Scaling Local Species-habitat Relations to the Larger Landscape with a Hierarchical Spatial Count Model

Much of what is known about avian species-habitat relations has been derived from studies of birds at local scales. It is entirely unclear whether the relations observed at these scales translate to the larger landscape in a predictable linear fashion. We derived habitat models and mapped predicted abundances for three forest bird species of eastern North America using bird counts, environmental variables, and hierarchical models applied at three spatial scales. Our purpose was to understand habitat associations at multiple spatial scales and create predictive abundance maps for purposes of conservation planning at a landscape scale given the constraint that the variables used in this exercise were derived from local-level studies. Our models indicated a substantial influence of landscape context for all species, many of which were counter to reported associations at finer spatial extents. We found land cover composition provided the greatest contribution to the relative explained variance in counts for all three species; spatial structure was second in importance. No single spatial scale dominated any model, indicating that these species are responding to factors at multiple spatial scales. For purposes of conservation planning, areas of predicted high abundance should be investigated to evaluate the conservation potential of the landscape in their general vicinity. In addition, the models and spatial patterns of abundance among species suggest locations where conservation actions may benefit more than one species.     

Environmental factors at different spatial scales governing soil fauna community patterns in fragmented forests

Spatial and temporal changes in community structure of soil organisms may result from a myriad of processes operating at a hierarchy of spatial scales, from small-scale habitat conditions to species movements among patches and large-sale landscape features. To disentangle the relative importance of spatial and environmental factors at different scales (plot, patch and landscape), we analyzed changes in Collembola community structure along a gradient of forest fragmentation, testing predictions of the Hierarchical Patch Dynamics Paradigm (HPDP) in different European biogeographic regions (Boreal, Continental, Atlantic, Mediterranean, Alpine). Using variance partitioning methods, based on partial CCAs, we observed that the independent effect of environmental processes was significantly explaining Collembola community variance in all regions, while the relative effect of spatial variables was not significant, due to the observed high levels of landscape heterogeneity along the gradient. Environmental factors at the patch and plot scales were generally significant and explained the larger part of community changes. Landscape variables were not significant across all study sites. Yet, at the landscape level, an increase in forest habitat and proximity of forest patches were showed to have an indirect influence on local community changes, by influencing microhabitat heterogeneity at lower spatial scales in all studied regions. In line with HPDP, large-scale landscape features influenced spatio-temporal changes in soil fauna communities by constraining small-scale environmental processes. In turn, these provided mechanistic understanding for diversity patterns operating at the patch scale, via shifts in community weighted mean of Collembola life-forms occurring in local communities along the fragmentation gradient.     

Associations of Bird Species Richness and Community Composition with Local and Landscape-scale Environmental Factors in Borneo

A comprehensive understanding of variables associated with spatial differences in community composition is essential to explain and predict biodiversity over landscape scales. In this study, spatial patterns of bird diversity in Central Kalimantan, Indonesia, were examined and associated with local-scale (habitat structure and heterogeneity) and landscape-scale (logging, slope position and elevation) environmental variables. Within the study area (c. 196 km²) local habitat structure and heterogeneity varied considerably, largely due to logging. In total 9747 individuals of 177 bird species were recorded. Akaike's information criterion (AIC) revealed that the best explanatory models of bird community similarity and species richness included both local- and landscape-scale environmental variables. Important local-scale variables included liana abundance, fern cover, sapling density, tree density, dead wood abundance and tree architecture, while important landscape-scale variables were elevation, logging and slope position. Geographic distance between sampling sites was not significantly associated with spatial variation in either species richness or similarity. These results indicate that deterministic environmental processes, as opposed to dispersal-driven stochastic processes, primarily structure bird assemblages within the spatial scale of this study and confirm that highly variable local habitat measures can be effective means of predicting landscape-scale community patterns.     

