Physiological tolerances account for range limits and abundance structure in an invasive slug

Despite the importance of understanding the mechanisms underlying range limits and abundance structure, few studies have sought to do so. Here we use a terrestrial slug species, Deroceras panormitanum, that has invaded a remote, largely predator-free, Southern Ocean island as a model system to do so. Across Marion Island, slug density does not conform to an abundant centre distribution. Rather, abundance structure is characterized by patches and gaps. These are associated with this desiccation-sensitive species'' preference for biotic and drainage line habitats that share few characteristics except for their high humidity below the vegetation surface. The coastal range margin has a threshold form, rapidly rising from zero to high density. Slugs do not occur where soil-exchangeable Na values are higher than 3000 mg kg⁻¹, and in laboratory experiments, survival is high below this value but negligible above it. Upper elevation range margins are a function of the inability of this species to survive temperatures below an absolute limit of −6.4°C, which is regularly exceeded at 200 m altitude, above which slug density declines to zero. However, the linear decline in density from the coastal peak is probably also a function of a decline in performance or time available for activity. This is probably associated with an altitudinal decline in mean annual soil temperature. These findings support previous predictions made regarding the form of density change when substrate or climatic factors set range limits.     

A neutral model of edge effects

In this paper a spatially implicit neutral model for explaining the edge effects between habitats is proposed. To analyze this model we use two different approaches: a discrete approach that is based on the Master equation for a one step jump process and a continuous approach based on the approximation of the discrete jump process with the Kolmogorov-Fokker-Planck forward and backward equations. The discrete and continuous approaches are applied to analyze the species abundance distributions and the time to species extinction. Moreover, with the aid of the continuous approach a realistic classification of the behavior of species in local communities is developed. The species abundance dynamics at the edge between two distinct habitats is compared with those located in the homogeneous interior habitats using species abundance distributions and the first time to species extinction. We show that the structure of the links between local community and the metacommunity plays an important role on species persistence. Specifically, species at the edge between two distinct metacommunities have higher extinction rate than those in the interior habitats connected only to one metacommunity. Moreover, the same species might be persistent in the homogeneous interior habitat, but its probability of extinction from the edge local community could be very high.     

Neutral Models with Generalised Speciation

Hubbell’s neutral theory claims that ecological patterns such as species abundance distributions can be explained by a stochastic model based on simple assumptions. One of these assumptions, the point mutation assumption, states that every individual has the same probability to speciate. Etienne et al. have argued that other assumptions on the speciation process could be more realistic, for example, that every species has the same probability to speciate (Etienne, et al. in Oikos 116:241–258, 2007). They introduced a number of neutral community models with a different speciation process, and conjectured formulas for their stationary species abundance distribution. Here we study a generalised neutral community model, encompassing these modified models, and derive its stationary distribution, thus proving the conjectured formulas.     

Community stability under different correlation structures of species’ environmental responses

The outcome of species interactions in a variable environment is expected to depend on how similarly different species react to variation in environmental conditions. We study community stability (evenness and species diversity) in competitive communities that are either closed or subjected to random migration, under different regimes of environmental forcing. Community members respond to environmental variation: (i) independently (IR), (ii) in a positively correlated way (CR), or (iii) hierarchically, according to niche differences (HR). Increasing the amplitude of environmental variation and environmental reddening both reduce species evenness in closed communities through a reduction in species richness and increased skew in species abundances, under all three environmental response scenarios, although autocorrelation only has a minor effect with HR. Open communities show important qualitative differences, according to changes in the correlation structure of species’ environmental responses. There is an intermediate minimum in evenness for HR communities with increasing environmental amplitude, explained by the interaction of changes in species richness and changes in the variance of within-species environmental responses across the community. Changes in autocorrelation also lead to qualitative differences between IR, CR and HR communities. Our results highlight the importance of considering mechanistically derived, hierarchical environmental correlations between species when addressing the influence of environmental variation on ecological communities, not only uniform environmental correlation across all species within a community.     

