Vegetation dynamics following resource manipulations in herb-rich woodland

Changes in species richness, turnover, composition and above-ground biomass of herb-rich woodland were documented following fertilizer application and water addition over three growing seasons. Addition of fertilizer significantly reduced species richness relative to unmanipulated control and water addition plots after 3 years. This change coincided with significant increases in biomass, which were largely due to increased growth of exotic annual grasses. The reductions in richness observed in the fertilized plots were a consequence of both lower rates of local colonization and enhanced rates of local extinction of the resident species. Species loss was not random; native species were lost after nutrient addition whereas exotic species were not. Nutrient limitation was more important for species coexistence in these communities than was water availability.     

Direct and indirect species interactions influencing within-season dynamics of apple rust mite, Aculus schlechtendali (Acari: Eriophyidae)

We conducted a series of path analyses to determine which direct and indirect species interactions were most important in determining the within-season dynamics of apple rust mite (Aculus schlechtendali) (Eriophyidae) in a series of Nova Scotian (Canada) apple orchards. Typhlodromus pyri (Phytoseiidae) was usually more important than Zetzellia mali (Stigmaeidae) in the direct reduction of the apple rust mite population growth rate. When both predators contributed to lowering apple rust mite growth, T. pyri acted earlier in the season than did Z. mali. Predation by T. pyri was not affected by plant quality (nitrogen content or cultivar), but predation by Z. mali was enhanced by high nitrogen levels and was influenced by cultivar. There was little evidence of direct competition between Panonychus ulmi and apple rust mite in these orchards, possibly due to the low densities of P. ulmi. The indirect effects were generally smaller than the direct effects, but were occasionally important. Apparent competition between P. ulmi and apple rust mite was observed at times in some orchards, more often mediated by Z. mali than by T. pyri. Interference between the predator species was only occasionally strong enough to affect apple rust mite population dynamics.     

The effect of the expansion of the clonal grass Calamagrostis epigejos on the species turnover of a semi‐arid grassland

Question: How does the dominance of Calamagrostis epigejos influence species turnover of a grassland? Location: Loess grassland at the foothills of Bükk Mountains, Hungary (47°54’ N., 20°35’ E). Methods: Presence/absence of vascular plants and different performance attributes of C. epigejos were recorded in a plot‐subplot system between 2002 and 2005. Appearance and disappearance rates of grassland species were calculated for pairs of consecutive years. 1. Mean appearance and disappearance rates were compared in grassland plots dominated by C. epigejos and in plots free from this species, based on Monte Carlo randomization. 2. Mean appearance rates were assessed for categories of C. epigejos performance and their confidence intervals were calculated via Monte Carlo randomization. For two performance variables (percentage cover and shoot number) analyses were performed at two spatial scales. Results: 1. C. epigejos.‐dominated plots differed from unaffected ones by significantly lower appearance rates. 2. Change in appearance rates was best explained by differences in percentage cover of C. epigejos. Coarse‐scale C. epigejos performance had a closer correspondence with appearance rate change than fine‐scale performance. Low level C. epigejos performance enhanced appearance rate compared to intact stands, while high level performance decreased it, regardless of the choice of performance measure. Conclusions: C. epigejos lowers species number by hindering reappearance of species of the original grassland. This is best explained by the increased shading effect at the coarse scale. The marked non‐linear initial enhancement in appearance rate, however, can also be taken as an early sign of future species loss.     

The Dynamic keyhole-key model of coexistence to explain diversity of plants in limestone and other grasslands

Abstract. This paper is a tribute to A.S. Watt who published his ‘Pattern and process in the plant community’ almost 50 years ago. Watt's interpretation of the plant community “as a working mechanism, which maintains and regenerates itself” is still highly relevant, although the keywords have changed. ‘Process’ in Watt's view involves both upgrade and downgrade aspects, whereas ‘Pattern’ was not specified, neither quantified. Nowadays. process is mainly approached as ‘disturbance’, that is natural disturbance and ‘pattern’ as patch structure. Together they make up the ‘patch dynamics’ of the community. Some implications of patch dynamics for phytosociology are discussed. A ‘Wattian’ concept of the plant community combines the Gleasonian idea of individualistic behaviour of species with the Clementsian (or rather Braun-Blanquetian) notion of community dynamics. Later work by Harper (demography), Grubb (regeneration niche) and earlier work of Sernander (forest gap dynamics) is significant for the understanding of the patch-dynamic nature of the community. Recent interest in plant species mobility can easily be linked to the concept of patch dynamics. Examples of mobility in a limestone grassland are given and a system of mobility types is proposed. Some perspectives for the study of patch dynamics are mentioned. Numerical pattern analysis should have a more prominent place in this type of study; the significance of the study of small permanent plots in a stand is emphasized, and unprejudiced demographic studies, as well as experimental studies of small-scale species replacement are recommended.     

