Tracking the effects of one century of habitat loss and fragmentation on calcareous grassland butterfly communities

Habitat loss and fragmentation are known to reduce patch sizes and increase their isolation, consequently leading to modifications in species richness and community structure. Calcareous grasslands are among the richest ecosystems in Europe for insect species. About 10% (1,150 ha) of the total area of a calcareous ridge region (Calestienne, Belgium) and its butterfly community was analysed over a timeframe of about 100 years. Since 1905 to present day (2005), the Calestienne region has undergone both calcareous grassland loss and fragmentation: not only did calcareous grassland size decrease and isolation increase, but also, the number of calcareous grassland patches within the landscape increased until 1965, and subsequently decreased, clearly reflecting the effects of fragmentation. These processes have had a profound effect on the butterfly community: extinction and rarefaction affected significantly more often specialist species, which means that generalist species are more and more overrepresented. This ecological drift, i.e. the replacement of specialists by generalists in species assemblages is likely to be a general effect of habitat loss and fragmentation on natural communities.     

When habitat management can be a bad thing: effects of habitat quality,isolation and climate on a declining grassland butterfly

The conservation of most temperate grassland habitats and their characteristic fauna and flora requires regular low-intensive forms of land-use to counteract natural succession. Although many species tolerate moderate disturbance regimes, some are known to be susceptible to grazing or mowing, thereby causing a management dilemma. One of these species is the Woodland Ringlet butterfly, Erebia medusa. In this study, we analysed which environmental factors determine the occurrence of E. medusa in the Diemel Valley (Central Germany). Furthermore, we conducted microclimatic measurements during the winter months to investigate the role of the litter layer as a microclimatic buffer. Patch occupancy in the Diemel Valley was well explained by the amount of litter present in a patch and connectivity to other inhabited patches. The role of local climatic conditions could not be clarified, due to inter-correlations with connectivity. During the winter, the air temperature inside the litter layer was significantly less variable than above it. We conclude that the current distribution of E. medusa in the Diemel Valley is caused by the combined effect of habitat quality and connectivity, and perhaps also by climatic factors. The importance of the litter layer reflects the dependence of E. medusa on low-intensive or absent land-use. In addition, the litter layer possibly constitutes an essential habitat element, as it buffers temperature fluctuations and thus probably reduces the energy consumption of overwintering larvae. Given the species’ preference for abandoned grasslands, the conservation of E. medusa requires a low-intensity habitat management, for example, by rotational grazing or mowing of small parts of the sites. On the landscape level, the preservation of well-connected habitat networks is important.     

生境破碎化对植物-昆虫及昆虫之间相互关系的影响

生境破碎化对生物多样性和生态系统功能影响是当前国内外生态学家研究的热点问题之一。文章针对生境破碎化的内涵、量度指标进行介绍,着重分析生境破碎化对植物-昆虫关系的影响,包括植物与植食性昆虫的关系、植物与传粉昆虫的关系、种子与种子捕食者的关系,植物及其分解者的关系,还分析生境破碎化对昆虫-昆虫关系的影响,包括昆虫及其拟寄生物的关系、捕食者与猎物的关系。通过对上述方面的阐述,旨在更好地理解生境破碎化对动植物群落相互关系产生的深刻影响,并提出今后研究中应注意的问题和研究热点。     

Experimental habitat fragmentation and invertebrate grazing in a herbaceous grassland species

