Effects of habitat loss and fragmentation on amphibians: A review and prospectus

Habitat loss and fragmentation are among the largest threats to amphibian populations. However, most studies have not provided clear insights into their population-level implications. There is a critical need to investigate the mechanisms that underlie patterns of distribution and abundance. In order to understand the population- and species-level implications of habitat loss and fragmentation, it is necessary to move from site-specific inferences to assessments of how the influences of multiple factors interact across extensive landscapes to influence population size and population connectivity. The goal of this paper is to summarize the state of knowledge, identify information gaps and suggest research approaches to provide reliable knowledge and effective conservation of amphibians in landscapes experiencing habitat loss and fragmentation. Reliable inferences require attention to species-specific ecological characteristics and their interactions with environmental conditions at a range of spatial scales. Habitat connectivity appears to play a key role in regional viability of amphibian populations. In amphibians, population connectivity is predominantly effected through juvenile dispersal. The preponderance of evidence suggests that the short-term impact of habitat loss and fragmentation increases with dispersal ability. However, species with limited dispersal abilities are likely to be equally imperiled by habitat loss and fragmentation over longer time periods. Rigorous understanding of the effects of habitat loss and fragmentation on amphibians will require species-specific, multi-scale, mechanistic investigations, and will be benefit from integrating large empirical field studies with molecular genetics and simulation modeling. Molecular genetic methods are particularly suited to quantifying the influences of habitat structure across large spatial extents on gene flow and population connectivity. Conservation strategies would benefit by moving from generalizations to species and process specific recommendations and by moving from site-specific actions to implementing conservation plans at multiple scales across broad landscapes.     

Body size, niche breadth, and ecologically scaled responses to habitat fragmentation: mammalian predators in an agricultural landscape

The ability to make a priori assessments of a species' response to fragmentation, based on its distribution in the landscape, would serve as a valuable conservation and management tool. During 1997-1999, we monitored 717 scent stations to examine seasonal use of forest patches, corridors, and crop fields by coyotes (Canis latrans), domestic cats (Felis catus), foxes (Vulpes vulpes and Urocyon cinereoargenteus), raccoons (Procyon lotor), striped skunks (Mephitis mephitis), opossums (Didelphis virginiana), and long-tailed weasels (Mustela frenata). For each species we developed landscape-based ecologically scaled landscape indices (ELSI), and we modeled species spatial distribution across three spatial scales (landscape-level, element-level, and local habitat-level). Our results suggest that these predators view landscape fragmentation at different spatial scales and demonstrate strong interspecific differences in their response to elements of the landscape. All species except coyotes and domestic cats avoided agricultural fields. In general, predator species that were more mobile (i.e. high ESLI for landscape connectivity; coyotes) were characterized by landscape- and element-based logistic models. In contrast, models including local habitat features generally were most appropriate for less mobile or more stenophagous predators (e.g. long-tailed weasels). Our analysis extends the application of the ESLI concept to species assemblages that do not appear to function as metapopulations, and it highlights the importance of examining spatial scale and species-specific responses to habitat fragmentation. We discuss the relevance of these findings for defining ecological landscapes, understanding predator-prey interactions at multiple spatial scales, and conserving predator and prey populations in fragmented landscapes.     

Forest moth taxa as indicators of lepidopteran richness and habitat disturbance: a preliminary assessment

The conservation of hyperdiverse taxa such as arthropods may prove frustrating for land managers who lack the taxonomic expertise to perform species-level identifications. Therefore, interest has turned towards selecting indicator taxa that can be monitored as surrogates for detecting changes in entire communities. In this study, we examined whether several moth families or subfamilies were significant indicators of lepidopteran species richness and habitat disturbance in Eastern deciduous forests of North America. We screened moth families to identify assemblages that are easily sampled and primarily comprised of species that can be identified by non-experts. Using existing field data collected over 3 years, we tested four taxa that met our initial requirements with regard to their performance as an indicator of total observed lepidopteran species richness and levels of forest disturbance. We determined that the Arctiidae and Notodontidae hold promise as potential indicator taxa in Eastern deciduous forests. The Arctiidae functioned best as an indicator of moth species richness, while the Notodontidae performed best as a coarse-scale disturbance indicator. Additional research will be required, however, before species within either family have practical significance as tools for assessing forest condition.     