Pattern and process in the plant community: Fifty years after A.S. Watt

Abstract. This paper is a tribute to A.S. Watt who published his ‘Pattern and process in the plant community’ almost 50 years ago. Watt's interpretation of the plant community “as a working mechanism, which maintains and regenerates itself” is still highly relevant, although the keywords have changed. ‘Process’ in Watt's view involves both upgrade and downgrade aspects, whereas ‘Pattern’ was not specified, neither quantified. Nowadays, process is mainly approached as ‘disturbance’, that is natural disturbance and ‘pattern’ as patch structure. Together they make up the ‘patch dynamics’ of the community. Some implications of patch dynamics for phytosociology are discussed. A ‘Wattian’ concept of the plant community combines the Gleasonian idea of individualistic behaviour of species with the Clementsian (or rather Braun-Blanquetian) notion of community dynamics. Later work by Harper (demography), Grubb (regeneration niche) and earlier work of Sernander (forest gap dynamics) is significant for the understanding of the patch-dynamic nature of the community. Recent interest in plant species mobility can easily be linked to the concept of patch dynamics. Examples of mobility in a limestone grassland are given and a system of mobility types is proposed. Some perspectives for the study of patch dynamics are mentioned. Numerical pattern analysis should have a more prominent place in this type of study; the significance of the study of small permanent plots in a stand is emphasized, and unprejudiced demographic studies, as well as experimental studies of small-scale species replacement are recommended.     

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.     

长江口沿岸碎波带仔稚鱼的种类组成及其多样性特征

为了进一步探明长江口沿岸碎波带仔稚鱼的种类组成和多样性特征,2006年7月-2007年6月的每月大潮期间,在长江口沿岸碎波带13个站位点的水深0.5-1.5m处,两人沿海岸平行方向步行拖曳小型拖网(1m×4m,网目1mm)采集仔稚鱼样本。周年共拖网397网次,采集到仔稚鱼49045尾。仔稚鱼隶属于31科,共84种,其中海洋性鱼类16种,河口性鱼类30种,洄游性鱼类4种,淡水性鱼类34种。洄游性鱼类刀鲚占总个体数的82.63%,为最优势种。仔稚鱼的物种数、个体数和丰度在冬季较低,以3月份最低,夏季最高。Margalef丰富度指数2006年11月—2007年4月相对较低,2006年10月最高。Shannon-Wiener多样性指数在10月最高,7月最低。Pielou均匀度指数全年较恒定,2006年7月和2007年6月相对较低。     

澜沧江下游/湄公河上游片断热带雨林物种多样性动态

 在澜沧江下游/湄公河上游的滇南西双版纳地区,通过样方法比较了热带雨林的连片与3个小片断的物种多样性变化趋势。与连续森林比较,片断热带雨林的植物物种丰富度和物种多样性指数都比较低,而且有相当低比例的大高位芽、中高位芽和附生等生活型植物,而藤本、小高位芽和矮高位芽等生活型植物的比例则较高;泛热带、热带亚洲至热带非洲的区系成分比例较高,而当地成分则减少;群落的上层树木比下层树木更加稳定。同样,动物的物种多样性指数和均衡度在片断热带雨林中都较低,与其密切相关的是片断热带雨林的环境质量,而不是片断的大小。此外,也探讨了片断热带雨林物种变化与森林小气候的关系,阐明了由凉湿向干暖转化的“林内效应”是其物种变化的重要原因之一。     

Fire-mediated effects of overstorey on plant species diversity and abundance in an eastern Australian heath

