Showing papers in "Perspectives in Plant Ecology Evolution and Systematics in 2008"

Predicting global change impacts on plant species' distributions: Future challenges

Abstract: Given the rate of projected environmental change for the 21st century, urgent adaptation and mitigation measures are required to slow down the on-going erosion of biodiversity. Even though increasing evidence shows that recent human-induced environmental changes have already triggered species' range shifts, changes in phenology and species' extinctions, accurate projections of species' responses to future environmental changes are more difficult to ascertain. This is problematic, since there is a growing awareness of the need to adopt proactive conservation planning measures using forecasts of species' responses to future environmental changes. There is a substantial body of literature describing and assessing the impacts of various scenarios of climate and land-use change on species' distributions. Model predictions include a wide range of assumptions and limitations that are widely acknowledged but compromise their use for developing reliable adaptation and mitigation strategies for biodiversity. Indeed, amongst the most used models, few, if any, explicitly deal with migration processes, the dynamics of population at the "trailing edge" of shifting populations, species' interactions and the interaction between the effects of climate and land-use. In this review, we propose two main avenues to progress the understanding and prediction of the different processes A occurring on the leading and trailing edge of the species' distribution in response to any global change phenomena. Deliberately focusing on plant species, we first explore the different ways to incorporate species' migration in the existing modelling approaches, given data and knowledge limitations and the dual effects of climate and land-use factors. Secondly, we explore the mechanisms and processes happening at the trailing edge of a shifting species' distribution and how to implement them into a modelling approach. We finally conclude this review with clear guidelines on how such modelling improvements will benefit conservation strategies in a changing world. (c) 2007 Rubel Foundation, ETH Zurich. Published by Elsevier GrnbH. All rights reserved.

Does phylogenetic relatedness influence the strength of competition among vascular plants

Abstract: A widely assumed but largely untested hypothesis central to ecology and evolutionary biology has been Charles Darwin's suggestion that closely related species will be more ecologically similar, and thus will compete more strongly with each other than they will with more distantly related species. We provide one of the first direct tests of the “competition-relatedness hypothesis” by combining two data sets: the relative competitive ability of 50 vascular plant species competing against 92 competitor species measured in five multi-species experiments, and measures of the phylogenetic relatedness of these species. In contrast to Darwin's assertion, there were weak relationships between the strength of competition and phylogenetic relatedness. Across all species studied, the competition-relatedness relationship was weak and not significant. This overall lack of pattern masked different responses of monocot and eudicot focal (phytometer) species. When monocots served as the focal (phytometer) species, the intensity of competition increased with the phylogenetic distance separating species, while competition decreased with phylogenetic distance for eudicot phytometers. These results were driven by the monocot-eudicot evolutionary split, such that monocots were poor competitors against eudicots, while eudicots are most strongly suppressed by other eudicots. There was no relationship between relatedness and competition for eudicots competing with other eudicots, while monocots did compete more intensely with closely related monocots than with distantly related monocots. Overall, the relationships between competition intensity and relatedness were weak compared to the strong and consistent relationships between competitive ability and functional traits such as plant size that have been reported by other studies. We suggest that Darwin's assertion that competition will be strongest among closely related species is not supported by empirical data, at least for the 142 vascular plant species in this study.

The 2004 tsunami in Aceh and Southern Thailand: A review on coastal ecosystems, wave hazards and vulnerability