Habitat loss and the habitat fragmentation threshold: an experimental evaluation of impacts on richness and total abundances using grassland invertebrates

Based on empirical analyses and computer-based modelling, it has been suggested that the impact of habitat loss is essentially independent of habitat fragmentation when >10-30% of the original habitat is left and that habitat fragmentation is influential only when less that this amount remains. This is the threshold effect of habitat fragmentation. In many systems, effects of habitat loss cannot be distinguished from those of habitat fragmentation in a way needed to critically evaluate the existence of the threshold effect. Therefore, we used an experimental model system (EMS) that was constructed to produce multiple micro-landscapes in which the habitat-loss and habitat-fragmentation impacts were potentially distinguishable. We used responses of terrestrial invertebrates to measure the impacts. We did not find an interaction between habitat-loss and habitat-fragmentation effects in the predicted fashion, although it is possible the threshold of habitat loss we used for the experiments (90%) may have still been above a critical level for the invertebrates. The only significant components were a strong ‘edge-centre’ difference in both richness and abundance, and a temporal change in both variables. Thus, in this EMS, there was little support for the threshold phenomenon or for general effects of habitat loss and fragmentation although this conclusion needs to be tempered by the limited duration of the experiment.     

Population demography and fecundity do not decline with habitat fragmentation in the rainforest tree Macadamia integrifolia (Proteaceae)

Habitat fragmentation is often associated with reduced levels of fitness and local extinction of plant species, and consequently poses a major threat to the persistence of species worldwide. The majority of demography-based fragmentation studies to date have focussed primarily on fragmentation impacts on individual plant fecundity. Here we investigate the impact of habitat fragmentation on the demography (plant height classes and density) and key population dynamic processes for the rainforest tree species Macadamia integrifolia (Proteaceae). Raceme and fruit production and seedling emergence across fragmented sites exceeded that in more intact sites with no apparent difference in short-term mortality rates. Fecundity of flowering trees did not appear to be affected by fragmentation. Instead, overall reproductive output in fragmented sites was enhanced relative to undisturbed sites due to a higher proportion of reproductively active individuals. The probability of flowering and fruiting was negatively correlated with the projected foliage cover (PFC) surrounding individual trees, and average PFC was significantly lower in small and medium fragments, suggesting light availability as a potential contributor to the trends observed here.This study demonstrates that the short-term effects of habitat fragmentation on population viability may not necessarily be detrimental for some species, and highlights the importance of assessing not only the fecundity of flowering individuals but also the proportion of individuals reproducing within fragments.     

Bryophytes in a changing landscape: The hierarchical effects of habitat fragmentation on ecological and evolutionary processes

Testing the myriad predictions associated with the community, demographic and genetic impacts of habitat fragmentation remains a high conservation priority. Many bryophyte taxa are ideal model systems for experimentally testing such metapopulation-based and population genetic predictions due to their relatively fast colonisation-extinction rates, high substrate specificity, dominant haploid condition, and diminutive size. Herein, we review the community, demographic and population genetic impacts of habitat fragmentation on bryophytes, highlight the present knowledge gaps, and offer ideas on how experimental studies utilizing bryophytes may be used to address the broader conservation implications associated with fragmented ecosystems. Previous research suggests that dispersal limitation best explains observed patterns of abundance and distribution of bryophytes in some fragmented habitats. However, edge effects influence bryophyte community structure of border habitats especially where abrupt differences in micro-climatic conditions between the matrix and the forest remnant exist, or where the species pool contains members with inherently restricted ecological amplitudes. Existing studies do not agree on the relationship between basic attributes of bryophyte community structure (i.e., species richness and local density), and habitat area and degree of spatial-isolation. Demographic studies are a critical step in structuring conservation strategies, however surprisingly little empirical information exists as to the impacts of habitat fragmentation on plant population dynamics. We propose that bryophytes offer great potential for testing predictions with respect to plant population persistence in spatially-structured landscapes.     

