Avian assemblages along a gradient of urbanization in a highly fragmented landscape

Our goal was to evaluate how avian assemblages varied along a gradient of urbanization in the highly fragmented landscape of coastal southern California. We measured species richness and abundance of birds within continuous blocks of habitat, within urban habitat fragments that varied in landscape and local habitat variables, and within the urban matrix at different distances from the wildland interface. These comparisons allowed us to characterize patterns of avifaunal response to a gradient of urban fragmentation. At the fragment scale, we found that fragment area was a strong, positive predictor of the total number of breeding species detected per fragment; total bird abundance per point count also increased with fragment size. Tree cover was higher in small fragments, as was the abundance of birds that typically occupy wooded habitats. Comparisons between core, fragment, and urban transects revealed differing patterns of response of individual bird species to urbanization. In unfragmented habitat, we recorded a relatively high diversity of urbanization-sensitive birds. In urban transects, these species were rare, and a relatively few species of non-native and anthropophilic birds were common. These urbanization-enhanced birds were also recorded in previous urban gradient studies in northern California and Ohio. Bird communities along the urban gradient reached their highest richness and abundance in fragments. The marked difference in vegetation structure between urban and natural landscapes in this arid shrubland system likely contributed to this pattern; the presence of native shrubs and exotic trees in fragments enabled both shrub and arboreal nesters to co-occur. As is characteristic of biotic homogenization, urban fragmentation in coastal southern California may increase local diversity but decrease overall regional avifaunal diversity.     

Does variation in garden characteristics influence the conservation of birds in suburbia?

Can enhancement of garden habitat for native birds have conservation benefits, or are garden bird assemblages determined by landscape and environmental characteristics? The relative roles of vegetation structure, floristics and other garden attributes, and environmental and landscape controls, on the abundance and richness of bird species in 214 back or front gardens in 10 suburbs of Hobart, Tasmania, Australia, are addressed to answer this question. Birds were counted in each garden and the resources they utilized noted. Vascular plant species and other attributes of the garden were noted, along with rainfall, altitude, distance from natural vegetation, distance from the city and garden size. Garden floristics and bird assemblages were ordinated, and garden groups characterized by particular assemblages of birds identified. General linear modelling was used to determine the combinations of independent variables that best predicted the richness of birds and the abundance of individual bird species and groups of species. The models for bird richness, bird species and groups of bird species were highly individualistic. Although native birds showed a preference for native plants, they also utilized many exotic plants. Exotic birds largely utilized exotic plants. Variation in garden characteristics does substantially affect the nature of garden bird assemblages in Hobart, with weaker environmental and landscape influences. The fact that gardens can be designed and managed to favour particular species and species assemblages gives gardeners a potentially substantial role in the conservation of urban native avifauna.     

Landscape-level effects on avifauna within tropical agriculture in the Western Ghats: Insights for management and conservation

A critical handicap to tropical biodiversity conservation efforts in agroecosystems is the unknowns regarding the influence of landscape-scale factors on the persistence of species. To address these uncertainties, we explored two essential landscape-scale questions, within India’s biologically-rich Western Ghats, examining two nearby human-dominated landscapes that dramatically differed in their pattern of land cover. First, how does the proximity of intact forest patches affect bird community composition within agricultural landscapes? Second, can simple remote sensing-derived measures (brightness, wetness, and NDVI) be used to estimate native bird species composition within those landscapes? In both landscapes, as distance to intact forest decreased, the similarity in bird community composition between agricultural areas and intact forest increased. This suggests that the retention of tropical forest bird communities within human-dominated landscapes critically depends on the maintenance of nearby intact forest. In an answer to the second question, the remote sensing measures correlated with forest-affiliated avian species richness in only one of the two landscapes, reflecting an ecological difference between the two in the response of forest bird species to local agricultural conditions. In the landscape where a correlation was found, there was high variation in vegetative structure, which strongly impacted both the remote sensing measures and forest bird species richness. In the other landscape, forest species richness strongly correlated with changes in tree species composition in the agriculture, a factor that could not be detected by the remote sensing metrics. In order to successfully conserve biodiversity in tropical agricultural landscapes, our findings show that it is essential to conserve intact forest within those landscapes and to understand the effect of local agricultural practices on species.     