Abstract: In December 2004, a huge tsunami in SE Asia claimed many lives and caused catastrophic damage. This event has stimulated a debate about the role played by coastal ecosystems such as mangrove forests and coral reefs in protecting low-lying coastal areas. While some observers claim that these ecosystems play an important role, others are more sceptical and fear that financial resources may be diverted away from tsunami preparedness programmes to ecosystem rehabilitation schemes. Here, we review the role of coastal ecosystems in mitigating sea wave hazards. In particular, we examine the influence of coastal vegetation in severely affected parts of Aceh and Southern Thailand during the 2004 tsunami, and include direct observations made during two field visits in 2006. Numerous scientific studies illustrate that coastal ecosystems are important in protecting the coast against normal and extreme wind-driven waves, including cyclonic storm surges. However, much less information is available concerning the role of coastal ecosystems in protecting against tsunamis, and most studies have been too limited in scope to be able to generalise over large geographic areas and diverse conditions. In particular, few studies have taken account of the high variability in the energy and speed of tsunami waves along different coastline stretches, making it difficult to compare the role of ecosystems in different regions. Other aspects that have been little studied include the spatial patterns of impact, e.g. flow diversion and channelling as a result of hydraulic resistance, and the effects of tree breaking and flow debris. However, despite these limitations, some tentative conclusions can be drawn about the protection provided by ecosystems against tsunami waves: (1) The most important predictor variables of the tsunami hazard (i.e. set-up and inundation) at the regional and landscape levels are distance to the tsunami source and coastal topography, in particular near-shore bathymetry. (2) The influence of coral reefs on tsunami waves is complex. While closed, intact reefs can provide some protection, water may be accelerated through channels in fragmented reefs, causing even greater destruction on land. (3) In contrast, seagrass beds appear to provide a more consistent buffering against tsunamis, though more data are needed to quantify this effect. (4) In several locations (particularly farther away from the tsunami source), mangroves and other vegetation probably provided some protection against the 2004 tsunami; in numerous other locations, however, vegetation provided no protection or may in some cases even have increased the hazard (e.g. by contributing to flow debris, or by channelling water flows). It seems that whether or not vegetation does provide protection depends on many factors, including stand size, density, species composition, structure and homogeneity. Overall, the value of vegetation as a potential tsunami buffer is probably fairly minor, though it cannot be completely dismissed. Many more detailed spatial and hydro-dynamic analyses are needed before the hazard-vegetation interactions can be realistically modelled and visualised, e.g. on risk maps. (5) Tsunami greenbelts should not be treated as alternatives to early warning systems. Greenbelts may only be considered as an economical (and multi-functional) means to provide relative hazard protection for material assets (e.g. infrastructure, agriculture). We recommend a more holistic view, with tsunami hazard mitigation being seen as one of several services provided by coastal ecosystems. Equally, the value of other ecosystem services may be seen within the frame of risk management as they foster social stability and, in case of disaster, robustness. To assess the overall value of these services and to develop more accurate indicators of risk will require much more transdisciplinary research.

Seed banks of invasive Australian Acacia species in South Africa: Role in invasiveness and options for management

Abstract: Despite impressive efforts at clearing stands of invasive Australian Acacia species in South Africa, insufficient attention has been given to understanding the role of seed banks in the invasiveness and long-term persistence of populations. We review information on seeds of these species, considering seed production, seed rain, and the dynamics of seeds in three layers: leaf litter, and upper and lower seed banks in the soil. Many factors affect the accumulation and susceptibility to destruction of seed banks and thus the opportunities for intervention to reduce seed numbers for each of these components. Reduction of seed banks is crucial for the overall success of the multi-million dollar management initiatives against these species. Classical biological control of buds, flower and young pods has reduced the seed production of many Australian acacias in South Africa. Fire can be applied to reduce seed numbers in the leaf litter and upper seed bank in some cases, although there are serious problems associated with high fire intensities in dense acacia stands. Other options, e.g. soil inversion and solarisation, exist to exercise limited reduction of seed numbers in some situations. There is little prospect of meaningful reduction of seed numbers in the lower seed bank. Preventing the accumulation of seed banks by limiting seed production through biological control is by far the most effective means, and in almost all cases the only practical means, of reducing seed numbers. This must be an integral part of management strategies. Several invasive Australian acacias are already under effective biological control, and further work to identify additional potential agents for all the currently invasive species and potentially invasive alien species is the top priority for improving the efficiency of management programmes.

Competition among plants: Concepts, individual-based modelling approaches, and a proposal for a future research strategy

