Landscape connectivity and the role of small habitat patches as stepping stones: an assessment of the grassland biome in South America

Connectivity losses lead to a reduction of the amount of habitat resources that can be reached and used by species, and hence to a decline in the ranges and abundance of multiple taxa. Despite the recognized important role of small habitat patches for many species inhabiting fragmented landscapes, their potential contribution as stepping stones for maintaining overall landscape connectivity has received less attention. Using connectivity metrics based on a graph-theoretic approach we (i) quantified the connectivity of grassland patches in a sector of the Pampa region in Argentina, using a range of dispersal distances (from 100 to 10,000 m) representative of the scale of dispersal of different species; (ii) identified the most relevant patches for maintaining overall connectivity; and (iii) studied the importance of small patches (defined for different area thresholds of 5, 20, and 50 ha) as connectivity providers in the landscape. Although grassland patches were in general poorly connected at all distances, some of them were critical for overall connectivity and were found to play different crucial roles in the patch network. The location of small patches in the grassland network allowed them to function as stepping stones, yielding significant connectivity gains for species that move large distances (>5000 m) for the three area thresholds considered. Thus, under the spatial pattern of the studied landscape, species that move long distances would benefit from stepping stones, while less mobile organisms would benefit from, and mostly rely on the largest patches. We recommend that future management activities should (i) aim at preserving the grassland patches with the highest potential as stepping stones to promote landscape-level connectivity; and (ii) pay more attention to the conservation of key small patches, particularly given that usually they are those more vulnerable to land clearing for agriculture.     

Biotic filtering and mass effects in small shrub patches: is arthropod community structure predictable based on the quality of the vegetation?

1. Community assembly is affected by four processes: dispersal, filtering effects (selection), ecological drift and evolution. The role of filtering relative to dispersal and drift should decline with patch size, hampering possibilities to predict which organisms will be observed within small‐sized patches. However, vegetation structure is known to have a marked impact on species assemblages, and plant quality may act as a biotic filter. This challenges the assumption of unpredictable species assemblages in small‐sized vegetation patches. 2. Using 32 stands of five shrub species in south‐west Finland, this study investigated whether biotic filtering effects caused by patch‐forming plants are strong enough to overcome the mixing of mobile arthropod assemblages across small patches. 3. Stochastic variation did not hide the signals of biotic filtering and dispersal in the small shrub patches. Habitat richness around the patches explained a three times larger share of variation in the species composition than did the identity of the patch‐forming plant, but it had less effect on the abundance of arthropods. A radius of 50–100 m around a patch explained the species composition best. 4. Abundance patterns varied between the feeding guilds; the patch‐forming shrub influenced the abundances of detritivores and leaf‐feeding herbivores, whereas the abundances of flower‐visiting herbivores appeared to track the flowering phenology of the plants. Shrub identity had little effect on omnivores or predators. Predator abundances were correlated with the abundance of potential prey. 5. The results of this study suggest that community composition within a vegetation patch may be predictable even if dispersal overrides local filtering effects, as suggested by the mass‐effects paradigm.     

The effect of plant litter on ecosystem properties in a Mediterranean semi‐arid shrubland

Abstract. We studied the distribution of litter in a shrubland of the Negev with a semi‐arid Mediterranean climate of less than 200 mm of rainfall per year. Our focus was on the effects of litter on properties of landscape patches relevant to ecosystem processes (water runoff and soil erosion), annual plant community responses (seedling density, biomass production and species richness), and animal activity (soil disturbance by termites). Three 60‐m transects, extending across a pair of opposing north‐ and south‐facing slopes and their drainage channel, showed that litter accumulates not only under shrubs, but to a lesser extent also on the crusted inter‐shrub open areas. We used 35 experimental units (‘cells’, 0.5m × 1 m), each containing a crust and a shrub patch. Because runoff flows from crusted patches and is intercepted by shrub patches, the latter were in the lower third of the cells. Leaf litter was added in single and double amounts providing ca. 0.5 and 1.0 cm litter depth, to either, both, or none of the patches. Litter addition significantly decreased the amount of runoff, regardless of the location and amount of litter applied. Litter on the crust increased species number and seedling density of species with low abundance. Adding a double litter layer increased annual plant biomass production, while a single amount had no effect. Litter addition to the shrub patch affected neither biomass nor species richness. Litter addition to both patches at both quantities caused a large increase in termite activity. Termites caused disturbance by disrupting the crust, which may contribute to the reduction in runoff amounts. In the open, flat crust patches, annual plant communities are limited in their productivity and species richness, as there are few structures stopping the outflow of water, soil and seeds. Litter adds such structures, but affects the plant communities only when added to litter‐free crust. Litter accumulation and its patchy distribution have large impacts on landscape patch properties affecting resource distribution, plant productivity and diversity, and animal activity. Therefore, understanding litter distribution in relation to the patchy structure of the landscape of semi‐arid shrubland should be viewed as an important component of shrubland management.     