Effects of spatial habitat heterogeneity on habitat selection and annual fecundity for a migratory forest songbird

Understanding how spatial habitat patterns influence abundance and dynamics of animal populations is a primary goal in landscape ecology. We used an information-theoretic approach to investigate the association between habitat patterns at multiple spatial scales and demographic patterns for black-throated blue warblers (Dendroica caerulescens) at 20 study sites in west-central Vermont, USA from 2002 to 2005. Sites were characterized by: (1) territory-scale shrub density, (2) patch-scale shrub density occurring within 25 ha of territories, and (3) landscape-scale habitat patterns occurring within 5 km radius extents of territories. We considered multiple population parameters including abundance, age ratios, and annual fecundity. Territory-scale shrub density was most important for determining abundance and age ratios, but landscape-scale habitat structure strongly influenced reproductive output. Sites with higher territory-scale shrub density had higher abundance, and were more likely to be occupied by older, more experienced individuals compared to sites with lower shrub density. However, annual fecundity was higher on sites located in contiguously forested landscapes where shrub density was lower than the fragmented sites. Further, effects of habitat pattern at one spatial scale depended on habitat conditions at different scales. For example, abundance increased with increasing territory-scale shrub density, but this effect was much stronger in fragmented landscapes than in contiguously forested landscapes. These results suggest that habitat pattern at different spatial scales affect demographic parameters in different ways, and that effects of habitat patterns at one spatial scale depends on habitat conditions at other scales.     

Does the scale of our observational window affect our conclusions about correlations between endangered salmon populations and their habitat?

Differences in the strength of species-habitat relationships across scales provide insights into the mechanisms that drive these relationships and guidance for designing in situ monitoring programs, conservation efforts and mechanistic studies. The scale of our observation can also impact the strength of perceived relationships between animals and habitat conditions. We examined the relationship between geographic information system (GIS)-based landscape data and Endangered Species Act-listed anadromous Pacific salmon (Oncorhynchus spp.) populations in three subbasins of the Columbia River basin, USA. We characterized the landscape data and ran our models at three spatial scales: local (stream reach), intermediate (6th field hydrologic units directly in contact with a given reach) and catchment (entire drainage basin). We addressed three questions about the effect of scale on relationships between salmon and GIS representations of landscape conditions: (1) at which scale does each predictor best correlate with salmon redd density, (2) at which scale is overall model fit maximized, and (3) how does a mixed-scale model compare with single scale models (mixed-scale meaning models that contain variables characterized at different spatial scales)? We developed mixed models to identify relationships between redd density and candidate explanatory variables at each of these spatial scales. Predictor variables had the strongest relationships with redd density when they were summarized over the catchment scale. Meanwhile strong models could be developed using landscape variables summarized at only the local scale. Model performance did not improve when we used suites of potential predictors summarized over multiple scales. Relationships between species abundance and land use or intrinsic habitat suitability detected at one scale cannot necessarily be extrapolated to other scales. Therefore, habitat restoration efforts should take place in the context of conditions found in the associated watershed or landscape.     

Comparative influence of isolation,landscape, and wetland characteristics on egg-mass abundance of two pool-breeding amphibian species

The distribution and abundance of species are shaped by local and landscape processes, but the dominant processes may differ with scale and increasing human disturbance. We investigated population responses of two pool-breeding amphibian species to differences in local and landscape characteristics in suburbanizing, southeastern New Hampshire, USA. In 2003 and 2004, we sampled 49 vernal pools for spotted salamander (Ambystoma maculatum) and wood frog (Lithobates sylvaticus) egg masses. Using egg masses as a proxy for breeding-female population size, we examined the relative influence of five land-use and three isolation variables at two scales (300 and 1000 m) and five wetland variables on egg-mass abundance. For both species, road density at the landscape scale (1000 m) and hydroperiod most strongly predicted egg-mass abundance, with abundance decreasing as roads became denser and hydroperiods shortened. Wetland isolation was also an important predictor, with abundance greatest at more isolated pools, suggesting that both species concentrate at isolated pools when alternative breeding sites are scarce. Surprisingly, no 300-m parameters were strongly associated with salamander egg-mass abundance, whereas several landscape parameters were. In suburbanizing areas, it is at least as important to consider landscape-scale road density as to consider hydroperiod when designing conservation plans for these species. Furthermore, both isolated and clustered pools provide these species important habitat and may require protection. Finally, the conceptual framework for spotted-salamander management must be expanded so that spatial configuration at the landscape scale becomes a regular, integrated component of conservation planning for this species.     

