Species In Fragmented Habitats Research Articles

Mapping conservation priorities for wild yak (Bos mutus) habitats on the Tibetan Plateau, China

The wild yak (Bos mutus) is a cold-tolerant herbivore native to the Tibetan Plateau and has been categorized as vulnerable by the International Union for Conservation of Nature and Natural Resources. Low population densities within currently fragmented habitats and unclear landscape conservation priorities warrant attention. Herein, we employed the maximum entropy (MaxEnt) model using over 900 wild yak occurrence records to model wild yak habitat suitability. Our analysis revealed unprotected wild yak landscapes covering 30.79 % of the habitat area, indicating a conservation gap between protected areas (PAs) and wild yak habitats. To protect metapopulation dynamics and mitigate high risks of poaching, habitat degradation and fragmentation, resource competition, and degenerated genetic characterization of wild yaks in fragmented and degraded habitat, we identified eight habitat patches as landscape conservation units (LCUs) and 14 linkages among the LCUs, enhancing the connectivity between LCUs to decrease negative effects of genetic threats. A centrality analysis demonstrated that Changtang, Arjinshan, and Hoh Xil national nature reserves and their linkages are all critical for the maintenance of habitat connectivity. Here, we suggest that habitat- and LCU-specific conservation strategies should be highlighted during the establishment of PAs and transboundary cooperation. Ultimately, our results can assist conservationists and land managers in comprehending wild yak distribution, movement, and habitat requirements, as well as for the development of effective protection strategies. Furthermore, the combined modeling method (MaxEnt–Zonation–InVEST) could be utilized as a component for identifying conservation priorities and linkages between core patches for species and assessing the efficiency of PAs, core habitats, and corridors in achieving conservation goals. Our study can provide a framework in identifying priority conservation and connectivity between habitat patches to facilitate effectively conservation and genetic resilience for endangered species in fragmented habitats.

Spread of networked populations is determined by the interplay between dispersal behavior and habitat configuration

Predicting the spread of populations across fragmented habitats is vital if we are to manage their persistence in the long term. We applied network theory with a model and an experiment to show that spread rate is jointly defined by the configuration of habitat networks (i.e., the arrangement and length of connections between habitat fragments) and the movement behavior of individuals. We found that population spread rate in the model was well predicted by algebraic connectivity of the habitat network. A multigeneration experiment with the microarthropod Folsomia candida validated this model prediction. The realized habitat connectivity and spread rate were determined by the interaction between dispersal behavior and habitat configuration, such that the network configurations that facilitated the fastest spread changed depending on the shape of the species' dispersal kernel. Predicting the spread rate of populations in fragmented landscapes requires combining knowledge of species-specific dispersal kernels and the spatial configuration of habitat networks. This information can be used to design landscapes to manage the spread and persistence of species in fragmented habitats.

Fragmented habitat compensates for the adverse effects of genetic bottleneck

In the face of the human-caused biodiversity crisis, understanding the theoretical basis of conservation efforts of endangered species and populations has become increasingly important. According to population genetics theory, population subdivision helps organisms retain genetic diversity, crucial for adaptation in a changing environment. Habitat topography is thought to be important for generating and maintaining population subdivision, but empirical cases are needed to test this assumption. We studied Saimaa ringed seals, landlocked in a labyrinthine lake and recovering from a drastic bottleneck, with additional samples from three other ringed seal subspecies. Using whole-genome sequences of 145 seals, we analyzed the distribution of variation and genetic relatedness among the individuals in relation to the habitat shape. Despite a severe history of genetic bottlenecks with prevalent homozygosity in Saimaa ringed seals, we found evidence for the population structure mirroring the subregions of the lake. Our genome-wide analyses showed that the subpopulations had retained unique variation and largely complementary patterns of homozygosity, highlighting the significance of habitat connectivity in conservation biology and the power of genomic tools in understanding its impact. The central role of the population substructure in preserving genetic diversity at the metapopulation level was confirmed by simulations. Integration of genetic analyses in conservation decisions gives hope to Saimaa ringed seals and other endangered species in fragmented habitats.