Birds as agents of seed dispersal in a human-dominated landscape in southern Costa Rica

Birds play vital roles as seed dispersers helping to maintain and restore plant communities. With restoration increasingly key to global conservation, it is important to understand the landscape attributes and bird community characteristics that most influence avian seed dispersal in human-altered landscapes. We examined bird community structure and seed-dispersal patterns in agricultural countryside in Costa Rica that is typical of much of the Neotropics. Contrary to expectations, bird abundance, not richness, best predicted the richness of bird-dispersed seeds. Neither forest patch size or proximity, nor total tree cover, influenced seed dispersal. The richness and abundance of dispersed seeds, however, was strongly correlated with “wetness,” a remotely-sensed metric of vegetation, at several scales. These results suggest that in this human-dominated tropical region: (1) bird abundance, not species richness or size, may drive seed dispersal, and (2) remote-sensing combined with field verification can detect landscape elements that are helpful for maintaining the option of bird-mediated reforestation.     

The influence of patch area and connectivity on avian communities in urban revegetation

Landscape restoration through revegetation is being increasingly used in the conservation management of degraded landscapes. To effectively plan restoration programs information is required on how the landscape context of revegetation influences biodiversity gains. Here, we investigate the relative influence of patch area and connectivity on bird species richness and abundance within urban revegetation patches in Brisbane, Australia. We carried out bird surveys at 20 revegetation sites, and used hierarchical partitioning and model selection to test the relative importance of patch area (the area of revegetation including all directly connected remnant vegetation) and landscape connectivity (the vegetated area connected by less than 10 m, 20 m, 30 m, 40 m and 50 m cleared gaps). We controlled for a number of possible confounding variables within the hierarchical partitioning procedure. Both the hierarchical partitioning and model selection procedures indicated that connectivity had an important influence on bird species richness. Patch area in combination with connectivity were important influencing factors on overall bird abundance. We also carried out the hierarchical partitioning procedure for bird abundance data within a range of feeding guilds, yielding results specific to species groups. Overall our data suggest that greater connectivity enhances the habitat area that colonists can arrive from (resulting in greater species richness), whereas increased patch area allows for increased abundance by expanding the habitat available to species already present in a patch. A combined approach where connectivity and overall habitat area is enhanced across the landscape is likely to be necessary to meet long-term conservation objectives.     

Long-term effects of fragmentation and fragment properties on bird species richness in Hawaiian forests

Forest fragmentation is a common disturbance affecting biological diversity, yet the impacts of fragmentation on many forest processes remain poorly understood. Forest restoration is likely to be more successful when it proceeds with an understanding of how native and exotic vertebrates utilize forest patches of different size. We used a system of forest fragments isolated by volcanic activity 153 years ago in Hawaii to examine how long-term fragmentation, as well as fragment size and structural features affect the richness of native and exotic bird species. The total number of bird species increased rapidly with forest fragment size, with most of the native species pool found in patches <3 ha. Smaller fragments were dominated by native bird species with several exotic bird species found only in the largest fragments, suggesting that exotic bird species in this landscape show greater area-sensitivity than native species. We used airborne scanning light detection and ranging (LiDAR) to assess whether fragment area was correlated with estimates of fragment vegetation volume as well as measures of tree height. Fragment area was highly correlated with vegetation volume, maximum tree height, and canopy height heterogeneity, and these variables were strong predictors of bird richness, demonstrating that remote sensing can provide key insights into the relationship between fragment structural attributes and biodiversity indicators. Overall, this work demonstrates the value of conserving small remnant mid-elevation forest patches for native birds in Hawaii. This work also provides insight into how newly created forest patches might be used by native and exotic bird species in Hawaii.     

Trends associated with residential development in riparian breeding bird habitat along the Snake River in Jackson Hole, WY, USA: Implications for conservation planning

Throughout North America, bird population declines may be attributable to loss of habitat on the breeding grounds. Human land uses such as residential development, greatly impact landscapes and biota, particularly breeding birds. Riparian areas, which are essential to breeding birds, are particularly vulnerable to development pressures. In North America, most studies have focused on eastern birds and it is unclear how western bird assemblages respond to the effects of residential development. We sampled bird community parameters and habitat variables at three spatial scales (microhabitat, macrohabitat, and landscape) along a residential development gradient within the Snake River riparian corridor in Jackson Hole, WY, USA. Principal component analysis and multiple linear regression statistical tests were used to determine the effects of housing densities on avian community parameters, guilds, individual species distributions, and environmental variables. Landscape-level features were most affected by residential development and trends associated with increasing housing densities, such as anthropogenic habitat fragmentation primarily structured local bird communities. Overall species richness and diversity declined with increasing residential development. Neotropical migrant species were most negatively impacted and consistently declined in proportional representation on forested plots as residential development densities increased. Food generalists, ground gleaners, and avian nest predators all increased with increasing residential development. Brood parasites, on the other hand, did not increase with increasing housing densities and their distribution may reflect the availability of nest host species. These results suggest that residential development within riparian habitats may be exerting a strong negative influence on western bird communities and at high densities may lead to a depauperate avian biota. The results of our study indicate that preserving forest structure may benefit certain species but will not likely conserve as diverse and rich an avian assemblage as minimizing fragmentation associated with residential development.     