A field experiment was conducted to examine the effects of habitat fragmentation on herbivore damage to individually tagged leaves of Betonica officinalis rosettes. Fragments of different size and corresponding control plots were established at three study sites in nutrient-poor calcareous grasslands in the northern Swiss Jura mountains. Leaf damage was recorded three times over the growing season (late spring, summer and early autumn). Five years after the initiation of the fragmentation, the density of rosettes did not differ between fragments and control plots. The number of leaves per rosette was higher in fragments than in control plots in summer but not in late spring and early autumn. The extent of leaf damage, expressed as proportion of leaf area removed by invertebrate herbivores, increased over the vegetation period. Leaf damage was greater in fragments than in control plots at two study sites, whereas the opposite (less strongly expressed) was found at the third site. Number of species and density (individuals per m²) of potential herbivores (gastropods and grasshoppers) were recorded in all fragments and control plots. Effects of fragmentation on the number of species and densities depended on plot size and differed between gastropods and grasshoppers. Leaf damage in fragments increased with increasing density of gastropods if the third site, which had lowest leaf damage, was excluded. Such a positive relationship was neither found in control plots nor for grasshopper densities. Thus, movement of gastropods in fragments was probably restricted which resulted in increased feeding pressure at least in two sites. However, even if our fragmentation experiment was well designed and replicated, the interpretation of these experimental results remains difficult because there was large site-to-site and seasonal variation.     

Estimating plant migration rates under habitat loss and fragmentation

Changes in the global environment are modifying the geographical locations of habitats suitable for plant growth. The capacity of plants to migrate to sites of suitable environmental quality will strongly influence future distributions of plant diversity. However, it is not well understood how rates of plant migration are influenced by the habitat loss and habitat fragmentation that characterise contemporary landscapes. In this study we develop a model that can predict migration rates in both intact landscapes (potential migration rate) and in fragmented landscapes (realised migration rates). Migration rates in fragmented landscapes might be slower for many reasons. In this study we focus on two, non‐exclusive reasons. First, the processes that move seeds may break down in fragmented landscapes causing seeds to be dispersed shorter distances. Second, in fragmented landscapes some proportion of seeds will not be deposited in habitats suitable for recruitment. We describe the breakdown of dispersal processes as a competing risk between the factors influencing dispersal in intact landscapes and the factors that may disrupt dispersal processes in fragmented landscapes. We show how the parameters that influence dispersal in fragmented landscapes can be estimated, and how these estimates can be used to forecast migration rates using an integrodifference equation (IDE). The forecasts of the IDE described the effects of reduced dispersal distances adequately. However, the IDE produced biased estimates of the effects of a reduction in plant habitat on migration rates. Model analyses showed that, although we can expect realised migration rates to be lower than potential migration rates, we can also expect the sensitivity of migration rate to habitat loss to vary. In addition, simulations showed that the qualitative nature of the responses of migration rate to habitat loss were variable – some model species responded non‐linearly to habitat loss, others responded linearly. While our method provides guidelines for empirical data collection and model parameterisation, we recognise that obtaining these data will be challenging.     

Modelling the effects of genetics and habitat on the demography of a grassland herb

There is growing evidence that genetic and ecological factors interact in determining population persistence. The demographic effects of inbreeding depression can largely depend on the ecological milieu. We used demographic data of the perennial herb Succisa pratensis from six populations in grazed and ungrazed sites with different soil moisture. We built an individual-based model assessing the demographic consequences of inbreeding depression in populations with different management and habitat. Today this plant has to cope with severe landscape fragmentation, deteriorating habitat conditions in terms of decreasing grazing intensity, and the effects of inbreeding depression. For each population we performed simulations testing two inbreeding depression hypotheses (partial dominance and overdominance) and three epistatic functions among loci. The results indicated stronger inbreeding depression effects for populations in unfavourable sites without grazing or in xeric habitats compared to populations in favourable mesic sites with grazing. Overall, we found stronger effects with overdominance, a result that emphasizes the importance of understanding the genetic mechanisms of inbreeding depression. Hence, management practices can interact with the genetic consequences of inbreeding depression in population dynamics, which may have important implications for plant population ecology and evolutionary dynamics of inbreeding depression.     

Vegetation structure influences foraging decisions in a declining grassland bird: the importance of fine-scale habitat and grazing regime

Capsule: Whinchat Saxicola rubetra foraging behaviour was significantly influenced by habitat structure and grazing.