Overstorey shrub species are known to influence the composition of theunderstorey in Southern Hemisphere heathlands. Overstorey densities aresusceptible to variations in fire frequency; thus, fire regimes may influenceoverstorey/understorey interactions and overall floristic composition. Wecompared patches of Banksia heath which had supported anoverstorey during a fire interval of about 30 y with patches wherethe overstorey was absent during the same period, and tested for differences inspecies composition as a function of overstorey presence. Floristic compositionvaried significantly between overstorey patches and open patches. Most specieswere less abundant in overstorey patches, however some were more abundant. Therelative abundance of species in relation to overstorey was unrelated to theirfire response, propagule longevity or propagule storage location. There wassignificantly less biomass in overstorey patches compared with open patches.Theeffect of the overstorey varied with soil moisture. In a dry area, the numberof species was lower in overstorey patches, with fewer herb and shrub speciespresent compared with open patches. Fewer species were recorded in a wetterarea, but overstorey had no effect on the number of species recorded. Reducedintensity of competition among understorey species in overstorey patches couldbe responsible for the higher abundance of some species in these patches. Wepostulate that full diversity will be maintained when the density of overstoreyshrubs fluctuates widely over a relatively short period of time. This is mostlikely when fire frequency is highly variable.     

What makes ecological systems reactive?

Although perturbations from a stable equilibrium must ultimately vanish, they can grow initially, and the maximum initial growth rate is called reactivity. Reactivity thus identifies systems that may undergo transient population surges or drops in response to perturbations; however, we lack biological and mathematical intuition about what makes a system reactive. This paper presents upper and lower bounds on reactivity for an arbitrary linearized model, explores their strictness, and discusses their biological implications. I find that less stable systems (i.e. systems with long transients) have a smaller possible range of reactivities for which no perturbations grow. Systems with more species have a higher capacity to be reactive, assuming species interactions do not weaken too rapidly as the number of species increases. Finally, I find that in discrete time, reactivity is determined largely by mean interaction strength and neither discrete nor continuous time reactivity are sensitive to food web topology.     

集合群落及其在河流鱼类群聚研究中的应用

集合群落(metacommunity)是指多个潜在相互作用的物种通过它们之间的扩散而连接在一起的一组局域群落,目前已成为斑块生境下生物群落结构、格局和动态的重要理论基础之一。斑块动态、物种排序、群体效应和中性模型等4种理论模型,可用于解释不同情形下集合群落内物种的迁移状况,描述集合群落的动态。可采用群落结构或生态学机制等途径,来阐述所研究的群落是属于哪一种特定的集合群落类型。集合群落可用于研究河流鱼类群聚,解释鱼类的群落结构等问题。另外本文还结合我国水域生态环境及水生生物现状,对今后集合群落的研究作了展望。     

Lost and found: Short-term dynamics of the flora on 100 small islands in the White Sea

The aim was to uncover factors that influence short-term (decade) flora dynamics and species richness of northern marine islets characterized by poor flora and weak anthropogenic pressure. The study used presence–absence data of vascular plant species on 100 small uprising islets of the Kandalaksha Gulf of White Sea (Northern Karelia, Russia). We investigated the influence of islands' attributes on species richness and rates of flora dynamics. Two island types were analyzed separately: younger, stone-like and older, islet-like (which generally are larger and have higher diversity of habitats). Sampled islands were studied via classical biogeographical per island approach and metapopulation per species approach. Stone-like islands had noticeably poorer flora with higher rates of immigration and extinction when compared to those of islet-like islands. The species number for islet-like islands correlated positively with number of habitats, abundance of different habitat types and island area. Species richness of stone-like islands correlated positively only with number of habitat types. Plant species associated with birds, crowberry thickets and coastal rocks were the most stable, and the species of disturbed habitats were significantly less stable. Floristic changes that have occurred have been caused by the massive establishment of new species rather than the extinction of pre-existing taxa. Thus, most of these islands are still in the colonization (assortative) stage. While we found no relationship between island area and species number for stone-like islands, this relationship was seen on islet-like islands.     

