Edge Effects and Isolation: Red-Backed Voles Revisited

Abstract:  We examined demographic responses of California red-backed voles ( Clethrionomys californicus ) to forest fragmentation in southwestern Oregon at sites where this species has previously shown negative responses to fragmentation. Voles were captured in live traps and released. Voles were rarely caught in clearcuts surrounding 11 forest fragments, but relative vole density did not decrease from the forest-fragment interiors to edges. The first result agrees with previous findings at these sites 6 years earlier, but the latter result does not. There was no evidence that vole response to edge changes with fragment age. Two years of intensive mark-recapture efforts at two forest-fragment sites and two unfragmented (control) sites did not show negative effects of fragmentation on vole survival, an important demographic rate. Vole capture probabilities varied greatly across space and time on these four sites, which may explain the differences in vole responses to edge seen between this and the previous study. These results suggest that reliable appraisal of edge effects may be difficult for many species on small fragments because the data necessary to apply population estimators require great efforts to obtain and the use of indices leads to a confounding of detection probabilities with demographic change.     

Response of Red-Backed Voles to Recent Patch Cutting in Subalpine Forest

We examined the response of southern red-backed voles ( Clethrionomys gapperi ) to patch cutting in a forested landscape in which 23% of the forest cover had been removed by timber harvest. We live trapped voles in and around 18 patchcuts in one watershed of southern Wyoming. Although we found a significant difference in capture rates between patchcut interior and forest habitats in 1 of 2 years, voles did not strongly avoid the interior of patchcuts. This result contrasts with results from most studies of voles in clearcut ecosystems, which report that red-backed voles are generally rare or absent from clearcuts. Capture rates were highest on both sides of the patchcut edge, which also contrasts with studies of voles at the edges of forest remnants. The use of patchcut interior and edge habitats could not be explained as a consequence of juvenile voles dispersing to those habitats or males moving through the habitat in search of mates. We suggest that, despite similar physiognomy in patchcut and clearcut sites, the differences in landscape structure in perforated versus fragmented landscapes lead to very different patterns of vole movement. Understanding the scales at which voles perceive landscapes as coarse- or fine-grained will be key to developing predictive models to aid managers in designing timber sales that maintain high vole populations. Our results emphasize the importance of the spatial pattern and scale of disturbance in determining the response of vertebrates to landscape change and the need for more refined investigations of the consequences of deforestation.     

Edge Effects on Lizards and Frogs in Tropical Forest Fragments

Abstract: We investigated whether forest-pasture edges affect the distribution of an assemblage of small vertebrate ectotherms in a consistent and predictable manner. We describe the abundance and distribution of two species of anoline lizards (   Norops ) and five species of leaf-litter frogs (   Eleutherodactylus ) along the edges and in the interiors of nine forest fragments near Las Cruces, Costa Rica. Over 4 months, we surveyed 44 pairs of plots by visual encounter. In each pair of plots, one was immediately adjacent to the pasture and the second was within the forest "interior." Both plots of a pair were searched simultaneously. This block design controlled for the effects of weather, topography, and searcher ability. The distribution of all species was highly variable with respect to edges. Only two species of frogs, Eleutherodactylus podiciferus and E. cruentus , were significantly more abundant in interior plots than in edge plots, although not consistently so. Both species of Norops lizards were more abundant along forest edges during the dry season. Both Norops species and several Eleutherodactylus species, however, appeared to become more abundant in the forest interior after the onset of the wet season, suggesting a seasonal edge effect. In Norops polylepis , the most abundant anole, rates of ectoparasitism were lower along edges than in forest interiors. The magnitude of the edge effect on any one species was not influenced by the size of fragments or by the distance of the interior plot from the nearest edge. We believe that edge effects should not be defined by the distance to which they are detected. Rather, they should be viewed as highly dynamic in space and time; taxa appear to respond to different components of edge effects according to their particular biological requirements.     

