Predicting which species will benefit from corridors in fragmented landscapes from population growth models

Connecting isolated patches of habitat in fragmented landscapes with corridors is a popular conservation strategy. This strategy is also controversial in large part because of uncertainty about what characteristics of a species and its environment promote corridor use. In this article we address the question, For what types of species will populations benefit from corridors? We asked this question using a model of two logistically growing populations connected by migration in which both emigration and migration success were determined by the presence or absence of a corridor. We found that in the short run (e.g., during recovery from disaster), corridors are most effective for species with fast-growing populations that have low survivorship when dispersing through unsuitable (matrix) habitat. We also found that emigration rates and habitat-specific mortality rates are key determinants of the effects of corridors on population size. In the long term, corridors are most likely to benefit species with slow-growing populations that have low survivorship when dispersing through matrix habitat. Our results confirm the major conclusions from previous empirical studies of corridor benefits. However, most studies fail to consider the most appropriate questions to determine the potential benefits of habitat corridors. First, what is the time scale of the conservation goal? Corridors have positive effects on different suites of species in the short and long term. Second, is the major threat of local extinction due to sustained population decline or boom-bust cycles? Third, what is the migration rate through the matrix? Fourth, what fraction of migrants dispersing through the matrix successfully immigrate to another patch?     

The effects of habitat fragmentation via forestry plantation establishment on spatial genotypic structure in the small marsupial carnivore, Antechinus agilis

Dispersal is an important influence on species' distributions, patch colonization and population persistence in fragmented habitat. We studied the impacts of habitat fragmentation resulting from establishment of an exotic pine plantation on dispersal of the marsupial carnivore, Antechinus agilis. We applied spatial analyses of individual multilocus microsatellite genotypes and mitochondrial haplotypes to study patterns of gene flow in fragmented habitat and natural habitat 'control' areas, and how this is affected by the spatial dispersion of habitat patches, the presence of corridors and a 'mainland' source of migrants. Spatial analysis of molecular variance and partial Mantel tests confirmed the absence of cryptic barriers to gene flow in continuous habitat, which if present would confound the comparison of genetic structures in fragmented vs. unfragmented habitats. Spatial genotypic structure suggested that although dispersal was male-biased in both habitat types, fragmentation restricted dispersal of males more than that of females and the degree of restriction of male dispersal was dependent on the geographical isolation of the patch. The scale of positive genotypic structure in fragmented habitat was restricted to the two closest patches for females and the three closest patches for males. Our results provide evidence for significantly increased gene flow through habitat corridors relative to that across the matrix and for significantly lower gene flow between 'mainland' unfragmented habitat and habitat patches relative to that within either habitat type, suggesting a behavioural barrier to crossing habitat interfaces.     

Forest fragmentation, the decline of an endangered primate, and changes in host-parasite interactions relative to an unfragmented forest

Forest fragmentation may alter host-parasite interactions in ways that contribute to host population declines. We tested this prediction by examining parasite infections and the abundance of infective helminths in 20 forest fragments and in unfragmented forest in Kibale National Park, Uganda. Over 4 years, the endangered red colobus (Procolobus rufomitratus) declined by 20% in fragments, whereas the black-and-white colobus (Colobus guereza) in fragments and populations of both colobines in unfragmented forest remained relatively stable. Seven nematodes (Strongyloides fulleborni, Strongyloides stercoralis, Oesophagostomum sp., an unidentified strongyle, Trichuris sp., Ascaris sp., and Colobenterobius sp.), one cestode (Bertiella sp.), and three protozoans (Entamoeba coli, Entamoeba histolytica/dispar, and Giardia sp.) were detected. Infection prevalence and the magnitude of multiple infections were greater for red colobus in fragmented than in unfragmented forest, but these parameters did not differ between forests for black-and-white colobus. Infective-stage colobus parasites occurred at higher densities in fragmented compared with unfragmented forest, demonstrating greater infection risk for fragmented populations. There was little evidence that the nature of the infection was related to the size of the fragment, the density of the host, or the nature of the infection in the other colobine, despite the fact that many of the parasites are considered generalists. This study suggests that forest fragmentation can alter host-parasite dynamics and demonstrates that such changes can correspond with changes in host population size in forest fragments.     