Species effects on ecosystem processes are modified by faunal responses to habitat composition

Heterogeneity is a well-recognized feature of natural environments, and the spatial distribution and movement of individual species is primarily driven by resource requirements. In laboratory experiments designed to explore how different species drive ecosystem processes, such as nutrient release, habitat heterogeneity is often seen as something which must be rigorously controlled for. Most small experimental systems are therefore spatially homogeneous, and the link between environmental heterogeneity and its effects on the redistribution of individuals and species, and on ecosystem processes, has not been fully explored. In this paper, we used a mesocosm system to investigate the relationship between habitat composition, species movement and sediment nutrient release for each of four functionally contrasting species of marine benthic invertebrate macrofauna. For each species, various habitat configurations were generated by selectively enriching patches of sediment with macroalgae, a natural source of spatial variability in intertidal mudflats. We found that the direction and extent of faunal movement between patches differs with species identity, density and habitat composition. Combinations of these factors lead to concomitant changes in nutrient release, such that habitat composition effects are modified by species identity (in the case of NH4-N) and by species density (in the case of PO4-P). It is clear that failure to accommodate natural patterns of spatial heterogeneity in such studies may result in an incomplete understanding of system behaviour. This will be particularly important for future experiments designed to explore the effects of species richness on ecosystem processes, where the complex interactions reported here for single species may be compounded when species are brought together in multi-species combinations.     

The trade-off between dispersability and longevity - an important aspect of plant species diversity

Abstract. Local presence of plant species is determined by population colonizations and extinctions. All traits that influence the capacity of individuals to colonize patches and survive within patches, are therefore important for community diversity. Spatial models can explain the coexistence of species provided that the inferior competitor has a greater spatial mobility and thereby can avoid competition. We searched the literature for empirical evidence for such trade-offs and included all available information on correlations between traits associated with the capacity to colonize and traits promoting the ability to survive. A lower reproductive effort of a species is associated with a longer life span and a higher competitive ability. Morphological adaptations for dispersal are less common in species which better tolerate stress, that are better competitors or possess seed dormancy. Such patterns suggest that species that are good survivors may have a limited ability to colonize new patches and vice versa. A negative correlation between dispersability and longevity has important effects on the regional dynamics of single species as well as on the coexistence of species. From a conservation perspective differences in the colonization capacity among species imply that restoration of plant biodiversity must not only focus on conditions within patches, but also consider the spatial arrangement of patches in order to enable plants to bridge gaps in time and space.     

Impact of resource availability on species composition and diversity in freshwater nematodes

This study investigates the long-term effects of resource availability in a freshwater nematode community. We carried out a mesocosm experiment where natural nematode communities were exposed to nutrient addition/depletion over 2 years. Compared to the nutrient-addition treatment, species richness and diversity were strongly reduced upon nutrient depletion. The functional group of bacterial feeders particularly suffered severely from nutrient depletion. The decrease in diversity of bacterial feeders was linked to reduced species richness and diversity of large omnivorous species, as predicted by trophic-dynamic models. Tilmans (1976) statement, that under low nutrient levels the best competitor dominates the system, was applicable in our system. Upon nutrient depletion, resource depletion led to a monoculture of 1 small bacterial feeder, but even after 2 years of resource depletion, up to 16 species still coexisted. Our results provide strong evidence that freshwater nematode systems can be regulated by nutrient competition.     

南京灵谷寺森林50年来的动态变化研究

 为进一步揭示北亚热带森林次生演替规律,于2002年5月在定点样带上沿用前人的方法,对南京灵谷寺森林进行了群落调查。通过比较1951、1981、2002年的定点样带资料,对灵谷寺森林的物种组成、多度、频度、重要值、种群结构以及物种多样性的动态变化进行了研究。50多年来,群落中立木株数相对基本稳定,但空间分布异质性逐渐增加,而林下苗木和灌木个体数波动幅度极大,由1951年的4 712株大幅上升至1981年的44 130株,到2002年又回落至7 372株。群落中物种数和建群种构成等都存在明显变化,物种数由1951年的75种下降到1981年的50种,2002年又上升为73种。物种多样性指数也存在波动,但目前尚未发现物种多样性指数有明显的上升或下降趋势。历经50多年的次生演替,早期人工针叶林中的马尾松（Pinus massoniana）逐渐衰退,首先被阳性落叶阔叶树,如黄连木（Pistacia chinensis）、枫香（Liquidambar formosana）等取代,之后又被相对耐荫的栓皮栎（Quercus variabilis）等树种所替代,一些常绿树种成功入侵,人工针叶林经针阔混交林、落叶阔叶林,逐渐向地带性植被落叶 常绿阔叶混交林方向演替,其中立地条件较好地段的群落演替速度也较快。     