Is bird incidence in Atlantic forest fragments influenced by landscape patterns at multiple scales?

The degree to which habitat fragmentation affects bird incidence is species specific and may depend on varying spatial scales. Selecting the correct scale of measurement is essential to appropriately assess the effects of habitat fragmentation on bird occurrence. Our objective was to determine which spatial scale of landscape measurement best describes the incidence of three bird species (Pyriglena leucoptera, Xiphorhynchus fuscus and Chiroxiphia caudata) in the fragmented Brazilian Atlantic forest and test if multi-scalar models perform better than single-scalar ones. Bird incidence was assessed in 80 forest fragments. The surrounding landscape structure was described with four indices measured at four spatial scales (400-, 600-, 800- and 1,000-m buffers around the sample points). The explanatory power of each scale in predicting bird incidence was assessed using logistic regression, bootstrapped with 1,000 repetitions. The best results varied between species (1,000-m radius for P. leucoptera; 800-m for X. fuscus and 600-m for C. caudata), probably due to their distinct feeding habits and foraging strategies. Multi-scale models always resulted in better predictions than single-scale models, suggesting that different aspects of the landscape structure are related to different ecological processes influencing bird incidence. In particular, our results suggest that local extinction and (re)colonisation processes might simultaneously act at different scales. Thus, single-scale models may not be good enough to properly describe complex pattern–process relationships. Selecting variables at multiple ecologically relevant scales is a reasonable procedure to optimise the accuracy of species incidence models.     

The Importance of Spatial Scale in Habitat Models: Capercaillie in the Swiss Alps

The role of scale in ecology is widely recognized as being of vital importance for understanding ecological patterns and processes. The capercaillie (Tetrao urogallus) is a forest grouse species with large spatial requirements and highly specialized habitat preferences. Habitat models at the forest stand scale can only partly explain capercaillie occurrence, and some studies at the landscape scale have emphasized the role of large-scale effects. We hypothesized that both the ability of single variables and multivariate models to explain capercaillie occurrence would vary with the spatial scale of the analysis. To test this hypothesis, we varied the grain size of our analysis from 1 to just over 1100 hectares and built univariate and multivariate habitat suitability models for capercaillie in the Swiss Alps. The variance explained by the univariate models was found to vary among the predictors and with spatial scale. Within the multivariate models, the best single-scale model (using all predictor variables at the same scale) worked at a scale equivalent to a small annual home range. The multi-scale model, in which each predictor variable was entered at the scale at which it had performed best in the univariate model, did slightly better than the best single-scale model. Our results confirm that habitat variables should be included at different spatial scales when species-habitat relationships are investigated.     

Influence of landscape structure on reef fish assemblages

Management of tropical marine environments calls for interdisciplinary studies and innovative methodologies that consider processes occurring over broad spatial scales. We investigated relationships between landscape structure and reef fish assemblage structure in the US Virgin Islands. Measures of landscape structure were transformed into a reduced set of composite indices using principal component analyses (PCA) to synthesize data on the spatial patterning of the landscape structure of the study reefs. However, composite indices (e.g., habitat diversity) were not particularly informative for predicting reef fish assemblage structure. Rather, relationships were interpreted more easily when functional groups of fishes were related to individual habitat features. In particular, multiple reef fish parameters were strongly associated with reef context. Fishes responded to benthic habitat structure at multiple spatial scales, with various groups of fishes each correlated to a unique suite of variables. Accordingly, future experiments should be designed to test functional relationships based on the ecology of the organisms of interest. Our study demonstrates that landscape-scale habitat features influence reef fish communities, illustrating promise in applying a landscape ecology approach to better understand factors that structure coral reef ecosystems. Furthermore, our findings may prove useful in design of spatially-based conservation approaches such as marine protected areas (MPAs), because landscape-scale metrics may serve as proxies for areas with high species diversity and abundance within the coral reef landscape.     