Determinants of bird community composition on patches in the suburbs of Paris, France

The ecological processes responsible for the spatial assemblages of breeding bird communities in urban landscapes are more and more investigated. Indeed, understanding these processes is imperative to plan relevant management policies. We investigated breeding bird communities on 67 patches in the suburbs of Paris, France. We examined the role of patch characteristics and geographic distance between patches in determining similarity between bird assemblages. To do this, we proposed a new SØrensen similarity index based on estimators of change in community composition taking into account the detection probability of species. The patch occupancy by sedentary and migratory species was also estimated to compare their sensitivity to urbanization. Patches close to each other supported more similar bird assemblages, suggesting an effect of the spatial distribution of patches on bird dispersal and a posteriori on local community composition. Accounting for spatial location of patches, bird assemblage similarity was related to the similarity of the surrounding level of urbanization but not to the similarity of patch size or to the similarity of patch vegetation. The mean estimated occupancy rate of sedentary species was higher than that of migratory species in the whole study area. While sedentary species occupied patches surrounded by both moderate and high levels of urbanization, migratory ones primarily occupied patches surrounded by moderate levels of urbanization. Human choices in degrees, styles, and extent of urbanization, including designation and design of patches within an urban matrix, affect the composition of local bird communities.     

Effects of age and intensity of urbanization on farmland bird communities

Urban sprawl is now occurring worldwide and considered as a major large-scale perturbation on ecosystems. Consequently, urban territory is replacing other habitats such as agricultural areas. As farmland biotic communities are already reported to be declining, it seems necessary to assess the urbanization impact on them. We conducted a bird survey on 92 plots of 1 × 1 km chosen after stratification on the proportion of urban area and farmland habitat (either 0%, 25%, 50%, 75%), focusing on farmland habitat. Two aspects of urbanization were studied: its intensity and its age. We found that farmland bird species richness did not vary with increasing proportion of urbanized habitat. Non-farmland bird species richness increased from 0% to 25% classes and was constant for other classes. No effect of the urbanization age on farmland bird species richness was found, whereas a positive one was found on the non-farmland birds’ species richness. Abundance of the most specialized farmland birds decreases with urbanization intensity and age. We also found that, the more urbanized and the more recently urbanized the plots, the more similar bird communities. A strong difference in farmland bird’s communities’ compositions was found between 0% and 25% of urbanization, whereas no distinction was found between 50% and 75%. Altogether, our results suggest that to maintain for farmland birds, it is better to add new urban habitat in place where it already exist, rather than to spread it in small lots throughout the landscape.     

Small isolated aspen stands enrich bird communities in southwestern ponderosa pine forests

Small aspen stands are disappearing from the landscape in the Southwest, so it is important to understand their contribution to the avian community. We sampled birds in 53 small, isolated aspen stands and 53 paired plots within the ponderosa pine forest in northern Arizona, during the 1996 and 1997 breeding seasons. Bird species richness and abundance were higher in aspen than in pine. However, bird species richness and abundance did not vary with size of the aspen patch or isolation index. In addition, direct ordination of species distributions with habitat factors suggested no distinct avian communities. This suggests that aspen stands do not harbor separate populations, but rather are locations where the regional avifauna reaches high local density and richness and may be crucial to birds in years of resource scarcity. Thus it is important for avian conservation to maintain many aspen stands across the landscape, encompassing a diversity of vegetation structure and composition.     

