Context

Predicting and managing edge effects requires an understanding of the mechanisms that drive them. However, analytical methods that dominate edge effects research are not well suited to discriminating mechanisms, because they do not measure ‘indirect’ edge effects: effects that are mediated by covariates in statistical models.

Objective

To discuss the value of indirect effects for improving mechanistic understanding of edge effects.

Methods

We explain how measuring indirect effects improves mechanistic understanding, and provide guidance on how to do so. We also conduct a literature review to examine awareness of indirect effects in empirical studies of mechanisms underpinning edge effects. Finally, we use a recent paper in Villaseñor et al. (Landscape Ecol 30:229–245, 2015) as a case study to discuss how failure to measure indirect edge effects may limit mechanistic understanding.

Results

Indirect effects provide a means to translate conceptual models of edge effects into mechanistic pathways that are testable and quantifiable. Moreover, failure to measure indirect edge effects can result in impacts of habitat edges being underestimated. However, few studies that we identified in our literature review quantified indirect effects (7 %, n = 72). Worryingly, 11 % of studies did not account for indirect effects despite using statistical models that potentially contained them, possibly resulting in incorrect inference.

Conclusions

A better awareness of indirect effects will help researchers to understand the mechanisms that underpin edge effects, while ensuring that impacts of habitat edges are not underestimated.

Context

Testing the influence of edges on animal distributions depends on our capacity to quantify ‘edge’, particularly in heterogeneous landscapes. Habitat quality is likely to differ in instances where edges are abrupt and anthropogenic in origin, versus diffuse, disturbance-created edges.

Objectives

We tested whether or not structurally distinct edge types influence northern spotted owl habitat selection and whether the relationship between edge type and use varied across spatial scales relevant to owl foraging (<3 ha) and home range selection (50–800 ha).

Methods

We used remotely sensed disturbance severity data to define two distinct edge types, ‘hard’ and ‘diffuse’, following a 11,000 ha fire and subsequent salvage logging in southern Oregon. The approach quantifies the steepness of gradients directly by measuring the ‘slope’ of change in disturbance severity. We tested the degree to which 23 radio-collared spotted owls responded to edge characteristics caused by fire and logging.

Results

Spotted owls showed a strong negative association with hard edge, even after accounting for habitat suitability and other confounding variables. However, this negative relationship was highly scale-dependent; spotted owls were resilient to hard edges at broad scales, but avoided the same feature at fine scales. On the other hand, spotted owls showed a positive association with diffuse edge, especially at broader scales.

Conclusions

Differential use of edge types indicates that owls favor disturbances that create diffuse edge habitat (e.g. low and mixed-severity fire) and rather than abrupt boundaries created by high severity disturbance.

Context

Farming practices influence the degree of contrast between adjoining habitats, with consequences for biodiversity and species movement. Little is known, however, on insect community responses to different kinds of edges over time, and the extent of cross-habitat movement in agricultural landscapes.

Objective

To determine temporal changes in beetle responses to different farmland-woodland edges, and document cross-habitat movement.

Methods

We examined species richness, abundance, and movement across edges between remnant woodlands and four farmland uses (plantings, fallow, annual crops, woody debris applied over crops post-harvest) in southeastern Australia. We used directional pitfall traps to infer movement, and sampled at edges, and 20 and 200 m on both sides of edges, during spring and summer.

Results

Detritivore and predator abundance varied between seasons across the edge between woodlands and all farmlands, but seasonal differences were weaker for fallow-woodland and woody debris-woodland edges. Detritivores moved from farmlands towards woodlands, but not across fallow-woodlands and woody debris-woodlands edges during summer. During summer, predators showed short-range movement towards edges from all farmlands except plantings, and towards woody debris from woodlands. Edges showed temporally stable predator richness and higher herbivore richness than adjoining habitats.

Conclusions

Farmland use and season interactively affect beetle abundance across farmland-woodland edges. Woody debris can reduce seasonal fluctuations in beetle edge responses and increase permeability for cross-habitat movement, while plantings provide habitat during summer. Edges provide important resources for beetles in adjoining habitats, however, seasonal movement of predators specifically into edges may affect prey assemblages—a link requiring further study.