Regulation of root vole population dynamics by food supply and predation: a two-factor experiment

This paper reports the effects of food supply, predation and the interaction between them on the population dynamics of root voles, Microtus oeconomus , by adopting factorial experiments in field enclosures. This two-factor experiment proved the general hypothesis that food supply and predation had independent and additive effects on population dynamics of root voles. The experimental results proved the following predictions: (1) predation reduced population density and recruitment significantly; (2) food supply increased population density; (3) predation and food supply influenced spacing behavior of root voles separately and additively: Exposure to predation reduced long movements of root voles between trapping sessions; additional food supply reduced aggression level and home range size of root voles. Less movement of individuals that exposed to predators possibly reduced their opportunity of obtaining food and lessened population survival rate, which led population density to decrease. Smaller home range and lower aggression level could make higher population density tolerable. The interactive effect of predation and food on home range size was highly significant (P=0.0082<0.01). The interactive effect of food and predation on dispersal rate was significant (P<0.01). From the experimental results, we conclude that the external factors (predation, food supply) were more effective than internal factors (spacing behavior) in determining population density of root voles – under the most favorable external conditions (−P, +F treatment), the mean density and mean recruitment of root vole population was the highest; under the most unfavorable external conditions (+P, −F treatment), the mean density and mean recruitment of root vole population was the lowest.     

Metapopulation extinction in fragmented landscapes: using bacteria and protozoa communities as model ecosystems

Extinction is notoriously difficult to study because of the long timescales involved and the difficulty in ascertaining that extinction has actually occurred. The effect of habitat subdivision, or fragmentation, on extinction risk is even harder to study, as it requires copious replication of habitat patches on large spatial scales and control of area effects between treatments. I used simple small-scale communities of bacteria and protozoa to study extinction in response to habitat loss and habitat fragmentation. I studied several different community configurations, each with three trophic levels. Unlike most metapopulation studies (experimental as well as theoretical), which have tended to deal with inherently unstable species interactions, I deliberately used community configurations that were persistent in large stock cultures. I recorded the time to extinction of the top predator in single habitat patches of different sizes and in fragmented systems with different degrees of subdivision but the same amount of available habitat. Habitat loss reduced the time to extinction of isolated populations. Fragmented systems went extinct sooner than corresponding unfragmented (continuous) systems of the same overall size. Unfragmented populations persisted longer than fragmented systems (metapopulations) with or without dispersal corridors between subpopulations. In fact, fragmented systems where the fragments were linked by dispersal corridors went extinctly significantly sooner than those where subpopulations were completely isolated from each other. If these results extend to more "natural" systems, it suggests a need for caution in management programs that emphasize widespread establishment of wildlife corridors in fragmented landscapes.     

Estimates of annual survival,growth, and recruitment of a white-tailed ptarmigan population in Colorado over 43 years

Long-term datasets for high-elevation species are rare, and considerable uncertainty exists in understanding how high-elevation populations have responded to recent climate warming. We present estimates of demographic vital rates from a 43-year population study of white-tailed ptarmigan (Lagopus leucura), a species endemic to alpine habitats in western North America. We used capture-recapture models to estimate annual rates of apparent survival, population growth, and recruitment for breeding-age ptarmigan, and we fit winter weather covariates to models in an attempt to explain annual variation. There were no trends in survival over the study period but there was strong support for age and sex effects. The average rate of annual growth suggests a relatively stable breeding-age population ( \( \bar{\lambda } \)  = 1.036), but there was considerable variation between years for both population growth and recruitment rates. Winter weather covariates only explained a small amount of variation in female survival and were not an important predictor of male survival. Cumulative winter precipitation was found to have a quadratic effect on female survival, with survival being highest during years of average precipitation. Cumulative winter precipitation was positively correlated with population growth and recruitment rates, although this covariate only explained a small amount of annual variation in these rates and there was considerable uncertainty among the models tested. Our results provide evidence for an alpine-endemic population that has not experienced extirpation or drastic declines. However, more information is needed to understand risks and vulnerabilities of warming effects on juveniles as our analysis was confined to determination of vital rates for breeding-age birds.     