Influence of landscape structure on reef fish assemblages

Management of tropical marine environments calls for interdisciplinary studies and innovative methodologies that consider processes occurring over broad spatial scales. We investigated relationships between landscape structure and reef fish assemblage structure in the US Virgin Islands. Measures of landscape structure were transformed into a reduced set of composite indices using principal component analyses (PCA) to synthesize data on the spatial patterning of the landscape structure of the study reefs. However, composite indices (e.g., habitat diversity) were not particularly informative for predicting reef fish assemblage structure. Rather, relationships were interpreted more easily when functional groups of fishes were related to individual habitat features. In particular, multiple reef fish parameters were strongly associated with reef context. Fishes responded to benthic habitat structure at multiple spatial scales, with various groups of fishes each correlated to a unique suite of variables. Accordingly, future experiments should be designed to test functional relationships based on the ecology of the organisms of interest. Our study demonstrates that landscape-scale habitat features influence reef fish communities, illustrating promise in applying a landscape ecology approach to better understand factors that structure coral reef ecosystems. Furthermore, our findings may prove useful in design of spatially-based conservation approaches such as marine protected areas (MPAs), because landscape-scale metrics may serve as proxies for areas with high species diversity and abundance within the coral reef landscape.

     

Communities in context: the influences of multiscale environmental variation on local ant community structure

We explored the ways in which environmental variation at multiple spatial scales influences the organization of ant species into local communities. Ground-dwelling ants were sampled in sandhill habitat at 33 locations throughout northern Florida, USA. Variance partitioning of local, landscape, and regional datasets using partial redundancy analysis indicates that ant community composition is significantly influenced by environmental variability across all scales of analysis. Habitat generalists appear to replace habitat specialists at sites with high proportions of matrix habitat in the surrounding landscape. Conversely, habitat specialists appear to replace habitat generalists at sites with more sandhill habitat in the surrounding landscape and greater amounts of bare ground locally. Local niche differentiation leading to species-sorting, combined with the effects of spatially structured dispersal dynamics at landscape scales, may explain this pattern of community structure. Regional influences on local ant communities were correlated with geographical and environmental gradients at distinct regional scales. Therefore, local ant communities appear to be simultaneously structured by different processes that occur at separate spatial scales: local, landscape, and regional scales defined by spatial extent. Our results illustrate the importance of considering multiscale influences on patterns of organization in ecological communities. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Constraints and time lags for recovery of a keystone species (Dipodomys spectabilis) after landscape restoration

Habitat restoration is typically focused on reestablishing suitable conditions at a local scale, but landscape constraints may be important for keystone species with limited dispersal. We tested for time lags and examined the relative importance of local and landscape constraints on the response of the banner-tailed kangaroo rat (Dipodomys spectabilis) to restoration of Chihuahuan Desert grasslands in New Mexico, USA. Dipodomys spectabilis is a keystone species that creates habitat heterogeneity and modifies the structure of plant and animal communities. We selected 21 sites and compared density of D. spectabilis between areas treated with herbicide to control shrubs (treated areas) and paired untreated areas. We evaluated whether density of D. spectabilis depended on treatment age, local habitat quality (vegetation and soil structure), and landscape factors (treatment area and spatial connectivity). Density was greater at treated areas than at untreated areas due to a direct effect of reduced shrub cover. However, the response of D. spectabilis to restoration was lagged by a decade or more. Structural equation modeling indicated the time lag reflected a dispersal constraint as opposed to a temporal change in habitat quality. This inference was corroborated by a positive relationship between density at treated areas and connectivity to source populations. Our results indicate that density of D. spectabilis depended strongly on the spatial configuration of treated areas, which supports a landscape mosaic approach to restoration. If keystone species commonly exhibit limited dispersal ability, landscape constraints may be broadly important for shaping ecosystem structure and function after habitat restoration.     