Context

In agricultural landscapes, small woodland patches can be important wildlife refuges. Their value in maintaining biodiversity may, however, be compromised by isolation, and so knowledge about the role of habitat structure is vital to understand the drivers of diversity. This study examined how avian diversity and abundance were related to habitat structure in four small woods in an agricultural landscape in eastern England.

Objectives

The aims were to examine the edge effect on bird diversity and abundance, and the contributory role of vegetation structure. Specifically: what is the role of vegetation structure on edge effects, and which edge structures support the greatest bird diversity?

Methods

Annual breeding bird census data for 28 species were combined with airborne lidar data in linear mixed models fitted separately at (i) the whole wood level, and (ii) for the woodland edges only.

Results

Despite relatively small woodland areas (4.9–9.4 ha), bird diversity increased significantly towards the edges, being driven in part by vegetation structure. At the whole woods level, diversity was positively associated with increased vegetation above 0.5 m and especially with increasing vegetation density in the understorey layer, which was more abundant at the woodland edges. Diversity along the edges was largely driven by the density of vegetation below 4 m.

Conclusions

The results demonstrate that bird diversity was maximised by a diverse vegetation structure across the wood and especially a dense understorey along the edge. These findings can assist bird conservation by guiding habitat management of remaining woodland patches.

Context

The study of habitat fragmentation is complex because multiple, potentially synergistic, ecological processes may be acting simultaneously. Further, edge effects themselves may be complex in that additivity from multiple edges can give rise to heterogeneous nearest–edge gradients.

Objectives

We used heat diffusion as a proxy for additive edge effects in two study landscapes in order to test whether two key observations recently attributed to synergy between edge and area effects could be more simply explained by additivity; namely, steeper edge gradients in larger fragments and variation in slopes of species–area relationships as a function of distances to fragment edges.

Methods

We sampled forest structure in northwestern Madagascar at various distances from the edge in fragments and continuous forest and used an inverse modelling approach to parameterize the model. In addition, we applied the model to data from a published study of beetle communities in fragmented forests in New Zealand.

Results

With increasing proximity to edges, woody stem densities decreased and, as predicted, smaller fragments had lower stem densities and less steep edge gradients than larger ones. The model successfully predicted shifts in species–area relationships as a function of nearest–edge distances for beetle species, although observed richness for forest specialists in the smallest fragments was lower than predicted.

Conclusions

Two key observations attributed to synergy between edge and area effects were explained by edge additivity. The model is particularly useful in that it can help to disentangle the complex sets of processes acting in fragmented landscapes.

Context

The biodiversity hotspot for conservation of New Caledonia has facing high levels of forest fragmentation. Remnant forests are critical for biodiversity conservation and can help in understanding how does forest fragmentation affect tree communities.

Objective

Determine the effect of habitat configuration and availability on tree communities.

Methods

We mapped forest in a 60 km² landscape and sampled 93 tree communities in 52 forest fragments following stratified random sampling. At each sampling point, we inventoried all trees with a diameter at breast height ≥10 cm within a radius of 10 m. We then analysed the response of the composition, the structure and the richness of tree communities to the fragment size and isolation, distance from the edge, as well as the topographical position.

Results

Our results showed that the distance from the forest edge was the variable that explained the greatest observed variance in tree assemblages. We observed a decrease in the abundance and richness of animal-dispersed trees as well as a decrease in the abundance of large trees with increasing proximity to forest edges. Near forest edges we found a shift in species composition with a dominance of stress-tolerant pioneer species.

Conclusions

Edge-effects are likely to be the main processes that affect remnant forest tree communities after about a century of forest fragmentation. It results in retrogressive successions at the edges leading to a dominance of stress-tolerant species. The vegetation surrounding fragments should be protected to promote the long process of forest extension and subsequently reduce edge-effects.

Context

Although forest fragmentation is generally thought to impact tree growth and mortality negatively, recent work suggests some forests are resilient. Experimental forests provide an opportunity to examine the timing and extent of forest tree resilience to disturbance from fragmentation.

Objectives

We used the Wog Wog Habitat Fragmentation Experiment in southeastern Australia to test Eucalyptus growth and survivorship responses to forest fragmentation over a 26 year period.

Methods

We measured 2418 tree diameters and used spline-regression techniques to examine non-monotonic fragmentation effect over two time periods.

