avian diversity when planning for broad-scale changes in land-use (i.e. clearing native vegetation for housing or industrial development), land planners should aim for a mosaic of variegated urban landscapes (>60 % vegetation retention) set amongst the fragmented and relictual urban landscapes (<60 % vegetation retention) that are characteristic of most cities and their suburbs. Management actions for conserving remnant biota within fragmented urban landscapes should concentrate on maintaining the integrity and quality of remnant native vegetation, and aim at building awareness among the general public of the conservation value of remnant native vegetation.     

Patterns of pollinator turnover and increasing diversity associated with urban habitats

The role of urban expansion on bee diversity is poorly understood, but it may play an important role in restructuring pollinator diversity observed in rural regions at the urban perimeter. We studied bee communities in two habitats essential for pollinators (residential gardens and semi-natural areas) at 42 sites situated at the edge of greater Montreal, Canada. Bee species richness, abundance and functional diversity all increased with urbanization in both habitat types, but gardens and semi-natural areas supported distinct bee communities with unique responses to urbanization in terms of species turnover. Compared to semi-natural sites, residential gardens supported bees that foraged from a greater number but a lower proportion of available plant species. Bees did not discriminate between exotic and indigenous plant species in either gardens or semi-natural sites and were attracted to flowers in either habitat irrespective of their origins. Protecting semi-natural ruderal areas and providing residential garden habitats for pollinators are both effective means of promoting regional bee diversity in urbanizing regions.     

Patterns of human disturbance and response by small mammals and birds in chaparral near urban development

We report on the extent of disturbance (including habitat alteration and road and trail proliferation) in chaparral near urban development and analyze the effects of disturbance on small mammal and resident bird species. Disturbance patterns were evaluated in a 6700 ha study area in southern California: effects on mammals and birds were investigated by analyzing relationships between vegetation structure and animal species richness and abundance. Disturbance was prevalent throughout the study area and included extensive human-altered habitat (from past human activities such as vegetation clearing, human-caused fires, refuse dumping, and vegetation trampling) and 157 km of roads and trails. A nonsignificant trend was found between human-altered habitat and proximity to development, but human-altered habitat was significantly associated with roadway proximity. Trails were also more frequent near urban development and roads. Small mammals responded strongly to disturbance-related vegetation changes, while birds showed little or no response. Mammals endemic to chaparral vegetation were less diverse and abundant in disturbed sites, whereas disturbance-associated species increased in abundance. Close proximity of urban development to natural areas resulted in alteration of natural habitat and proliferation of roads and trails. Variation in life history traits between birds and mammals may affect response to disturbance and influence persistence if disturbance continues. Conservation efforts must recognize the potential for habitat damage and associated declines in native animal species caused by disturbance near urbanization and implement strategies to reduce these threats.     

Are the closed landfills recovered habitats for small rodents? A case study in a riparian site,Buenos Aires,Argentina

The establishment of landfills in urban areas leads to extensive disturbances. Their development after landfill closure depends on the characteristics of the soil cover, the surrounding communities and the dispersal of plants and animals. This study was carried out in a landfill closed in 2004, surrounded by an urban area, freshwater marshes and a riparian forest. The aim of this study was to determine the role that the closed landfill may play in maintaining rodent communities typical of this zone and its relation to characteristics of the sites. Four rodent and plant samplings were carried out from December 2005 to September 2006 at five sites inside the landfill: three filled cells and two areas of the riparian margin. We recorded a total of 433 individual rodents. The rodent community of the closed landfill included species typical of rural, riparian and rural habitats: Akodon azarae (358), Oligoryzomys flavescens (32), Deltamys kempi (14), Rattus rattus (14), Cavia aperea (11) and Scapteromys aquaticus (4). Rodent species composition varied among sites, but A. azarae was usually the dominant species. We found a rich rodent community mostly composed of wild species. The relictual riparian margin may have served as a major refuge for native rodent community while the landfill was in operation, and after closure it possibly acted as a source for some species to colonize the covered cells.     