MICROGEOGRAPHIC VARIATION IN MITOCHONDRIAL DNA OF MEADOW VOLES (MICROTUS PENNSYLVANICUS) IN RELATION TO POPULATION DENSITY

We examined mitochondrial-DNA (mtDNA) sequence heterogeneity on four adjacent trapping grids in an island population of meadow voles (Microtus pennsylvanicus) at two different population densities. Four restriction endonucleases revealed 20 different mtDNA composite phenotypes in samples totaling 198 meadow voles. There were significant heterogeneities in the distribution of four common mtDNA composite phenotypes among the four trapping grids, suggesting that there is population subdivision on a fine scale. Genetic distances between grids, mtDNA diversity within grids, and G_ST also varied during the study period. We found a decrease in genetic distance and an increase in diversity when the population density was high and vice versa when the population density was low. When population density was high, the coefficient of gene differentiation was smaller than the same coefficient observed when the population density was low. These changes in population subdivision and diversity are consistent with theoretical expectations of population structure in which effective female population size and dispersal are the critical variables. The data also support the hypothesis of maintenance of mtDNA diversity by population subdivision, rapid population growth rate, and dispersal.     

Estimating Population Growth and Recruitment Rates Across the Range of American Common Eiders

Sound management of bird populations rests upon an adequate understanding of their population dynamics. Our study evaluated recruitment and population growth rates of 14 American common eider (Somateria mollissima dresseri) colonies from Labrador, Nova Scotia, Quebec, Canada, and Maine, USA, during various periods between 1970 and 2019. We used Pradel mark-recapture models to estimate colony-specific growth rates and the relative contributions of survival and recruitment on growth. We also validated this approach using annual nest counts (~8,000 pairs) conducted between 2003 and 2019 during down harvest operations in 3 colonies located in the Saint Lawrence estuary in Quebec. There was generally a good agreement between estimates derived using the 2 approaches. We considered that capture-recapture data were suitable to estimate population trends of common eiders in other colonies, especially for colonies where accurate nest monitoring is impaired by dense vegetation. The breeding abundance declined at major colonies in Maine and Nova Scotia and increased or was stable in Quebec and Labrador. Female survival contributed the most to population growth, but variation in recruitment among colonies was more important than variation in survival to explain population growth. Management measures should thus strive to maximize local recruitment in colonies with declining populations. The assumption that apparent survival probabilities were homogeneous throughout an individual capture history was violated at several colonies in Quebec and Labrador. Using recaptures and band recoveries, we showed that the lower apparent survival for newly marked individuals compared to females that had been recaptured at least once was caused by a difference in site fidelity rather than true survival. But <1% of recaptured females dispersed to another colony for breeding, indicating that the lower site fidelity could be related to heterogeneity in capture probability among individuals. © 2021 The Wildlife Society.     

Density-dependent reproduction causes winter crashes in a common vole population

Common voles in western France exhibit three-year population cycles with winter crashes after large outbreaks. During the winter of 2011–2012, we monitored survival, reproduction, recruitment and population growth rate of common voles at different densities (from low to outbreak densities) in natura to better understand density dependence of demographic parameters. Between October and April, the number of animals decreased irrespective of initial density. However, the decline was more pronounced when October density was higher (loss of ≈54 % of individuals at low density and 95 % at high density). Using capture-mark-recapture models with Pradel's temporal symmetry approach, we found a negative effect of density on recruitment and reproduction. In contrast, density had a slightly positive effect on survival indicating that mortality did not drive the steeper declines in animal numbers at high density. We discuss these results in a population cycle framework, and suggest that crashes after outbreaks could reflect negative effects of density dependence on reproduction rather than changes in mortality rates.     