Results

Over the first 4 years after fragmentation, individual eucalypt tree growth was greater than in continuous forest for large trees and mortality rates were higher only within 10 m of edges. Over the following 22 years only the effects on tree growth remained and on average all fragments rebounded so that their biomass and mortality rates were equivalent to continuous forest. Importantly non-monotonic patterns were observed in growth and mortality with respect to area and distance from edge in both study periods, demonstrating that fragmentation impacts on trees can be strong in localized areas (greatest in 3 ha fragments and 0–30 m edges) and over short time periods.

Conclusions

Dry-sclerophyll eucalypt forests join the set of forest types that display resilient growth dynamics post fragmentation. Moreover, persistent non-monotonic impacts on tree growth with respect to tree size, fragment area, and fragment distance from edge, highlighting landscape fragmentation as a driver of habitat heterogeneity within remnant forest fragments.

Context

Quantifying gene flow in natural populations is a key topic in both evolutionary and conservation biology. Understanding the extent to which the landscape matrix facilitates or impedes gene flow is becoming a high priority in a context of worldwide habitat loss and fragmentation.

Objectives

Unexpectedly, a lower genetic diversity and a higher genetic structure have been previously observed in the less fragmented and the most forested habitat across four pine marten (Martes martes) populations in France. Our aim was to quantify the effect of landscape on the spatial distribution of genetic diversity in two populations in contrasting habitats.

Methods

We conducted an individual-based landscape genetics analysis in a highly fragmented rural plain (Bresse, n = 126) and in a highly forested (50 %) mountainous area (Ariège, n = 88) in France. We tested for isolation-by-resistance using least-cost distances and used a causal modeling approach on 16,384 landscape and 104 elevation resistance scenarios.

Results

Landscape structure influenced the genetic differentiation in Bresse, with vegetation providing more genetic connectivity over the study area than open areas, while roads and human buildings showed unexpected low resistance to gene flow. In Ariège, genetic differentiation was mainly associated with changes in elevation, with an optimal elevation for gene flow of around 1700 m, likely associated with changes in vegetation structure.

Conclusions

The pine marten seems to be able to cope with human-dominated landscapes and with fragmented forest landscapes, whereas elevation is the major driver of genetic differentiation in our mountainous landscape. Additionally, we highlight the importance of spatial replication in landscape genetics for deriving reliable conservation and management measures over the species distribution.

Context

The prevalence of edges is increasing due to anthropogenic landscape change. Edge responses can vary considerably between and within species. Understanding species’ responses to edges, and the causes of variation in such responses is central to managing biodiversity in contemporary landscapes.

Objective

A resource distribution model predicts that species that require complementary resources in different land cover types will be most abundant at edges, displaying a positive edge response. Eastern tiger (Papilio glaucus) and spicebush (P. troilus) swallowtail butterflies use forest plant species for oviposition sites but open-habitat plants for nectar. They are excellent models for testing the positive edge response and exploring sources of variability in edge responses, such as species-specific traits or temporal effects.

Methods

In southwestern Ontario, we examined both the abundance and flight orientation of these species in relation to forest/meadow edges and at different times of day. We used a transect method similar to the Pollard walk and a catch and release method, respectively.

Results

The distribution and flight behaviour of these butterfly species were overall consistent with a positive edge response. Both species were most abundant at the edge and oriented their flight towards the edge from the forest and meadow. However, P. glaucus demonstrated a much stronger positive edge response, while P. troilus showed temporal variation in its response.

Conclusions

Our results confirm the ability of the resource distribution model to predict species edge responses and movement behaviours, but also indicate that species-specific traits and time of sampling can influence such responses.

Context

Intensive agricultural management practices and landscape homogenisation are the main drivers of biodiversity loss in agricultural landscapes. Agricultural fields are regularly disturbed and provide unstable habitats due to crop management regimes. This may lead to movement of arthropods into neighbouring non-arable habitats, as natural and semi-natural habitats provide suitable overwintering sites.

Objectives

Here we assessed the effect of landscape composition and configuration on the overwintering spider and carabid fauna of grassy field margins and hedgerows.

Methods

We sampled ground-dwelling arthropods at field edges of different types (grassy field margin and hedgerows), landscape composition (diverse and simple) and configuration (mosaic and large-scale agricultural landscapes).