Island biogeography theory predicts plant species richness of remnant grassland patches in the agro-pastoral ecotone of northern China

Habitat loss and fragmentation are critical threats that cause biodiversity loss. However, the driving factors of species diversity in fragmented habitats are complex. This study was conducted in the agro-pastoral ecotone of northern China, a region that has suffered from severe habitat fragmentation due to agricultural activities. Based on the plant species richness, patch characteristics (patch area, fractal dimension, and isolation), landscape characteristics (grassland area ratio within buffer zones with different radii (50–500 m) around each patch), and environmental characteristics (mean annual precipitation, mean annual temperature, altitude, extremely high temperature, extremely low temperature, slope, and aspect) of 82 remnant grassland patches, we first explored the appropriate buffer zone with the most powerful impact on species richness of remnant grassland patches (SRP). Furthermore, we analyzed the relationships between a series of predictors and SRP. Finally, we confirmed the critical driving factors of SRP through a random forest analysis. The results showed that the landscape scale with the most powerful impact on SRP was 150 m. The patch area, grassland area ratio of a buffer zone with a 150 m radius, mean annual precipitation, and slope all had significantly positive effects on SRP. Among these factors, patch area and patch isolation were the top two driving factors of SRP. Our study suggests that large and highly connected patches should be protected for the construction of natural reserves. Meanwhile, the habitat around patches within a suitable range should also not be ignored. We highlight that the island biogeography theory could be used to predict plant species richness of remnant grassland patches in the agro-pastoral ecotone of northern China.     

Distance models in ecological network management: A case study of patch connectivity in a grassland network

Landscape connectivity is a key issue of nature conservation and distance parameters are essential for the calculation of patch level metrics. For such calculations the so-called Euclidean and the least cost distance are the most widespread models. In the present work we tested both distance models for landscape connectivity, using connectivity metrics in the case of a grassland mosaic, and the ground beetle Pterostichus melas as a focal species. Our goal was to explore the dissimilarity between the two distance models and the consequent divergence from the calculated values of patch relevance in connectivity. We found that the two distance models calculated the distances similarly, but their estimations were more reliable over short distances (circa 500 m), than long distances (circa 3000 m). The variability in the importance of habitat patches (i.e. patch connectivity indices) was estimated by the difference between the two distance models (Euclidean vs. least cost) according to the patch size. The location of the habitat patches in the matrix seemed to be a more important factor than the habitat size in the estimation of connectivity. The uncertainty of three patch connectivity indices (Integral Index of Connectivity, Probability of Connectance and Flux) became high above a habitat size of 5 ha. Relevance of patches in maintaining connectivity varied even within small ranges depending on the estimator of distance, revealing the careful consideration of these methods in conservation planning.     

Interactive effects of grazing and shrubs on the annual plant community in semi‐arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi‐arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south‐facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above‐ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north‐facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species‐specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi‐arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species‐specific facilitation/ interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.     