神农架山地河岸带中珍稀植物群落特征

以领春木(Euptelea pleiospermum)、连香树(Cercidiphyllum japonicum)为优势种的珍稀植物群落是神农架山地中海拔地段河岸带中的典型群落类型.在神农架地区4大水系(沿渡河、香溪河、南河和堵河)的河岸带中共调查了48个样地,采用植物群落学方法,分析了该区山地河岸带中珍稀植物群落特征.结果表明:(1)群落中植物种类丰富,科属组成比较分散.共记录到维管束植物749种,隶属144科412属,其中每科仅记录到1个属的有72个科,每属仅记录到1个种的有256个属.(2)在科水平上,热带、亚热带成分与温带成分所占比例几乎相同,在属水平上温带成分占主导.东亚分布成分、中国特有成分所占比例较高,共记录了6个东亚特有科、50个东亚特有属,2个中国特有科、27个中国特有属.(3)中小型草质单叶的落叶阔叶高位芽和地面芽植物是决定该珍稀植物群落外貌的主要成分.(4)群落垂直结构复杂,成层现象明显.第一乔木亚层树冠层分布不连续,第一、第二乔木亚层和灌木层从高到低郁闭度逐渐增大,草本层盖度各样地差别较大,层间植物丰富.该山地河岸带中丰富的珍稀、特有和/或孑遗植物表现出该群落的古老、孑遗性.虽然该区多数珍稀植物为温带性质,但该珍稀植物群落整体上在种类组成、区系分布、生活型谱、叶片性质等方面表现出亚热带亚高山垂直带谱上常绿落叶阔叶混交林的过渡性特征.自上而下逐渐密集的群落垂直结构为幼苗稍耐荫、成年趋于喜光的珍稀植物的不同发育阶段提供了利于其生长的光环境.特殊的地理位置、独特的地形地貌和良好的水热条件使亚热带山地河岸带成为珍稀植物(尤其是孑遗植物)就地保护中一个关键而特殊的区域.     

科尔沁沙地不同沙漠化成因区域的景观动态比较

在3S技术支持下,比较分析了科尔沁沙地不同沙漠化成因区域的沙地景观动态变化过程。结果表明：近20年来,以人为活动干扰为主的区域（八仙筒镇）,沙漠化土地变化幅度较大,各种沙漠化土地的转化主要表现在沙漠化土地中流动和半固定沙地向固定沙地转化、固定沙地向农田和林地转化;以自然干扰为主的区域（苇莲苏乡）,沙漠化土地变化幅度较小,主要表现为各种沙地类型之间的转换;在人为活动为主的区域固定沙地的空间格局变化显著,1985—1995年固定沙地的联合度降低,人为活动干扰较强,而1995—2005年联合度增高,人为活动干扰相对较弱;在自然干扰为主的区域,同一时期空间格局的变化明显的是流动沙地,在1985—1995年,流动沙地斑块破碎度减小,流动沙地发育显著,在1995—2005年,流动沙地的破碎度继续减小,但其面积得到控制,表明治沙效果在这一时期是显著的。     

Land use improves spatial predictions of mountain plant abundance but not presence‐absence

Question: Does a land‐use variable improve spatial predictions of plant species presence‐absence and abundance models at the regional scale in a mountain landscape? Location: Western Swiss Alps. Methods: Presence‐absence generalized linear models (GLM) and abundance ordinal logistic regression models (LRM) were fitted to data on 78 mountain plant species, with topo‐climatic and/or land‐use variables available at a 25‐m resolution. The additional contribution of land use when added to topo‐climatic models was evaluated by: (1) assessing the changes in model fit and (2) predictive power, (3) partitioning the deviance respectively explained by the topo‐climatic variables and the land‐use variable through variation partitioning, and (5) comparing spatial projections. Results: Land use significantly improved the fit of presence‐absence models but not their predictive power. In contrast, land use significantly improved both the fit and predictive power of abundance models. Variation partitioning also showed that the individual contribution of land use to the deviance explained by presence‐absence models was, on average, weak for both GLM and LRM (3.7% and 4.5%, respectively), but changes in spatial projections could nevertheless be important for some species. Conclusions: In this mountain area and at our regional scale, land use is important for predicting abundance, but not presence‐absence. The importance of adding land‐use information depends on the species considered. Even without a marked effect on model fit and predictive performance, adding land use can affect spatial projections of both presence‐absence and abundance models.     