Assessing the effect of developed habitat on waterbird behaviour in an urban riparian system in Ottawa,Canada

Waterbird species were used to assess the consequences of developed habitat on wildlife behaviour in an urban riparian system along the Rideau River in Ottawa, Canada. Fourteen developed sites and 14 undeveloped sites were surveyed from October 2004 to February 2005. Each site was approached on foot and the first observed behaviour (i.e., fleeing, foraging, resting or swimming) of each individual was recorded. A total of 10,604 behavioural observations were made across 12 taxa. Overall, the fleeing behaviour was observed more often at undeveloped sites. Foraging, resting and swimming behaviours were observed more often at developed sites. We found that seven species fled more often at undeveloped sites, including hooded merganser. This species was also more abundant at undeveloped sites, suggesting that hooded merganser may be more sensitive to anthropogenic disturbance in urban environments than other species that are habituated toward humans. For sensitive species, undeveloped shoreline may serve as a refuge from human disturbance in urban ecosystems.     

Wild bee abundance declines with urban warming,regardless of floral density

As cities expand, conservation of beneficial insects is essential to maintaining robust urban ecosystem services such as pollination. Urban warming alters insect physiology, fitness, and abundance, but the effect of urban warming on pollinator communities has not been investigated. We sampled bees at 18 sites encompassing an urban warming mosaic within Raleigh, NC, USA. We quantified habitat variables at all sites by measuring air temperature, percent impervious surface (on local and landscape scales), floral density, and floral diversity. We tested the hypothesis that urban bee community structure depends on temperature. We also conducted model selection to determine whether temperature was among the most important predictors of urban bee community structure. Finally, we asked whether bee responses to temperature or impervious surface depended on bee functional traits. Bee abundance declined by about 41% per °C urban warming, and temperature was among the best predictors of bee abundance and community composition. Local impervious surface and floral density were also important predictors of bee abundance, although only large bees appeared to benefit from high floral density. Bee species richness increased with floral density regardless of bee size, and bee responses to urban habitat variables were independent of other life-history traits. Although we document benefits of high floral density, simply adding flowers to otherwise hot, impervious sites is unlikely to restore the entire urban pollinator community since floral resources benefit large bees more than small bees.     

Relation between green spaces and bird community structure in an urban area in Southeast Brazil

Increased urbanization typically leads to an increase in abundance of a few species and a reduction in bird species richness. Understanding the structure of biotic communities in urban areas will allow us to propose management techniques and to decrease conflicts between wild species and human beings. The objective of this study was to describe the structure of the bird community in an urban ecosystem. The study was carried out in the city of Taubaté in southeastern Brazil. Point-counts were established in areas with different levels of tree density ranging from urban green spaces to predominantly built-up areas. We looked for a correlation between the richness/abundance of birds and the size of the area surveyed, the number of houses, the number of tree species and the number of individual trees. The results of multiple regression showed that bird richness had a direct relationship with vegetation complexity. The abundance and diversity of tree species were better predictors of bird species than the number of houses and size of the area surveyed. We discuss implications of this study for conservation and management of bird diversity in urban areas, such as the need to increase green areas containing a large diversity of native plant species.     

Urbanisation factors impacting on ant (Hymenoptera: Formicidae) biodiversity in the Perth metropolitan area, Western Australia: Two case studies

Two synchronous projects undertaken in 2011 examined the likely impact of increasing urban densification on invertebrate populations within urban settlement in Perth, Western Australia. One project analysed the ant fauna found in 20 gardens and lawns in small to very small properties (these having a bungalow or duplex (semi-detached) as the main residential building, and a lawn or garden area of 43 m²–332 m²) east, south, north and west of the Central Business District (CBD). The other project examined the ant fauna at 14 sites, principally in native regrowth along the Kwinana Freeway, a major artery that runs north to south through Perth’s suburbs. The gardens and lawns produced a very depauperate fauna of 26 ant species, of which a maximum of 20 were native and at least six species were exotic. The ant fauna from regrowth adjacent to the Kwinana Freeway and at two additional sites (one a bush control) was more than twice as rich, the 56 species collected including only two exotics. In the garden project, ant richness, evenness and abundance were not significantly correlated with size of the garden area. The same applied even when the exotic Pheidole megacephala-dominated gardens were removed from the analysis. Ordination analysis combining the two sets of data revealed a distinct clustering of most of the regrowth sites, whereas the bush control stood alone and garden or lawn sites exhibited a much looser pattern of association. We suggest that increasing the density of Perth suburbs is resulting in drastic loss of native invertebrate fauna, of which ants are a useful bioindicator. However, native vegetation regrowth along major arterial roads could act as a reservoir for invertebrate species that might otherwise disappear entirely from the Perth metropolitan area.