Grasslands as movement barriers for a forest-associated salamander: Migration behavior of adult and juvenile salamanders at a distinct habitat edge

The persistence of amphibian populations in fragmented landscapes requires dispersal and recolonization of habitat patches after local extinction. These processes entail individuals crossing habitat edges. Edge permeability integrates the behavior of individuals with the vegetative structure of the habitat edge and may influence the dispersal rates between habitat patches. We used drift fences, radio telemetry, and an experimental displacement to examine the movement behavior of juvenile and adult spotted salamanders (Ambystoma maculatum) at a pond located in continuous forest and a pond located on a distinct forest-grassland edge. At the pond on the habitat edge, adult salamanders migrated to and from the forested side of the pond. Resident adults with transmitters migrated to forested habitat without approaching the habitat edge. Displaced adults with transmitters halted emigration movements when they approached the habitat edge. None of the radio-tagged adults were observed more than a few meters into the grassland. Recently metamorphosed juveniles exiting the grassland side of the pond changed their direction of movement and migrated into the forested habitat. We conclude that salamanders oriented movements with respect to features in the terrestrial habitat, detected the habitat edge, and behaviorally avoided the grassland. Exploring the permeability of habitat edges will improve our understanding of dispersal within fragmented landscapes and enhance efforts to conserve regional populations of amphibians.     

Plant species richness in the Chaco Serrano Woodland from central Argentina: Ecological traits and habitat fragmentation effects

The Chaco Serrano Woodland from central Argentina has been dramatically reduced during the past 30 years, and is currently confined to several isolates of different size. In this study, we evaluated the effects of forest size, isolation and edge formation on plant species richness. Furthermore, we tested whether plants species with particular ecological traits were differentially affected by habitat fragmentation. Habitat area showed the highest explanatory value for plant species richness in stepwise multiple regressions. The effect of area was most pronounced for rare species, suggesting that large forests are necessary to preserve species with low local or regional abundance. Differences between edge and interior of Chaco Serrano were more pronounced for native and shrub species richness. The analysis of individual species cover revealed that native and biotically pollinated plants were less abundant in woodland edges. Our results showed that forest transformation into smaller remnants has lead to an impoverishment of plant communities, with particular subsets of species defined by ecological traits (rarity, origin and pollination mode) being more susceptible.     

Habitat fragmentation in marine landscapes: relative effects of habitat cover and configuration on juvenile crab survival in California and North Carolina seagrass beds

Processes that dictate faunal abundance (e.g. predation) may be linked primarily to the configuration of habitat (e.g. patch size, patch isolation, proximity to edge) or simply to the amount of habitat (i.e. habitat cover) in marine and terrestrial systems. I asked whether juvenile crab survival in marine seagrass habitats is linked to habitat cover or to habitat configuration, and whether relationships between crab survival and habitat features were similar in two widely separated seagrass landscapes (Bodega Harbor, California and Back Sound, North Carolina, USA). I also evaluated the independent effects of seagrass structural complexity (shoot density and shoot biomass per unit area) and habitat fragmentation on crab survival. Juvenile red rock crabs (Cancer productus: 1+year class) were tethered in Bodega Harbor and juvenile blue crabs (Callinectes sapidus: 0+year class) were tethered in Back Sound. Seagrass cover in both landscapes ranged from <10 to 100% over 10×10 m areas. Crab survival was correlated with elements of landscape structure and structural complexity in both landscapes, but relationships between crab survival and specific habitat features differed between the two landscapes. In California, juvenile red rock crab survival was negatively correlated with seagrass cover and was positively correlated with seagrass shoot density, and was marginally (P=0.06) negatively correlated with seagrass shoot biomass. In North Carolina, juvenile blue crab survival was positively correlated with log₁₀ patch area and was negatively correlated with seagrass shoot biomass. The results indicate that (1) both seagrass cover and configuration may influence crab survival; (2) seagrass structural complexity influences crab survival independently of landscape structure; (3) the relative influence of cover, configuration and structural complexity on survival likely differ among seagrass landscapes; and (4) increasing structural complexity (e.g. shoot biomass) may not always lead to increased faunal survival. The results correspond to those of recent studies in marine and terrestrial landscapes showing widely variable faunal responses to landscape structure among species, geographic location and through time, and imply that seagrass restoration and conservation strategies should incorporate species-specific responses to habitat structure at multiple scales.     