Abstract: Competition is a key process in plant populations and communities. We thus need, if we are to predict the responses of ecological systems to environmental change, a comprehensive and mechanistic understanding of plant competition. Considering competition, however, only at the population level is not sufficient because plant individuals usually are different, interact locally, and can adapt their behaviour to the current state of themselves and of their biotic and abiotic environment. Therefore, simulation models that are individual-based and spatially explicit are increasingly used for studying competition in plant systems. Many different individual-based modelling approaches exist to represent competition, but it is not clear how good they are in reflecting essential aspects of plant competition. We therefore first summarize current concepts and theories addressing plant competition. Then, we review individual-based approaches for modelling competition among plants. We distinguish between approaches that are used for more than 10 years and more recent ones. We identify three major gaps that need to be addressed more in the future: the effects of plants on their local environment, adaptive behaviour, and below-ground competition. To fill these gaps, the representation of plants and their interactions have to be more mechanistic than most existing approaches. Developing such new approaches is a challenge because they are likely to be more complex and to require more detailed knowledge and data on individual-level processes underlying competition. We thus need a more integrated research strategy for the future, where empirical and theoretical ecologists as well as computer scientists work together on formulating, implementing, parameterization, testing, comparing, and selecting the new approaches.

Dispersal, demography and spatial population models for conservation and control management

Abstract: Spatial population dynamics can seldom be ignored in management aimed at conserving or controlling plant species in a spatial context. Therefore, spatial population models, that bring together knowledge about a species’ local demography and dispersal behavior, are of growing applied importance. Here, we survey increasingly complex analytical and simulation models that are being developed to describe both demography and dispersal in applied studies. Local population dynamics can be modeled in an unstructured way, by specifying age- or stage-structure or by modeling each individual. Dispersal is often summarized in population-spread models with descriptive and simple statistical models. Mechanistic models that incorporate the physical or behavioral dynamics of dispersal vectors, however, provide more insight and can more readily be applied to novel situations. Importantly, mechanistic models provide a tool for linking variation in species traits and environments to dispersal and population spread. Spatial population models span a wide range: from diffusion models, metapopulation models, integrodifference equation models, and Neubert–Caswell models, to spatially explicit individual-based models. The complexity (and biological realism) of such models often trades off with tractability: for instance, individual-based simulation models allow for unlimited incorporation of biological detail, but rarely for analytical exploration of the model dynamics. We discuss the advantages and disadvantages of these various spatial population models; the choice of the most appropriate model will depend on the management objective, the biological complexity, available data and the principle of parsimony. We present five case studies of endangered and invasive species for which spatial population models have been developed to inform management, for instance to decrease the spread rate of invasive species or to improve the regional persistence of endangered species. We also anticipate exciting new developments in both spatial analytical and spatial simulation models with increasing demographic, dispersal and spatial sophistication.

The state of plant population modelling in light of environmental change

Abstract: Plant population modelling has been around since the 1970s, providing a valuable approach to understanding plant ecology from a mechanistic standpoint. It is surprising then that this area of research has not grown in prominence with respect to other approaches employed in modelling plant systems. In this review, we provide an analysis of the development and role of modelling in the field of plant population biology through an exploration of where it has been, where it is now and, in our opinion, where it should be headed. We focus, in particular, on the role plant population modelling could play in ecological forecasting, an urgent need given current rates of regional and global environmental change. We suggest that a critical element limiting the current application of plant population modelling in environmental research is the trade-off between the necessary resolution and detail required to accurately characterize ecological dynamics pitted against the goal of generality, particularly at broad spatial scales. In addition to suggestions how to overcome the current shortcoming of data on the process-level we discuss two emerging strategies that may offer a way to overcome the described limitation: (1) application of a modern approach to spatial scaling from local processes to broader levels of interaction and (2) plant functional-type modelling. Finally we outline what we believe to be needed in developing these approaches towards a ‘science of forecasting’.

Relationships among levels of biodiversity and the relevance of intraspecific diversity in conservation - a project synopsis

Abstract: The importance of the conservation of all three fundamental levels of biodiversity (ecosystems, species and genes) has been widely acknowledged, but only in recent years it has become technically feasible to consider intraspecific diversity, i.e. the genetic component to biodiversity. In order to facilitate the assessment of biodiversity, considerable efforts have been made towards identifying surrogates because the efficient evaluation of regional biodiversity would help in designating important areas for nature conservation at larger spatial scales. However, we know little about the fundamental relationships among the three levels of biodiversity, which impedes the formulation of a general, widely applicable concept of biodiversity conservation through surrogates. Here, we present the set-up of an international, interdisciplinary project, I ntra B io D iv ( http://www.intrabiodiv.eu ), which studied vascular plant biodiversity at a large scale, i.e. across the European Alps and the Carpathians. Our assessment comprises species richness (high-mountain flora), genetic variation (amplified fragment length polymorphisms, AFLPs) and environmental diversity (modelled potential habitat diversity). Our primary aims were to test for correlations between intra- and interspecific diversity and to identify possible environmental surrogates to describe biodiversity in the two study regions. To the best of our knowledge, I ntra B io D iv represents the first multispecies study on intraspecific, molecular-genetic variation in relation with species and habitat diversity. Here, we outline the theoretical background, our sampling scheme, the technical approaches and the feasibility of a concentrated and standardized sampling effort. We further show exemplary results. Our three data sets will be made freely available and will provide a playground for further hypothesis testing in conservation, ecology or evolution open to the scientific community.