Scale-dependent homogenization: Changes in breeding bird diversity in the Netherlands over a 25-year period

Changes in breeding bird diversity in the Netherlands between 1973-1977 and 1998-2000 were evaluated by testing three hypotheses related to the loss of biodiversity: (1) species diversity is declining, (2) biotic homogenization is increasing and (3) rare species are declining more severely than abundant species. Using data collected for two successive national breeding bird atlases, changes in diversity were assessed at different spatial scales (local, regional and national) and among species characteristic for different landscapes (farmland, woodland, heathland, wetland, coastal habitats and urban habitats). National species richness, diversity and equitability had increased between the two atlas periods, with more species increasing than decreasing in range and abundance. Most species in the large groups of woodland and wetland birds showed positive trends, whereas most in the smaller groups of heathland, reed-breeding and meadow birds showed negative trends. However, findings varied between regions and localities. Increases in species richness occurred mainly in regions in the low-lying, western part of the country which were previously relatively poor in species. By contrast, species richness decreased in some previously species-rich regions in the eastern part of the country. This has resulted in a homogenization of breeding bird communities between regions. We advocate the conservation and restoration of regional identity as a priority for landscape planning in the Netherlands. We did not find a clear relation between species abundance and trends, although both rare and very abundant species tended to decrease on average.     

Do regional gradients in land-use influence richness, composition and turnover of bird assemblages in small woods?

Small patches of natural or semi-natural habitat have an important role in the conservation of biodiversity in human-dominated environments. The values of such areas are determined by attributes of the patch as well as its context in the surrounding land mosaic. There is a need for better understanding of the ways in which assemblages are influenced by patch context and the scale over which this occurs. Here we examine the influence of regional environmental gradients on the richness, annual turnover and composition of breeding bird species in small woods in south-eastern England. Regional gradients were defined independently of woods by an ordination of attributes for 5 km × 5 km landscape units across a 2100 km² region. Patch-level attributes, particularly area, were the most important predictors for most bird variables. For woodland migrants and woodland-dependent species, variables representing the context of each wood, either at a local or regional scale, explained significant additional variance in species richness after accounting for wood area, but did not do so for species turnover. Significant context effects for woodland-dependent species related to the extent of hedges and woodland cover in the local vicinity (<1 km radius), whereas for woodland seasonal migrants the best predictors of richness after patch area were two regional environmental gradients. The initial cue to settlement for migrants may be at a coarse regional scale, with selection for suitable landscapes that have a greater extent of woodland cover. Edge species showed different responses: they were influenced by the diversity of structural features in woods, and were a more-dominant component of the avifauna in isolated woods in open fenland environments of the region. Significant relationships between coarse regional gradients (25 km² units) and bird assemblages in small woods (0.5-30 ha) suggest that population and community processes in the avifauna operate across a broader scale than local patch neighbourhoods. They also highlight the importance of adopting a landscape or regional perspective on potential changes to land-use in rural environments, and on the conservation management of small reserves.     

Landscape and patch-level factors influence bird communities in an urbanized tropical island

As human population continues to increase and intensification of human land use escalates, it is important to address the role of urban forest patches in supporting bird communities. We related bird species richness and community assemblage to landscape- and patch- level factors in 40 forest patches in the densely populated metropolitan area of San Juan, Puerto Rico. In total, 54 bird species were observed including 26 resident, 10 endemic, 12 migratory, and 6 introduced species. Patch size, level of urbanization in the surrounding matrix, and vertical heterogeneity of forest patches were the most important variables in explaining species richness. Patch size had the highest predictive power in explaining species richness for all groups except migrants, which were best predicted by patch-level factors (vegetation heterogeneity). The degree of matrix urbanization was correlated negatively with endemic species richness, but positively with introduced species. Endemic species were particularly sensitive to landscape factors (patch size, matrix urbanization, and canopy texture). Introduced species richness was not dependent on any patch-level factors. Eight species were relatively unaffected by urbanization, whereas two species were only found in large forest patches, and two species were sensitive to urban development. Seven species demonstrated a preference for small patches. Recommendations for land managers and conservation agencies to maintain a high bird species richness and diversity include: (1) preserve both large and small forest patches, (2) limit urban development near forest patches, (3) manage forest patch structure to maintain vertical heterogeneity, and (4) maintain forest patches with different vegetation types.     

The age and amount of regrowth forest in fragmented brigalow landscapes are both important for woodland dependent birds

