Robert B. Blair

Land Use and Avian Species Diversity Along an Urban Gradient

I examined the distribution and abundance of bird species across an urban gradient, and concomitant changes in community structure, by censusing summer resident bird populations at six sites in Santa Clara County, California (all former oak woodlands). These sites represented a gradient of urban land use that ranged from relatively undisturbed to highly developed, and included a biological preserve, recreational area, golf course, residential neighborhood, office park, and business district. The composition of the bird community shifted from predominantly native species in the undisturbed area to invasive and exotic species in the business district. Species richness, Shannon diversity, and bird biomass peaked at moderately disturbed sites. One or more species reached maximal densities in each of the sites, and some species were restricted to a given site. The predevelopment bird species (assumed to be those found at the most undisturbed site) dropped out gradually as the sites became more urban. These patterns were significantly related to shifts in habitat structure that occurred along the gradient, as determined by canonical correspondence analysis (CCA) using the environmental variables of percent land covered by pavement, buildings, lawn, grasslands, and trees or shrubs. I compared each formal site to four additional sites with similar levels of development within a two-county area to verify that the bird communities at the formal study sites were rep- resentative of their land use category.

Butterfly diversity and human land use: Species assemblages along an urban gradient

Abstract We examined the distribution and abundance of butterfly species across an urban gradient and concomitant changes in community structure by censusing the butterfly and skipper populations at 48 points within six sites near Palo Alto, California, USA (all former oak woodlands). These sites represent a gradient of urban land use running from relatively undisturbed to highly developed and include a nature preserve, recreational area, golf course, residential neighborhood, office park and business district. The species richness and Shannon diversity of butterflies peaked at moderately disturbed sites while the relative abundance decreased from the natural to the urban areas. Butterfly species thought to be most representative of the original, predevelopment butterfly fauna progressively disappear as the sites become more urban. These patterns are significantly related to shifts in habitat structure that occur along the gradient as determined by canonical correspondence analysis (CCA) using the environmental variables of percent land covered by pavement, buildings, lawn, grasslands, and trees or shrubs. The mechanisms behind these patterns may be related to life history and resource use by the individual butterfly species.

BIRDS AND BUTTERFLIES ALONG AN URBAN GRADIENT: SURROGATE TAXA FOR ASSESSING BIODIVERSITY?

This study examines whether birds and butterflies may be used as surrogates for one another in assessing biodiversity at the community level. To do this, I compared the distribution and abundance of bird and butterfly species across an urban gradient by surveying six sites near Palo Alto, California, USA (all former oak woodlands) to see if these taxa have responded similarly to urbanization. The sites represent a gradient of urban land use ranging from relatively undisturbed to highly developed and include nature preserves, recreational areas, golf courses, residential neighborhoods, office parks, and business districts. At the community level, the two taxa displayed similar patterns across the gradient: species richness and Shannon diversity peak at intermediate levels of development, and the oak-woodland species gradually drop out at more developed sites. These measures are highly correlated between the two groups. The two taxa differed in their patterns of total abundance, however. Butterfly abundance...

Birds and Butterflies Along Urban Gradients in Two Ecoregions of the United States: Is Urbanization Creating a Homogeneous Fauna?

Humans are transforming Earth’s landscape to an unprecedented degree in both magnitude and rate (Meyer and Turner 1992). We alter the landscape through the extraction of resources, the development of industry, the practice of agriculture, the pursuit of recreation, and the building of structures. These alterations affect every ecosystem on Earth (Vitousek et al. 1997).

Suburban habitats and their role for birds in the urban–rural habitat network: points of local invasion and extinction?

Suburban habitats in naturally forested areas present a conundrum in the urban–rural habitat network. Typically, these habitats contain less than half of the native woodland bird species that would exist at these sites if they were not developed. They also contain more total bird species than if these sites were left in a natural state. This apparent contradiction raises the question of “How do suburban habitats function in the urban–rural habitat network?” In this study, we analyze bird distributions on three rural-to-urban gradients in different ecoregions of the United States: Oxford, Ohio; Saint Paul, Minnesota; and Palo Alto, California. All three gradients exhibit similar patterns of extinction of native species followed by invasion of common species and subsequent biotic homogenization with urbanization. This patterning suggests that suburban land uses, those represented by the intermediate levels of development on the gradients, are a point of extirpation for woodland birds as well as an entry point for invasive species into urban systems. Furthermore, there are consistent patterns in the functional characteristics of the bird communities that also shift with intensifying urbanization, providing insight on the possible mechanisms of homogenization and community structure in urban ecosystems including an increase in the number of broods per year, a shift in nesting strategies, a decrease in insectivorous individuals, an increase in granivorous individuals, and a decrease in territoriality. Consequently, it appears that there are specific traits that drive the shift in community composition in response to urban and suburban land use. These results have significant implications for improving understanding of the mechanisms of suburban community ecology and conserving birds in urban habitat networks.