Species richness and identity affect the use of aboveground space in experimental grasslands

Abstract: Complementary resource use is regarded as a mechanism that contributes to positive relationships between biodiversity and ecosystem functioning. Here, we used a biodiversity experiment composed of nine potentially dominant species (grasses: Alopecurus pratensis, Arrhenatherum elatius, Dactylis glomerata, Phleum pratense, Poa trivialis; legumes: Trifolium pratense, T. repens; non-legume herbs: Anthriscus sylvestris, Geranium pratense) to test for differences among monocultures and mixtures and for effects of species richness and the presence of particular species on the use of aboveground space. The number of rooting shoots determined in a line transect increased from monocultures to mixtures. Particularly, the presence of A. elatius in mixtures caused a higher shoot density at the community level. The number of pin contacts per sampling point (cumulative cover) at the community level, analysed with the point intercept method, was higher in mixtures than monocultures, and higher in mixtures with than without A. elatius. The effect was attributable to increased densities across the strata of the vertical stand profile as well as to an increase in community height. The impact of species richness on the use of aboveground space differed considerably between individual species. A. elatius achieved increased densities across all strata of the stand profile, while D. glomerata reached higher densities with a more pronounced use of space in the upper strata with increasing species richness of mixtures. Cumulative cover of P. pratense and A. pratensis was not affected by species richness, while the remaining species decreased space use mostly in the upper strata with increasing species richness or in mixtures with the competitively superior A. elatius. Our study shows that potentially dominant species are limited in their ability for adaptive responses to canopy shading. Nevertheless, the differential responses to species richness of individual species with regard to vertical niche occupation resulted in positive diversity effects on aboveground space use at the community level.

Correlates of naturalization and occupancy of introduced ornamentals in Germany.

Abstract: Invasions are multistage processes and the performance of a species at different stages depends on socio-economic, biogeographical, ecological and evolutionary factors. Most studies addressing the factors that determine invasion success focus on one particular stage, usually by examining data on introduced species that have successfully naturalized, whereas species that fail to naturalize are often not considered. In this study, we examined naturalization success (whether a species escaped from cultivation and became naturalized in the wild) and occupancy (the number of grid cells of 6′ longitude×10′ latitude in which it is recorded) of up to 8018 ornamental plant species introduced into botanical gardens in Germany. Data on these introductions were extracted from the SYSTAX database (Information System of German Botanical Gardens), information on successful naturalization in Germany from the BiolFlor database and data on species traits from SYSTAX and the European Garden Flora. The effect of propagule pressure, biogeography, winter hardiness, life strategy, morphology and genetic variability on the probability of naturalization and the number of grid cells occupied was tested using regression models. The influence of phylogenetic dependence was considered within simple single variable models as a nested random effect. All traits that appeared significant in these simple models were combined in a multivariable model. The simplified multivariable model revealed an increasing probability of naturalization for species with a higher winter hardiness, a wider native range and a higher planting frequency in botanical gardens (Nagelkerke- R 2 of 0.196). Moreover, interactions between plant height and planting frequency and between growth form and winter hardiness also affected the probability of naturalization. The number of grid cells occupied was best explained by the winter hardiness (pseudo- R 2 of 0.61). The stratified pre-selection of ornamental plants by gardeners may hold the key to their successful escape from cultivation and subsequent naturalization.