A Comparison of Grizzly Bear Demographic Parameters Estimated from Non-Spatial and Spatial Open Population Capture-Recapture Models

Capture-recapture studies are frequently used to monitor the status and trends of wildlife populations. Detection histories from individual animals are used to estimate probability of detection and abundance or density. The accuracy of abundance and density estimates depends on the ability to model factors affecting detection probability. Non-spatial capture-recapture models have recently evolved into spatial capture-recapture models that directly include the effect of distances between an animal’s home range centre and trap locations on detection probability. Most studies comparing non-spatial and spatial capture-recapture biases focussed on single year models and no studies have compared the accuracy of demographic parameter estimates from open population models. We applied open population non-spatial and spatial capture-recapture models to three years of grizzly bear DNA-based data from Banff National Park and simulated data sets. The two models produced similar estimates of grizzly bear apparent survival, per capita recruitment, and population growth rates but the spatial capture-recapture models had better fit. Simulations showed that spatial capture-recapture models produced more accurate parameter estimates with better credible interval coverage than non-spatial capture-recapture models. Non-spatial capture-recapture models produced negatively biased estimates of apparent survival and positively biased estimates of per capita recruitment. The spatial capture-recapture grizzly bear population growth rates and 95% highest posterior density averaged across the three years were 0.925 (0.786–1.071) for females, 0.844 (0.703–0.975) for males, and 0.882 (0.779–0.981) for females and males combined. The non-spatial capture-recapture population growth rates were 0.894 (0.758–1.024) for females, 0.825 (0.700–0.948) for males, and 0.863 (0.771–0.957) for both sexes. The combination of low densities, low reproductive rates, and predominantly negative population growth rates suggest that Banff National Park’s population of grizzly bears requires continued conservation-oriented management actions.     

A graph theoretic approach for modelling tiger corridor network in Central India-Eastern Ghats landscape complex,India

Wildlife habitat corridors are components of landscapes, which facilitate the movement of organisms and processes between areas of intact habitat, and thus provide landscape corridor as well as serve as an ideal component to study and understand physiological ecology. Corridors are thus regions within a given landscape that generally comprise native vegetation, and connect otherwise fragmented, disconnected, non-contiguous wildlife habitat patches in the landscape. The purpose of designing corridors as a conservation strategy is primarily to counter, and to the extent possible, mitigate the impacts of habitat fragmentation and loss on the biodiversity of the landscape, as well as support continuance of land use for essential local and global economic activities in the region of reference.In this paper, we use game theory and graph theory to model and design a wildlife corridor in the Central India – Eastern Ghats landscape complex, with tiger (Panthera tigris tigris) as the focal species. We construct a graph using the habitat patches supporting wild tiger populations in the landscape complex as vertices and the possible paths between these vertices as edges. A cost matrix is constructed to indicate the cost incurred by the tiger for passage between the habitat patches in the landscape (based on Shelford's Law of Tolerance), by modelling a two-person Prisoner's Dilemma game. A minimum spanning tree is then obtained by employing Kruskal's algorithm, which would suggest a feasible tiger corridor network for the tiger population within the landscape complex. Additionally, analysis of the graph is done using various centrality measures, in order to identify and focus on potentially important habitat patches, and their potential community structure. Correlation analysis is performed on the centrality indices to draw out interesting trends in the network.     

