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To estimate invasiveness of exotic plant species, many studies have used the frequency of occurrence within a defined region. This measure is informative on how widespread exotics are, however, it does not inform on their local dominance, which is crucial for conservation of biodiversity and ecosystem functioning. The aim of the present study is to determine if regional frequency of occurrence of exotic plant species indeed is indicative of their local dominance. We also determined which plant traits and other factors predict regional and local frequencies best. The Netherlands. We used information on exotic plant species established in The Netherlands and compared traits relating to their frequency of occurrence regionally (the entire country) and their frequency of dominance locally (in 1-100 m quadrats). We created minimal adequate models with factors explaining regional frequency and frequency of local dominance of 111 exotic plant species in The Netherlands. The model that used plant traits to explain regional frequency of exotic plant species differed from the models that best explained their frequency of local dominance. Regionally, the factors that correlated with frequency were: life form, height, polyploidy, length of flowering season, residence time, human use and origin. The factors that correlated to frequency of local dominance were lateral vegetative spread and residence time. We conclude that plant traits relating to the regional frequency of exotic plant species differ from those that relate to their frequency of local dominance. The implication of our results is that predictive studies on plant invasiveness based on regional frequencies may not be indicative of the local impacts. Since the prediction of local impacts is crucial for conservation and risk assessment, our study emphasized the need for better information on the local abundance of exotic invaders. [ABSTRACT FROM AUTHOR]

Aim Although urban areas only occupy c. 2.8% of the earth's land surface, urbanization threatens biodiversity as areas of high human population density often coincide with high biodiversity. Therefore, nature conservation should concentrate on both remote areas and densely populated regions. Protecting rare plant species in rural and urban areas can contribute to the protection of biodiversity. We therefore need to understand why species are rare. Studies on causes of rarity often concentrate on either plant traits or extrinsic threats (such as habitat fragmentation or nitrogen enrichment). However, there are only a few studies that combine traits and extrinsic threats, although such analyses might clarify causes of rarity. We assessed how the affinity of vascular plant species to urban land use (‘urbanity’) interacts with plant traits in determining species frequency. Location Germany, resolution c. 12 km × 11 km. Methods Species with a low frequency may be rare because they occur in rare habitats or because of other reasons, although their habitat is frequent. Therefore, we calculated the frequency of species corrected for habitat frequency, i.e. relative species frequency. We explained relative species frequency by the interactions of species traits and species affinity to urban land use using generalized linear models. Simultaneous autoregressive error models controlled for phylogenetic relationships of species. Results Relative species frequency depends on species affinity to urban land use, independent of the different interactions between traits and urbanity used as predictors. The higher the urbanity the higher is species frequency. Urbanity interacts with species preferences towards temperature and soil acidity. Moreover, dispersal, nitrogen preferences and origin explain relative species frequency, amongst others. Main conclusions Many rare species, especially those preferring cool or acidic habitats might already have disappeared from urban areas. Analyses that combine species traits and environmental effects can explain the causes of rarity and help to derive better conservation strategies. [ABSTRACT FROM AUTHOR]

Long-distance seed dispersal is generally assumed to be important for the regional survival of plant species. In this study, we quantified the importance of long-distance seed dispersal for regional survival of plant species using wind dispersal as an example. We did this using a new approach, by first relating plant species’ dispersal traits to seed dispersal kernels and then relating the kernels to regional survival of the species. We used a recently developed and tested mechanistic seed dispersal model to calculate dispersal kernels from dispersal traits. We used data on 190 plant species and calculated their regional survival in two ways, using species distribution data from 36,800 1 km2-grid cells and 10,754 small plots covering the Netherlands during the largest part of the 20th century. We carried out correlation and stepwise multiple regression analyses to quantify the importance of long-distance dispersal, expressed as the 99-percentile dispersal distance of the dispersal kernels, relative to the importance of median-distance dispersal and other plant traits that are likely to contribute to the explanation of regional survival: plant longevity (annual, biennial, perennial), seed longevity, and plant nutrient requirement. Results show that long-distance dispersal plays a role in determining regional survival, and is more important than median-distance dispersal and plant longevity. However, long-distance dispersal by wind explains only 1–3% of the variation in regional survival between species and is equally important as seed longevity and much less important than nutrient requirement. In changing landscapes such as in the Netherlands, where large-scale eutrophication and habitat destruction took place in the 20th century, plant traits indicating ability to grow under the changed, increasingly nutrient-rich conditions turn out to be much more important for regional survival than seed dispersal. [ABSTRACT FROM AUTHOR]