Biodiversity maintenance in food webs with regulatory environmental feedbacks

Although the food web is one of the most fundamental and oldest concepts in ecology, elucidating the strategies and structures by which natural communities of species persist remains a challenge to empirical and theoretical ecologists. We show that simple regulatory feedbacks between autotrophs and their environment when embedded within complex and realistic food-web models enhance biodiversity. The food webs are generated through the niche-model algorithm and coupled with predator-prey dynamics, with and without environmental feedbacks at the autotroph level. With high probability and especially at lower, more realistic connectance levels, regulatory environmental feedbacks result in fewer species extinctions, that is, in increased species persistence. These same feedback couplings, however, also sensitize food webs to environmental stresses leading to abrupt collapses in biodiversity with increased forcing. Feedback interactions between species and their material environments anchor food-web persistence, adding another dimension to biodiversity conservation. We suggest that the regulatory features of two natural systems, deep-sea tubeworms with their microbial consortia and a soil ecosystem manifesting adaptive homeostatic changes, can be embedded within niche-model food-web dynamics.     

Nutrient ratios influence variability in Pseudo-nitzschia species diversity and particulate domoic acid production in the Bay of Seine (France)

The population dynamics of different Pseudo-nitzschia species, along with particulate domoic acid (pDA) concentrations, were studied from May 2012 to December 2013 in the Bay of Seine (English Channel, Normandy). While Pseudo-nitzschia spp. blooms occurred during the two years of study, Pseudo-nitzschia species diversity and particulate domoic acid concentrations varied greatly. In 2012, three different species were identified during the spring bloom (P. australis, P. pungens and P. fraudulenta) with high pDA concentrations (∼1400 ng l⁻¹) resulting in shellfish harvesting closures. In contrast, the 2013 spring was characterised by a P. delicatissima bloom without any toxic event. Above all, the results show that high pDA concentrations coincided with the presence of P. australis and with potential silicate limitation (Si:N < 1), while nitrate concentrations were still replete. The contrasting environmental conditions between 2012 and 2013 highlight different environmental controls that might favour the development of either P. delicatissima or P. australis. This study points to the key role of Pseudo-nitzschia diversity and cellular toxicity in the control of particulate domoic acid variations and highlights the fact that diversity and toxicity are influenced by nutrients, especially nutrient ratios.     

An evaluation of randomization models for nested species subsets analysis

Randomization models, often termed “null” models, have been widely used since the 1970s in studies of species community and biogeographic patterns. More recently they have been used to test for nested species subset patterns (or nestedness) among assemblages of species occupying spatially subdivided habitats, such as island archipelagoes and terrestrial habitat patches. Nestedness occurs when the species occupying small or species-poor sites have a strong tendency to form proper subsets of richer species assemblages. In this paper, we examine the ability of several published simulation models to detect, in an unbiased way, nested subset patterns from a simple matrix of site-by-species presence-absence data. Each approach attempts to build in biological realism by following the assumption that the ecological processes that generated the patterns observed in nature would, if they could be repeated many times over using the same species and landscape configuration, produce islands with the same number of species and species present on the same number of islands as observed. In mathematical terms, the mean marginal totals (column and row sums) of many simulated matrices would match those of the observed matrix. Results of model simulations suggest that the true probability of a species occupying any given site cannot be estimated unambiguously. Nearly all of the models tested were shown to bias simulation matrices toward low levels of nestedness, increasing the probability of a Type I statistical error. Further, desired marginal totals could be obtained only through ad-hoc manipulation of the calculated probabilities. Paradoxically, when such results are achieved, the model is shown to have little statistical power to detect nestedness. This is because nestedness is determined largely by the marginal totals of the matrix themselves, as suggested earlier by Wright and Reeves. We conclude that at the present time, the best null model for nested subset patterns may be one based on equal probabilities of occurrence for all species. Examples of such models are readily available in the literature. Received: 3 February 1997 / Accepted: 21 September 1997