Results

We detected larger spiders in hedgerows than in grassy field margins and in complex landscapes rather than in simple landscapes. We found a significant effect of interaction between landscape composition and edge type on ballooning propensity of spiders. Agrobiont carabids were more abundant in field edges of compositionally simple and large-scale agricultural landscapes. Furthermore, we showed an effect of interaction between landscape composition and edge type on agrobiont spiders. We collected larger carabids in grassy field margins than in hedgerows and carabids were smaller in simple landscapes than in diverse landscapes. The spider community was affected by edge type, and landscape composition had a significant effect on the carabid community.

Conclusions

Small-scale agricultural landscapes may have higher overall densities of ground-dwelling spiders and carabids than large scale landscapes due to the relatively high edge density and the higher quantity of available overwintering sites.

Context

Linear transportation infrastructures traverse and separate wildlife populations, potentially leading to their short- and long-term decline at local and regional scales. To attenuate such effects, we need wildlife crossings suitable for a wide range of species.

Objectives

We propose a method for identifying the best locations for wildlife crossings along linear infrastructures so as to improve the connectivity of species with varying degrees of mobility and living in different habitats. We evaluate highway impacts on mammal species.

Methods

The study area is the Grésivaudan Valley, France. We used allometric relationships to create eight virtual species and model their connectivity networks, developing a nested method defining populations by daily travel distances and connecting them by dispersal. We tested the gain in connectivity for each species produced by 100 and 600 crossing locations respectively in crossable, i.e. with crossing infrastructures, and uncrossable highway scenarios. We identified the crossings that optimize the connectivity of the maximum number of species combining the results in multivariate analyses.

Results

Highly mobile species needing a large habitat area were the most sensitive to highways. The importance of locomotive performance in structuring the graphs decreased with highway impermeability. Depending on the species, the best locations improved connectivity by 0–10 and 2–75 % respectively in the crossable and uncrossable scenarios. Compromise locations were found for seven of the eight species in both scenarios.

Conclusions

This method could guide planners in identifying crossing locations to increase the connectivity of different species at regional scales over the long term.

Context

Woodland and agricultural expansion are major causes of grassland fragmentation. Fire and rainfall play important roles in maintaining grasslands, however, fire activity has been reduced in fragmented landscapes.

Objectives

Quantify the degree to which basic landscape fragmentation metrics could be used as drivers of woody cover potential.

Methods

Woody plant percent cover was calculated between 2004 and 2008 at?>?2000 sites. At each site, we calculated these fragmentation metrics for grassland cover type (classified by the National Land Cover Database); # patches, landscape proportion, edge density, largest patch index, effective mesh size and patch cohesion index within 3 circular areas (10 km², 360 km² and 3600 km²) surrounding the sampling site. A quantile regression was performed to identify which metrics were useful at predicting the 25th, 50th, 75th or 95th quantile of woody cover distribution.

Results

Grassland proportion and edge density were significant predictors of the woody plant potential (75th and 95th quantile). Woody cover potential was positively associated with edge density suggesting that fragmented areas (i.e., areas with high number of edges) maintained higher woody cover, while grassland proportion was negatively associated with woody plant potential.

Conclusion

We propose that in addition to a lack of fire, fragmented landscapes may facilitate further woodland expansion by reducing natural land and restricting grasslands to smaller, less connected patches, which can maintain higher woody cover. Given current trends in woodland expansion, special attention should be given to areas that are found within a fragmented landscape and climatically prone to woodland expansion.

Context

Edge effects due to habitat loss and fragmentation have pervasive impacts on many natural ecosystems worldwide.

Objective

We aimed to explore whether, in tandem with the resource-based model of edge effects, species feeding-guild and flight-capacity can help explain species responses to an edge.

Methods

We used a two-sided edge gradient that extended from 1000 m into native Eucalyptus forest to 316 m into an exotic pine plantation. We used generalised additive models to examine the continuous responses of beetle species, feeding-guild species richness and flight-capable group species richness to the edge gradient and environmental covariates.

Results

Phytophagous species richness was directly related to variation in vegetation along the edge gradient. There were more flight-capable species in Eucalyptus forest and more flightless species in exotic pine plantation. Many individual species exhibited multiple-peaked edge-profiles.

