Exotic plant species in a C4-dominated grassland: invasibility, disturbance, and community structure

We used data from a 15-year experiment in a C₄-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C₃ plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80–90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C₄ grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C₄-dominated grassland can also be directly related to community structure independent of disturbance. Received: 9 February 1999 / Accepted: 12 May 1999     

Pine invasions in treeless environments: dispersal overruns microsite heterogeneity

Understanding biological invasions patterns and mechanisms is highly needed for forecasting and managing these processes and their negative impacts. At small scales, ecological processes driving plant invasions are expected to produce a spatially explicit pattern driven by propagule pressure and local ground heterogeneity. Our aim was to determine the interplay between the intensity of seed rain, using distance to a mature plantation as a proxy, and microsite heterogeneity in the spreading of Pinus contorta in the treeless Patagonian steppe. Three one‐hectare plots were located under different degrees of P. contorta invasion (Coyhaique Alto, 45° 30′S and 71° 42′W). We fitted three types of inhomogeneous Poisson models to each pine plot in an attempt for describing the observed pattern as accurately as possible: the “dispersal” models, “local ground heterogeneity” models, and “combined” models, using both types of covariates. To include the temporal axis in the invasion process, we analyzed both the pattern of young and old recruits and also of all recruits together. As hypothesized, the spatial patterns of recruited pines showed coarse scale heterogeneity. Early pine invasion spatial patterns in our Patagonian steppe site is not different from expectations of inhomogeneous Poisson processes taking into consideration a linear and negative dependency of pine recruit intensity on the distance to afforestations. Models including ground‐cover predictors were able to describe the point pattern process only in a couple of cases but never better than dispersal models. This finding concurs with the idea that early invasions depend more on seed pressure than on the biotic and abiotic relationships seed and seedlings establish at the microsite scale. Our results show that without a timely and active management, P. contorta will invade the Patagonian steppe independently of the local ground‐cover conditions.     

Vulnerability of benthic habitats to the aquatic invasive species

A comparative vulnerability analysis of 16 selected benthic habitat types in the SE Baltic Sea waters and the Curonian lagoon, including Klaipeda strait, was performed using long-term monitoring datasets (1980–2003) and results of several other surveys in the lagoon and the sea. Results indicated that invasive species richness (number of alien species per habitat) in lagoon habitats was significantly higher than in the sea. Habitats formed by artificial rock and stone, sand, mud, and habitats modified by zebra mussel shell deposits appeared to be the most invaded. Highest invasive species richness occurred in habitats with high native species richness indicating that the main factors driving native species distribution (such as favourable physical conditions, habitat alterations generated by human or/and biotic activities) are also driving aquatic invaders. Physical factors distinguished to be the most important for native and invasive species distribution were salinity, depth range (expressed by the maximal and minimal depths difference within a habitat), shallowness of a habitat (expressed by a minimal depth), and availability of a hard substrate.     

Invasion by Fallopia spp. in a French upland region is related to anthropogenic disturbances

Within Europe, mountain ecosystems are generally less invaded by exotic plant species than are lowland areas. This pattern is commonly attributed to climatic harshness, which limits invasive species presence, and higher propagule pressure and rates of disturbance in lowlands, which favours dissemination. However, the extent to which anthropogenic and natural disturbances contribute to invasive species presence in mountain and lowland environments remains unclear. We conducted field observations in a lowland and an upland region in France and measured environmental variables, estimated the natural and anthropogenic disturbance of plots invaded by Fallopia spp. and compared them to non-invaded plots. Based on generalised linear mixed models, the predictors of Fallopia spp. presence in the upland area only included anthropogenic elements such as the presence of a road or trail and frequentation by humans, whereas both anthropogenic parameters and natural components (light penetration, slope, presence of a road and of a watercourse) were retained as predictors for the lowland region. We calculated the odds of Fallopia spp. presence for the increase of one unit of each predictor. We conclude that the spread of Fallopia spp. in upland areas was mainly linked to human activity whereas dissemination of the species occurred both through humans and in natural ways in lowland areas, and this may be due to a more recent colonisation in the mountains. We therefore advise stakeholders to undertake actions in mountain areas to specifically limit the dissemination of exotic species by humans and to monitor areas of high invasion risk by exotic species, such as areas neighbouring trails and roads highly frequented by humans.     