Small mammals respond to extreme habitat fragmentation in the Brazilian Atlantic Forest according to the landscape continuum model

There are several factors that can determine the pattern of habitat use by species in fragmented habitats. Here we investigated the applicability of the continuum model, which predicts that habitat use varies in a continuous way and not categorically like habitat and not-habitat, for small mammal habitat use. We also investigated if habitat-related guilds of species can change their habitual pattern of habitat use in an extremely altered landscape. This study was conducted in a highly fragmented area of Atlantic Forest in Brazil, where forest fragments are immersed in a pasture matrix. We captured mammals using pitfall traps placed in six sampling sites. In each of these sites, we selected four of five different habitats that were available in the local landscape: high-elevation forests without stream, low-elevation forests with stream, forest edges, pasture matrix without stream, and pasture matrix with stream. This study revealed that the theory of the continuum model can be applied for investigating patterns of use of fragmented forests by small mammals. We also observed, in general, low tolerance of forest species to the most disturbed habitat, the pasture matrix without stream. However, a small increase in the complexity of the vegetation, as observed in riparian pasture matrices, seems to mitigate this negative effect, making the abundance, richness, and diversity of small mammals comparable to forest habitats. These results indicate that the presence of streams may facilitate the use of the pasture matrix, contributing for the maintenance of the original community in the fragmented forest.

Effects of population size and structure on reproductive success of a Pistacia chinensis population

The Thousand-Island Lake region, a highly fragmented landscape in Zhejiang Province, China, was selected as a natural laboratory to examine how the reproductive success of a Pistacia chinensis (Anacardiaceae) population was affected by population size and structure. Thirty local island populations and one large mainland population were studied. Pollen density during the flowering season differed significantly among four differently sized populations. The number of pollen grains on the female stigma was positively related to the size of the male population. The 100-seed weight and seed-setting rate of fruits were also significantly correlated with the size of the male population. These results indicated that pollen limitation caused by a reduction in the size of the male population in this fragmented landscape was the main factor that reduced the female reproductive success of the P. chinensis population. In the small population, artificial pollination resulted in significant increases in the 100-seed weight and the seed-setting rate compared with those of untreated individuals. However, artificial pollination did not significantly increase the 100-seed weight or seed-setting rate in the large population. Pollen limitation caused by a reduction in the size of male population was the main factor that reduced the female reproductive success of the P. chinensis population in this fragmented landscape. Thus, the size of populations is an important factor in developing conservation and protection strategies for dioecious plant species in fragmented habitats.

Landscape connectivity among remnant populations of guanaco (Lama guanicoe Müller, 1776) in an arid region of Chile impacted by global change.

Connectivity between populations plays a key role in the long-term persistence of species in fragmented habitats. This is of particular concern for biodiversity preservation in drylands, since water limited landscapes are typically characterized by little suitable habitat cover, high habitat fragmentation, harsh matrices, and are being rapidly degraded at a global scale. In this study, we modelled landscape connectivity between 11 guanaco Lama guanicoe populations in Chile’s arid Norte Chico, a region that supports the last remnant coastal populations of this emblematic herbivore indigenous to South America. We produced a habitat suitability model to derive a regional surface resistance map, and used circuit theory to map functional connectivity, investigate the relative isolation between populations, and identify those that contribute most to the patch connectivity network. Predicted suitable habitat for L. guanicoe represented about 25% of the study region (i.e., 29,173 km2) and was heterogeneously distributed along a continuous stretch along the Andes, and discontinuous patches along the coast. As a result, we found that high connectivity current flows in the mid and high Andes formed a wide, continuous connectivity corridor, enabling connectivity between all high Andean populations. Coastal populations, in contrast, were more isolated. These groups demonstrate no inter-population connectivity between themselves, only with higher altitude populations, and for two of them, animal movement was linked to the effectiveness of wildlife crossings along the Pan-American highway. Our results indicate that functional connectivity is an issue of concern for L. guanicoe in Chile’s Norte Chico, implying that future conservation and management plans should emphasize strategies aimed at conserving functional connectivity between coastal and Andean populations, as well as the protection of habitat patches likely to act as stepping stones within the connectivity network.