Conclusions

The resource based model for edge effects can be used in tandem with traits such as feeding-guild and flight-capacity to understand drivers of large scale edge responses. Some trait-groups can show generalisable responses that can be linked with drivers such as vegetation richness and habitat structure. Many trait-group responses, however, are less generalisable and not explained by easily measured habitat variables. Difficulties in linking traits with resources along the edge could be due to unmeasured variation and indirect effects. Some species’ responses reached the limits of the edge gradient demonstrating the need to examine edge effects at large scales, such as kilometres.

Context

The analysis of individual movement choices can be used to better understand population-level resource selection and inform management.

Objectives

We investigated movements and habitat selection of 13 bobcats in Vermont, USA, under the assumption individuals makes choices based upon their current location. Results were used to identify “movement-defined” corridors.

Methods

We used GPS-collars and GIS to estimate bobcat movement paths, and extracted statistics on land cover proportions, topography, fine-scale vegetation, roads, and streams within “used” and “available” space surrounding each movement path. Compositional analyses were used to determine habitat preferences with respect to landcover and topography; ratio tests were used to determine if used versus available ratios for vegetation, roads, and streams differed from 1. Results were used to create travel cost maps, a primary input for corridor analysis.

Results

Forested and scrub-rock land cover were most preferred for movement, while developed land cover was least preferred. Preference depended on the composition of the “available” landscape: Bobcats moved?>?3 times more quickly through forest and scrub-rock habitat when these habitats were surrounded by agriculture or development than when the available buffer was similarly composed. Overall, forest edge, wetland edge and higher stream densities were selected, while deep forest core and high road densities were not selected. Landscape-scale connectivity maps differed depending on whether habitat suitability, preference, or selection informed the travel cost map.

Conclusions

Both local and landscape scale land cover characteristics affect habitat preferences and travel speed of bobcats, which in turn can inform management and conservation activities.

Context

The positive correlation between landscape area of semi-natural habitat and wild pollinator richness and abundance in agroecosystems has been well studied. However, we lack a deep understanding of local scale floral resource and nest provisioning for wild bees necessary to optimize implementation of pollinator conservation practices.

Objectives

The primary objective of this study was to use a spatially interactive landscape pollination model (hereafter, the Lonsdorf model) to represent field scale spatial patterns of wild bee abundance and richness within a heterogeneous landscape in the mid-Atlantic USA.

Methods

We parameterized the Lonsdorf model with high resolution aerial imagery and insight from a previously published floristic study. To test the Lonsdorf model predictions, field studies were conducted to measure wild bee abundance and species richness in apple orchards as a function of distance from a forest edge.

Results

Field measurements indicated apple pollinator abundance and species richness significantly decreased with increasing distance from the forest edge. The Lonsdorf model pollination service score was highly sensitive to changes in resource provisioning in orchard and non-crop areas, and including resource rich forest and forest edge habitats in the model significantly improved pollination service estimates.

Conclusions

We demonstrated a novel application of the Lonsdorf model at a field scale to predict trends in pollination service provisioning as a factor of local habitat features. With sufficiently detailed inputs, the Lonsdorf model is a promising tool to quantify field scale pollination service deficits, guiding more cost effective habitat supplementation and other conservation efforts.

Context

Although multi-scale approaches are commonly used to assess wildlife-habitat relationships, few studies have examined selection at multiple spatial scales within different hierarchical levels/orders of selection [sensu Johnson’s (1980) orders of selection]. Failure to account for multi-scale relationships within a single level of selection may lead to misleading inferences and predictions.

Objectives

We examined habitat selection of the federally threatened eastern indigo snake (Drymarchon couperi) in peninsular Florida at the level of the home range (Level II selection) and individual telemetry location (Level III selection) to identify influential habitat covariates and predict relative probability of selection.

Methods

Within each level, we identified the characteristic scale for each habitat covariate to create multi-scale resource selection functions. We used home range selection functions to model Level II selection and paired logistic regression to model Level III selection.

Results

At both levels, EIS selected undeveloped upland land covers and habitat edges while avoiding urban land covers. Selection was generally strongest at the finest scales with the exception of Level II urban edge which was avoided at a broad scale indicating avoidance of urbanized land covers rather than urban edge per se.