Overcrowding,food and phosphorus limitation effects on ephipphia production and population dynamics in the invasive species <Emphasis Type="Italic">Daphnia lumholtzi</Emphasis>

Daphnia lumholtzi has been very successful in colonizing North America since its appearance in Texas in 1990. Although previous studies have sought to link its success as an invasive species with various aspects of its population biology, there is little experimental data linking the invasion success of D. lumholtzi with its autecology, specifically its reproduction strategy. In this study we sought to link food quality and quantity to diapause in D. lumholtzi through a variation in phosphorus (P) content of algae, food quantity, and light level. We also assessed the effect of Daphnia peak population densities on reproductive rates and production of resting eggs. We found that when food is abundant, per capita ephippia production may be limited by P, but under food limitation conditions, there is no significant effect of food quality on ephippia production. Our results suggest that a combination of food quality/quantity and population density may work together to induce the production of resting eggs in this invasive species. Handling editor: S. Declerk     

Asexual spread versus sexual reproduction and evolution in Japanese Knotweed s.l. sets the stage for the “Battle of the Clones”

The invasion of NW Europe by a single clone of male-sterile Fallopia japonica var. japonica by purely vegetative reproduction has demonstrated that reproduction by seed and the genetic diversity associated with it may not be an essential feature of plant invasions, at least in the short term. What is less well known is that a significant proportion of the Japanese Knotweed s.l. involved is not F. japonica var. japonica, but the hybrid between it and F. sachalinensis–F. × bohemica. This hybrid is able to backcross to either parent with the potential to replace the missing male F. japonica; by the same process, the hybrid is generating the genetic diversity so conspicuously lacking in F. japonica. In terms of understanding the population structure in a particular country, it is important to be able to identify hybrids and putative back-crosses. We bring together a mixture of published and unpublished information to provide a comprehensive section on morphological and anatomical aids to identification. Regeneration of plants from stem and rhizome fragments varies by taxon, as do responses to control techniques, underlining the importance of correct identification of these plants. In this paper we look closely at this group of taxa, with special emphasis on the role played by hybridization, with an examination of the genetic make up of seedlings produced in the wild or by artificial hybridizations, and the implications that this will have on the future directions of the invasion process.     

Immigration and zooplankton community responses to nutrient enrichment: a mesocosm experiment

Dispersal can be an important determinant of local diversity and species composition, but evidence for effects of the regional species pool on local zooplankton communities has been mixed. Theory and experiments suggest that immigration will be necessary for maintenance of community diversity and functioning during periods of environmental change; conversely, fluctuating resource levels may increase the likelihood of invasion success. We conducted a factorial-design mesocosm experiment to test the effects of a nutrient pulse and weekly immigration from other lakes on the diversity and composition of a pelagic zooplankton community. Contrary to expectations, there were no interactive effects of nutrient enrichment and immigration on any measure of diversity, and the initial shift in community composition in response to the nutrient pulse did not depend on the introduction of new species or genotypes from more productive lakes. Although immigration increased species richness in enclosures, success of most colonising species was poor. However, the dispersal treatment appears to have enabled a stronger predator response to increased herbivore numbers in nutrient-pulsed enclosures, leading to an eventual decline in the abundance of some herbivorous species in response to immigration. We conclude that community invasibility was not influenced by productivity, and that dispersal limitation did not strongly constrain the response of the zooplankton community to our applied disturbance. This indicates an unexpected resistance to change in species composition and diversity in spite of disturbance, and suggests that, in our study system, changes in the abundance of resident species are more important than introductions of new species in the community response to short-term environmental change.Electronic Supplementary Material Supplementary material is available to authorized users in the online version of this article at .     