Spatial modelling of succession-disturbance dynamics in forest ecosystems: Concepts and examples

Abstract: Over the last few decades it has become increasingly obvious that disturbance, whether natural or anthropogenic in origin, is ubiquitous in ecosystems. Disturbance-related processes are now considered to be important determinants of the composition, structure and function of ecological systems. However, because disturbance and succession processes occur across a wide range of spatio-temporal scales their empirical investigation is difficult. To counter these difficulties much use has been made of spatial modelling to explore the response of ecological systems to disturbance(s) occurring at spatial scales from the individual to the landscape and above, and temporal scales from minutes to centuries. Here we consider such models by contrasting two alternative motivations for their development and use: prediction and exploration, with a focus on forested ecosystems. We consider the two approaches to be complementary rather than competing. Predictive modelling aims to combine knowledge (understanding and data) with the goal of predicting system dynamics; conversely, exploratory models focus on developing understanding in systems where uncertainty is high. Examples of exploratory modelling include model-based explorations of generic issues of criticality in ecological systems, whereas predictive models tend to be more heavily data-driven (e.g. species distribution models). By considering predictive and exploratory modelling alongside each other, we aim to illustrate the range of methods used to model succession and disturbance dynamics and the challenges involved in the model-building and evaluation processes in this arena.

Herbaceous species responses to long-term effects of prescribed fire, grazing and selective tree cutting in the savanna-woodlands of West Africa

Abstract: Grazing, fire and selective tree cutting are major disturbances that shape species diversity in savanna ecosystems, yet their effects are highly variable. We carried out a factorial experiment with two levels to examine the effects of grazing, fire and selective tree cutting on herbaceous species richness, abundance and diversity on two sites in the Sudanian savanna-woodlands of Burkina Faso for 10 years (1994–2003). The results showed significant inter-annual variation in species richness, abundance and diversity at both sites (p<0.001), while main or combined effects of fire, grazing and selective cutting were very limited and varied between life forms and sites. Grazing tended to favour the diversity of perennial grasses; fire tended to influence the richness of annual grasses and abundance and diversity of perennial grasses while selective tree cutting had no effect on any of the vegetation attributes assessed. The combined effect of grazing, fire and selective cutting tended to increase the diversity of forbs. In many cases, the responses of herbaceous species to treatments were clearer on the site with deeper soils than the one with shallow soils. Depending on the site and treatments, the inter-annual variation in vegetation attributes was partly related to amount and/or frequency of rainfall and partly to inter-annual variation in grazing or fire intensity. It can be concluded that both disturbances and climatic condition influence the structure and diversity of herbaceous flora in the Sudanian savanna-woodland ecosystem. The responses were site-specific, which accentuates the importance of landscape-scale approaches to understand the impacts of disturbances on composition, structure and diversity of savanna ecosystems.

Which factors determine plant invasions in man-made habitats in the Czech Republic?

Abstract: Factors determining the invasibility of different types of anthropogenic vegetation were studied in the Czech Republic. A data set of 3420 vegetation plots recorded between 1945 and 2005, containing 913 species, was used. A set of climatic variables (mean annual temperature and precipitation, together with elevation), propagule pressure (substituted by human population density) and local habitat conditions (substituted by values of CSR life strategies and Ellenberg indicator values of native species) was obtained for each plot. All species were classified as native, archaeophytes (i.e. alien species introduced before 1500), and neophytes (i.e. aliens introduced after 1500) and their relative proportion was calculated for each plot. Regression tree models were used to determine the ecological characteristics of the most invasible man-made habitats in the Czech Republic. The plots contained on average 31.9% archaeophytes and 7.3% neophytes. Correlation between the proportions of archaeophytes and neophytes was positive and significant. Both archaeophytes and neophytes were found predominantly in strongly disturbed habitats with a high nutrient supply located at low elevations in warmer climatic areas of the Czech Republic. Archaeophytes are more influenced by local habitat conditions and preferentially colonize sunny and dry man-made habitats with higher soil reaction. Neophytes have no special preferences for local habitat conditions and their highest proportion was found mainly in disturbed habitats at low elevations. Our results show that for anthropogenic vegetation in the Czech Republic, ecological and habitat characteristics are more important factors for plant invasions than different land use in the surrounding area.