Conclusions

Our study illustrates how characteristic scales may vary within a single level of selection and demonstrates the utility of multi-level, scale-optimized habitat selection analyses. We emphasize the importance of maintaining large mosaics of natural habitats for eastern indigo snake conservation.

Context

The relative influence of habitat loss versus configuration on avian biodiversity is poorly understood. However, this knowledge is essential for developing effective land use strategies, especially for grassland songbirds, which have experienced widespread declines due to land use changes. Habitat configuration may be particularly important to grassland songbirds as configuration of habitat affects the extent of edge effects on the landscape, which strongly influences habitat use by grassland birds.

Objectives

We examined the relative influence of grassland amount and a measure of grassland configuration per se (Landscape Shape Index; LSI) on the relative abundance and richness of grassland songbirds.

Methods

In 2013, 361 avian point counts were conducted across 47, 2.4 km radii landscapes in south-west Manitoba, Canada, selected to minimize the correlation between grassland amount and configuration. We used generalized linear mixed-effects models within a multi-model inference framework to determine the relative importance of grassland amount and configuration on songbird response variables.

Results

Effects of grassland amount and configuration were generally weak, but effects of configuration were greater than grassland amount for most species. Relative abundance and richness of obligate species, and Savannah sparrows, showed a strong negative response to LSI, while grasshopper sparrows responded positively to grassland amount.

Conclusion

Our results suggest that habitat configuration must be considered when managing landscapes for conservation of grassland songbirds. Maintaining large, intact tracts of grasslands and limiting development of roads that bisect grassland parcels may be an effective means of maintaining grassland songbird diversity and abundance in northern mixed-grass prairies.

Context

In tropical landscapes, dominant land-use changes involve conversion of intact forest to an agricultural matrix with embedded fragments of remnant forest. However, most research to date has focused on how these land-use changes affect species within the fragmented ecosystem, rather than the flux of energy and nutrients within these different landscape elements.

Objectives

We examined how forest fragmentation and conversion to orange fields impact the potential for litter decomposition in a Costa Rican landscape, in particular via effects on macroinvertebrates (MIs) and microclimate.

Methods

We measured mass losses of a standard leaf litter in four habitats: orange fields, small forest fragments, large forest fragments and intact forest. Litter bags were constructed of mesh that either excluded or allowed MIs. Decomposition rates were measured in wet and dry seasons, and at different distances from the forest edge.

Results

Forest fragmentation and forest conversion had divergent effects on decomposition rates. Decomposition rates were 7 % slower in forest fragments during the dry season than in intact forest, and this result was mediated by forest fragmentation effects on MIs. Decomposition rates were 9 % higher in orange fields during the wet season, relative to intact forest, and this pattern was explained by effects of the litter microenvironment on leaching rates or smaller invertebrates. Fragment area and distance from forest edge had minor or undetectable effects on decomposition in fragments.

Conclusions

We conclude that land-use changes affect decomposition processes in both forest and agroecosystems, and these effects can vary in mechanism and direction across disturbed landscapes.

Context

North American grassland songbird populations have declined significantly due to habitat loss and fragmentation. Understanding the influence of the surrounding landscape on prairie fragment occupancy is vital for predicting the fate of grassland birds in these heavily altered landscapes.

Objectives

We examined the relative importance of local and landscape variables on grassland bird occupancy of prairie fragments using a focal-patch study. We also investigated the spatial scale at which landscape variables were most influential.

Methods

We surveyed birds on 29 unplowed prairie fragments in western Minnesota and eastern North and South Dakota. We quantified local habitat on the fragment using vegetation surveys and aerial photographs and the landscape surrounding the fragment out to 4 km using aerial photographs. We analyzed occupancy using multi-model approaches applied to multiple logistic regression.

Results

Of 38 species encountered, nine were neither too rare nor too abundant to be analyzed. Predictors of patch occupancy were unique for each bird species, yet general patterns emerged. For eight species, landscape variables were more important than local variables. Mostly, those landscape variables measured configuration (e.g., edge density) and not composition (e.g., percent cover of a particular matrix element). Landscape effects were mostly from variables measured at the greatest extents from the prairie fragment.

Conclusions

Using a focal-patch study design we demonstrated the importance of the surrounding landscape, often out to 4 km from the fragment edge, on prairie occupancy by grassland birds. Effective management of grassland songbirds will require attention to the landscape context of prairie fragments.