Offspring performance and recruitment of the pioneer tree Acacia caven (Fabaceae) in a fragmented subtropical dry forest

AbstractThe process of habitat fragmentation results in the breaking apart of originally continuous habitats, causing multiple changes in biotic and abiotic interactions. Alterations in resource availability and in mutualistic and antagonistic plant–animal interactions may impact plant offspring quantity and quality. Currently, several old fragmented systems evidence a process of flora homogenization, where shade‐tolerant species are replaced by pioneer light‐demanding species. Notably, the relationship between quantity and quality parameters of plant offspring production and the successful recruitment of pioneer species in fragmented forests has been poorly explored. Here, we assess population size, sapling recruitment and offspring performance of one of the most widespread tree species of subtropical South America, the native pioneer Acacia caven (Fabaceae). Population size of adults and saplings increased from small to continuous forests, whereas the sapling recruitment per adult tree (sapling/adult ratio) showed no significant differences among forests of different size. Seedling performance was negatively related to forest area and population size, implying potential superior competitive ability of seedlings produced in smaller populations compared to larger ones. Our results show that A. caven is resilient to habitat fragmentation effects, which may be ascribed to a set of advantageous ecological traits such as outcrossing, massive flowering, generalist pollination, drought resistance, rapid growth and re‐sprouting. Thus, this pioneer tree benefits from the availability of vacant sites and resources released by declining plant populations of other species, eventually becoming the dominant species in fragmented habitats. Pioneer native plant species with ecological traits such as A. caven may represent the silent successful survivors and new colonizers of fragmented habitats, the ubiquitous landscapes of the future.

Limited gene flow and high genetic diversity in the threatened Betic midwife toad (Alytes dickhilleni): evolutionary and conservation implications

Habitat fragmentation may involve a loss of genetic diversity and increments the vulnerability to species persistence. It could be a particular issue when coupled with other negative factors as the predicted climatic changes and the emergence of infectious diseases. In Southern Iberian Peninsula several endemic amphibian species have confined and fragmented distributions, including the Betic midwife toad Alytes dickhilleni. Herein, we present the first range-wide assessment of genetic diversity and structure in this species, using mitochondrial and microsatellite data. A mitochondrial fragment of the ND4 gene was amplified for 65 individuals and a set of 20 microsatellite loci, specifically developed for this species, was genotyped for 490 individuals from several sampling sites distributed across the species entire range. While both markers revealed high genetic diversity, only for microsatellites a marked genetic substructure was apparent. Our results evidence low levels of gene flow, suggesting the persistence of the species in fragmented habitats for several generations and a very limited connectivity between most of mountain ranges. The high diversity within A. dickhilleni populations could help to respond to the emergence of new diseases and to the predicted effects of climatic changes in Southeastern Iberian Peninsula. We hypothesize that the lack of gene flow is due to the absence of available breeding habitats and recommend that future management efforts of A. dickhilleni include the creation and maintenance of aquatic breeding habitats in a way that most of genetic diversity is preserved.

Pollinator limitation and the effect of breeding systems on plant reproduction in forest fragments