Spatial processes that maintain biodiversity in plant communities

Abstract: The composition of communities of sessile organisms, and the change in species diversity with time, is a spatially explicit phenomenon. Three spatial factors clearly affect diversity: (1) the structure and heterogeneity of the landscape that limits species immigration and ultimate community size; (2) neighborhood interactions that determine colonization and extinction rates and influence residence times of local populations; and (3) disturbances that open spatially contiguous areas for recolonization by less abundant species. The importance of these three factors was first reviewed and then examined with a spatially explicit, multi-species model of plant dispersal, competition and establishment, with an assumption of neutrality (all species had equivalent life histories) that reduced the initial dimensionality of the problem. The simulations assumed that the probability of immigration was a linear function of mainland abundance and distance to islands, similar to the equilibrium theory of island biogeography and the unified neutral theory of biodiversity. The rate of increase in species richness was not constant across island sizes, declining as island area became very large. This pattern was explained by the spatial dynamics of colonization and establishment, a non-random process that cannot be explained by passive sampling alone. Simulations showed that population establishment depended critically on rare long-distance dispersal events while population persistence was achieved by the formation of aggregated species distributions that developed through restricted dispersal and local competitive interactions. Nevertheless, species richness always declined to a single species in the absence of disturbances, while up to 40 species could persist to 10,000 years when spatially dependent mortality was added. Further explorations with spatially explicit models will be required to fully appreciate the consequence of land use change and altered disturbance regimes on patterns of species distribution and the maintenance of diversity.

Multi-proxy evidence for competition between savanna woody species

Abstract: Coexistence of trees and grasses in savannas should be possible if competition between the woody and the grassy components is less intense than the competition within each component. Although several studies have investigated competition between trees and grasses, little is known about tree–tree interactions. We used a multi-proxy approach to examine the spatial pattern of Acacia mellifera and other savanna woody species in a semi-arid savanna in South Africa. Spatial analysis of the point patterns of young and reproductively mature shrubs detected decreasing aggregation with size/age over all spatial scales. This indicated the prevalence of competition although the overall spatial shrub pattern was aggregated. In contrast to point pattern statistics that detect changes only when competition has led to the death of the inferior competitor, we also applied methods identifying the competitive effect on sizes of individual trees. Competition should lead to a negative spatial autocorrelation in size, which we observed in half of the studied cases. Quantile regressions show that nearest-neighbour distance increased steeply with combined size of the target shrub and its neighbours indicating strong competitive effects. The medians of the distributions of maximum root lengths of A. mellifera, of the scale of regular patterns, and of negative autocorrelations were not significantly different, suggesting that overlapping root systems mediate competitive interactions. A competitor removal experiment did not lead to increased shrub sizes, which may be due to the limited duration of the experiment. From the nearest neighbour and autocorrelation analyses, we conclude that competition had a strong impact on growth rates of savanna woody species. Competition-induced mortality only becomes obvious when analysing the shift towards less aggregated spatial patterns when shrubs become reproductively mature. As the overall clustered spatial pattern masks the perceptible effect of competition, a time component should always be included in spatial pattern-based inference of competition.

Disentangling the roles of climate, propagule pressure and land use on the current and potential elevational distribution of the invasive weed Oxalis pes-caprae L. on Crete.

Abstract: Climatic warming and land use change are likely to facilitate range expansions in invasive plant species, although the ability to predict such changes requires a better mechanistic understanding of the biological limits of populations. The introduced weed Oxalis pes-caprae, a significant pest of cultivation in many Mediterranean-type ecosystems, presents a suitable case study. The species distribution in the Mediterranean Basin closely follows that of olive cultivation, limited to below 600 m; yet its potential to colonise vulnerable areas at higher elevations has yet to be adequately assessed. To investigate the possibility, plant performance was assessed by experimentally sowing O. pes-caprae bulbils along an altitudinal gradient in the Lefka Ori mountains, Crete. The survivorship and bulbil biomass of the resulting plants all declined significantly with elevation, irrespective of soil type, initial bulbil size or seasonal variation. Whilst plants survived vegetatively up to 1400 m, seasonal bulbil productivity, likely to be critical to population viability, exceeded that of the sown bulbil biomass only below 750 m. These data indicate that the current elevation of O. pes-caprae is close to, but not at, its current climatic limit, and that low propagule pressure and scarcity of suitable habitat probably also act to limit the altitudinal distribution. Plant performance was correlated strongly with the duration of spring snow cover. Despite a 2 °C difference in mean spring temperatures in the 2 years of study, the predicted elevational change was only 37 m higher in the milder conditions. Overall, our results suggest that while O. pes-caprae performance is strongly linked to climate and is currently close to its climatic limit on Crete, there is limited scope for further spread unless land use and/or propagule pressure change at higher elevations. For this species, these elements are likely to be more significant drivers of invasion risk than the predicted changes of future climates.