Conserving a variety of ancient forest patches maintains historic arthropod diversity

Forests are naturally extensive tracts. However, in South Africa natural fires over many millennia have reduced forested areas into small remnants spread throughout a grassland matrix. Small patches, especially distant patches, are generally considered to be adverse for forest specialists, owing to decreased forest interior and increased edge. Here we test this assumption by determining the impact of forest interpatch distance and patch size on epigaeic arthropod diversity in this globally rare vegetation type. Forty patches were selected: ten large (100–435 m diameter) that are distant (500–645 m) from other patches, ten large that are close to other patches (38–97 m), ten small (30–42 m) that are distant, and ten small-close patches. Each patch had two plots: edge and interior. Arthropods were sampled using pitfall traps, Berlese-Tullgren funnels and active searches. Interiors and edges had similar species richness and composition, excluding spiders, which were richer in interiors. Patch size significantly influenced species richness of predatory beetles and arthropod assemblages, excluding spiders. Effect of the interaction between patch size and interpatch distance on species richness and composition varied among taxa. Furthermore, large patches supported similar assemblages regardless of interpatch distance. Arthropod response, particularly ants to patch size and interpatch distance, was partly shaped by the matrix type. The percentage of surrounding grassland had little effect on arthropod diversity. We can conclude that large and close patches are important for arthropod conservation. Nevertheless, it is also important to conserve a variety of patch sizes at various distances to maximize overall arthropod composition.     

Landscape connectivity of the grassland biome in Mpumalanga,South Africa

The South African grassland biome is one of the most threatened biomes in South Africa. Approximately 45% of the grassland biome area is transformed, degraded or severely invaded by alien plants and the remaining natural areas are highly fragmented. In this fragmented landscape, the connectivity between habitat patches is very important to maintain viable populations. In this study we aimed to quantify connectivity of the grassland biome in Mpumalanga using graph theory in order to identify conservation priorities and to direct conservation efforts. Graph theory‐based connectivity indices have the ability to combine spatially explicit habitat data with species specific dispersal data and can quantify structural and functional connectivity over large landscapes. We used these indices to quantify the overall connectivity of the study area, to determine the influence of abandoned croplands on overall connectivity, and to identify the habitat patches and vegetation types most in need of maintaining overall connectivity. Natural areas were identified using 2008 land cover data for Mpumalanga. Connectivity within the grassland biome of Mpumalanga was analysed for grassland species with dispersal distances ranging from 50 to 1000 m. The grassland habitat patches were mostly well connected, with 99.6% of the total habitat area connected in a single component at a threshold distance of 1000 m. The inclusion of abandoned croplands resulted in a 33% increase in connectivity at a threshold distance of 500 m. The habitat patches most important for maintaining overall connectivity were the large patches of continuous habitat in the upper and lower centres of the study area and the most important vegetation types were the Wakkerstroom Montane Grassland and the Eastern Temperate Freshwater Wetlands. These results can be used to inform management decisions and reserve design to improve and maintain connectivity in this biome.     

Alpine glacial relict species losing out to climate change: The case of the fragmented mountain hare population (Lepus timidus) in the Alps

Alpine and Arctic species are considered to be particularly vulnerable to climate change, which is expected to cause habitat loss, fragmentation and—ultimately—extinction of cold‐adapted species. However, the impact of climate change on glacial relict populations is not well understood, and specific recommendations for adaptive conservation management are lacking. We focused on the mountain hare (Lepus timidus) as a model species and modelled species distribution in combination with patch and landscape‐based connectivity metrics. They were derived from graph‐theory models to quantify changes in species distribution and to estimate the current and future importance of habitat patches for overall population connectivity. Models were calibrated based on 1,046 locations of species presence distributed across three biogeographic regions in the Swiss Alps and extrapolated according to two IPCC scenarios of climate change (RCP 4.5 & 8.5), each represented by three downscaled global climate models. The models predicted an average habitat loss of 35% (22%–55%) by 2100, mainly due to an increase in temperature during the reproductive season. An increase in habitat fragmentation was reflected in a 43% decrease in patch size, a 17% increase in the number of habitat patches and a 34% increase in inter‐patch distance. However, the predicted changes in habitat availability and connectivity varied considerably between biogeographic regions: Whereas the greatest habitat losses with an increase in inter‐patch distance were predicted at the southern and northern edges of the species’ Alpine distribution, the greatest increase in patch number and decrease in patch size is expected in the central Swiss Alps. Finally, both the number of isolated habitat patches and the number of patches crucial for maintaining the habitat network increased under the different variants of climate change. Focusing conservation action on the central Swiss Alps may help mitigate the predicted effects of climate change on population connectivity.     