Aims: To assess how foraging habitats selected by breeding Whinchats differed from wider territory attributes under contrasting grazing management in multiple upland areas in Scotland: principally sheep grazed, Red Deer grazed or ungrazed, and to identify how differing land use may limit suitable foraging areas.

Methods: We compared fine-scale vegetation structure in patches chosen for foraging by Whinchats in contrasting grazing management regimes.

Results: Whinchats were less likely to forage in patches with a greater cover of bracken and tall non-bracken vegetation, regardless of grazing regime. Grass cover influenced foraging behaviour in ungrazed habitats only, where Whinchats were less likely to forage in areas with high grass cover.

Conclusion: Whinchats appear to require a mosaic or range of sward structures within breeding territories, highlighting the importance of establishing how vegetation structure influences breeding birds at different spatial scales. Our results suggest that suitable foraging patches were plentiful within grazed habitats but potentially limited in ungrazed habitats. Further work is needed to identify management regimes and interventions to maintain conditions suitable for breeding Whinchats that are compatible with other land use and conservation objectives.     

Negative effects of habitat drying and prior exploitation on the detritus resource in an ephemeral aquatic habitat

Ephemeral aquatic habitats are characterized by cycles of drying and subsequent inundation, and by production of sequential non-overlapping cohorts of organisms. Both processes may alter the quantity or quality of resources, and may therefore affect survival and development of cohorts that subsequently colonize ephemeral habitats. We examined these effects of habitat drying and non-overlapping cohorts on experimental cohorts of the tree hole mosquito Aedes triseriatus, testing specifically whether the value of leaf litter as a food resource is altered by cycles of inundation and drying, or by exploitation by a prior non-overlapping cohort. We created four treatments of leaf litter: (1) no prior cohort, continuously wet; (2) no prior cohort, one␣wet/dry cycle; (3) prior cohort, continuously wet, and (4) prior cohort, one wet/dry cycle, and tested for effects on individual fitness components (survivorship, mean dry mass at, and median days to eclosion) and on population growth (estimated finite rate of increase –λ′). Both resource drying and the presence of a prior cohort negatively affected individual fitness components in tires, increasing days to eclosion, and decreasing mean dry mass at eclosion for both sexes. Resource drying also negatively affected estimated rates of increase (λ′) in tree holes. A prior cohort had no significant effects on λ′. These results indicate that intraspecific interactions among mosquito larvae may include amensalistic effects of earlier, non-overlapping cohorts, and that resource drying reduces resource quality. The latter effect indicates that enhanced production of A. triseriatus from recently filled containers is not due to resource drying per se, and may result from more complex community-level effects of habitat drying. Extreme cycles of drying and inundation seem likely to increase intraspecific resource competition among drought-adapted species like A. triseriatus. Received: 5 May 1997 / Accepted: 20 January 1998     

Demographic response of plant populations to habitat fragmentation and temporal environmental variability

Many plant species currently exist in fragmented populations of different sizes, while they also experience unpredictable climatic fluctuation over time. However, we still understand little about how plant demography responds to such spatial and temporal environmental variability. We studied population dynamics of an understory perennial herb Trillium camschatcense in the Tokachi plain of Hokkaido, Japan, where a significant effect of forest fragmentation on seedling recruitment was previously reported. Four populations across a range of fragment sizes were studied for 6 years, and the data were analyzed using matrix population models. Per capita fecundity (the number of recruits per plant) varied greatly among populations, but the variation in population growth rates (λ) was mainly driven by the variation in stasis and growth rates, suggesting that the general trend of reduced fecundity in fragmented populations may not be readily translated into subsequent dynamics. Temporal variation in λ among years was more than 2 times larger than spatial variation among populations, and this result was likely attributable to the contrasting response of correlation structures among demographic rates. The among-population variation in λ was dampened by negative covariation between matrix elements possibly due to density-dependent regulation as well as an inherent constraint that some elements are not independent, whereas positive covariation between matrix elements resulted in large temporal variation in λ. Our results show that population dynamics responded differently to habitat fragmentation and temporal variability of the environment, emphasizing the need to discriminate these spatial and temporal variations in demographic models. Although no populations were projected to be declining in stochastic simulations, correlation between current habitat size and plant density implies historical λ is positively related to habitat size.     