How do introduction characteristics influence the invasion success of Mediterranean alien plants

Abstract: Invasive plant species are becoming increasingly widespread following accelerated anthropogenic activity in the Mediterranean region. Humans have played a central role in the expansion process, and it is important to incorporate such considerations into management plans. Using generalized linear models, our first aim was to describe how the invasion success of 862 prominent alien plant species on Mediterranean islands is related to characteristics of the introduction process: introduction frequency, date and region of origin, range size and purpose of import. The importance of each was measured by the numbers of species present and their average invasiveness. The main findings were: (a) accidental imports and ornamentals accounted for a high proportion of all aliens, although neither group had particularly high average invasiveness; (b) introduction frequency had a comparatively modest influence, with the most commonly-introduced species naturalized only three times more widely than those rarely-introduced; (c) rates of species introduction appear to have increased dramatically in the last century, although aliens which have been present in the region for more than 200 years were most widespread, indicating that it may be centuries before some species fill their potential range; (d) there were small tendencies for successful invaders to originate in the Neotropics or in regions with Mediterranean climate biomes and to have large range sizes. Our second aim was to determine whether the number or average invasiveness of species introduced via a given pathway had the most influence on the overall probability of invasion on a given island. An elasticity analysis suggested that the number of species was substantially the best predictor of the two. This finding arises largely because invasion events are rare and remain unpredictable, and has significant implications for assessing invasion risk. We discuss how substantial sources of error and intrinsic variability in invasiveness within species groups limit the potential for developing accurate risk models.

Consequences of invasion by the alien plant Mimulus guttatus on the species composition and soil properties of riparian plant communities in Scotland

Abstract: Invasive plant species are widely recognised to have severe ecological impacts in a wide range of ecosystems throughout the world, yet there are few experimental studies measuring community-level effects of invasive plant species. Thus most evidence is from correlative studies, and as such often cannot easily disentangle cause and effect. Through a combination of an addition and removal experiment and a correlative approach (multi-site comparisons), this study aimed to quantify the effects of a widespread invasive species, Mimulus guttatus, on species richness and soil properties of riparian plant communities. The marked negative association between Mimulus cover and plant species richness identified through correlative multi-site comparisons was consistent with experimental removal studies which indicate Mimulus significantly alters the structure of riparian plant communities. Total C and N and soil moisture were marginally higher in invaded than in uninvaded disturbed sediment plots. Following Mimulus removal, there was an increase in the occurrence and abundance of another non-native species, Claytonia sibirica, as well as germination and establishment of Mimulus seedlings. This highlights that, although removal increased richness, bringing the plant community closer structurally to uninvaded vegetation, the application of removal as a management tool needs to be undertaken with caution, as it may create opportunities for other invaders. The impact of Mimulus appeared restricted to disturbed sediment communities, as addition experiments into herb–grass communities were relatively unsuccessful in establishing Mimulus. These patterns were consistent with the distribution of the species in riparian plant communities. The addition experiments highlight that, as well as competition from the resident vegetation community, mollusc herbivory further hinders the establishment of Mimulus. Many manipulation studies have removed invasive plant species from heavily invaded communities, and it is often thought that invasive species usually affect plant community structure only where their cover is high. This study is unique in demonstrating impacts where cover of the invasive plant is relatively low.

Biological Flora of Central Europe: Baldellia ranunculoides (Alismataceae)

Abstract: Baldellia ranunculoides (L.) Parl. (Alismataceae) is a taxonomically problematic aquatic plant with an historically ill-defined distribution and global conservation status. This paper finds morphological, ecological and molecular evidence for two distinct taxa, probably best described as subspecies: (1) B. ranunculoides subsp. ranunculoides and (2) B. ranunculoides subsp. repens and provides detailed distribution data on their overlapping range, in different habitats, across the cool, high rainfall areas of western Europe and west Mediterranean. The two subspecies are amongst the relatively large number of threatened European and north Africa aquatic plants and this paper provides a systematic review of their relative conservation pressures and management needs, with particular emphasis on the status of both taxa in central Europe. Other observations indicate probable evolutionary relationships within B. ranunculoides s.l. and its associated taxa and the review points out where these and other research topics could potentially be pursued.

