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Central European temperate forests are – with the exception of floodplain forests – relatively little invaded by alien plants. However, despite substantial recent progress, there is still a lack of using vege-tation plot data for analyzing spatio-temporal patterns of alien tree species invasions.
We calculated relevé-based metrics of tree species’ ecological preferences using 19,413 phytosociological forest relevés of the Austrian vegetation database. We focused on the five most widely distributed alien trees, i.e. two archaeophytes (Castanea sativa, Juglans regia) and three neophytes (Acer negundo, Ailanthus altissima, Robinia pseudoacacia). For each of these species we analyzed the mean cover in the tree layer and the occurrence in the herb and shrub layers in relevés colonized by adult trees as a measure for persistence. Further, we evaluated the intergenerational ecological plasticity (= the ability of young trees to grow under different site conditions than adults) for the tree species, and the mean relevé indicator values for light, nutrients, moisture and hemeroby. We then compared these alien and native tree species metrics.
We found that A. altissima and R. pseudoacacia build up high mean cover values in invaded forests, but this was not the case for the other alien trees. Thus, both species strongly affected forest communities of invaded sites. Similarly, the two species were common in the lower vegetation layers indicating recruitment under the canopy of adult conspecifics; this was facilitated by their ability to produce root suckers. Highest values of inter-generational ecological plasticity occurred in native pioneer trees and species of softwood floodplain forests, while alien trees had moderately high (A. negundo, A. altissima, J. regia) to low values (C. sativa, R. pseudoacacia). With the exception of C. sativa, all alien species showed high mean Ellenberg indicator values for light and nutrients, and were more common in sites with high hemeroby and high mean Ellenberg indicator values for temperature. Distinct from the ecological preferences of alien trees, and thus rarely invaded, were montane beech forests, coniferous mountain forests and forests at extremely dry sites, as well as swamp and bog forests dominated by willows and ash.
We conclude that relevé-based metrics of the behavior of alien tree species allow new insights into the spatio-temporal dynamics of invasion of woody species in forests. Future work should expand this approach, e.g., by considering the role of life history traits and actual site conditions.
Many recent studies in invasion science have identified species traits that determine either invasiveness or impact. Such analyses underpin risk assessments and attempts to prioritise management actions. However, the factors that mediate the capacity of an introduced species to establish and spread (i.e. its invasiveness) can differ from those that affect the nature and severity of impacts. Here we compare those traits correlated with invasiveness with those correlated with impact for Cactaceae (“cacti”) in South Africa. To assess impact magnitude, we scored 70 cacti (35 invasive and 35 non-invasive species) using the Generic Impact Scoring System (GISS) and identified traits correlated with impact using a decision tree approach. We then compared the traits correlated with impact with those identified in a recent study as correlated with invasiveness (i.e. native range size and growth form). We found that there is a significant correlation between native range size and both invasiveness and impact. Cacti with larger native ranges were more likely to become invasive (p=0.001) and cause substantial impacts (p=0.01). These results are important for prioritising efforts on the management of cactus species. Understanding when and why impact and invasiveness are correlated (as they appear to be for Cactaceae) is likely to be an important area of future research in risk assessment.
Trichopsis vittata (Cuvier, 1831) is a small, freshwater gourami (Fam: Osphronemidae) native to southeast Asia. It was first detected in Florida in the 1970s and seems to have persisted for decades in a small area. In this study, we documented T. vittata’s ecophysiological tolerances (salinity and low-temperature) and qualitatively compared them to published values for other sympatric non-native species that have successfully invaded much of the Florida peninsula. Trichopsis vittata survived acute salinity shifts to 16 psu and was able to survive up to 20 psu when salinity was raised more slowly (5 psu per week). In a cold-tolerance experiment, temperature was lowered from 24 °C at 1 °C hr-1 until fish died. Mean temperature at death (i.e., lower lethal limit) was 7.2 °C. Trichopsis vittata seems as tolerant or more tolerant than many other sympatric non-native fishes for the variables we examined. However, T. vittata is the only species that has not dispersed since its introduction. Species other than T. vittata have broadly invaded ranges, many of which include the entire lower third of the Florida peninsula. It is possible that tolerance to environmental parameters serves as a filter for establishment, wherein candidate species must possess the ability to survive abiotic extremes as a first step. However, a species’ ability to expand its geographic range may ultimately rely on a secondary set of criteria including biotic interactions and life-history variables.
Invasion ecology has made considerable progress in identifying specific mechanisms that potentially determine success and failure of biological invasions. Increasingly, efforts are being made to interrelate or even synthesize the growing number of hypotheses in order to gain a more comprehensive and integrative understanding of invasions. We argue that adopting an eco-evolutionary perspective on invasions is a promising approach to achieve such integration. It emphasizes the evolutionary antecedents of invasions, i.e. the species’ evolutionary legacy and its role in shaping novel biotic interactions that arise due to invasions. We present a conceptual framework consisting of five hypothetical scenarios about the influence of so-called ‘eco-evolutionary experience’ in resident native and invading non-native species on invasion success, depending on the type of ecological interaction (predation, competition, mutualism, and commensalism). We show that several major ecological invasion hypotheses, including ‘enemy release’, ‘EICA’, ‘novel weapons’, ‘naive prey’, ‘new associations’, ‘missed mutualisms’ and ‘Darwin’s naturalization hypothesis’ can be integrated into this framework by uncovering their shared implicit reference to the concept of eco-evolutionary experience. We draft a routine for the assessment of eco-evolutionary experience in native and non-native species using a food web-based example and propose two indices (xpFocal index and xpResidents index) for the actual quantification of eco-evolutionary experience. Our study emphasizes the explanatory potential of an eco-evolutionary perspective on biological invasions.