Patch quality and connectivity influence spatial dynamics in a dune wolfspider

The spatial population dynamics of the wolfspider Pardosa monticola, inhabiting patchily distributed grasslands in the Flemish coastal dunes of Belgium and Northern France were investigated with incidence function models using field survey data from 1998 and 2000. Vegetation height and patch size were related to habitat quality. Mark-recapture experiments revealed maximum cursorial dispersal distances of 280 m for moss dunes and 185 m for higher dune grassland. Higher shrub vegetation appeared to be dispersal barriers. These habitat-dependant cursorial distances and the theoretically estimated ballooning distance were included with patch distances into a connectivity index for both dispersal modes. Forward multiple regression indicated that patch occurrence was influenced by habitat quality and ballooning connectivity. Habitat quality and cursorial connectivity explained patterns in short-term colonisation. Extinction appeared to be stochastic and not related to habitat quality and connectivity. Genetic differentiation and variability was low. The discrepancy between the estimated low dispersal capacity and the indirect estimate of gene flow ( F(ST)) indicates that historical population dynamics and/or historical ballooning dispersal influence the genetic structure in this species.     

The effects of plant traits on species' responses to present and historical patch configurations and patch age

Patch configuration is viewed as an important factor affecting the distributions of plant species. Although a number of studies have explored the relationship between plant life‐history traits and species’ distributions in fragmented landscapes, the effect of individual traits on the dependence of species on historical versus current landscape configurations remains unclear. We identified the extent to which present (2000s) and historical (1843, 1954, 1980s) patch configurations (area and connectivity) and patch age affected the distributions of 99 species inhabiting dry calcareous grasslands. We used traits related to dispersal, survival, growth and habitat preferences to explain the dependence of 60 of these species on present and historical configurations, and the age of grassland patches. We found that most of the species had an affinity to currently or historically large, older and more isolated patches. This suggests that many dry grassland species are not in equilibrium with the current landscape, as their distributions still reflect past landscape structures. Rapidly growing species with higher seed bank longevity and nutrient requirements primarily occur in young, large grassland patches whereas species with the opposite traits occur in older, historically large and currently more isolated smaller patches. We hypothesise that patch quality is the reason why different species occupy patches of different age. Species occupying young, large patches commonly disperse by endozoochory. By contrast, no dispersal trait is associated with species occupying old, usually isolated patches. Our results suggest that species occupying old patches are exposed to higher potential risk of extinction, as their distributions are probably limited by the low number and connectivity of available suitable patches and poor dispersal ability. We thus suggest that the dynamics of these species can effectively be supported by improving the quality of young grassland patches.     

Do seed and microsite limitation interact with seed size in determining invasion patterns in flooding Pampa grasslands?

Additive influences of the invasion ability of species (invasiveness) and the characteristics of the habitat which make it invasible (invasibility) cannot fully explain grassland invasion patterns. We tested the hypothesis that different species assemblages of grassland communities may partly result from interactive influences between the relative invasiveness of available species and community invasibility. During 10 months, we evaluated seed and microsite limitation of seedling emergence, survival, and recruitment of plants belonging to species with different seed size (large-seeded species vs. small-seeded species), with in a two-phase community mosaic typical of semi-natural grasslands in the southern flooding Pampa of Argentina. Seeds of large versus small-seeded species were sown either in species-poor patches dominated by a tall tussock grass (“pajonal”) or in species-rich patches dominated by short grasses (“matrix”), subjected to different levels of canopy disturbance (cut vs. uncut). Seed addition promoted seedling emergence for 7 out of the 10 species sown, and this effect was higher for large than for small-seeded species. After seed limitations were removed, interactive effects among seed size, community state and canopy disturbance reflected a strong positive influence of seed size on plant recruitment only in cut pajonal patches. Therefore, according to the stage of invasion process, relative species success may depend on non-interactive (seed-size effect on seed limitation to seedling emergence) or interactive influences among species invasibility and community invasiveness (from seedling emergence to plant recruitment). As a general conclusion, different assemblages of species are expected to successfully colonize spatially close grassland patches, according to both the available invasible species (seed size) and the community state and stage (species composition and canopy disturbance).     