红池坝地区草地红三叶草的空间分布型

植物种群的空间分布型是生物在特定的条件下形成的,它反映了植物种群个体的散布状态以及对环境的适应性和生境对植物影响的生态反映的生态学关系,是一条可以了解凭种类组成研究演替的研究途径 森下正明(Morisita,1959)提出了扩散指数Io(Morisita's index);美国的Lloyd,(1967)提出了平均拥挤度(Mean crowding,m)这个颇有兴趣     

Experimental simulations about the effects of overexploitation and habitat fragmentation on populations facing environmental warming

Populations of many species are dramatically declining worldwide, but the causal mechanism remains debated among different human-related threats. Coping with this uncertainty is critical to several issues about the conservation and future of biodiversity, but remains challenging due to difficulties associated with the experimental manipulation and/or isolation of the effects of such threats under field conditions. Using controlled microcosm populations, we quantified the individual and combined effects of environmental warming, overexploitation and habitat fragmentation on population persistence. Individually, each of these threats produced similar and significant population declines, which were accelerated to different degrees depending upon particular interactions. The interaction between habitat fragmentation and harvesting generated an additive decline in population size. However, both of these threats reduced population resistance causing synergistic declines in populations also facing environmental warming. Declines in population size were up to 50 times faster when all threats acted together. These results indicate that species may be facing risks of extinction higher than those anticipated from single threat analyses and suggest that all threats should be mitigated simultaneously, if current biodiversity declines are to be reversed.     

Effects of inbreeding and interpopulation crosses on performance and plasticity of two generations of offspring of a declining grassland plant

? Premise of the study: Inbreeding depression is a major evolutionary force and an important topic in conservation genetics because habitat fragmentation leads to increased inbreeding in the populations of many species. Crosses between populations may restore heterozygosity, resulting in increased performance (heterosis), but may also lead to the disruption of coadapted gene complexes and to decreased performance (outbreeding depression). ? Methods: We investigated the effects of selfing and of within and between population crosses on reproduction and the performance of two generations of offspring of the declining grassland plant Saxifraga granulata (Saxifragaceae). We also subjected the first generation of offspring to a fertilization and two stress treatments (competition and defoliation) to investigate whether the effects of inbreeding and interpopulation gene flow depend on environmental conditions. ? Key results: Inbreeding depression affected all traits in the F(1) generation (δ = 0.07-0.55), but was stronger for traits expressed late during development and varied among families. The adaptive plasticity of offspring from selfing and from interpopulation crosses in response to nutrient addition was reduced. Outbreeding depression was also observed in response to stress. Multiplicative fitness of the F(2) generation after serial inbreeding was extremely low (δ > 0.99), but there was heterosis after crossing inbred lines. Outbreeding depression was not observed in the F(2). ? Conclusions: Continuous inbreeding may drastically reduce the fitness of plants, but effects may be environment-dependent. When assessing the genetic effects of fragmentation and interpopulation crosses, the possible effects on the mean performance of offspring and on its adaptive plasticity should be considered.     