Reproduction of plants in fragmented habitats may be limited because of lower diversity or abundance of pollinators, and/or variation in local plant density. We assessed natural fruit set and pollinator limitation in ten species of woody plants in natural and restored fragments in the Pondicherry region of southern India, to see whether breeding system of plants (self-compatible and self-incompatible) affected fruit set. We tested whether the number of flowering individuals in the fragments affected the fruit set and further examined the adult and sapling densities of self-compatible (SC) and self-incompatible (SI) species. We measured the natural level of fruit set and pollinator limitation (calculated as the difference in fruit set between hand cross-pollinated and naturally pollinated flowers). Our results demonstrate that there was a higher level of pollinator limitation and hence lower levels of natural fruit set in self-incompatible species as compared to self-compatible species. However, the hand cross-pollinated flowers in SC and SI species produced similar levels of fruit set, further indicating that lower fruit set was due to pollinator limitation and not due to lack of cross-compatible individuals in the fragments. There was no significant relation between number of flowering individuals and the levels of natural fruit set, except for two species Derris ovalifolia, Ixora pavetta. In these species the natural fruit set decreased with increasing population size, again indicating pollinator limitation. The adult and sapling densities in self-compatible species were significantly higher than in self-incompatible species. These findings indicate that the low reproductive output in self-incompatible species may eventually lead to lower population sizes. Restoration of pollinator services along with plant species in fragmented habitats is important for the long-term conservation of biodiversity.

Population genetics at three spatial scales of a rare sponge living in fragmented habitats

BackgroundRare species have seldom been studied in marine habitats, mainly because it is difficult to formally assess the status of rare species, especially in patchy benthic organisms, for which samplings are often assumed to be incomplete and, thus, inappropriate for establishing the real abundance of the species. However, many marine benthic invertebrates can be considered rare, due to the fragmentation and rarity of suitable habitats. Consequently, studies on the genetic connectivity of rare species in fragmented habitats are basic for assessing their risk of extinction, especially in the context of increased habitat fragmentation by human activities. Sponges are suitable models for studying the intra- and inter-population genetic variation of rare invertebrates, as they produce lecitotrophic larvae and are often found in fragmented habitats.ResultsWe investigated the genetic structure of a Mediterranean sponge, Scopalina lophyropoda (Schmidt), using the allelic size variation of seven specific microsatellite loci. The species can be classified as "rare" because of its strict habitat requirements, the low number of individuals per population, and the relatively small size of its distribution range. It also presents a strong patchy distribution, philopatric larval dispersal, and both sexual and asexual reproduction. Classical genetic-variance-based methods (AMOVA) and differentiation statistics revealed that the genetic diversity of S. lophyropoda was structured at the three spatial scales studied: within populations, between populations of a geographic region, and between isolated geographic regions, although some stochastic gene flow might occur among populations within a region. The genetic structure followed an isolation-by-distance pattern according to the Mantel test. However, despite philopatric larval dispersal and fission events in the species, no single population showed inbreeding, and the contribution of clonality to the population makeup was minor (only ca. 4%).ConclusionsThe structure of the S. lophyropoda populations at all spatial scales examined confirms the philopatric larval dispersal that has been reported. Asexual reproduction does not seem to play a relevant role in the populations. The heterozygote excess and the lack of inbreeding could be interpreted as a hitherto unknown outcrossing strategy of the species. The envisaged causes for this strategy are sperm dispersal, a strong selection against the mating of genetically related individuals to avoid inbreeding depression or high longevity of genets combined with stochastic recruitment events by larvae from other populations. It should be investigated whether this strategy could also explain the genetic diversity of many other patchy marine invertebrates whose populations remain healthy over time, despite their apparent rarity.

Conserving Species in Fragmented Habitats: Population Dynamics of the Flat-Tailed Horned Lizard, Phrynosoma mcallii

Understanding what drives population dynamics is a key tool for conserving species in fragmented habitats. We evaluate hypotheses explaining fluctuations in populations of flat-tailed horned lizards, Phrynosoma mcallii, in the Coachella Valley, California. This species has the smallest range of any horned lizard in the United States. Within portions of its range, there are potentially conflicting activities, such as suburban development, agriculture, off-road recreation, and activities along the international border. We analyzed fluctuations in populations with respect to resource variables during 2001–2007. Unlike results for other desert lizards, population dynamics of horned lizards did not correlate positively with rainfall. Compaction of sand, abundance of harvester ants, annual rainfall, and annual cover of plants all contributed to parsimonious models, but contributions of these variables varied between year and criteria for the dependent variable (occurrence of horned lizard). We also examined demographic characteristics of increasing and decreasing populations. Rates of growth of hatchlings and age at sexual maturity provided a mechanistic explanation for the observed population dynamics, and were consistent with a food-limiting hypothesis.