Habitat factors related to wild rabbit conservation in an agricultural landscape

Populations of European wild rabbit (Oryctolagus cuniculus) have been decreasing since the 1950s. Changes in agricultural practices have been suggested as reasons for their decline in Mediterranean landscapes. We evaluated the environmental variables affecting rabbit distribution in a semiarid agricultural landscape of Northeastern Spain. Sampling was performed in 147 sites randomly distributed across Zaragoza province. At each site, data were recorded in five 100 m segments along a 1 km transect, following ecotones between crops and natural-vegetation areas. A rabbit abundance index was estimated from latrine count, pellet density and number of plots with pellets. In addition to environmental variables that have been shown to be related to rabbit abundance in other habitats, as climate, soil hardness and topography of the site, we measured landscape components related to agricultural use, such as structure of natural vegetation in remaining areas non-devoted to agricultural use and distances to different types of crops and to ecotone between crop and natural vegetation. Our results showed that rabbit abundance was positively correlated to yearly mean temperature, February and May mean rainfall, and negatively correlated to September and November mean rainfall, hardness of soil, and site topography. In relation to agricultural use, rabbit abundance was positively correlated to the scrub structure of natural-vegetation areas and negatively correlated to distance to edge between cultivated unirrigated cereal crops (wheat or barley) and yearly resting cereal crops. Rabbit abundance increased only when the edge between alternate cereal crops was less than 50 m from the ecotone between crops and natural vegetation.This revised version was published online in May 2005 with corrections to the Cover Date.     

Influence of landscape structure and stand age on species density and biomass of a tropical dry forest across spatial scales

Three central related issues in ecology are to identify spatial variation of ecological processes, to understand the relative influence of environmental and spatial variables, and to investigate the response of environmental variables at different spatial scales. These issues are particularly important for tropical dry forests, which have been comparatively less studied and are more threatened than other terrestrial ecosystems. This study aims to characterize relationships between community structure and landscape configuration and habitat type (stand age) considering different spatial scales for a tropical dry forest in Yucatan. Species density and above ground biomass were calculated from 276 sampling sites, while land cover classes were obtained from multi-spectral classification of a Spot 5 satellite imagery. Species density and biomass were related to stand age, landscape metrics of patch types (area, edge, shape, similarity and contrast) and principal coordinate of neighbor matrices (PCNM) variables using regression analysis. PCNM analysis was performed to interpret results in terms of spatial scales as well as to decompose variation into spatial, stand age and landscape structure components. Stand age was the most important variable for biomass, whereas landscape structure and spatial dependence had a comparable or even stronger influence on species density than stand age. At the very broad scale (8,000–10,500 m), stand age contributed most to biomass and landscape structure to species density. At the broad scale (2,000–8,000 m), stand age was the most important variable predicting both species density and biomass. Our results shed light on which landscape configurations could enhance plant diversity and above ground biomass.     

Role of past and present landscape structure in determining epiphyte richness in fragmented Mediterranean forests

Context

The anthropocene is characterised by global landscape modification, and the structure of remnant habitats can explain different patterns of species richness. The most pervasive processes of degradation include habitat loss and fragmentation. However, a recovery of modified landscape is occurring in some areas.

Objectives

The main goal is to know how lichen and bryophyte epiphytic richness growing on Mediterranean forests is influenced not only by fragments characteristics but also by the structure of the landscape. We introduce a temporal dimension in order to evaluate if the historical landscape structure is relevant for current epiphytic communities.

