Invasion by Heracleum mantegazzianum in different habitats in the Czech Republic

Abstract. Heracleum mantegazzianum, a tall forb from the western Caucasus invaded several different habitats in the Czech Republic. The relation between invasion success and type of recipient habitat was studied in the Slavkovskù les hilly ridge, Czech Republic. The vegetation of 14 habitat types occurring in an area of ca. 25 km² was analysed using phytosociological relevés, and the invasion success of Heracleum (in terms of number of localities, area covered and proportion of available area occupied) was recorded separately in each of them. Site conditions were expressed indirectly using Ellenberg indicator values. The hypothesis tested was that Heracleum spreads in the majority of vegetation types regardless of the properties of the recipient vegetation. Community invasibility appeared to be affected by site conditions and the composition of the recipient vegetation. The species is not found in acidic habitats. Disturbed habitats with good possibilities of dispersal for Heracleum seeds are more easily invaded. Communities with a higher proportion of phanerophytes and of species with CS (Competitive/Stresstolerating) strategy were more resistant to invasion. The invasion success was bigger in sites with increased possibilities of spread for Heracleum diaspores. Communities invaded by Heracleum had a lower species diversity and a higher indicator value for nitrogen than not-invaded stands. It appears that species contributing to community resistance against invasion of Heracleum, or capable of persisting in Heracleum-invaded stands, have similar ecological requirements but a different life strategy to the invader.     

Predicting the distribution of the invasive alien Heracleum mantegazzianum at two different spatial scales

Modelling the distribution of invasive alien species is widely used for predicting future dispersal, response to climate change, and effects of management, but little information is available on the scale dependence of spatial models. This study is focused on Heracleum mantegazzianum , a problematic invasive plant in central and north-western Europe. The main objective was to model the current distribution of this species at national (43,000 km²) and regional scale (4900 km²) using autologistic regression with a Danish data set. Presence–absence data were used in a grid system with 5 × 5 km² or 2 × 2 km² as basic units. To avoid misleading presence–absence models and unreliable probability values due to unbalanced data, the prevalence was used as cut-off value, and a favourability function was applied to the model predictions. The national model showed a widespread distribution of H. mantegazzianum with highest habitat suitability in the eastern and northern parts of the country where human population density is high, winters more severe and/or loamy soils more common. At a regional scale the distribution of H. mantegazzianum is associated with alluvial sand cover, high human population density, spring precipitation, and presence of the species in neighbour grid units. The observed widespread national distribution is likely the result of anthropogenic spread of this ornamental plant, while the locally clumped distribution suggests that H. mantegazzianum naturally spreads mainly over short distances. The current distribution in Denmark resembles an intermediate invasion stage where long-distance dispersal is less important, while spread from suitable neighbour habitats is significant. The study demonstrates that the favourability function leads to improved mapping standards for invasive species.     

Biological Invasions Across Spatial Scales: Intercontinental, Regional, and Local Dispersal of Cladoceran Zooplankton

The frequency of dispersal of invertebrates among lakes depends upon perspective and spatial scale. Effective passive dispersal requires both the transport of propagules and the establishment of populations large enough to be detected. At a global scale, biogeographic patterns of cladoceran zooplankton species suggest that effective dispersal among continents was originally rare, but greatly increased in the past century with expanded commerce. Genetic analysis allows some reconstruction of past dispersal events. Allozyme and mitochondrial DNA comparisons among New World and Old-World populations of several exotic cladocerans have provided estimates for likely source populations of colonists, their dispersal corridors, and timing of earlier dispersal events. Detecting the Old-World tropical exotic Daphnia lumholtzi early in its invasion of North America has allowed detailed analysis of its spatial spread. Twelve years of collection records indicate a rapid invasion of reservoirs in the United States, by both regional spread and long-distance jumps to new regions. Combining landscape features with zooplankton surveys from south-central US reservoirs revealed higher colonization rates of D. lumholtzi at lower landscape positions, a result which can be explained by either greater propagule load or by higher susceptibility of these downstream reservoirs. Because invaded reservoirs provide a source of propagules for nearby floodplain ponds, the rarity of this species in ponds suggests limitation by local environments. Such analyses of invading species over multiple spatial scales allow a better understanding of ecological processes governing invasion dynamics.     