Spatial (a)synchrony in population fluctuations of five plant species in fragmented habitats

Abstract The spatial scale at which populations show synchronous temporal fluctuations in abundance, relative to the spatial scale over which they can disperse, may influence the persistence of local and regional populations. There have been frequent demonstrations of spatial synchrony in population dynamics of animal populations. But few studies have investigated the degree of spatial synchrony in less mobile taxa, e.g. plants, where life history, dispersal and interaction with the environment would be different due to a sessile phase. This study has during three years investigated the synchrony in local population size changes in four short-lived species, and during a nine-year period for one long-lived species, in a semi-natural grassland landscape in southern Sweden. The spatial scale of this study was less than 15 km, which is quite small in comparison with other studies, but the temporal scale was of similar magnitude as the few studies on plant abundances and synchrony. When using detrended estimates of population size change, a significant pattern of decreasing synchrony with increasing distance was found for the two short-lived species that were most confined to manage semi-natural grasslands. Spatial synchrony was detected up to a few km. However, the species displayed synchrony in different years. The degree of synchrony can thus vary considerably across years and among species. Spatially autocorrelated weather conditions could partly explain the spatial scale of synchrony found during certain time intervals. However, the prevailing asynchrony suggests that local factors dominate the dynamics of the populations at the investigated scale.

Limited genetic diversity and high differentiation among the remnant adder (Vipera berus) populations in the Swiss and French Jura Mountains

Although the adder (Vipera berus) has a large distribution area, this species is particularly threatened in Western Europe due to high habitat fragmentation and human persecution. We developed 13 new microsatellite markers in order to evaluate population structure and genetic diversity in the Swiss and French Jura Mountains, where the species is limited to only a few scattered populations. We found that V. berus exhibits a considerable genetic differentiation among populations (global FST = 0.269), even if these are not geographically isolated. Moreover, the genetic diversity within populations in the Jura Mountains and in the less perturbed Swiss Alps is significantly lower than in other French populations, possibly due to post-glacial recolonisation processes. Finally, in order to minimize losses of genetic diversities within isolated populations, suggestions for the conservation of this species in fragmented habitats are proposed.

Potential impacts of global climate change on freshwater fisheries

Despite uncertainty in all levels of analysis, recent and long-term changes in our climate point to the distinct possibility that greenhouse gas emissions have altered mean annual temperatures, precipitation and weather patterns. Modeling efforts that use doubled atmospheric CO2 scenarios predict a 1–7°C mean global temperature increase, regional changes in precipitation patterns and storm tracks, and the possibility of “surprises” or sudden irreversible regime shifts. The general effects of climate change on freshwater systems will likely be increased water temperatures, decreased dissolved oxygen levels, and the increased toxicity of pollutants. In lotic systems, altered hydrologic regimes and increased groundwater temperatures could affect the quality of fish habitat. In lentic systems, eutrophication may be exacerbated or offset, and stratification will likely become more pronounced and stronger. This could alter food webs and change habitat availability and quality. Fish physiology is inextricably linked to temperature, and fish have evolved to cope with specific hydrologic regimes and habitat niches. Therefore, their physiology and life histories will be affected by alterations induced by climate change. Fish communities may change as range shifts will likely occur on a species level, not a community level; this will add novel biotic pressures to aquatic communities. Genetic change is also possible and is the only biological option for fish that are unable to migrate or acclimate. Endemic species, species in fragmented habitats, or those in east–west oriented systems will be less able to follow changing thermal isolines over time. Artisanal, commercial, and recreational fisheries worldwide depend upon freshwater fishes. Impacted fisheries may make it difficult for developing countries to meet their food demand, and developed countries may experience economic losses. As it strengthens over time, global climate change will become a more powerful stressor for fish living in natural or artificial systems. Furthermore, human response to climate change (e.g., increased water diversion) will exacerbate its already-detrimental effects. Model predictions indicate that global climate change will continue even if greenhouse gas emissions decrease or cease. Therefore, proactive management strategies such as removing other stressors from natural systems will be necessary to sustain our freshwater fisheries.