Woody plant invasion at a semi‐arid/arid transition zone: importance of ecosystem type to colonization and patch expansion

Question: How do patterns in colonization and patch expansion of an invasive woody plant (Larrea tridentata, Zygo‐phyllaceae) differ between two grassland ecosystems at a biome transition zone? Location: Semi‐arid/arid transition zone in central New Mexico. Methods: Frequency of occurrence, height, and surface area of saplings (n= 134) and patches of adult plants (n= 247) of the invasive shrub, L. tridentata, were measured within a mosaic of ecosystems dominated either by the Chihuahuan Desert species, Bouteloua eriopoda (Poaceae), or the shortgrass steppe species, B. gracilis, located within 1 km of the L. tridentata‐dominated ecosystem. Distances between L. tridentata patches and patch area were used to estimate connectivity as a measure of propagule pressure. Sapling age (estimated from height using previously established relationships) and distance to the L. tridentata‐dominated ecosystem was used to evaluate patterns in dispersal. Cover by species or functional group inside each L. tridentata patch was compared with surrounding vegetation to estimate changes in species composition with patch expansion. Results: L. tridentata saplings (< 1%) and adult patches (15%) occurred less frequently in B. gracilis‐dominated ecosystems than expected based on areal extent of this ecosystem type. Propagule pressure did not differ with distance from the core ecosystem dominated by L. tridentata. Evidence for both local and long‐distance dispersal events was found. Similar relationships between number of plants and patch area in both grassland types indicate similar patterns in patch expansion. Cover of perennial forbs was higher and cover of dominant grasses was lower in L. tridentata patches compared with the surrounding vegetation for both ecosystem types. Conclusions Spatial variation in L. tridentata saplings and patches at this biome transition zone is related to the different susceptibilities to invasion by two grassland ecosystems. The persistence of grasslands at this site despite region‐wide expansion by L. tridentata may be related to the spatial distribution of B. gracilis‐dominated ecosystems that resist or deter invasion by this woody plant.     

Relative influence of size,connectivity and disturbance history on plant species richness and assemblages in fragmented grasslands

Questions: What is the relative influence of size, connectivity and disturbance history on plant species richness and assemblages of fragmented grasslands? What is the contribution of small fragments to the conservation of native species pool of the region? Location: Tandilia's Range, Southern Pampa, Argentina. Methods: Cover of plants was registered within 24 fragments of tall‐tussock grassland remnants within an agricultural landscape using modified Whittaker nested sampling. We analysed the influence of site variables related to disturbance history (canopy height, litter thickness) and fragment variables (size, connectivity) on species richness (asymptotic species richness, slope of the species–area curve) as well as on species assemblages by multiple regressions analysis and canonical correspondence analyses, respectively. Cumulative area was used for analysing whether small fragments or large fragments are more important to species diversity in the landscape. Results: Asymptotic species richness was significantly influenced by site variables, in particular by Paspalum quadrifarium's canopy height, but not by fragment variables. Species assemblages were also affected by site variables (12.2% of total variation), but no additional portion of the species assemblage variability was significantly explained by fragment size and connectivity. Sampling of several small fragments rendered more exotic and native species than sampling of few large fragments of the same total area. Conclusions: Our results agree with previous studies reporting low sensitivity of species diversity to size and isolation of grassland fragments in fragmented landscapes and high sensitivity of species diversity to local variables. The higher capture of regional native species pool by small grassland fragments than by few larger ones of equivalent accumulated area highlights the value of small fragments for conservation.     