Methods

40 well-preserved forest fragments were selected in a landscape with a large habitat loss over decades, but with a recovery of forest surface in the last 55 years. The most relevant fragment and landscape-scale attributes were considered. Some of the variables were measured in three different years to incorporate a temporal framework.

Results

The results showed that variables at fragment scale had a higher influence, whereas variables at the landscape scale were irrelevant. Among all the historical variables analyzed, only the shift in forest fragment size had influence on species richness.

Conclusions

Mediterranean forests had suffered fragmentation along centuries. Their epiphytic communities also suffer the hard conditions of Mediterranean climate. Our results indicate that Mediterranean epiphytic communities may be in a threshold since it they will never be similar to those communities existing previous fragmentation process even a recovery habitat occur or, they may require more time to response to this habitat recovery.

     

Habitat variables explain Loggerhead Shrike occurrence in the northern Chihuahuan Desert,but are poor correlates of fitness measures

Conservation efforts should be based on habitat models that identify areas of high quality and that are built at spatial scales that are ecologically relevant. In this study, we developed habitat models for the Loggerhead Shrike (Lanius ludovicianus) in the Chihuahuan Desert of New Mexico to answer two questions: (1) are highly used habitats of high quality for shrikes in terms of individual fitness? and (2) what are the spatial scales of habitat associations relevant to this species? Our study area was Fort Bliss Army Reserve (New Mexico). Bird abundance was obtained from 10 min point counts conducted at forty-two 108 ha plots during a 3-year period. Measures of fitness were obtained by tracking a total of 73 nests over the 3 years. Habitat variables were measured at spatial scales ranging from broad to intermediate to local. We related habitat use and measures of fitness to habitat variables using Bayesian model averaging. We found a significant relationship between bird abundance and measures of fitness averaged across nesting birds in each plot (correlation up to 0.61). This suggests that measures of habitat use are indicative of habitat quality in the vicinity of Fort Bliss. Local- and intermediate-scale variables best explained shrike occurrence. Habitat variables were not related to any measures of fitness. A better understanding of the factors that limit individual bird fitness is therefore necessary to identify areas of high conservation value for this species.     

What size is a biologically relevant landscape?

The spatial extent at which landscape structure best predicts population response, called the scale of effect, varies across species. An ability to predict the scale of effect of a landscape using species traits would make landscape study design more efficient and would enable landscape managers to plan at the appropriate scale. We used an individual based simulation model to predict how species traits influence the scale of effect. Specifically, we tested the effects of dispersal distance, reproductive rate, and informed movement behavior on the radius at which percent habitat cover best predicts population abundance in a focal area. Scale of effect for species with random movement behavior was compared to scale of effect for species with three (cumulative) levels of information use during dispersal: habitat based settlement, conspecific density based settlement, and gap-avoidance during movement. Consistent with a common belief among researchers, dispersal distance had a strong, positive influence on scale of effect. A general guideline for empiricists is to expect the radius of a landscape to be 4?C9 times the median dispersal distance or 0.3?C0.5 times the maximum dispersal distance of a species. Informed dispersal led to greater increases in population size than did increased reproductive rate. Similarly, informed dispersal led to more strongly decreased scales of effect than did reproductive rate. Most notably, gap-avoidance resulted in scales that were 0.2?C0.5 times those of non-avoidant species. This is the first study to generate testable hypotheses concerning the mechanisms underlying the scale at which populations respond to the landscape.     

Effects of increasing landscape heterogeneity on local plant species richness: how much is enough?