EFFECTS OF SUCCESSIONAL DYNAMICS ON METAPOPULATION PERSISTENCE

The classical (Levins) metapopulation scenario envisions a species persisting in a network of habitat patches through a balance between frequent local (within-patch) extinctions and recolonizations. Although this is the dominant paradigm for species in fragmented habitats, empirical support is limited, and it has been argued that very restrictive conditions on migration rates are required: high enough for recolonization to balance extinctions, but low enough that local populations do not fluctuate in synchrony. Through simulation and analysis of a stochastic spatial model, we argue that the likelihood of persistence via the classical scenario is strongly affected by some basic properties of within-patch successional dynamics whose importance has not been emphasized in metapopulation theory: the distribution of successional stage durations, and whether patches are “refractory” vs. immediately available for recolonization after an extinction has occurred. These properties are tied to the biological causes o...

Avian persistence in fragmented rainforest.

What factors determine the persistence of species in fragmented habitats? To address this question, we studied the relative impacts of forest deterioration and fragmentation on bird species in 12 rainforest fragments in Kenya, combining 6 years of individual capture-recapture data with measurements of current captures and museum specimens. Species mobility, as estimated from species-specific dispersal rates, and tolerance to habitat deterioration, as estimated from change in fluctuating asymmetry with increasing habitat disturbance, explained 88% of the variation in patch occupancy among eight forest bird species. Occupancy increased with mobility and with tolerance to deterioration, where both variables contributed equally to this relationship. We conclude that individual-level study, such as of dispersal behavior and phenotypic development, can predict patterns of persistence at the species level. More generally, for conservation tactics to stand a high chance of success, they should include action both within sites, to minimize habitat deterioration, and across landscapes, to maximize dispersal.

Order and disorder in ectoparasite communities: the case of congeneric gill monogeneans ( Dactylogyrus spp.)

The measure of order and disorder in the distribution of species in fragmented habitats proposed by Atmar and Patterson (Oecologia, 96 (1993) 373–82) was applied to investigate nested patterns of Dactylogyrus species parasitising the gills of roach. Organisation in dactylogyrid assemblages was investigated at three levels: (1) host populations between localities; (2) local host populations over seasons; and (3) individual hosts over one season within a local host population. Dactylogyrid assemblages showed nested patterns when analyses were conducted at the level of localities (among host populations) and at the level of seasons (among host populations within localities). The analysis at the level of hosts (infracommunities of parasites) revealed that nested pattern is not common. We suggest that nestedness may have a variety of causes and does not necessarily imply competition.

The effect of the Argentine ant on the threatened valley elderberry longhorn beetle