In highly-modified agricultural landscapes, regrowth (secondary) forest on abandoned farmland offers the potential for passive landscape restoration for biodiversity conservation. While numerous studies have investigated the ecological values of regrowth for fauna recovery at the local-level (1-10 ha), there is a dearth of studies quantifying the contribution of regrowth forest at the landscape-level (100-1000s ha). To address this critical knowledge gap we question how the age and amount of regrowth forest in the landscape influence species richness and abundance of mature forest dependent species? Using woodland dependent birds in fragmented sub-tropical brigalow landscapes in southern Queensland, Australia, we applied model averaging and hierarchical partitioning analyses to test and rank the relative importance of the amount of regrowth forest in the landscape in three age classes (<15, 15-30, and >30 years) compared to local (grazing disturbance, abundance of aggressive miners, mistletoe abundance and patch age) and landscape measures of habitat (amount of mature forest and number of mature forest patches). Response variables included the species richness of woodland dependent birds and sub-groupings of foraging guilds, and the abundance of selected individual species. The importance of explanatory variables differed considerably among response groups. Local attributes, such as patch age and the abundance of mistletoe plants, had the strongest influences on woodland dependent birds. However, we found that the amount of regrowth forest, particularly >30 years, also had a strong influence on both species richness and abundance. This study confirms that regrowth, especially older regrowth, can make an important contribution to landscape restoration in highly-modified agricultural landscapes.     

Land use and habitat gradients determine bird community diversity and abundance in suburban, rural and reserve landscapes of Minnesota, USA

Bird species’ community responses to land use in the suburbanizing Twin Cities, Minnesota, USA, were contrasted among reserves, rural lands, and suburbs. For each land use type, bird composition, diversity, and abundance were recorded for 2 years in ≈99 plots in three sampling units (each ≈4500 ha). A habitat gradient defined by canopy structure (grasslands to savannas to forests) was influenced by land use, so ≈300 plots were used to characterize simultaneous variation in bird communities along land use and habitat gradients. At broad scales (aggregate of 33 plots covering ≈4500 ha) suburbs supported the lowest bird richness and diversity and rural landscapes the most, with reserves slightly below rural. Although reserves were like rural lands in diversity of bird communities, they supported more species of conservation concern, particularly of grasslands and savannas. Differences among land use types varied with habitat structure. Suburbs, rural lands, and reserves had similar forest bird communities, but differed in grassland and savanna bird communities. The extensive rural forests are important for the region’s forest birds. Suburban grasslands and savannas had low shrub abundance, low native bird richness and high non-native bird richness and abundance. However, total bird richness and diversity were as high in suburban as in rural and reserve plots because high native richness in suburban forests and high non-native species richness in suburban grasslands and savannas compensated for lower native richness in suburban grasslands and savannas. Bird conservation here and in the Midwest USA should protect rural forests, expand grasslands and savannas in reserves, and improve habitat quality overall.     

Changes in Collembola richness and diversity along a gradient of land-use intensity: A pan European study

Changes in Collembola richness and diversity along a land-use intensity gradient were studied in eight European countries (Portugal, Spain, France, Switzerland, Hungary, UK, Ireland and Finland). In each country a set of six 1 km² land-use units (LUUs) were selected forming a gradient ranging from natural forest to agricultural dominated landscapes, passing through mixed-use ones. In addition to data on Collembola, detailed information regarding landscape diversity and structure was collected for each LUU. A total of 47,774 individuals were identified from 281 species. Collembola reacted not only to changes in the diversity of the landscape, but also to the composition of that diversity and the area occupied by each land-use type at each LUU. Although species richness patterns were not concordant among the different countries, the total number of species per LUU (landscape richness) was generally higher in natural forests and mixed-used landscapes, and lower in agricultural dominated landscapes. Moreover, high richness and diversity of Collembola at each LUU were associated with a diverse landscape structure, both in terms of number of patches and patch richness. Despite this comparable species richness between mixed-use landscapes and those dominated by natural forests, average species richness on forested areas (local richness) decreased along the gradient, showing that forest patches on mixed-use landscapes support a lower richness than in landscapes dominated by forest. This aspect is important when addressing the role of native forests in structuring biodiversity in disturbed and fragmented landscapes. Although a diverse landscape can support a high biodiversity, the results suggest that intensive fragmentation should be avoided with the risk of collapsing local species richness with the consequent result for regional biodiversity.     