The Cerrado into-pieces: Habitat fragmentation as a function of landscape use in the savannas of central Brazil

Habitat fragmentation and land conversion by humans for agricultural purposes are constant threats to conservation of biodiversity in the Cerrado biome. These landscapes dominated by agricultural activities became dynamic mosaics, which are formed by different land uses. Thus, understanding how the properties of these mosaics affect species’ persistence is one urgent necessity. In this study, the landscape structure of the Cerrado in Goiás State, Central Brazil, was quantified by the use of fragmentation indices, analysed at the class level. The objective of this study was to assess if land use for crop production or for pasture produces different fragmentation patterns, which can result in different pressures for the Cerrado biodiversity. The study showed that landscapes dominated by crops are more fragmented than landscapes dominated by pastures. These crop-dominated landscapes also presented a smaller number of fragments that could maintain populations of threatened mammal species in Cerrado. Regions with more preserved natural areas are in the northeast of Goiás, where there are rough relief and soil unsuitable for agriculture. Our results indicate that croplands generate a landscape structure more damaging for the conservation of biodiversity in the Cerrado biome. Otherwise, they support the importance to preserve natural remnants, even in areas occupied by agriculture, mainly due to its potential to maintain ecosystem services, and suggest that landscapes dominated by pastures should have more current value for conservation, since they showed larger fragments.     

Patch and landscape characteristics associated with the distribution of woodland amphibians in an agricultural fragmented landscape: an information-theoretic approach

In the Midwestern United States, agricultural landscapes with scattered patches of fragmented forest are common. To investigate the relationship between amphibian distributions and wetland, woodlot, and landscape characteristics, we studied the pond-breeding amphibians within a 15,450-ha plot in rural north-central Ohio. We surveyed 25 woodlots and one area of continuous riparian forest for amphibians, and each surveyed woodland contained at least one temporary wetland. We used Akaike's Information Criterion (AIC) to evaluate the effectiveness of 13 a priori models in predicting total amphibian species richness, anuran richness, caudate richness, and the presence of individual species in woodlots. We identified 13 species of amphibians within the study plot, and every woodlot contained at least one amphibian species. The most important variable in predicting total amphibian and anuran species richness was hydroperiod. For caudates, woodlot edge-to-area ratio, hydroperiod, pH, and ammonia were important characteristics in predicting species richness. Woodlots within agricultural landscapes are important refuges for amphibians.     

Long-term impacts of even-aged timber management on abundance and body condition of terrestrial amphibians in Northwestern California

Conservation needs for amphibians in managed timberlands may differ based upon the species present and the timber harvesting methods employed. Clearcuts have been documented to be detrimental to amphibians but the impacts of associated silvicultural edges and alternative harvesting treatments are not well understood. The primary objective of this study was to determine if amphibian abundances and body condition differed in thinned forests and intact forests, and in clearcuts and associated silvicultural edges. We also examined which environmental attributes were important in explaining observed differences. We sampled clearcuts, silvicultural edges, and adjacent late-seral forests at 10 sites in northwestern California from October 1999 to July 2002. Clearcuts at these sites ranged in age from 6 to 25 years. Five of these forest stands were intact and five had been commercially thinned at least 10 years prior to our study. Amphibian abundances were similar in thinned and unthinned forests, but body condition of the most common species was lower in thinned forests. Abundances of amphibians were nearly twice as high in forests and at silvicultural edges than in clearcuts. Clearcutting at these sites appears to have affected amphibian numbers up to 25 years post-harvest, however, silvicultural edges were suitable habitats for amphibians. While commercial thinning did not reduce amphibian numbers, it is an intermediate treatment followed by clearcutting. Where conservation of amphibians is a concern, even-aged silvicultural systems may not provide the most appropriate method for maintaining viable populations on managed forestlands in the northwestern US.     

