Estimating the spatial scales of landscape effects on abundance

Context

Spatial variation in abundance is influenced by local- and landscape-level environmental variables, but modeling landscape effects is challenging because the spatial scales of the relationships are unknown. Current approaches involve buffering survey locations with polygons of various sizes and using model selection to identify the best scale. The buffering approach does not acknowledge that the influence of surrounding landscape features should diminish with distance, and it does not yield an estimate of the unknown scale parameters.

Objectives

The purpose of this paper is to present an approach that allows for statistical inference about the scales at which landscape variables affect abundance.

Methods

Our method uses smoothing kernels to average landscape variables around focal sites and uses maximum likelihood to estimate the scale parameters of the kernels and the effects of the smoothed variables on abundance. We assessed model performance using a simulation study and an avian point count dataset.

Results

The simulation study demonstrated that estimators are unbiased and produce correct confidence interval coverage except in the rare case in which there is little spatial autocorrelation in the landscape variable. Canada warbler abundance was more highly correlated with site-level measures of NDVI than landscape-level NDVI, but the reverse was true for elevation. Canada warbler abundance was highest when elevation in the surrounding landscape, defined by an estimated Gaussian kernel, was between 1300 and 1400 m.

Conclusions

Our method provides a rigorous way of formally estimating the scales at which landscape variables affect abundance, and it can be embedded within most classes of statistical models.

     

The representation of landscapes in global scale assessments of environmental change

Landscape ecology has provided valuable insights in the relations between spatial structure and the functioning of landscapes. However, in most global scale environmental assessments the representation of landscapes is reduced to the dominant land cover within a 0.5 degree pixel, disregarding the insights about the role of structure, pattern and composition for the functioning of the landscape. This paper discusses the contributions landscape ecology can make to global scale environmental assessments. It proposes new directions for representing landscape characteristics at broad spatial scales. A contribution of landscape ecologists to the representation of landscape characteristics in global scale assessments will foster improved information and assessments for the design of sustainable earth system governance strategies.     

Genetic and environmental variation for corolla traits in Portulaca grandiflora

A commercial seed lot of Portulaca grandiflora was sampled and the variation for several flower traits was analysed. A genotypic component of variance was estimated for the traits petal length, petal width, and petal number. The heritability for petal number was high (64 · 3), for petal width was intermediate (31 · 3), and for petal length was low (12–3). The environmental variance was partitioned into three components, but more than 90% of it was unaccountable. A significant positive correlation between petal length and petal width was found. A covariance analysis was used to test for genetic variance for petal length and petal width after the effects of covariance had been removed. No relationship was found between total corolla area and flower production. Thus, selection for larger flowers is not expected to reduce the number of flowers produced. Selection responses are predicted for different selection differentials and the assumptions are discussed. The results suggest that selection for increased petal number will be very effective.     

Current velocity and spatial scale as determinants of the distribution and abundance of two rheophilic herbivorous insects

Organisms frequently show marked preferences for specific environmental conditions, but these preferences may change with landscape scale. Patterns of distribution or abundance measured at different scales may reveal something about an organism's perception of the environment. To test this hypothesis, we measured densities of two herbivorous aquatic insects that differed in body morphology and mobility in relation to current velocity measured at different scales in the upper Colorado River (Colorado, USA). Streambed densities of the caddisfly larva Agapetus boulderensis (high hydrodynamic profile, low mobility) and mayfly nymph Epeorus sp. (low hydrodynamic profile, high mobility) were assessed at 3 spatial scales: whole riffles, individual cobbles within riffles, and point locations on cobbles. Riffles were several meters in extent, cobbles measured 10–30 cm in size, and the local scale was within a few centimeters of individual larvae (themselves ca. 0.5–1.0 cm in size). We also quantified the abundance of periphytic food for these herbivores at the cobble and riffle scales. Agapetus favored slow current (<30 cm s^–1) across all scales. Epeorus, by contrast, favored fast current (60–80 cm s^–1) at the local and riffle scale, but not at the cobble scale. Only Agapetus showed a significant relationship to current at the cobble scale, with greatest larval densities occurring at velocities near 30 cm s^–1. We had predicted an inverse correlation between grazer density and periphytic abundance; however, this occurred only for Agapetus, and then only at the cobble scale. These data suggest that organisms respond to environmental gradients at different spatial scales and that the processes driving these responses may change with scale, e.g., shifting from individual habitat selection at local and cobble scales to population responses at the riffle scale. This study also highlights the importance of using the appropriate scale of measurement to accurately assess the relationship between organisms and environmental gradients across scale.     

