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# Edge Effects in Fragmented Forests: Implications for Conservation

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## Abstract

Edges are presumed to have deleterious consequences for the organisms that remain in forest fragments. However, there is substantial discrepancy among recent studies about the existence and intensity of edge effects. Most studies have focused on seeking simplistic and static patterns. Very few have tested mechanistic hypotheses or explored the factors that modulate edge effects. Consequently,studies are very site-specifci and their results cannot be generalized to produce a universal theory of edges. Although estimates of the intensity and impact of edge effects in fragmented forests are urgently required, little can be done to ameliorate edge effects unless their mechanics are better understood.
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Thesis
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Article
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Article
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1 Forest herbs, shrubs, and tree seedlings were censused at five distances from human-generated edges at 14 sites in south-eastern Pennsylvania and northern Delaware, USA. Forest edges varied in orientation and in the degree of canopy closure, allowing comparisons of vegetation by aspect and successional stage. Soil depth and pH were measured at vegetation sampling points at a subset of sites; microclimate data were available from a parallel study. 2 Edge-related pattern was observed in overall species composition, and in distributions of 15 individual species. At recently created forest edges, species abundances were correlated with steep climatic gradients. Although most edge-orientated species were clustered $\leq 5$ m from the edge, some species consistently reached peak densities at distances up to 40 m, the greatest distance surveyed. 3 Edge-related pattern was most frequently observed at recently created edge sites, but persistent pattern was also observed at older edges, including those closed for 55 years by succession. Species distributions at older edges showed fewer instances of edge-related pattern, modal distances were farther from the edge, and the strength of residual pattern was lower than at recent sites. 4 Edge-related pattern was often observed at north-facing sites. North-facing edges were not generally distinguishable from other recently exposed edges on the basis of edge effect strength (Gini coefficient) or mean distance of individual species from the edge. 5 These observations suggest a cycle of pattern imposition and relaxation as the edge ages, and are consistent with the great clonal longevity observed in many forest species. A Piedmont forest reserve designed to protect interior habitat from vegetational changes at forest edges should include points further than 92 m from the nearest edge, and should be comprised of forest more than 100 years old.
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Some ground nesting bird species, which have partially covered nests (eg Anthus trivialis, Miliaria calandra), breed close to woodland edges, whereas others, which have open nests (eg Alauda arvensis, Anthus pratensis, Motacilla flava), always breed at some distance from edges differential nest predation in relation to nest type affected nest site selection. Eggs mimicking those of skylarks were placed in open or partially covered nests at distances of 200, 25 and 0 m outside woodland edges and at distances of 25 and 100 m inside woodlots. Partially covered nest were preyed upon less often than open nests near the edge, but not at 100 m distances inside and 200 m distances outside woodlots. Magpies Pica pica preyed on open nests more often than hooded crows Corvus cornix and jays Garrulus glandarius. -from Author
Chapter
Human management of the land frequently creates or sharpens landscape boundaries between natural and managed ecosystems. These boundaries often correspond approximately to natural landscape features, such as changes in slope, soil types, or riparian zones. Managed systems usually export very large fluxes of soils, nutrients, pesticides, and inorganic ions. Steep gradients of material concentrations and process rates develop across and adjacent to these boundaries. The resulting alteration in rates of processes in the natural ecosystem ultimately has secondary unanticipated effects upon soils, biota, and atmospheric exchange rates. In the below ground environment, pH, Eh, ionic composition, nutrient status, toxic metal levels, and organic matter pathways are altered. These impacts are especially important for remnant or relict natural ecosystems such as wetlands which are downslope from intensively managed ecosystems.
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Territories of breeding forest birds in a rectangular, 21·4-ha forest plot adjacent to 800 m of a power-line corridor in East Tennessee were mapped in 1975, 1977, and 1979. Trends in density from the corridor edge to 268 m into the forest were examined for the bird community as a whole and for edge, deep forest, and unaffected species. Analysis of computer-generated, randomly distributed 'species' indicated that most trends observed in individual bird species were real. Apparently due to a tendency for some birds to establish territories in a row along the straight corridor edge, peaks in total density occurred at the edge and again in deeper forest. In each year, total density was higher at some distance into the forest than at or near the edge. The contribution of five edge species to bird density on the plot as a whole was negated by lower densities of nine deep forest species in areas near the edge. Considered as a group, thirteen forest species that individually appeared unaffected by the corridor showed a significant decrease in density with increasing distance from the corridor edge; this may have been caused by higher bird density in a small amount of mixed forest habitat near the corridor.
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In forest, birds were relatively more common than mammals as nest robbers than in farmland. Differences in the role of different predator species as nest robbers in forest vs. farmland habitats reflected their choice of habitat. No predator seemed to have developed specific nest-robbing skills. Loss of individual nests was considered as a random event. Predation rates were higher when nests could be detected from a distance. The main factor affecting the rate of predation in patchy environments is apparently the steepness of productivity gradients between an habitat island and the surrounding matrix rather than patch size itself. -from Author
Article
I used artificial nests to examine predation on birds' nests in lowland wet forest in Costa Rica. My purpose was to assess patterns of latitudinal variation in nest predation intensity and to predict the effects of further fragmentation of this forest on its associated avifauna. Nest loss did not vary among sites or between dry and wet seasons within the primary forest interior during 1988. Nests near forest/second growth edges were destroyed more frequently than nests in the forest's interior. Nest loss near forest/pasture edges, however, was similar to that in the forest interior. Nest loss was higher at five smaller (< 100 ha) than at three larger forest patches. Predation intensity at this site was comparable to that observed in three similar studies conducted in temperate forests. Comparative data from this site indicate that predation on nests of ground-dwelling birds at Barro Colorado Island, Panama, may be unusually high due to elevated densities of nest predators. High mortality rates of birds' nests previously reported for tropical forests could be an artifact of studying birds in disturbed or isolated forests.
Article
We tested factors associated with predation near forest-field edge and with the ecological trap hypothesis using artificial bird nests containing Japanese quail (Coturnix japonica) eggs. Predation rates were determined for 759 nests placed in a systematic pattern within plots perpendicular to a forest-field ecotone created by commercial timber harvest in northern Idaho. One plot contained an abrupt edge and 1 contained a wide feathered edge of partial timber removal. Nests of 2 diameters (80 and 100 mm) were located on the ground and above ground in shrubs, and were placed in high- (20 nests/ha) and low-density (9 nests/ha) patterns. No difference was detected for predation rate with respect to nest location or size. Nests placed in a low-density pattern received higher percent predation than nests in a high-density pattern. The high-density plot had greater shrub cover, which may have restricted predator effectiveness. Predation rates were >4× higher in forest plots than field plots; these results may reflect relatively infrequent use of field subplots by avian predators requiring perch sites. Our data do not support the ecological trap hypothesis; we found no relationship between distance from edge and predation rate. However, our abrupt-edge subplots had higher predation than the feathered-edge subplots. These data support earlier hypotheses that birds are poorly adapted to abrupt, artificial-edge habitats, and that these habitats may have a barrier effect and create a travel lane for predators.