Modelling effects of habitat fragmentation on the ability of trees to respond to climatic warming

The ability of trees to migrate in response to climatic warming was simulated under various conditions of habitat availability. The model uses Holocene tree migration rates to approximate maximum migration rates in a forested landscape. Habitat availability and local population size was varied systematically under two dispersal and colonization models. These dispersal models varied in the likelihood of long-distance dispersal events. The first model used a negative exponential function that severely limited the probability of long-distance dispersal. The results of this model indicate that migration rate could decline an order of magnitude where the habitat availability is reduced from 80 to 20% of the matrix. The second model, using an inverse power function, carried a higher probability of long-distance dispersal events. The results from this model predict relatively small declines in migration rates when habitat availability is reduced to 50% of the simulation matrix. Below 50% habitat availability, mean migration rate was similar to the negative exponential model. These results predict a failure of many trees to respond to future climatic change through range expansion.     

Direct versus indirect effects of habitat fragmentation on community patterns in experimental landscapes

Habitat area and fragmentation are confounded in many ecological studies investigating fragmentation effects. We thus devised an innovative experiment founded on fractal neutral landscape models to disentangle the relative effects of habitat area and fragmentation on arthropod community patterns in red clover (Trifolium pratense). The conventional approach in experimental fragmentation studies is to adjust patch size and isolation to create different landscape patterns. We instead use fractal distributions to adjust the overall amount and fragmentation of habitat independently at the scale of the entire landscape, producing different patch properties. Although habitat area ultimately had a greater effect on arthropod abundance and diversity in this system, we found that fragmentation had a significant effect in clover landscapes with ≤40 % habitat. Landscapes at these lower habitat levels were dominated by edge cells, which had fewer arthropods and lower richness than interior cells. Fragmentation per se did not have a direct effect on local-scale diversity, however, as demonstrated by the lack of a broader landscape effect (in terms of total habitat area and fragmentation) on arthropods within habitat cells. Fragmentation—through the creation of edge habitat—thus had a strong indirect effect on morphospecies richness and abundance at the local scale. Although it has been suggested that fragmentation should be important at low habitat levels (≤20–30 %), we show that fragmentation per se is significant only at intermediate (40 %) levels of habitat, where edge effects were neither too great (as at lower levels of habitat) nor too weak (as at higher levels of habitat).     

Lethal effects of habitat degradation on fishes through changing competitive advantage

Coral bleaching has caused catastrophic changes to coral reef ecosystems around the world with profound ecological, social and economic repercussions. While its occurrence is predicted to increase in the future, we have little understanding of mechanisms that underlie changes in the fish community associated with coral degradation. The present study uses a field-based experiment to examine how the intensity of interference competition between juveniles of two species of damselfish changes as healthy corals degrade through thermal bleaching. The mortality of a damselfish that is a live coral specialist (Pomacentrus moluccensis) increased on bleached and dead coral in the presence of the habitat generalist (Pomacentrus amboinensis). Increased mortality of the specialist was indirectly owing to enhanced aggression by the generalist forcing the specialist higher up and further away from shelter on bleached and dead coral. Evidence from this study stresses the importance of changing interspecific interactions to community dynamics as habitats change.     

Synergistic effects of an extreme weather event and habitat fragmentation on a specialised insect herbivore

Habitat fragmentation is considered to be one of the main causes of population decline and species extinction worldwide. Furthermore, habitat fragmentation can decrease the ability of populations to resist and to recover from environmental disturbances such as extreme weather events, which are expected to occur at an increasing rate as a result of climate change. In this study, we investigated how calcareous grassland fragmentation affected the impact of the climatically extreme summer of 2003 on egg deposition rates, population size variation and survival of the blue butterfly Cupido minimus, a specialist herbivore of Anthyllis vulneraria. Immediately after the 2003 summer heat wave, populations of the host plant declined in size; this was paralleled with decreases in population size of the herbivore and altered egg deposition rates. In 2006 at the end of the monitoring period, however, most A. vulneraria populations had recovered and only one population went extinct. In contrast, several butterfly populations had gone extinct between 2003 and 2006. Extinction probability was significantly related to initial population size, with small populations having a higher risk of extinction than large populations. These results support the prediction that species of higher trophic levels are more susceptible to extinction due to habitat fragmentation and severe disturbances.     