Population age structure and reproductive behavior of the monocarpic perennial Heracleum mantegazzianum (Apiaceae) in its native and invaded distribution ranges

Many invasive species are benign in their native region-are there interactions between their key traits and the new habitats that explain invasion success? The giant perennial herb Heracleum mantegazzianum is a problematic invader in Europe and is also naturalized in North America. We compared its population structure and reproductive behavior in the native (W. Caucasus) and invaded (Czech Republic) areas in managed (pastures) and unmanaged sites. The age structure of the populations and age at flowering were analyzed using herb-chronology, a method based on counting annual rings in the secondary xylem of roots. The species was strictly monocarpic; most plants in unmanaged sites in the invaded range flowered in the third and fourth yr (maximum 12 yr). In unmanaged habitats, plants from the native range flowered later than those from the invaded range. In both ranges, flowering was delayed in managed sites where the population density was higher and most plants flowered around the fifth year. Reproductive output of individual plants was neither related to population density nor to age at flowering. More favorable climatic conditions in the invaded region, together with increased chances for dispersal in a densely colonized central Europe, seemed to allow the massive invasion.     

A simulation model of plant invasion: long-distance dispersal determines the pattern of spread

Mechanisms and consequences of biological invasions are a global issue. Yet, one of the key aspects, the initial phase of invasion, is rarely observed in detail. Data from aerial photographs covering the spread of Heracleum mantegazzianum (Apiaceae, native to Caucasus) on a local scale of hectares in the Czech Republic from the beginning of invasion were used as an input for an individual-based model (IBM), based on small-scale and short-time data. To capture the population development inferred from the photographs, long-distance seed dispersal, changes in landscape structures and suitability of landscape elements to invasion by H. mantegazzianum were implemented in the model. The model was used to address (1) the role of long-distance dispersal in regional invasion dynamics, and (2) the effect of land-use changes on the progress of the invasion. Simulations showed that already small fractions of seed subjected to long-distance dispersal, as determined by systematic comparison of field data and modelling results, had an over-proportional effect on the spread of this species. The effect of land-use changes on the simulated course of invasion depends on the actual level of habitat saturation; it is larger for populations covering a high proportion of available habitat area than for those in the initial phase of invasion. Our results indicate how empirical field data and model outputs can be linked more closely with each other to improve the understanding of invasion dynamics. The multi-level, but nevertheless simple structure of our model suggests that it can be used for studying the spread of similar species invading in comparable landscapes.     

The role of long‐distance seed dispersal in the local population dynamics of an invasive plant species

Aim Long‐distance dispersal is important for plant population dynamics at larger spatial scales, but our understanding of this phenomenon is mostly based on computer modelling rather than field data. This paper, by combining field data and a simulation model, quantifies the fraction of the seed of the alien species Heracleum mantegazzianum that needs to disperse over a long distance for successful invasion. Location Central Europe, Czech Republic. Methods To assess the role of random dispersal in long‐term population dynamics of the studied species, we combined longitudinal data covering 50 years of the invasion of this plant from its very start, inferred from a series of aerial photographs of 60‐ha plots, with data on population dynamics at a fine scale of 10‐m² plots. Results A simulation model based on field data indicates that the fraction of seed that is dispersed from source plants not described by the short‐distance dispersal kernel ranges from 0.1 to 7.5% of the total seed set. The fraction of long‐distance dispersed seed that provides the best prediction of the observed spread was significantly negatively correlated with the percentage of habitats suitable for invasion. Main conclusions Our results indicate that the fraction of seeds that needed to be dispersed over long distances to account for the observed invasion dynamics decreased with increasing proportion of invasible habitats, indicating that the spatial pattern of propagule pressure differs in landscapes prone to invasion. Long‐distance dispersal is an important component of the population dynamics of an invasive species even at relatively small scales.     

Fronts,jumps and secondary introductions suggested as different invasion patterns in marine species,with an increase in spread rates over time

Not all introduced (invasive) species in a region will spread from a single point of introduction. Long-distance dispersal or further introductions can obscure the pattern of spread, but the regional importance of such processes is difficult to gauge. These difficulties are further compounded when information on the multiple scale process of invasive species range expansion is reduced to one-dimensional estimates of spread (e.g. km yr⁻¹). We therefore compared the results of two different metrics of range expansion: maximum linear rate of spread and accumulation of occupied grid squares (50 × 50 km) over time. An analysis of records for 54 species of introduced marine macrophytes in the Mediterranean and northeast Atlantic revealed cases where the invasion process was probably missed (e.g. Atlantic Bonnemaisonia hamifera) and suggested cases of secondary introductions or erratic jump dispersal (Dasysiphonia sp. and Womersleyella setacea). A majority of species analysed showed evidence for an accumulation of invaded sites without a clear invasion front. Estimates of spread rate are increasing for more recent introductions. The increase is greater than can be accounted for by temporally varying search effort and implies a historical increase in vector efficiency and/or a decreased resistance of native communities to invasion.     