Herb layer response to broadleaf tree species with different leaf litter quality and canopy structure in temperate forests

Question: How do broadleaf tree species affect humus characteristics, herb layer composition and species diversity through their leaf litter quality and canopy structure? Location: Mixed broadleaf forests in Brandenburg, NE Germany. Methods: We studied the herb and tree layer composition in 129 undisturbed stands using a 10‐degree cover‐abundance and percentage scale, respectively. The main floristic gradients were extracted by non‐metric multidimensional scaling. Effects of tree species on the herb layer were analysed with partial Spearman rank correlation. We assessed affinities for specific tree species using indicator species analysis. Results: Both beech and oak influenced herb layer composition mainly through their litter quality, which resulted in deep Ol and Of horizons, respectively. The less dense canopy of oak, in contrast to the dense beech canopy, enhanced species diversity in favour of indifferent herb species (species not closely tied to forests). Lime was correlated with a distinct floristic gradient, but a direct effect on the herb layer cannot be proven with the available data. Effects of hornbeam were less pronounced. Conclusions: The relationship between the tree and herb layer must be partly attributed to pH differences. However, tree species effects on humus characteristics and on light flux to the ground were largely responsible as well. The results suggest that tree species can influence herb layer composition and diversity, but the missing correlation with lime and hornbeam raise questions requiring further detailed investigation.     

Small-scale plant species distribution in snowbeds and its sensitivity to climate change

Alpine snowbeds are characterized by a long-lasting snow cover and low soil temperature during the growing season. Both these key abiotic factors controlling plant life in snowbeds are sensitive to anthropogenic climate change and will alter the environmental conditions in snowbeds to a considerable extent until the end of this century. In order to name winners and losers of climate change among the plant species inhabiting snowbeds, we analyzed the small-scale species distribution along the snowmelt and soil temperature gradients within alpine snowbeds in the Swiss Alps. The results show that the date of snowmelt and soil temperature were relevant abiotic factors for small-scale vegetation patterns within alpine snowbed communities. Species richness in snowbeds was reduced to about 50% along the environmental gradients towards later snowmelt date or lower daily maximum temperature. Furthermore, the occurrence pattern of the species along the snowmelt gradient allowed the establishment of five species categories with different predictions of their distribution in a warmer world. The dominants increased their relative cover with later snowmelt date and will, therefore, lose abundance due to climate change, but resist complete disappearance from the snowbeds. The indifferents and the transients increased in species number and relative cover with higher temperature and will profit from climate warming. The snowbed specialists will be the most suffering species due to the loss of their habitats as a consequence of earlier snowmelt dates in the future and will be replaced by the avoiders of late-snowmelt sites. These forthcoming profiteers will take advantage from an increasing number of suitable habitats due to an earlier start of the growing season and increased temperature. Therefore, the characteristic snowbed vegetation will change to a vegetation unit dominated by alpine grassland species. The study highlights the vulnerability of the established snowbed vegetation to climate change and requires further studies particularly about the role of biotic interactions in the predicted invasion and replacement process.     

Significance of intra-inflorescence variation on flowering time of a spring ephemeral, <Emphasis Type="Italic">Gagea lutea</Emphasis> (Liliaceae), under seasonal fluctuations of pollinator and light availabilities

I studied the relationship between seed-set patterns within inflorescences and temporal variations in light and pollinator availabilities for 2 years in the spring ephemeral species Gagea lutea in a deciduous forest. Timing of canopy closure and seasonal trend of pollinator frequency did not synchronize with the annual fluctuation in flowering phenology. In the early snowmelt year, seed-set success reflected the seasonal pollinator abundance from early to middle flowering periods. In the late snowmelt year, however, seed-set rates were independent of pollinator activity and decreased with canopy closing even after hand pollination. The restricted seed production by defoliation and the increase in seed-set rates at the forest edge suggested that seed production was supported by current photosynthetic carbon gain. Thus, annual fluctuations of reproductive success can explain the variation in flowering phenology within a population although seasonal light deterioration would serve as a selective force for flowering in the early season.