Woodpeckers as reliable indicators of bird richness, forest health and harvest

Woodpeckers (family Picidae) show promise as indicators of avian diversity in forests because their populations can be reliably monitored, and their foraging and nesting activities can positively influence the abundance and richness of other forest birds. A correlation between woodpecker richness and richness of forest birds is known to exist at the landscape scale, but uncertainty remains whether this correlation occurs at the smaller stand-level spatial scales where forest management activities take place. We used data collected under a diverse range of forest types, harvest treatments, and forest health conditions during a long-term study of bird communities in interior British Columbia, Canada, to examine two basic questions: (1) at the level of individual forest stands, is woodpecker richness correlated with bird richness (measured as richness of all other bird species)? and (2) do woodpecker richness and bird richness have similar habitat correlates? Bird richness was positively correlated with woodpecker richness (β = 0.59, SE = 0.22, 95% CI = [0.14 1.03]). Richness of both woodpeckers and all other birds were positively correlated with tree species richness and negatively correlated with density of pines, and the effect for forest harvest type was similar for both measures of avian richness (uncut < partial harvest < clearcut with reserves). The effect of density of lodgepole pines killed by mountain pine beetles differed between the two richness measures, being positive for woodpecker richness and negative for forest bird richness. We conclude that the richness of woodpeckers is indeed correlated with the richness of other birds at the stand-level, and can serve as a reliable indicator of overall bird richness in most forest stands and conditions, except during insect outbreaks when differential responses by woodpeckers and the rest of the avian community may decouple the relationship between bird richness and woodpecker richness.     

Spatial and temporal variations in the use of rice-paddy dominated landscapes by birds in Japan

Agricultural landscapes with spatial and temporal variations interact with each other to affect the existing biodiversity. Though rice fields provide important habitats for birds all over the world, studies so far have rarely explored the effects of landscape heterogeneity on bird species in rice paddy areas. This study investigated the effects of habitat cover and landscape variables on the species richness and the abundance of birds in rice paddy areas in Japan. Data on bird occurrence and the environment were collected at 32 grid squares (1 × 1 km) in the Tone River basin. The richness and the abundance of agricultural wetland species were particularly high in landscapes with large areas of rice fields in summer, when rice fields were irrigated, but in those with large areas of open water in winter, when rice fields were drained. It is important to maintain a combination of rice fields and open water to satisfy multiple habitat requirements by agricultural wetland species throughout the year. Grassland species were positively associated with a rich diversity of land cover including fallow fields and open water, indicating the importance of a simultaneous existence of multiple landscape elements. Forest cover in landscapes positively affected edge species and woodland species. Since forest cover had a relatively strong correlation with edge density, the responses of bird species to changes in forest cover and edge density need to be explored further. This study illustrates the importance of spatial and temporal landscape complementation for bird species in rice paddy areas.     

Environmental effects related to the local absence of exotic fish

Given the extent of biological invasions in industrialized countries, our understanding of the determinants of overall patterns of biological invasions could gain most from consideration of why exotic species are absent from some areas, rather than from distribution patterns of exotic species. Fish communities were sampled at 381 sites representing 221 rivers in the Adour-Garonne stream system (116 000 km², SW France). Very few rivers were not colonized by exotic fish species, however, on a local basis, only 33% of the sampling sites hosted exotics. Using General Linear Modelling, we found that patterns of exotic fish (occurrence, number of species, proportion within assemblage) responded to both land-use and physical variables, whereas patterns of native fish only responded to the local meso-scale characteristics of each stream reach from headwaters to mouth. All fish communities were susceptible to invasion regardless of native species richness, and higher native species richness did not decrease the opportunity for establishment by exotic species. The likelihood that exotic fish are absent primarily increased with elevation and with lower human influence upon the land cover, while human-impacted landscapes (agricultural and urban areas) were more likely to host exotic fish and higher numbers of exotic species. In light of urban and agricultural development, our ability to detect responses of exotic species to landscape alterations using a combination of simple physical and land cover variables exemplifies a cost-effective technique for assessing areas at greater invasion risk in large stream systems.     

Predicting juniper encroachment and CRP effects on avian community dynamics in southern mixed-grass prairie, USA

The probability of occurrence of 30 bird species was modeled as a function of landscape covertype in northwestern Oklahoma, USA. This grassland region has been extensively fragmented by agricultural activity, and remnant grassland patches are undergoing severe degradation from encroaching juniper (Juniperus virginiana L.). In addition, many marginal or highly erodable croplands have been placed into perennial pasture dominated by exotic grasses under the Conservation Reserve Program (CRP). Based on temporal patterns of landscape change observed between 1965 and 1995, we estimated the covertype composition of the landscapes in the year 2015 under various CRP administrative and juniper expansion/control scenarios. We then used logistic regression to predict bird responses to these landscape composition estimates. Our estimates suggest that at the current rate of expansion, juniper will overtake substantial areas of remnant grassland even with extensive control measures. As a result, some obligate and facultative grassland birds are projected to decline, while numerous species tolerant of or partially reliant on woody vegetation will increase. Landscape dynamics due to changes in the CRP might be significant and could be designed to benefit declining grassland birds, but these benefits thus far are relatively minor compared to the effects encroaching juniper woodlands will have on the landscape and the avian community.