This paper describes the reproductive characteristics of 93 neophytes (alien species introduced after 1500 A.D.) of the flora of the Czech Republic and compares trait values between naturalized invasive and naturalized non-invasive neophytes. Species were sampled and seed collected in the field from multiple localities in the Czech Republic. Traits related to seed production (propagule number per plant and per population), dispersal (propagule size, length/width ratio and weight; buoyancy; epizoochory; terminal velocity) and establishment (germination; seedling relative growth rate; seedling establishment) were measured for each species either in the field, in a common garden experiment or in the laboratory. Invasive species significantly differ from naturalized non-invasive species in propagule length/width ratio (by having lower ratio, i.e. more rounded propagules) and fecundity (invasive species are more fecund, both per individual plant and in terms of the population propagule production). Invasive species have proportionally fewer seedlings establishing in the autumn and better capacity for dispersal by wind than non-invasive species. The results for several traits differ depending on whether or not the effect of phylogeny is included in analytical models. Considering species relatedness expressed as a taxonomic hierarchy, invasive species have lighter propagules and higher population propagule numbers, and marginally significantly differ in producing more propagules per plant and having higher capacity for dispersal bywater.We found that most variation in invasiveness is linked to variation among species within genera. This distribution of relatedness means that predictions of whether a species will become invasive cannot be based on traits of the relatives of the given species at higher taxonomic levels. The distinction made in this paper, i.e. invasive species vs. naturalized but non-invasive species, can potentially contribute to a deeper understanding of the role of traits associated with invasiveness because the crucial transition from the naturalized to invasion stage is rarely addressed in invasion ecology.
We investigated the effects of different temperature regimes and dry storage on germination of H. mantegazzianum (Apiaceae, native to Caucasus) seeds in the laboratory and linked the results with studies of seasonal seed bank depletion in a common garden experiment and under field conditions. Seeds were collected at seven sites in the Slavkovský les region, Czech Republic, cold-stratified for 2 months and germinated at seven temperature regimes. Under all temperature regimes, fresh seeds germinated to significantly higher percentages than older (1, 2, 3 years) seeds. For all storage lengths, seeds germinated best at alternating day/night temperatures of 20/5 °C. The length of the germination period had a significant effect only at low constant temperatures of 2 and 6 °C, where germination percentage increased between 2 and 6 months. Seasonal germination exhibited a distinct pattern, with rapid depletion of seed bank by the first spring after seed burial. Non-dormant seeds were present in the soil early in spring and late in autumn. The higher summer temperatures prevented dormancy breaking and another cold period of at least two months below 10 °C was needed to bring non-germinated seeds out of dormancy. The results suggest that (1) seed dormancy of H. mantegazzianum was not completely broken until the first spring, but that some seeds re-enter or retain dormancy during high summer temperatures and that (2) the threshold needed for breaking the dormancy was achieved gradually during the cold autumn and winter months. However, in a small fraction of seeds the dormancy breaking process took several years. Of seeds buried in 10 different regions of the Czech Republic, on average 8.8% survived 1 year, 2.7% 2 years and 1.2% remained viable and dormant after 3 years of burial. The ability of even small fraction of H. mantegazzianum seeds to survive for at least 3 years can result in re-invasion of this species into controlled sites.
Invasions by alien plant species significantly affect biodiversity and ecosystem functioning. Investigations of the soil seed banks of invasive plant species and changes in the composition and structure of resident seed banks following plant invasions can provide valuable insight into the long-term implications of plant invasions. Soil seed banks play a major role as reservoirs of species and genetic diversity and allow for the persistence of a species at a locality, buffering environmental changes that may occur over time. Despite the emerging body of literature on ecological impacts of invasive plants on the diversity of resident communities, the long-term implications of impoverished soil seed banks for vegetation dynamics and ecosystem functioning have only recently begun receiving attention. Evidence has so far indicated that there is a correlation between the invasiveness of a species and the characteristics of its seed bank, and that changes in the seed banks of resident communities associated with plant invasions affect their biotic resistance to primary and secondary invasions. To promote the study of soil seed banks in the context of invasive species, we (i) summarize the functional roles of soil seed banks; (ii) describe how the capacity to form a seed bank may contribute to a species’ invasiveness using data from the flora of the Czech Republic, showing an increasing representation of species capable of forming long-term persistent seed bank from casual to naturalized to invasion stage; (iii) assess the impact of invasive plants on seed banks of resident communities, including the potential creation of conditions that favour secondary invasions by other alien species or native weeds, and long-term implications of such impact; and (iv) describe the potential effects of climate change on the soil seed bank in the context of plant invasions. We conclude with highlighting promising avenues for future research on invaded soil seed banks, and emphasize the importance of this knowledge in the development of control programs and restoration strategies.
The citation frequency of papers on invasion ecology published between 1981 and 2003 and that had accumulated at least 30 citations on the Web of Science on 9 August 2006 was analysed. The dataset comprised 329 papers and 27,240 citations. For each paper, the total number of citations was recorded and the annual citation rate (number of citations per year) was calculated. Papers were classified into broad research fields: plant invasions, animal invasions, biological control, and general papers (reviews and syntheses). Eight papers were cited more than 300 times, five of them dealt with general topics, and the mean value of the total number of citations across the whole data set is 82.8±73.1. The mean annual citation rate is 11.5±11.3 citations per year; six studies received on average at least 50 citations each year. About a half (50.8%) of papers in the data set deal with plant invasions. General papers are significantly more cited than papers from the other categories. The annual citation rate increased with time over the analysed period (1981–2003), by 1.0 citations per year. To compare the trends in invasion ecology with those in other fields of ecology, comparable data were compiled for population ecology and dynamics, and global change. The annual citation rate for invasion ecology as a whole increased faster than that for population ecology and dynamics, but not exponentially as is the case with studies on global change. The best-cited papers on invasion ecology were distributed among most of the top ecology journals. Those published in Oikos, Journal of Ecology, Ecological Applications and BioScience are cited 3.8–5.8 times more than the average for these journals (based on the impact factor). Papers on biodiversity, community ecology, impact, invasibility, dispersal, population ecology, competition, resources, genetical issues, biological control and species invasiveness received the highest total number of citations. However, measured by the annual citation rate, the hottest current topics in invasion ecology are the effect of global change on invasions, the role of natural enemies, character of the invasion process, evolutionary aspects, invasibility of communities and ecosystem processes. Some topics are disproportionally more cited than studied and vice versa. Studies on plant and animal invasions differ in focus: the topics of invasibility, biodiversity, resources, species invasiveness and population genetics are more emphasized in botanical studies, dispersal, competition, impact and pathways in papers dealing with animal invasions. Studies of grasslands and marine environment are most frequently cited in botanical and zoological studies, respectively. Most of the highly cited papers deal with multiple species; only 14 plant species and four animal species are the primary focus of one or more of the highly-cited papers. Twenty-two authors (4.5% of the total involved in the papers analysed), each with seven or more contributions cited at least 30 times, together contributed 49.4% of the most-cited papers, and attracted 55.6% of the total number of citations.