Distribution of plant species at a biome transition zone in New Mexico

Question: Is there a difference in plant species and life form composition between two major patch types at a biome transition zone? Are subordinate species associated with different patch types at the shortgrass steppe — Chihuahuan desert grassland transition zone? Is this association related to differences in soil texture between patch types and the geographic range of associated species? Location: central New Mexico, USA. Methods: Patches dominated by either Bouteloua gracilis, the dominant species in the shortgrass steppe, or Bouteloua eriopoda, dominant species in the Chihuahuan desert grasslands, were sampled for the occurrence of subordinate species and soil texture within a 1500‐ha transitional mosaic of patches. Results: Of the 52 subordinate species analysed, 16 species were associated with B. gracilis‐dominated patches and 12 species with B. eriopoda‐dominated patches. Patches dominated by B. gracilis were richer in annual grasses and forbs, whereas patches dominated by B. eriopoda contained more perennials forbs and shrubs. Soils of B. gracilis‐dominated patches had higher clay and lower rock contents compared with soils of B. eriopoda‐dominated patches. Differences in species characteristics of the dominant species as well as differences in soil texture between patch types contribute to patch‐scale variation in composition. The association of species to patch types was not related to their geographic range and occurrence in the adjacent biomes. Conclusions: Patch types at this biome transition zone have characteristic life‐form and species composition, but species are associated to patch types due to local constraints, independently from their affinity to the adjacent biomes.     

Relative importance of habitat area, connectivity, management and local factors for vascular plants: spring ephemerals in boreal semi-natural grasslands

Using species and environmental data from an extensive grassland area in south-western Finland, we investigated the effect of patch area and connectivity, management and local habitat variables on the occurrence of spring-flowering vascular plants and their richness in boreal agricultural landscapes. Generalized linear models (GLM) and variation partitioning were used to study the explanatory power of the three groups of variables and their combined contributions on the richness and occurrence of six spring-flowering plant species. Generalized additive models (GAMs) and associated cross-validation tests were used to evaluate the predictability of the species occurrence and richness patterns. Present-day grassland patch area and connectivity were important predictors for occurrence and richness of the studied plant species. In addition, local habitat factors, especially radiation, accounted for major fractions of occurrence patterns of the studied species. Hybrid models including variables from all three variable groups had higher explanatory power and predictive capability than partial models. However, performance of the separate single-species models varied considerably between the six study species. Exclusion of radiation or connectivity from the hybrid models decreased their predictive performance, suggesting that these factors are of particular importance for grassland plant species at their northern range margins. When developing conservation and management planning for grassland plant species in Northern Europe, attention should be paid to well-connected networks of grassland patches including large, steeply-sloped patches with a favorable microclimate.     

Isolation determines patterns of species presence in highly fragmented landscapes

In fragmented landscapes, changes in habitat availability, patch size, shape and isolation may affect survival of local populations. Proposing efficient conservation strategies for such species relies initially on distinguishing the particular effects of those factors. To address these issues, we investigated the occurrence of 3 bird species in fragmented Brazilian Atlantic Forest landscapes. Playback techniques were used to collect presence/absence data of these species inside 80 forest patches, and incidence models were used to infer their occupancy pattern from landscape spatial structure. The relative importance of patch size, shape and surrounding forest cover and isolation was assessed using a model selection approach based on maximum likelihood estimation. The presence of all species was in general positively affected by the amount of surrounding habitat and negatively affected by inter‐patch distances. The joint effects of patch size and the surrounding landscape characteristics were important determinants of occupancy for two species. The third species was affected only by forest cover and mean patch isolation. Our results suggest that local species presence is in general more influenced by the isolation from surrounding forests than by patch size alone. We found evidence that, in highly fragmented landscapes, birds that can not find patches large enough to settle may be able to overcome short distances through the matrix and include several nearby patches within their home‐ranges to complement their resource needs. In these cases, patches must be defined as functionally connected habitat networks rather than mere continuous forest segments. Bird conservation strategies in the Atlantic forest should focus on increasing patch density and connectivity, in order to implement forest networks that reduce the functional isolation between large remnants with remaining core habitat.     