Large-scale effects on the spatial distribution of seabirds in the Northwest Atlantic

Scale questions are particularly important for organisms which range over large areas, as pelagic seabirds do. The investigations of scale are of practical importance for describing patch size of predator and prey, determining the appropriate scale of study and correcting survey transects. We conducted this study in order to explore a substantially wider diversity of spatial scales than has previously been attempted in the pelagic bird literature. As an example of large monitoring datasets dealing with seabirds, we use the PIROP (Programme intégré pour le recherche des oiseaux pélagiques) data set to investigate relevant large scale issues for these species in the Northwestern Atlantic. We analyzed autocorrelation within selected winter and summer transects, and for 1 degree analysis units (‘bins’) for data collected June–August 1966–1992. We also investigated effects of the analysis unit on counting results and on the links between seabirds and their environment (depth, sea surface salinity and temperature). We selected scales of 1, 2, 5 and 10 degrees analysis units; an ecological mapping scale (‘Banks’ not deeper than 200 m) and a political scale (management convention zones of the North Atlantic Fisheries Organization, NAFO) were also included. Using ‘binning’ of various scales, our results show that the Coefficient of Variation for seabird abundances varies among aggregation scales, and that seabird associations with their environment can show scale effects. Autocorrelation of analysis units indicated some distinct larger scale patch sizes for particular species during the breeding season.     

Interactions between past land use, life-history traits and understory spatial heterogeneity

Past land use has contributed to variability in the distribution of herbaceous species by reducing plant abundance and altering species’ chances of recolonizing suitable habitat. Land use may also influence plant heterogeneity by changing environmental conditions within stands. We compared the variability of understory herb abundance in southern Appalachian forests with different land-use histories to examine how past land use influenced plant heterogeneity. The cover of eleven focal species or genera was estimated and mineral soil concentrations were determined during 2001 and 2002 in eight stands that were farmed, logged, or had no disturbance history (reference) in western North Carolina. Analysis of the coefficients of variation revealed that the abundance of understory plants was more heterogeneous in disturbed stands compared with reference stands. However, when nutrient availability differences were accounted for by detrending the plant cover data, understory variability within stands declined, and no differences between disturbed and reference stands could be distinguished. This finding suggests that nutrient availability has important effects on plant heterogeneity, which depend on past land use. Species dispersal, seed size, and phenology also explained variability in the spatial heterogeneity of plants, but generally only before soil nutrient differences were statistically controlled. In addition to demonstrating that past land use has long-term effects on plant heterogeneity, these results indicate that soil nutrients may play different roles in determining vegetation patterns in historically altered and unaltered forests.     

Partitioning spatial,environmental, and community drivers of ecosystem functioning

Landscape Ecology - Community composition, environmental variation, and spatial structuring can influence ecosystem functioning, and ecosystem service delivery. While the role of space in...     

Modeling the effects of forest harvesting on landscape structure and the spatial distribution of cowbird brood parasitism