An Edge Effect Caused by Adult Corn-Rootworm Beetles on Sunflowers in Tallgrass Prairie Remnants

Abstract: The once-extensive tallgrass prairie community of North America has been reduced to small remnants, many of which are surrounded by intensive corn (   Zea mays ) agriculture. We investigated adult corn-rootworm beetles (Chrysomelidae : Diabrotica spp.), important pests of corn, on sunflowers (Asteraceae : Helianthus spp . ) in prairie remnants in southeast Minnesota. Large numbers of beetles invaded the prairie from surrounding corn fields in late summer. D. barberi and D. virgifera were captured on sticky traps in all locations in the prairie, but abundance was much greater near the edge adjacent to corn. We observed D. barberi ( but not D. virgifera ) feeding extensively on sunflower pollen and occasionally on other flower parts, such as petals. Sunflowers located nearer corn fields sustained more floral damage than those farther from corn. To determine the effect of beetle damage on seed set, we enclosed sunflower heads in bags with either zero, two, or four D. barberi adults. Seed set was reduced in heads enclosed with D. barberi . Thus, this agricultural pest may interfere with the successful reproduction of sunflowers and possibly other prairie composites that flower in late summer. Given the small size of most prairie remnants and the abundance of this flower-feeding beetle in landscapes dominated by corn agriculture, D. barberi may affect the sustainability of prairie plant populations.     

Edge Effects on the Understory Bird Community in a Logged Forest in Uganda

Abstract: Understanding how the fauna of logged tropical rainforests responds to fragmentation and the creation of edges is vital to ensure conservation of biodiversity. We studied the composition of the understory bird community from the edge of a 15-ha clearing toward the interior of the forest in a part of Budongo Forest Reserve, Uganda, that was selectively logged about 45 years ago. Mist netting was conducted along five transects from the edge and 500 m into the interior. The total number of individuals captured did not change with distance from the edge, but there was a significant increase in the number of species. We sampled fewer, but more common species near the edge, whereas the interior of the forest had more, and less common species. Guild composition also changed with distance from the edge. Frugivore-insectivores and nectarivores were most common close to the edge. Among insectivores, ground foragers, bark-gleaners, and leaf-gleaners were most common in the interior of the forest, whereas sallying insectivores favored the edge. Graminivores were unaffected by the edge. Analysis of common species showed that Ispidina picta , Andropadus curvirostris , A. latirostris , Camaroptera brachyura , Terpsiphone rufiventer , and Nectarinia olivacea were associated with the edge, but no species showed significant avoidance of the edge. This finding may be explained by the generally low sample sizes of interior species. Our results show that even bird communities in logged forests respond to edges. Estimates of edge effects suggested that changes in bird densities may have occurred several hundred meters from the edge. In conclusion, logged forests provide habitat for bird species avoiding forest edges, and this should be considered in the management of such forests for conservation.     

Effects of Warming Conditions in Eastern North American Forests on Red-Backed Salamander Morphology

Abstract:  Several studies have reported climate-associated changes in phenotypically plastic traits of amphibians, yet it remains unknown whether amphibians can manifest an evolutionary response to global climate change at the rate and magnitude that it is occurring. To assess this issue, we examined temporal change in the morphology of the red-backed salamander ( Plethodon cinereus ), a small, abundant woodland salamander distributed widely in eastern North America with two distinct morphotypes: striped individuals associated with cooler microclimates and unstriped individuals associated with warmer microclimates. We compiled morph frequencies for 50,960 individual salamanders from 558 sites as recorded in the published literature and in unpublished field notes of herpetologists between 1908 and 2004. We observed that striping probability increased with increasing latitude, longitude, and elevation and decreased (from 80% to 74% range wide) with time. The combined forces of regional climate warming and, particularly, forest disturbance have evidently been sufficient to cause morphological evolution in this amphibian over the last century.     

Effects of Habitat Loss and Fragmentation on the Behavior and Demography of Gray-Tailed Voles