Genetic and morphological divergence reveals local subdivision of perch (Perca fluviatilis L.)

The level of gene flow is an important factor influencing genetic differentiation between populations. Typically, geographic distance is considered to be the major factor limiting dispersal and should thus only influence the degree of genetic divergence at larger spatial scales. However, recent studies have revealed the possibility for small-scale genetic differentiation, suggesting that the spatial scale considered is pivotal for finding patterns of isolation by distance. To address this question, genetic and morphological differentiation were studied at two spatial scales (range 2–13 km and range 300 m to 2 km) in the perch ( Perca fluviatilis L.) from the east coast archipelago of Sweden, using seven microsatellite loci and geometric morphometrics. We found highly significant genetic differentiation between sampled locations at both scales. At the larger spatial scale, the distance per se was not affecting the level of divergence. At the small scale, however, we found subtle patterns of isolation by distance. In addition, we also found morphological divergence between locations, congruent with a spatial separation at a microgeographic scale, most likely due to phenotypic plasticity. The present study highlights the importance of geographical scale and indicates that there might be a disparity between the dispersal capacity of a species and the actual movement of genes. Thus, how we view the environment and possible barriers to dispersal might have great implications for our ability to fully understand the evolution of genetic differentiation, local adaptation, and, in the end, speciation.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 746–758.     

Modelling the introduction and spread of non‐native species: international trade and climate change drive ragweed invasion

Biological invasions are a major driver of global change, for which models can attribute causes, assess impacts and guide management. However, invasion models typically focus on spread from known introduction points or non‐native distributions and ignore the transport processes by which species arrive. Here, we developed a simulation model to understand and describe plant invasion at a continental scale, integrating repeated transport through trade pathways, unintentional release events and the population dynamics and local anthropogenic dispersal that drive subsequent spread. We used the model to simulate the invasion of Europe by common ragweed (Ambrosia artemisiifolia), a globally invasive plant that causes serious harm as an aeroallergen and crop weed. Simulations starting in 1950 accurately reproduced ragweed's current distribution, including the presence of records in climatically unsuitable areas as a result of repeated introduction. Furthermore, the model outputs were strongly correlated with spatial and temporal patterns of ragweed pollen concentrations, which are fully independent of the calibration data. The model suggests that recent trends for warmer summers and increased volumes of international trade have accelerated the ragweed invasion. For the latter, long distance dispersal because of trade within the invaded continent is highlighted as a key invasion process, in addition to import from the native range. Biosecurity simulations, whereby transport through trade pathways is halted, showed that effective control is only achieved by early action targeting all relevant pathways. We conclude that invasion models would benefit from integrating introduction processes (transport and release) with spread dynamics, to better represent propagule pressure from native sources as well as mechanisms for long‐distance dispersal within invaded continents. Ultimately, such integration may facilitate better prediction of spatial and temporal variation in invasion risk and provide useful guidance for management strategies to reduce the impacts of invasion.     

Coexistence patterns of two invasive thistle species, Carduus nutans and C. acanthoides, at three spatial scales

To better understand the competitive processes involved in invasion by congeners, we examine coexistence patterns of two invasive species, Carduus nutans and C. acanthoides, at three spatial scales. A roadside survey of 5 × 5 km blocks in a previously identified overlap zone provided information about the regional scale. At smaller scales, we surveyed four fields of natural co-occurrence, quantifying the spatial patterns at the field scale by randomly placed 1 × 1 m quadrats and at the smallest scale by detailing plant position within the quadrats. The patterns observed are strikingly different at the different scales. At the regional scale, there is positive local autocorrelation in both species but negative cross-correlation between them, consistent with previous surveys. However, at the field scale, there is positive local autocorrelation in both species, and we generally see a positive association between the two species. At the plot scale, when excluding areas of joint absence, there is again a negative association between the two species. This pattern can also be seen at the field scale when excluding plots with joint absence. These results suggest that, at the scale of a field, the strongest factor determining location is aggregation in favorable habitats, which is a stronger force than the competition-induced segregation evidenced at small scales. Lottery competition for spatially aggregated safe sites thus appears to drive the patterns observed at the field scale, while the regional scale pattern may be a result of restricted natural dispersal and invasion history.