Timber harvesting affects both composition and structure of the landscape and has important consequences for organisms using forest habitats. A timber harvest allocation model was constructed that allows the input of specific rules to allocate forest stands for clearcutting to generate landscape patterns reflecting the look and feel of managed landscapes. Various harvest strategies were simulated on four 237 km² study areas in Indiana, USA. For each study area, the model was applied to simulate 80 years of management activity. The resulting landscape spatial patterns were quantified using a suite of landscape pattern metrics and plotted as a function of mean harvest size and total area of forest harvested per decade to produce response surfaces. When the mean clearcut size was 1 ha, the area of forest interior remaining on the landscape was dramatically reduced and the amount of forest edge on the landscape increased dramatically. The potential consequences of the patterns produced by the model were assessed for a generalized neotropical migrant forest bird using a GIS model that generates maps showing the spatial distribution of the relative vulnerability of forest birds to brood parasitism by brown-headed cowbirds. The model incorporates the location and relative quality of cowbird feeding sites, and the relation between parasitism rates and distance of forest from edge. The response surface relating mean harvest size and total area harvested to the mean value of vulnerability to cowbird brood parasitism had a shape similar to the response surfaces showing forest edge. The results of our study suggest that it is more difficult to maintain large contiguous blocks of undisturbed forest interior when harvests are small and dispersed, especially when producing high timber volumes is a management goal. The application of the cowbird model to landscapes managed under different strategies could help managers in deciding where harvest activity will produce the least negative impact on breeding forest birds.     

Plant and animal diversity in a region of the Southern Alps: the role of environmental and spatial processes

Different organisms respond to landscape configuration and spatial structure in different terms and across different spatial scales. Here, regression models with variation partitioning were applied to determine relative influence of the three groups of variables (climate, land use and environmental heterogeneity) and spatial structure variables on plant, bird, orthopteran and butterfly species richness in a region of the Southern Alps, ranging in elevation from the sea level to 2,780 m. Grassland and forest cover were positively correlated with species richness in both taxonomic groups, whilst species richness decreased with increasing urban elements and arable land. The variation was mainly explained by the shared component between the three groups in plants and between landscape and environmental heterogeneity in birds. The variation was related to independent land use effect in insects. The distribution in species richness was spatially structured for plants, birds and orthopterans, whilst in butterflies, no spatial structure was detected. Plant richness was associated with linear trend variation and broad-scale spatial structure in the northern part of the region, whilst bird richness with broad-scale variation which occurs on the external Alpine ridge. Orthopteran diversity was strongly related to fine-scale spatial structure, generated by dynamic processes or by unmeasured spatially structured abiotic factors. Although the study was carried out in relatively small area, the four taxonomic groups seem to respond to biodiversity drivers in a surprisingly different way. This has considerable implications for conservation planning as it restricts the usefulness of simple indicators in prioritizing areas for conservation purposes.     

Underestimating the effects of spatial heterogeneity due to individual movement and spatial scale: infectious disease as an example

Many ecological and epidemiological studies occur in systems with mobile individuals and heterogeneous landscapes. Using a simulation model, we show that the accuracy of inferring an underlying biological process from observational data depends on movement and spatial scale of the analysis. As an example, we focused on estimating the relationship between host density and pathogen transmission. Observational data can result in highly biased inference about the underlying process when individuals move among sampling areas. Even without sampling error, the effect of host density on disease transmission is underestimated by approximately 50 % when one in ten hosts move among sampling areas per lifetime. Aggregating data across larger regions causes minimal bias when host movement is low, and results in less biased inference when movement rates are high. However, increasing data aggregation reduces the observed spatial variation, which would lead to the misperception that a spatially targeted control effort may not be very effective. In addition, averaging over the local heterogeneity will result in underestimating the importance of spatial covariates. Minimizing the bias due to movement is not just about choosing the best spatial scale for analysis, but also about reducing the error associated with using the sampling location as a proxy for an individual’s spatial history. This error associated with the exposure covariate can be reduced by choosing sampling regions with less movement, including longitudinal information of individuals’ movements, or reducing the window of exposure by using repeated sampling or younger individuals.

     

A multi-scale assessment of human and environmental constraints on forest land cover change on the Oregon (USA) coast range