We monitored the short term behavioral and demographic responses of gray-tailed voles (Microtus canicaudus) to the reduction and fragmentation of their habitat. Our objectives were (1) to test whether animals perished or moved into remaining fragments after 70% of their habitat was removed; and (2) to test the null hypothesis that the social structure and demography of animals would not differ between habitats consisting of one large continuous fragment (625 m²), a mosaic of 25 small fragments (each 25 m²) separated by 4 m of bare ground, and control, unmanipulated habitats (1850 m²). We conducted the experiment in 12, 0.2-ha enclosures planted with alfalfa with four replicates for each of two manipulated treatments and a control. A 70% reduction in habitat did not adversely affect adult survival, reproductive rate, juvenile recruitment, or population size. However, an influx of unrelated females into habitat fragments resulted in decreased juvenile recruitment in those fragments. Voles from cleared habitat moved into the remaining habitat and did not measurably affect the resident population. Similarly, the demography of voles did not differ significantly among the large-fragment, small-fragment, and control enclosures. Peak density estimates based on the amount of habitat in each enclosure were 545 animals per hectare in control, 1056 in large-fragment, and 2880 in small-fragment enclosures. Reduced movement of animals among the small fragments was the most obvious effect of habitat fragmentation. Six percent of females and 15% of males moved among small fragments within a week compared to approximately 60% moving comparable distances in large-fragment and control enclosures. Rates of juvenile dispersal and sexual maturation declined throughout the summer on all treatments, were associated with season and density, and were only marginally associated with habitat loss and fragmentation. We conclude that at the time of habitat removal and fragmentation, populations were small enough to accommodate a 70% reduction in habitat and still continue to increase in numbers. The social system of gray-tailed voles was sufficiently flexible to accommodate an influx of animals to withstand densities> 1000 voles per ha. The behavioral and demographic features of gray-tailed voles are similar to those reported for other small mammals, thus confirming the use of voles for ecological model systems in habitat fragmentation studies.     

Effect of Edge Structure on the Flux of Species into Forest Interiors

Abstract: A key goal of conservation biology is to prevent the spread of exotic species. Previous work on exotic invasion has two limitations: (1) the lack of a spatially explicit approach and (2) a primary focus on the net effect of invasion by examining invasive species already present in the community. We address these limitations by focusing on the arrival of a potential invader into a community and use a spatially explicit approach to quantify the flow of seeds from the surrounding landscape into the interior of a forest. We hypothesize that the structure of forest-edge vegetation influences how the edge mediates seed flux. To test our hypothesis, we experimentally altered vegetation structure within 20 m of the edge to create two edge treatments: thinned and intact. We quantified the flux of seeds moving into the forest interior across the two treatments. We used seed traps randomly arrayed on transects from 5 to 50 m into the forest. More seeds crossed the thinned treatment than crossed the intact treatment to reach the forest interior. In addition, seeds that crossed the thinned treatment dispersed farther into the forest than those that crossed the intact treatment. These results were consistent throughout the period of maximum autumn dispersal, including periods before and after leaf drop. Our results show that the structure of vegetation on the edge interacts with the flux of wind-dispersed seed across the edge. We demonstrated that an edge with intact vegetation can function as a physical barrier to seed dispersal. Therefore, the structure of vegetation on edges can influence the function of edges as barriers to seed flux into the forest interior.     

Evaluating Effects of Habitat Loss and Land-Use Continuity on Ant Species Richness in Seminatural Grassland Remnants

Abstract:  Seminatural grasslands in Europe are susceptible to habitat destruction and fragmentation that result in negative effects on biodiversity because of increased isolation and area effects on extinction rate. However, even small habitat patches of seminatural grasslands might be of value for conservation and restoration of species richness in a landscape with a long history of management, which has been argued to lead to high species richness. We tested whether ant communities have been negatively affected by habitat loss and increased isolation of seminatural grasslands during the twentieth century. We examined species richness and community composition in seminatural grasslands of different size in a mosaic landscape in Central Sweden. Grasslands managed continuously over centuries harbored species-rich and ecologically diverse ant communities. Grassland remnant size had no effect on ant species richness. Small grassland remnants did not harbor a nested subset of the ant species of larger habitats. Community composition of ants was mainly affected by habitat conditions. Our results suggest that the abandonment of traditional land use and the encroachment of trees, rather than the effects of fragmentation, are important for species composition in seminatural grasslands. Our results highlight the importance of considering land-use continuity and dispersal ability of the focal organisms when examining the effects of habitat loss and fragmentation on biodiversity. Landscape history should be considered in conservation programs focusing on effects of land-use change.     

Acid Precipitation Effects on Forest Habitats: Implications for Wildlife

Abstract: Forests ecosystems, with their stratified structure, provide a diversity of habitats for a variety of animals. The influence of acidic deposition on forests is being studied intensely, and preliminary research indicates that some forest resources are being affected. These effects and the potential for subsequent change in forest structure and function imply that an impact on the wildlife inhabitants is occurring. In most cases, these impacts are not direct, but instead reflect modifications of habitat and changes in trophic relationships. Detecting changes distinguishable from normal spatiotemporal variations in animal populations is difficult. The net result over time may be reductions or shifts in populations and species composition. This paper provides an overview of current information on the impact of acidic deposition on forest resources as it relates to animals.