Prehistoric human influence on the abundance and distribution of deadwood in alpine landscapes

Abstract: Scientists have long inferred the locations of past treelines from the distribution of deadwood above modern tree boundaries. Although it is recognized that deadwood above treeline may have decayed, the absence of such wood is routinely taken to imply the absence of trees for periods ranging from the past few millennia to the entire Holocene. Reconstructed treeline histories are then explained in terms of such variables as slope, drainage, temperature, solar insolation, and precipitation. While these variables certainly help determine where deadwood is to be found above treeline today, we suggest that they cannot always explain where it is not to be found. In the alpine environments of the western United States, archeological work has established a human presence during nearly the entire Holocene in portions of the Rocky Mountains and for over 5000 radiocarbon years in the Great Basin and Sierra Nevada. We suggest that prehistoric occupations may have stripped deadwood from the landscape in all of these areas. To the extent that this is true, reconstructions of past treelines from deadwood may reflect the human prehistory of an area as much as it reflects treeline history itself. We encourage evaluation of this hypothesis in areas of active dendrochronological and archeological research.

A plant diversity×water chemistry experiment in subalpine grassland

Abstract: Plant diversity has been shown to drive important ecosystem functions such as productivity. At the same time, plant diversity and species composition are altered in alpine ecosystems by human impacts such as skiing. Therefore, we investigated impacts of decreased species richness and ski piste treatments on ecosystem functions in subalpine grassland. Species richness manipulations were combined with nutrient input from snow cover treated with snow additives that are commonly used on ski pistes. Three different species richness levels containing 1, 3 or 9 species randomly selected from a larger pool plus unmanipulated meadow plots were treated with four water types to simulate melt water. One water type contained the snow additive ammonium nitrate. Invasion into the communities was prevented by weeding during 2 years and allowed in three subsequent years. Higher species richness increased plant cover and biomass and decreased their variation. The number of functional groups in a plant assemblage had a positive effect on plant growth. Ammonium nitrate strongly increased biomass and plant cover after a single application but decreased species richness in originally diverse meadow plots. There was no significant interaction between species richness and water-type treatments. After the cessation of weeding, the species richness of different plot types converged within 3 years due to invasion. Nevertheless, relationships between initial species richness and plant cover remained positive. The results suggest that the diversity and species composition of alpine vegetation are important factors influencing cover and biomass, in particular during re-colonization of bare ground after disturbances such as ski-piste construction. In slow-growing alpine vegetation, initially positive diversity effects may remain even after successional convergence of species richness due to invasion. The negative effect of ammonium nitrate on species richness suggests the snow additives should only be used with care.

Biological flora of Central Europe: Atriplex tatarica L.

Abstract: Atriplex tatarica is a heterocarpic annual native to a wide area of Middle and western Central Asia, Asia Minor, North Africa, and Eastern Europe. It is an early successional species of disturbed habitats, in Central Europe mostly occupying road margins and waste places. This paper deals with taxonomic status, morphology, distribution, ecology, and population biology of this species with special emphasis on its population genetics.

Fungal and animal associates of Toxicodendron spp. (Anacardiaceae) in North America

Abstract: Toxicodendron spp. (Anacardiaceae; the poison ivies, oaks, and sumacs) are regarded by some as noxious, tenacious weeds in forests, grasslands, and waste places across the United States, despite playing numerous important ecological roles. Biological control of Toxicodendron spp. is a virtually unexplored option for ecosystem managers. The purpose of this review is twofold: (1) to synthesize and consider what is known about the biological relationships of Toxicodendron spp. from an ecological standpoint; and subsequently (2) to consider these associations from a managerial standpoint. Fungal, arthropodal, mammalian, and avian relationships are detailed, and their potential utility as biological control agents for Toxicodendron spp. are evaluated based on effectiveness, selectivity, practicality, and indirect or side effects. Fungi, and to a lesser extent arthropods, represent the most feasible agents for Toxicodendron spp. biological control.