Habitat invasion research: where vegetation science and invasion ecology meet

In the last decade, habitat‐oriented studies of plant invasions, performed at broad scales and using large data sets of vegetation plots, have focused on quantifying the representation of alien species in vegetation or habitat types, identifying factors underlying invasions, and exploring the pools of species available for invasion into particular habitats. In this essay we summarize what we have learned, discuss constraints associated with this kind of data and outline promising research topics to which a macroecological perspective of habitat invasions can contribute. Such topics include, among others: integrating species‐specific information on invasion status, residence time in the region, biological and ecological traits and phylogenetic relationships into habitat invasion research to better capture the context‐dependence of invasions; focusing on the functional role that alien species, relative to natives, play in plant communities; and obtaining insights into the role of pre‐adaptation for invasion by comparing the functional composition of habitat species pools in the native range. There is still a strong geographic bias, with detailed assessments across broader ranges of habitat types in large regions available only from Europe, the United States and New Zealand, which call for extension of this research to other continents.     

Impact of invasions by alien plants on soil seed bank communities: Emerging patterns

Much of our current understanding of the impact of invasive species on plant communities is based on patterns occurring in the above-ground vegetation, while only few studies have examined changes in soil seed banks associated with plant invasions, despite their important role as determinants of vegetation dynamics. Here, we reviewed the literature on the impact of plant invasions on the seed bank and we provide a quantitative synthesis using a meta-analysis approach. Specifically, (1) we quantified the impact of 18 invasive alien plants on (i) species richness and (ii) density of the seed banks of invaded communities, based on 58 pair-wise invaded-uninvaded comparisons (cases); we identified (2) the invasive taxa that are responsible for the largest changes in the seed bank; and (3) the habitats where substantial changes occur. Our study showed three major findings: (1) species richness (68% of cases) and density (58% of cases) were significantly lower in native seed banks invaded by alien plants; (2) species richness and density of native and alien species were remarkably lower in seed banks invaded by large, perennial herbs compared to uninvaded sites; and (3) invaded seed banks were often associated with a larger richness and/or abundance of alien species. This study indicates a need for additional seed bank data in invasion ecology to characterize species-specific and habitat-specific impacts of plant invasions, and to determine whether changes in the seed banks of native and alien species are a symptom of environmental degradation prior to a plant invasion or whether they are its direct result. The findings of this study help improve our capacity to predict the long-term implications of plant invasions, including limitations in the recruitment of native species from the seed bank and the potential for secondary invasions by seeds of other alien species.     

Evolution of cannibalism: referring to costs of cannibalism

A rational explanation for cannibalism is that it would be favored under conditions of crowding of conspecific individuals and/or low availability of alternative prey with the fear of starvation, so as to maximize individual fitness. Cannibalism has, however, not evolved and is not maintained by a simple individual optimization, while it has evolved and is maintained as a game among population members. We analysed the attainable state of an evolutionary cannibalism game within a framework that reflects the minimum essence of cause-effect in the cannibalism phenomenon. Cannibalism is predator-prey interaction among conspecifics. Immediate direct payoffs (survival in the interaction among conspecifics) and indirect payoffs (growth results in potential productivity and survival against the threat of starvation) would be included. No morphological specialization and no size priority of cannibalism individuals are assumed as conservative situations in which we analyse the possibility of cannibalism. Cannibalism would be possible under the conservative condition, if initially the wild population's cannibalism rate is not sufficiently lower than a threshold value. Crowding and/or low availability of alternative prey with the fear of starvation facilitates cannibalism evolution. Energy gain from conspecific prey would be attenuated by costs of counterattacks by conspecific victims and by challenge cost of its own. Discounting net intake energy required in the arms race for cannibalism challenge result in a relative disadvantage of having a high cannibalism rate and makes an evolutionary equilibrium of low cannibalism rate, even when potential profitability of conspecific prey is high.