The extinction of species from island and fragmented mainland ecosystems due to exotic species has resulted in a significant loss of species over the last 300 years (Courtenay and Meffe 1989; Diamond 1984; Minckley and Deacon 1991; Savidge, 1987; Simberloff 1981; Simberloff et al. 1997; Sinclair et al. 1998). Here, I present evidence suggesting an impact of an introduced species, the Argentine ant Linepithema humile Mayr (Hymenoptera: Formicidae) on the threatened valley elderberry longhorn beetle (VELB) Desmocerus californicus dimorphus (Coleoptera: Cerambycidae). Holway and others have shown that a loss of intraspecific aggression among nests in invaded areas has contributed to the success of Argentine ant (Holway 1998b; Holway et al. 1998). L. humile has been spreading in riparian woodlands adjacent to permanent streams since its introduction into California circa 1907 (Holway 1998; Ward 1987). These riparian woodlands are important habitat for blue elderberry Sambucus mexicana (Caprifoliacae), the host plant of the VELB. Because the VELB is an obligate specialist on elderberry, any further reduction in the amount or quality of suitable riparian woodland habitat could significantly impact the VELB. Riparian woodland habitat has been largely reduced and severely fragmented (from a preGold Rush 775,000 acres to approximately 12,000) by flood control, intensive agricultural production, and urbanization (Barbour et al. 1993; Kucera and Barrett 1995; Smith 1980). The potential interaction between VELB and L. humile may be most important in fragmented riparian woodland habitat, such as along Putah Creek and American River watersheds. Where it has spread, the Argentine ant has impacted native ants and other ground-dwelling arthropods (Cole et al. 1992; Holway 1998a, b; Human and Gordon, 1997; Ward 1987). For example, Suarez et al. (1998) found that the Argentine ant had a greater impact on native ant species in fragmented habitats than in more continuous habitat. The extreme rarity and threatened status of the VELB does not allow for manipulative experimentation. Therefore, using the ‘natural experiment’ of L. humile invasions, I explored whether the presence of L. humile was correlated with the absence of VELB. Specifically, I use a comparative approach to determine whether the invasion of L. humile has impacted the distribution of D. californicus dimorphus along the Putah Creek and the American River watersheds.

Dispersal and spatial scale affect synchrony in spatial population dynamics

A large body of theoretical studies has shown that synchrony among populations is critical for the long‐term persistence of species in fragmented habitats. Although the effects of dispersal and environmental factors on synchrony have been investigated theoretically, empirical studies of these relationships have been lacking. We explored the interplay between environmental and demographic factors (fecundity, survival, dispersal) on population synchrony for 53 species of birds. We show that the interspecific differences in mean synchrony were determined by global environmental factors whose action was probably mediated by the abundance of each species. After removing the effect of these global factors on synchrony, the residual synchrony was strongly correlated with dispersal distance. The relationship between dispersal and synchrony was stronger for the species nesting in wet habitats than for those nesting in dry habitats. Our results indicate that different factors synchronize bird populations at different spatial scales, thus highlighting the role of scale in understanding spatial population dynamics and extinction risks.

Effects of forest fragmentation on the winter body condition and population parameters of an habitat generalist, the wood mouse Apodemus sylvaticus: a test of hypotheses

Three main causal hypotheses have been proposed to explain the inverse relationships between habitat patch size and density of gen- eralist mouse species in fragmented habitats: 1) enhanced habitat conditions as habitat patch size decreases; 2) inhibited emigration of excess indi- viduals in small and isolated habitat patches; and 3). reduced territoriality in small patches because they are occupied temporarily by non- reproductive individuals. From the mechanism underlying each hypothesis, we derived predictions on the effects of fragment size on the body condition of individuals (measured both as absolute body size and as body mass relative to body size) and some demographic parameters of mouse populations redated to reproductive output (sex-ratio and proportions of sexually active and recently-born individuals), and we tested such predic- tions with data from wood mice Apodemus sylvaticus wintering in three Spanish forest archipelagos in which the inverse relationship between for- est patch size and mouse abundance had been previously proven. No differences in average body size or in average body mass relative to body size were detected among fragments. Mouse populations wintering in small fragments showed more male-biased sex-ratios, a larger proportion of sex- ually active adults and fewer juveniles as compared to mouse populations wintering in large fragments nearby. Results clearly rejected the third hypothesis and did not support the second one. It thus seemed that habitat conditions for mice improved as forest fragment size decreased, although the expected positive effects on individuals could have been prevented by relaxed territoriality and increased food resource depletion by denser mouse populations. Bearing in mind the negative effects of dense wood mice populations on the distribution, abundance and population dynamics of forest species, this apparent enhancement of habitat conditions for mice in small forest fragments could have far-reaching conse- quences for the long-term persistence of such fragments. 0 Elsevier, Paris