Human modification of forest habitats is a major component of global environmental change. Even areas that remain predominantly forested may be changed considerably by human alteration of historical disturbance regimes. To better understand human influences on the abundance and pattern of forest habitats, we studied forest land cover change from 1936 to 1996 in a 25000 km² landscape in the Oregon (USA) Coast Range. We integrated historical forest survey data and maps from 1936 with satellite imagery and GIS data from 1996 to quantify changes in major forest cover types. Change in the total area of closed-canopy forests was relatively minor, decreasing from 68% of the landscape in 1936 to 65% in 1996. In contrast, large-conifer forests decreased from 42% in 1936 to 17% in 1996, whereas small-conifer forests increased from 21% of the landscape in 1936 to 39% in 1996. Linear regression models were used to predict changes in the proportion of large conifer forest as a function of socioeconomic and environmental variables at scales of subbasins (mean size = 1964 km², n=13), watersheds (mean size = 302 km², n=83), and subwatersheds (mean size = 18 km², n=1325). The proportion of land in private ownership was the strongest predictor at all three spatial scales (partial R² values 0.57–0.76). The amounts of variation explained by other independent variables were comparatively minor. Results corroborate the hypothesis that differing management regimes on private and public ownerships have led to different pathways of landscape change. Furthermore, these distinctive trajectories are consistent over a broad domain of spatial scales.     

Twelve-year change in tree diversity and spatial segregation in the Mediterranean city of Santiago,Chile

Tree diversity is one of the most important components of urban ecosystems, because it provides multiple ecological benefits and contributes to human well-being. However, the distribution of urban trees may be spatially segregated and change over time. To provide insights for a better distribution of tree diversity in a socially segregated city, we evaluated spatial segregation in the abundance and diversity of trees by socioeconomic group and their change over a 12-year period in Santiago, Chile. Two hundred vegetation plots were sampled across Santiago in 2002 and 2014. We found that overall abundance and diversity of urban trees for the entire city were stable over 12 years, whereas species richness and abundance of native tree species increased. There was segregation in tree species richness and abundance by socioeconomic group, with wealthier areas having more species and greater abundance of trees (for all tree species and native species) than poorer ones. Tree community composition and structure varied with socioeconomic group, but we found no evidence of increased homogenization of the urban forest in that 12 years. Our findings revealed that although tree diversity and abundance for the entire city did not change in our 12-year period, there were important inequities in abundance and diversity of urban trees by socioeconomic group. Given that 43% of homes in Santiago are in the lower socioeconomic areas, our study highlights the importance of targeting tree planting, maintenance and educational programs in these areas to reduce inequalities in the distribution of trees.     

Effects of land-cover change on spatial pattern of forest communities in the Southern Appalachian Mountains (USA)

Landscape Ecology - Understanding the implications of past, present and future patterns of human land use for biodiversity and ecosystem function is increasingly important in landscape ecology. We...     

Contrasting roles of environmental and spatial processes for common and rare urban butterfly species compositions

Context

Environmental processes and dispersal are primary determinants of metacommunity dynamics. The relative importance of these effects may vary between species of different abundance classes, given variation in life history traits. Under high disturbance conditions, rare species may be more easily eliminated from their optimal habitats and their distribution may therefore be more heavily dependent upon dispersal from nearby habitat patches than common species.

Objectives

We tested if metacommunity dynamics vary between abundance classes in a high disturbance environment.

Methods

Standardized butterfly sampling was conducted in the urban parks of Hong Kong. To estimate the strength of environmental processes, we measured an array of environmental variables for all sampled parks. Spatial predictors were generated to estimate the effect of dispersal.

Results

For shaping common species compositions, we found environmental processes (and specifically environmental variables including floral density and surrounding woody plant cover) slightly more important than spatial processes. For rare species, only spatial processes were significant while environmental processes were insignificant. Our result contrasts previous studies in natural metacommunities, which have shown that both common and rare species compositions are shaped by environmental processes and similar variables.

Conclusions

Our results demonstrate that high disturbance conditions may inhibit rare species establishment and persistence in urban landscapes. Local habitat management may not be sufficient in conserving rare species in urban environments—spatial context and configuration should be considered in planning for biodiversity. We also highlight the utility of community deconstruction analysis in providing insights into rare species metacommunity dynamics.