小相岭山系大熊猫廊道规划

廊道能有效地增加栖息地斑块间的大熊猫种群交流,降低种群灭绝的风险。小相岭山系是现存大熊猫种群数量最少的山系之一。通过MAXENT(Maximum Entropy)分析,发现小相岭山系存在7个相对隔离的栖息地斑块,栖息地破碎化严重。为了增加小相岭山系大熊猫种群交流,利用最小代价路径原理和电流理论,从保护管理角度出发,在该区域划定了9条连接大熊猫栖息地斑块的廊道。其中紫马-麻麻地廊道连接该区域面积最大、大熊猫痕迹点最多的两个栖息地斑块,生态意义重大,建议优先建设该廊道。此外,研究划定的多个廊道彼此邻近,甚至存在一定程度的重叠。将这些极其邻近或者重叠的廊道合并,并提出"廊道群"的概念。廊道群是两个或两个以上极其相邻的廊道构成的网状结构,廊道群作为景观上的网状结构,能同时连接多个栖息地斑块,更有效地增加多个栖息地斑块间的种群交流。由于农田的挤压,部分廊道难以达到大熊猫廊道的最低宽度需求(2230 m),制约了其生态效应的发挥,建议在以后的保护工作中探索农田区域的栖息地恢复方式。     

The effects of habitat fragmentation on the social kin structure and mating system of the agile antechinus, Antechinus agilis

Habitat fragmentation is one of the major contributors to the loss of biodiversity worldwide. However, relatively little is known about its more immediate impacts on within-patch population processes such as social structure and mating systems, whose alteration may play an important role in extinction risk. We investigated the impacts of habitat fragmentation due to the establishment of an exotic softwood plantation on the social kin structure and breeding system of the Australian marsupial carnivore, Antechinus agilis. Restricted dispersal by males in fragmented habitat resulted in elevated relatedness among potential mates in populations in fragments, potentially increasing the risk of inbreeding. Antechinus agilis nests communally in tree hollows; these nests are important points for social contact between males and females in the mating season. In response to elevated relatedness among potential mates in fragmented habitat, A. agilis significantly avoided sharing nests with opposite-sex relatives in large fragment sites (but not in small ones, possibly due to limited nest locations and small population sizes). Because opposite-sex individuals shared nests randomly with respect to relatedness in unfragmented habitat, we interpreted the phenomenon in fragmented habitat as a precursor to inbreeding avoidance via mate choice. Despite evidence that female A. agilis at high inbreeding risk selected relatively unrelated mates, there was no overall increased avoidance of related mates by females in fragmented habitats compared to unfragmented habitats. Simulations indicated that only dispersal, and not nonrandom mating, contributed to inbreeding avoidance in either habitat context. However, habitat fragmentation did influence the mating system in that the degree of multiple paternity was reduced due to the reduction in population sizes and population connectivity. This, in turn, reduced the number of males available to females in the breeding season. This suggests that in addition to the obvious impacts of reduced recruitment, patch recolonization and increased genetic drift, the isolation of populations in habitat patches may cause changes in breeding behaviour that contribute to the negative impacts of habitat fragmentation.     

Effects of female kin groups on reproduction and demography in the gray-tailed vole, Microtus canicaudus

I conducted an experiment with gray-tailed voles, Microtus canicaudus , to test the hypothesis proposed by Charnov and Finerty that populations of voles comprised of female kin groups would grow more rapidly and reach higher densities more quickly than populations in which female kin groups were disrupted. The experiment was conducted in 0.2-ha semi-natural enclosures planted with mixed grasses. In four enclosures, females were unmanipulated (control) and in four enclosures all newly caught females were removed from their natal enclosures and replaced with females of comparable age from another enclosure, such that juvenile females did not settle near their siblings or parents (treatment). I found no significant differences in survival, reproduction, juvenile recruitment, population growth rates, or population size between control and treatment populations. The only difference was the time to sexual maturation for young females, which was 3.1 weeks for control enclosures compared with 4.2 weeks for treatment enclosures. I could not measure reproductive success for individual females, but my results did not support the hypothesis that the presence or absence of kin groups resulted in any biologically meaningful population-level effects. Female voles that have nesting territories adjacent to relatives may accrue some individual benefits, but these benefits are unlikely to contribute to population regulation in gray-tailed voles.     