Creating a homogeneous avifauna

I compared birds on urban gradients in two ecoregions of the United States by censusing summer resident bird populations at six sites in central California’s coastal chaparral and southwest Ohio’s eastern broadleaf forest. These sites represented comparable gradients of urban land-use which ranged from relatively undisturbed to highly developed and included biological preserves, recreational areas, golf courses, residential neighborhoods, office parks (or apartment complexes), and business districts. Species richness and Shannon diversity peaked at moderately disturbed sites and were significantly correlated between comparable land-use types in the two ecoregions. Bird abundance and biomass peaked at moderately disturbed sites as well but were not significantly correlated between ecoregions. The pre-development bird species (assumed to be those found at the most undisturbed sites) dropped out gradually as the sites became more urban and the number of remaining species was significantly correlated between ecoregions with only three (CA) and one (OH) species remaining in the most urban sites. Taxonomically, the bird communities at the least urbanized sites (the preserves and recreational areas) were very different with an average Jaccard’s index of species similarity of 0.065 while the most disturbed sites (the business district, apartment complexes/office park, residential areas and golf courses) had an average similarity of 0.185. The species assemblages along the gradient shifted gradually, demonstrating local extinction of and local invasion by different species as the sites become more urban.

Nestedness analysis and conservation planning: the importance of place, environment, and life history across taxonomic groups

Abstract. We used nested subsets analysis to examine distribution patterns of birds and butterflies in the same set of 83 locations in canyons of three mountain ranges in the Great Basin of western North America. We tested whether the same environmental variables influenced nestedness among taxonomic groups and among mountain ranges within taxonomic groups. We also examined whether nestedness of birds and butterflies appeared to be sensitive to human use of riparian areas in the ecoregion. Site area and topography did not appear to differ in their influence on nestedness of birds. By contrast, area and topography differed in how strongly they affected nestedness of butterflies, but their respective influence varied among mountain ranges. Riparian dependence had little discernible effect on nested distribution patterns of either taxonomic group. Because processes influencing distribution patterns can differ among taxonomic groups, and the relative importance of those processes may vary spatially even within a taxonomic group, we urge restraint in using birds and butterflies as surrogates of other taxa for conservation planning.

Woody vegetation and canopy fragmentation along a forest-to-urban gradient

To identify patterns that can be used to predict vegetation and landscape characteristics in urban environments, we surveyed the species composition and size of woody plants, as well as the landscape structure of forest canopies, along a forest-to-urban gradient near Oxford, Ohio, USA. The gradient included six sites of increasingly urban land-use: a preserve, a recreational area, a golf course, a residential subdivision, apartment complexes, and a business district. We recorded species identity and stem diameter for all woody plants greater than 3 cm diameter at breast height (DBH) to examine the distribution of individual species as well as overall community composition. We used digitized aerial photographs to compare the spatial characteristics of the forest canopy at each site. We found predictable patterns in species diversity (Shannon index), spatial heterogeneity in species composition (mean percent dissimilarity), and all measures of patch fragmentation (canopy cover and patch number and size). There were clear differences in tree density and total basal area between forested sites and developed sites, but there was little resolution among developed sites. Species richness and average DBH showed no clear pattern, suggesting that landscaping preference largely determined these values. We present a modified version of an intermediate heterogeneity model that can be used to predict diversity patterns in urban areas. We discuss probable mechanisms that led to these patterns and the potential implications for animal communities in urban environments.

Effects of spatial scale and taxonomic group on partitioning of butterfly and bird diversity in the Great Basin, USA

Different taxonomic groups perceive and respond to the environment at different scales. We examined the effects of spatial scale on diversity patterns of butterflies and birds in the central Great Basin of the western USA. We partitioned the landscape into three hierarchical spatial levels: mountain ranges, canyons, and sites within can yons. We evaluated the relative contribution of each level to species richness and quantified changes in species composition at each level. Using additive partitioning, we calculated the contribution of spatial level to overall species diversity. Both canyon and mountain range had significant effects on landscape-level species richness of butterflies and birds. Species composition of butterflies was more similar in space than species composition of birds, but assemblages of both groups that were closer together in space were less similar than assemblages that were further apart. These results likely reflect differences in resource specificity and the distribution of resources for each group. Additive partitioning showed that alpha diversity within canyon segments was the dominant component of overall species richness of butterflies but not of birds. As the size of a sampling unit increased, its contribution to overall species richness of birds increased monotonically, but the relationship between spatial scale and species richness of butterflies was not linear. Our work emphasizes that the most appropriate scales for studying and conserving different taxonomic groups are not the same. The ability of butterflies and birds to serve as surrogate measures of each other’s diversity appears to be scale-dependent.

Reduced Density and Nest Survival of Ground-Nesting Songbirds Relative to Earthworm Invasions in Northern Hardwood Forests

European earthworms (Lumbricus spp.) are spreading into previously earthworm-free forests in the United States and Canada and causing substantial changes, including homogenization of soil structure, removal of the litter layer, and reduction in arthropod abundance and species richness of understory plants. Whether these changes affect songbirds that nest and forage on the forest floor is unknown. In stands with and without earthworms in the Chequamegon-Nicolet National Forest, Wisconsin (U.S.A.), we surveyed for, monitored nests of, and measured attributes of habitat of Ovenbirds (Seiurus aurocapillus) and Hermit Thrushes (Catharus guttatus), both ground-dwelling songbirds, and we sampled earthworms at survey points and nests. Bird surveys indicated significantly lower densities of Ovenbirds and Hermit Thrushes in relation to Lumbricus invasions at survey point and stand extents (3.1 and 15-20 ha, respectively). Modeling of Ovenbird nest survival (i.e., the probability that nestlings successfully fledge) indicated that lower survival probabilities were associated with increased sedge cover and decreased litter depth, factors that are related to Lumbricus invasions, possibly due to reduced nest concealment or arthropod abundance. Our findings provide compelling evidence that earthworm invasions may be associated with local declines of forest songbird populations.