     

Simulated effects of climate change on the production pattern of winter cauliflower in the UK

Models of the phases of juvenility, curd induction and curd growth, driven by temperature, were developed for Roscoff, Walcheren and Roscoff×Walcheren winter cauliflower types and were linked to enable prediction of the time of curd maturity. Because some crop types in Cornwall (west of UK); Lincolnshire (east of UK) responded differently to weather, separate models were developed for each location. These models were then applied to each cauliflower type at four locations in the UK (Cornwall and Pembrokeshire (west); Kent and Lincolnshire (east)) using four climate impact emissions scenarios and three time-slices (2011–2040, 2041–2070 and 2071–2100). In all forecasts temperatures increased relative to baseline data (1961–1990). The phases of juvenility and curd growth were shortened by increased temperature, while in most cases that of curd induction was increased. The net effect was to advance maturity in all situations except for Roscoff types grown in Cornwall. Maturity was particularly advanced in Pembrokeshire because temperatures were closest to the optimum for curd induction. The effects of location were greater than those of time-slice and scenario and effects on Roscoff×Walcheren types were greater than those on Roscoff or Walcheren types.     

Land-use and land-cover change processes in the Upper Uruguay Basin: linking environmental and socioeconomic variables

Landscape Ecology - Land-use and land-cover change affects both ecological and socioeconomic processes, motivating the integration of environmental and socioeconomic data to help understand this...     

Contemporary spatial and environmental factors determine vascular plant species richness on highly fragmented meadows in Central Finland

Context

Habitat loss is a major threat to biodiversity. It can create temporal lags in decline of species in relation to destruction of habitat coverage. Plant species specialized in semi-natural grasslands, especially meadows, often express such extinction debt.

Objectives

We studied habitat loss and fragmentation of meadows and examined whether the changes in meadow coverage had caused an extinction debt on vascular plants. We also studied whether historical or present landscape patterns or contemporary environmental factors were more important determinants of species occurrence.

Methods

We surveyed the plant species assemblages of 12 grazed and 12 mown meadows in Central Finland and detected the meadow coverages from their surroundings on two spatial scales and on three time steps. We modelled the effects of functional connectivity, habitat amount, and isolation on species richness and community composition.

Results

We observed drastic and dynamic meadow loss in landscapes surrounding our study sites during the last 150 years. However, we did not find explicit evidence for an extinction debt in meadow plants. The observed species richness correlated with contemporary factors, whereas both contemporary factors and habitat availability during the 1960s affected community composition.

Conclusions

Effective conservation management of meadow biodiversity builds on accurate understanding of the relative importance of past and present factors on species assemblages. Both mown and grazed meadows with high species richness need to be managed in the future. The management effort should preferably be targeted to sites located near to each other.

     

The influence of sampling scheme and interpolation method on the power to detect spatial effects of forest birds in Ontario (Canada)

Spatial ecology is becoming an increasingly important component of resource management, and the general monitoring of how human activities affect the distribution and abundance of wildlife. Yet most work on the reliability of sampling strategies is based on a non-spatial analysis of variance paradigm, and little work has been done assessing the power of alternative spatial methods for creating reliable maps of animal abundance. Such a map forms a critical response variable for multiple scale studies relating landscape structure to biotic function. The power to reconstruct patterns of distribution and abundance is influenced by sample placement strategy and density, the nature of spatial auto-correlation among points, and by the technique used to extrapolate points into an animal abundance map. Faced with uncertainty concerning the influence of these factors, we chose to first synthesize a model reference system of known properties and then evaluate the relative performance of alternative sampling and mapping procedures using it. We used published habitat associations of tree nesting boreal neo-tropical birds, a classified habitat map from the Manitou Lakes area of northwestern Ontario, and point count means and variances determined from field studies in boreal Canada to create 4 simulated models of avian abundance to function as reference maps. Four point sampling strategies were evaluated by 4 spatial mapping methods. We found mixed-cluster sampling to be an effective point sampling strategy, particularly when high habitat fragmentation was avoided by restricting samples to habitat patches >10 ha in size. We also found that of the 4 mapping methods, only stratified ordinary point kriging (OPK) was able to generate maps that reproduced an embedded landscape-scale spatial effect that reduced nesting bird abundance in areas of higher forest age-class fragmentation. Global OPK was effective only for detecting broader, regional-scale differences. This revised version was published online in July 2006 with corrections to the Cover Date.