Scent marking by common volesMicrotus arvalis in the presence of a same-sex neighbour

Several vole species use scent marked runways radiating from their burrows for foraging and dispersion. These marks are probably used for social communication. This 4-day laboratory study investigated the environmental and social causations of marking inside pre-existing corridors in male and female common volesMicrotus arvalis (Pallas, 1778). Firstly I tested the novelty and the reinforcement hypotheses in isolated voles, predicting respectively a habituation or a continuous increase in mark deposition. I then confronted with each other two same-sex voles for two days to investigate the differences between males and females in the pattern of marks inside three corridors, one of which runs along the common partition with the neighbour. I tested the self-advertisement and territorial-defence hypotheses, respectively predicting in the presence of a neighbour either a similar marking between the three corridors or a greater marking in the corridor close to the neighbour than in the two other corridors. The results showed no habituation in marking, even in a familiar environment, confirming the reinforcement hypothesis. After the addition of a neighbour, only the females left more marks in the corridor that ran alongside the common border than in the two other corridors. The territorial-defence hypothesis was thus confirmed in (territorial) females while the self-advertisement was supported in (non-territorial) males. Finally, I tested the competitive-ability hypothesis in females, stating that the abundance of scent marks of an individual before a social interaction can predict its degree of intolerance in a future social interaction. The results from female pairs physically interacting for four days support the hypothesis.     

Enigmatic Near-Extirpation in a Boreal Toad Metapopulation in Northwestern Montana

North America's protected lands harbor biodiversity and provide habitats where species threatened by a variety of stressors in other environments can thrive. Yet disease, climate change, and other threats are not limited by land management boundaries and can interact with conditions within protected landscapes to affect sensitive populations. We examined the population dynamics of a boreal toad (Anaxyrus boreas boreas) metapopulation at a wildlife refuge in northwestern Montana, USA, over a 16-year period (2003–2018). We used robust design capture-recapture models to estimate male population size, recruitment, and apparent survival over time and in relation to the amphibian chytrid fungus (Batrachochytrium dendrobatidis). We estimated female population size in years with sufficient captures. Finally, we examined trends in male and female toad body size and condition. We found no evidence of an effect of disease or time on male toad survival but detected a strong negative trend in recruitment of new males to the population. Estimates of male and female abundance decreased over time. Body size of males and females was inversely related to estimated population size, consistent with reduced recruitment to replace adults, but body condition of adult males was only weakly associated with abundance. Together, these results describe the demography of a near-extirpation event, and point to dramatic decreases in the recruitment of new individuals to the breeding population as the cause of this decline. We surmise that processes related to the restoration of historical hydrology within the refuge adversely affected amphibian breeding habitat, and that these changes interacted with disease, life history, and other factors to restrict the recruitment of new individuals to the breeding population over time. Our results point to challenges in understanding and predicting factors that influence population change and highlight that current metrics for assessing population status can have limited predictive ability. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.     

Initiation of early breeding in a population of Microtus townsendii (Rodentia) with the secondary plant compound 6-MBOA

Summary Feeding the secondary plant compound 6-Methoxybenzoxazolinone (6-MBOA) during winter to a free living population of Microtus townsendii accelerated breeding in females. Both the recruitment of young and sexual maturation of these young were advanced by four weeks in comparison with a control population. Overwintered females supplied with 6-MBOA matured at a lower body weight than control females. But there was no such weightat-maturity difference between the grids for males. During April and May 72% of all juveniles captured came from the experimental population. Winter reproduction and early recruitment of young stimulated by 6-MBOA could have important population consequences for these voles.     