基于结构方程模型分析森林草原带草本物种丰富度对景观因子的响应

受损景观是由不同比例的植被斑块组成的镶嵌体,阐明植被斑块的景观结构特征对物种多样性的影响,有助于提高受损景观物种多样性保护。在塞罕坝自然保护区选取38个天然植被斑块,其中包括12个草地、11个灌木林和15个天然次生林。根据光合作用的不同途径将草本物种划分为C_3和C_4功能群。选取斑块面积、形状指数、隔离度指数及每个斑块500 m缓冲区内的森林和草地比例作为景观因子。通过结构方程模型探讨C_3、C_4草本物种丰富度与景观因子的相互关系。斑块面积(2.18—74.06 hm~2)与C_3、C_4草本及总物种丰富度分布格局均具有显著的正相关关系(P0.05),且对C_3草本的影响最大;形状指数(1.06—3.11)、隔离度指数(33.51—327.65)对C_3、C_4草本及总物种丰富度影响不显著(P0.05);C_3、C_4草本及总物种丰富度与毗邻斑块草地比例(4.20%—64.95%)呈正相关,而与森林比例(35.05%—95.80%)呈负相关。研究区斑块面积和毗邻斑块植被构成是影响C_3、C_4草本植物的主要因素。在破碎化景观中保存面积大的天然植被斑块及提高毗邻斑块草地比例是保护关键C_3、C_4草本植物物种的有效途径。     

Between‐patch natal dispersal declines with increasing natal patch size and distance to other patches in the endangered Southern Dunlin Calidris alpina schinzii

Natal dispersal has profound consequences for populations through the movement of individuals and genes. Habitat fragmentation reduces structural connectivity by decreasing patch size and increasing isolation, but understanding of how this impacts dispersal and the functional connectivity of landscapes is limited because many studies are constrained by the size of the study areas or sample sizes to accurately capture natal dispersal. We quantified natal dispersal probability and natal dispersal distances in a small migratory shorebird, the Southern Dunlin Calidris alpina schinzii, with data from two extensively monitored endangered metapopulations breeding in Sweden and Finland. In both metapopulations philopatry was strong, with individuals returning to or close to their natal patches more often than expected by chance, consistent with the patchy distribution of their breeding habitat. Dispersal probabilities were lower and dispersal distances were shorter in Sweden. These results provide a plausible explanation for the observed inbreeding and population decline of the Swedish population. The differences between Sweden and Finland were explained by patch‐specific differences. Between‐patch dispersal decreased with increasing natal patch size and distance to other patches. Our results suggest that reduced connectivity reduces movements of the philopatric Dunlin, making it vulnerable to the effects of inbreeding. Increasing connectivity between patches should thus be one of the main goals when planning future management. This may be facilitated by creating a network of suitably sized patches (20–100 ha), no more than 20 km apart from each other, from existing active patches that may work as stepping stones for movement, and by increasing nest success and pre‐fledging survival in small patches.     

Factors related to the inter‐annual variation in plants' pollination generalization levels within a community

The number of pollinators of a plant species is considered a measure of its ecological generalization and may have important evolutionary and ecological implications. Many pollination studies report inter‐annual fluctuations in the composition of pollinators to particular species. However, the factors causing such variation are still poorly understood. Here we investigate how flowering duration and plant and pollinator assemblages influenced the inter‐annual changes in the functional generalization level of the 20 most common plant species of a semi‐natural meadow in southern Norway. We also studied the extent to which changes in generalization levels were controlled by flower‐shape and flowering time. Large inter‐annual changes in generalization levels were common and there was no relationship between the generalization level one year and the following. Generalization level of particular plant species increased with flowering duration, sampling effort, and the abundance of managed honeybees in the community. Generalization level decreased with the flowering synchrony between the focal plant species and the rest of the plant community and with the focal species’ own abundance, which we attribute to inter‐specific competition for pollinator attraction and foraging decisions made by pollinators. Plants with different flower‐shapes and flowering times did not differ in the extent of inter‐annual variation in generalization levels. Most studies do not consider the effect of the plant community on the generalization level of particular plant species. We show here that both pollinator and plant assemblages can affect the inter‐annual variation in generalization levels of plant species. Studies like ours will help to understand how pollination interactions are structured at the community level, and the ecological and evolutionary consequences that these inter‐annual changes in generalization levels may have.