Contemporary landscape ecology continues to explore the causes and consequences of landscape heterogeneity across a range of scales, and demands for the scientific underpinnings of landscape planning and management still remains high. The spatial distribution of resources can be a key element in determining habitat quality, and that in turn is directly related to the level of heterogeneity in the system. In this sense, forest habitat mosaics may be more affected by lack of heterogeneity than by structural fragmentation. Nonetheless, increasing spatial heterogeneity at a given spatial scale can also decrease habitat patch size, with potential negative consequences for specialist species. Such dual effect may lead to hump-backed shape relationships between species diversity and heterogeneity, leading to three related assumptions: (i) at low levels of heterogeneity, an increase in heterogeneity favours local and regional species richness, (ii) there is an optimum heterogeneity level at which a maximum number of species is reached, (iii) further increase in spatial heterogeneity has a negative effect on local and regional species richness, due to increasing adverse effects of habitat fragmentation. In this study, we investigated the existence of a hump-shaped relationship between local plant species richness and increasing forest landscape heterogeneity on a complex mosaic in the French Alps. Forest landscape heterogeneity was quantified with five independent criteria. We found significant quadratic relationships between local forest species richness and two heterogeneity criteria indicators, showing a slight decrease of forest species richness at very high heterogeneity levels. Species richness–landscape heterogeneity relationships varied according to the heterogeneity metrics involved and the type of species richness considered. Our results support the assumption that intermediate levels of heterogeneity may support more species than very high levels of heterogeneity, although we were not able to conclude for a systematic negative effect of very high levels of heterogeneity on local plant species richness.     

Multi-scale landscape and seascape patterns associated with marbled murrelet nesting areas on the U.S. west coast

Habitat for wide-ranging species should be addressed at multiple scales to fully understand factors that limit populations. The marbled murrelet (Brachyramphus marmoratus), a threatened seabird, forages on the ocean and nests inland in large trees. We developed statistical relationships between murrelet use (occupancy and abundance) and habitat variables quantified across many spatial scales (statewide to local) and two time periods in California and southern Oregon, USA. We also addressed (1) if old-growth forest fragmentation was negatively associated with murrelet use, and (2) if some nesting areas are more important than others due to their proximity to high quality marine habitat. Most landscapes used for nesting were restricted to low elevation areas with frequent fog. Birds were most abundant in unfragmented old-growth forests located within a matrix of mature second-growth forest. Murrelets were less likely to occupy old-growth habitat if it was isolated (> 5 km) from other nesting murrelets. We found a time lag in response to fragmentation, where at least a few years were required before birds abandoned fragmented forests. Compared to landscapes with little tono murrelet use, landscapes with many murrelets were closer to the ocean's bays, river mouths, sandy shores, submarine canyons, and marine waters with consistently high primary productivity. Within local landscapes (≤ 800ha), inland factors limited bird abundance, but at the broadest landscape scale studied (3200 ha), proximity to marine habitat was most limiting. Management should focus on protecting or creating large, contiguous old-growth forest stands, especially in low-elevation areas near productive marine habitat. This revised version was published online in July 2006 with corrections to the Cover Date.     

Broad-scale heterogeneity influences nest selection by Brown-headed Cowbirds

Increasing habitat heterogeneity is widely considered to improve conditions for biodiversity. Yet benefits for native species depend on scale and the effect of heterogeneity on key processes influencing survival and reproduction. We examined the relationship between habitat heterogeneity and brood parasitism at multiple scales in a region characterized by (1) relatively high cowbird abundance, (2) high abundance of our focal species, the grassland obligate Grasshopper Sparrow (Ammodramus savannarum), (3) variation in the structure and composition of grassland habitats, and (4) a gradient of woodland cover in the landscape matrix. Tree cover at broad scales was found to have the greatest impact on parasitism while factors at finer scales were relatively unimportant. We found that for every 1 % increase in tree cover within 1 km of Grasshopper Sparrow nests, the probability of parasitism decreases by 3 %. Parasitism reduced clutch sizes and the number of Grasshopper Sparrows fledged, but survival rates were similar between non-parasitized and parasitized nests. Furthermore, simple population projection models indicated that parasitism has the greatest impact at moderate survival levels and can inhibit the resiliency of this population. Our results support the hypothesis that cowbirds prefer forest hosts, which may reduce parasitism rates on grassland birds in heterogeneous landscapes. Collectively, our findings suggest that the effect of cowbird parasitism may be greater for Grasshopper Sparrows than was previously thought.