黄土沟壑区小流域不同地形条件下土壤锰的形态分布及其有效性

黄土高原复杂的地形条件因改变了土壤的基本性质而成为影响土壤锰素有效性的主要原因,本文研究了黄土沟壑区不同地形条件对锰的形态分布及其有效性的影响。结果表明,研究区土壤有效锰的含量因地形条件不同差异很大,有效锰在土壤剖面呈下降趋势,在塬面土壤含量最高,沟道土壤含量最低。交换态锰在塬面、梯田土壤含量较低,在坡地和沟道土壤含量较高;塬面、梯田和坡地土壤碳酸盐结合态和有机结合态锰的含量高于沟道土壤,氧化物结合态锰的含量显著低于沟道土壤;矿物态锰以塬面土壤最低,沟道和梯田土壤较高,坡地土壤居中。土壤交换态锰是有效锰的直接来源,但因其含量低而对锰的有效性贡献很小;有机结合态锰是有效锰的潜在来源,对锰的有效性贡献最大。     

Population viability of Cape mountain zebra in Gamka Mountain Nature Reserve, South Africa: the influence of habitat and fire

The small Cape mountain zebra population in Gamka Mountain Nature Reserve represents a third of the entire gene pool of this endangered species and is thus vital for it’s conservation. Presently, management of this population is largely hands off, with the belief that it will grow to levels which will allow it to form a source for the mixing of mountain zebra stocks in the future. The growth of this population however, has been slow and we investigated the influence of habitat and fire on this growth. Firstly, we used a diffusion model to perform a population viability analysis. This analysis indicated that the population had a low probability of attaining quasi-extinction in the next 50 years (G = 0.0032). However, our findings indicated that less than 30% of the reserve was suitable for mountain zebra and that the preferred habitat would have to be burnt at unnaturally short intervals to sustain the present growth. We therefore argue that the risk of quasi-extinction to this population is greater than predicted and suggest that management options need to be implemented to reduce this risk. These options include; translocation to another protected area; acquisition of adjacent land; burning preferred habitat at unnaturally short intervals; forming a conservancy with adjacent landowners; leasing cultivated land for pasture. We suggest that only the latter two options are likely to stimulate mountain zebra population growth in the short term and that these should receive immediate attention.     

Fire is a key element in the landscape-scale habitat requirements and global population status of a threatened bird: The Mallee Emu-wren (Stipiturus mallee)

The Mallee Emu-wren (Stipiturus mallee) is a threatened, narrow-range passerine endemic to south-eastern Australia. To inform future conservation measures for this poorly known species, we used ecological niche factor analysis, habitat suitability modelling and distance sampling to determine landscape-scale habitat requirements and estimate the population size. Using GIS software, we integrated digital layers of ecogeographic variables with; (1) presence-only observations to derive and validate a habitat suitability model using ecological niche factor analysis, and (2) distance sampling to determine population distribution and densities across vegetation types. We detected populations in only five of seven reserves which they had occupied in 2000. We estimate the global population size to be 16,821 individuals (range 8431-39, 104), 68% greater than the previously estimated 10,000 individuals, with a single large reserve containing the majority (∼92%) of the global population. The Mallee Emu-wren is a habitat specialist, primarily occurring in mallee-Triodia vegetation that has not been burnt for at least 15 years. The highest densities were in vegetation associations containing at least a 15% cover of Triodia, however, time since the habitat was last burnt was the overriding factor in determining densities. Large-scale wildfires are a pervasive threat to the global status of the Mallee Emu-wren, and the risk to remaining populations is exacerbated by the adverse impact of prolonged drought and the potential for altered fire regimes caused by global warming. Evaluation of the global population status, and the continued wildfire threat warranted recent reclassification of the Mallee Emu-wren from Vulnerable to Endangered according to IUCN Red List categories and criteria.     