Metapopulation processes and persistence in remnant water vole populations

We examined the spatial distribution of water vole populations in four consecutive years and investigated whether the regional population processes of extinction, recolonisation and migration influence distribution and persistence. We examined how such regional processes are influenced by spatial variation in habitat quality. In addition, we assessed the relevance of metapopulation concepts for understanding the dynamics of species that deviate from classical metapopulation assumptions and developing conservation measures for them. Populations were patchy and discrete, and the patchy distribution was not static between years. Population turnover occurred even in the absence of predatory mink, which only influenced the network of populations at the end of the study. Most populations were clustered close together in the upper tributaries. Local population persistence was predominantly influenced by population size: large populations were more persistent. Recolonisation rates were influenced by isolation and habitat quality. The isolation estimates which best explained the distribution of water vole populations incorporated straight‐line distances, suggesting water voles disperse overland. The distribution of recolonised sites indicated that dispersing voles actively selected habitat on the basis of its quality. Water voles depart from some of the assumptions made by frequently used metapopulation models. In particular there is no clear binary distinction between suitable and non‐suitable habitat. Accounting for variation in habitat quality before investigating temporal changes in population distribution allowed us to demonstrate that the key metapopulation processes were important. The significance of regional population processes relative to local population processes may have increased in declining, fragmented populations compared to pristine regional populations. We hypothesise that although mink predation is likely to eventually cause regional extinction in many areas, metapopulation processes have delayed this decline. Consequently, conservation measures should take into account mink predation rates and regional population processes, before considering aspects of habitat quality.     

Do forest-dwelling plant species disperse along landscape corridors?

Woody corridors in fragmented landscapes have been proposed as alternative habitats for forest plants, but the great variation in species-specific responses blurs the overall assessment. The aim of this study was to estimate the dispersal success of forest-dwelling plants from a stand into and along an attached woody corridor, and to explain the observed patterns from the point of view of species’ dispersal traits and corridor properties. We sampled 47 forest–corridor transects in the agricultural landscapes of southeastern Estonia. Regionally common forest-dwelling species (observed in at least 10 % of seed-source forests) were classified on the basis of their ecological response profile—forest-restricted species (F-type) and forest-dwelling generalists (G-type). Species richness and the proportion of F-type species decreased sharply from the seed-source forest core to the forest edge and to the first 10–15 m of the corridor, while G-type species richness remained constant throughout the transect. Corridor structure had a species-specific effect—F species were promoted by old (≥50 years) and wide (≥10 m) corridors, while G species were supported by young and narrow corridors with ditch-related soil disturbances. Moderate shade (canopy cover <75 %) was optimal for all forest-dwelling species. Large dispersule weight, and not seed weight, dispersal vector or Ellenberg’s indicator values, was the trait that differentiated F species from G species. We conclude that most woody corridors are only dispersal stepping-stone habitats for habitat generalist species, and not for specialists. Only century old corridors can relieve the dispersal limitation of forest-restricted species.     

Changes in population structure and reproduction during a 3-yr population cycle of voles

Cyclic changes in population growth rate are caused by changes in survival and/or reproductive rate. To find out whether cyclic changes in reproduction are an important part of the mechanism causing cyclic fluctuations in small mammal populations, we studied changes in the population structure and reproduction of field voles ( Microtus agrestis ), sibling voles ( M. rossiaemeridionalis ), bank voles ( Clethrionomys glareolus ), and common shrews ( Sorex araneus ) in western Finland during 1984–1992, in an area with 3-yr vole cycles. We also modelled the population growth of voles using parameter values from this study. The animals studied were collected by snap trapping in April, May, June, August, September, and, during 1986–1990, also in October. We found several phase-related differences in the population structure (age structure, sex ratio, proportion of mature individuals) and reproduction (litter size, length of the breeding season) of voles. In non-cyclic common shrews, the only significant phase-related difference was a lower proportion of overwintered individuals in the increase phase. According to the analyses and the vole model, phase-related changes in litter size had only a minor impact on population growth rate. The same was true for winter breeding in the increase phase. The length and intensity of the summer breeding season had an effect on yearly population growth but this impact was relatively weak compared to the effect of cyclic changes in survival. The population increase rates of Microtus were delayed dependent on density (8–12-month time lag). Our results indicate that cyclic changes in reproduction are not an important part of the mechanism driving cyclic fluctuations in vole populations. Low survival of young individuals appeared to play an important role in the shift from the peak to the decline phase in late summer and early autumn.