Land-use and edge effects unbalance seed dispersal and predation interactions under habitat fragmentation

The process of fragmentation can greatly influence plant–animal interactions. To assess the degree to which it affects the balance between two interactions of opposite sign, namely seed dispersal and post-dispersal seed predation, we selected 16 patches of chestnut forest in O Courel and El Bierzo, northwestern Spain. We assessed the effect of fragmentation over two different seed dispersal–predation systems using Helleborus foetidus and Ilex aquifolium as model species. In the first case, field experiments consisted of seed-offering trays with selective exclusion of rodents and ants in a two-way orthogonal design. In the second experiment, we placed experimental branches and trays on the floor to assess seed dispersal and predation. The interactions between several fragment traits and the relative contribution of rodents, ants and birds to seed removal were analyzed by means of generalized linear mixed models. Results show that for H. foetidus, differences in seed dispersal–predation were accounted for by patch shape, which affected mainly the dispersal phase. Major seed dispersal took place in patches with a smaller edge to core ratio and high plant cover (abandoned patches), whilst the latter also showed maximum seed predation. For I. aquifolium, fragmentation effects were significant only for seed predation, which was increased in abandoned patches. This shows that the effects of habitat fragmentation can emerge at different phases depending on specific traits of the interacting animals. It also highlights the importance of traditional land-use practices in species interactions.     

Combined effects of overwintering temperature and habitat degradation on the survival of boreal wood ant

The survival of insects during diapause may be affected by overwintering temperature and other environmental stress, such as anthropogenic habitat degradation. We experimentally studied the effects of overwintering temperature (+1 and +7°C) and commercial forest clear-cutting on the overwintering survival of the forest-dwelling wood ant Formica aquilonia. We found that both the higher overwintering temperature and clear-cutting lowered the overwintering survival and body fat resources of Formica aquilonia. The survival and body fat resources were highest in lower temperature treatment forest nests and lowest in higher temperature treatment clear-cut nests. The overall survival of ants increased with higher body fat resources. It is possible that both forest clear-cutting and higher winter temperature due to possible climate warming may increase the wintertime mortality of wood ants and other forest-dwelling ants.     

Experimental effects of habitat fragmentation on old-field canopy insects: community, guild and species responses

We examined the effects of habitat fragmentation on the species distributions, guild membership, and community structure of old-field insects using a fine-scale experimental approach. A continuous 1-ha goldenrod field was fragmented into four treatments that varied in both patch size and degree of isolation. Each treatment was replicated four times and arranged in a Latin square design. Canopy insects in fragmented patches were sampled with sweep nets during early and late summer 1995. The species richness of insects was significantly lower in fragmented than in unfragmented treatments during July, but was similar among treatments in September. Overall community abundance showed no treatment effect during either month. We also found significant row and column effects, suggesting there was spatial heterogeneity in species richness and abundance apart from treatment effects. Differences in species richness during July were primarily due to the loss of rare species in highly fragmented plots. Overall abundance was less responsive to community change because deletions of rare species in fragmented areas were not detected in abundance analyses. Four feeding guilds showed different responses to fragmentation: the species richness of sucking herbivores and the abundance of parasitoids were significantly reduced by fragmentation but predators and chewing herbivores were largely unaffected. Analyses of a subset of individual species within guilds suggest that the greater effects of fragmentation on sucking herbivores and parasitoids may be due to the degree of habitat specificity of guild members. The effects of small-scale habitat fragmentation were therefore detectable at the level of community, guild, and individual species. Changes in species richness, guild structure and species distributions were likely due to differential effects of habitat alteration on individual movements and patch selection rather than dispersal or demographic change. Nonetheless, the selective loss of rare species, differential guild effects and changes in species occupancy that we found in this small-scale experiment are also factors that are likely to operate in fragmented habitats over broader spatial scales. Received: 11 May 1998 / Accepted: 27 September 1998