Assessing habitat use of the endangered marine mollusc Patella ferruginea (Gastropoda, Patellidae) in northern Africa: preliminary results and implications for conservation

The limpet Patella ferruginea Gmelin, 1791, endemic to the Mediterranean, is the most endangered marine species on the list of the European Council Directive 92/43/EEC on the Conservation of Natural Habitat of Wild Fauna and Flora (1992). Its Mediterranean range has progressively contracted to a few restricted areas and the species is now threatened with extinction. Seventeen stations were sampled along the littoral zone on the coast of Ceuta, North Africa, for quantifying environmental factors (water movement, siltation and suspended solids), and 70 transects were selected for P. ferruginea sampling. The study revealed the presence of a well-established population of P. ferruginea with values of density and size of 0.67 individuals m⁻¹ ±0.96 and 48.94 mm±11.61 respectively (mean±standard error of mean). The biggest densities of P. ferruginea were found inside the harbour of Ceuta. The limpets were more abundant on artificial harbour stones than on natural rocky shores, and the areas under the highest human pressure were characterised by the lowest densities and the smallest specimens probably due mainly to the predation for food and fishing. Human pressure is probably the main contributing factor to the currently endangered status of P. ferruginea. Programmes of environmental education to avoid its collection for fishing, food or for fun as decorative objects should be conducted, and further experimental studies dealing with the reproductive biology of this species are needed to properly assess the future programmes of conservation.     

Grazing as a management tool in dune grasslands: Evidence of soil and scale dependence of the effect of large herbivores on plant diversity

In nature management, the introduction of large herbivores into human-influenced grasslands is thought to be effective to maintain or enhance plant diversity. In order to test the validity of this assumption, we studied the effect of grazing by large herbivores on plant species richness and community heterogeneity across a soil acidity gradient at different spatial scales in dry coastal dune grasslands in western Belgium and north-western France. The effect of grazing on plant richness varied with scale and soil acidity. Grazing had a predominantly positive effect on plant species richness in all habitats at the small scale (0.25 × 0.25 m). However, at site scale (8 × 8 m) it had only positive effects in grasslands with higher soil pH (6-7.4). Similarly, grazing resulted in a homogenization of grassland vegetation at lower pH, while heterogeneity increased with grazing on soil with higher pH. In general, grazing increased the number of rare species, independent of soil pH. The results confirm that the impact of grazing on plant diversity depends on the scale considered and that the effects further depend on soil acidity which was correlated to biomass production at the given soil pH range in this study. Although grazing seems an appropriate management tool to maintain and even enhance plant biodiversity under many circumstances, it may negatively affect plant species richness, where soil resources limit plant biomass production.     

Landscape changes as visible indicators in the social,economic and political process of soil erosion: A case study of the municipality of puebla de valles (Guadalajara Province), Spain

This paper proposes a methodological approach to data gathering as a sound basis for understanding land degradation processes, breaking down barriers of distrust between Government officials and land users, and promoting participation in planning for soil and water conservation. It examines how landscape changes, of value as indicators of erosion, are perceived by land users and can be used in planning. It is proposed that the participation of land users in the construction of questionnaires, data gathering, analysis and writing up of the findings are examples of the participatory initial stages in planning.     

Large-scale changes in the abundance of common higher plant species across Britain between 1978, 1990 and 1998 as a consequence of human activity: Tests of hypothesised changes in trait representation

Presence of higher plant species was recorded in 1455 permanently marked quadrats located across Britain in 1978, 1990 and 1998 in a stratified, random sample of 259 1 km squares. Significant increases and decreases in frequency of each species were summarised as changes in the representation of simple traits, each of which had an established relationship with varying levels of fertility or disturbance. By testing the null hypothesis that the trait values represented among increasing or decreasing species were a random draw from the 1978 species pool, we determined the consistency of botanical change with changes in land-use in different vegetation types and landscape locations across Britain.Overall, 63% of significant changes in species occupancy were decreases and 37% increases. Trait changes were largely consistent with the impact of increased nutrient availability across vegetation types associated with inherently low fertility, such as infertile grassland, heath, bog and moorland. Linear habitats in lowland Britain saw trait changes consistent with secondary succession. Although trait changes were highly consistent with eutrophication in upland vegetation, the identity of the changing species left open the possibility that increased N deposition, sheep grazing intensity and local improvement could all have played a part.Recent changes in common plant species across Britain suggest that objectives for large-scale restoration and maintenance of plant communities must address four problems: (a) the increasing scarcity of stress-tolerant species in lowland species pools, (b) exploitation and maintenance of species populations on habitat fragments and linear features in the lowlands, (c) the spread and persistence of generalist species in upland Britain, (d) systemic nutrient enrichment in both lowland and upland environments.