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.

A widespread but controversial idea in ecology states that the number of invaders of a species assemblage depends on its species richness. Both negative and positive relationships have been reported. We examined whether a simple neutral model where assemblages are generated by drawing individuals from two pools of identical species (native and alien) can predict this relationship. We performed a meta-analysis of published data on this relationship.The neutral model showed that in communities with low and fixed numbers of individuals, the relationship between the number of aliens and native species is strong and negative. This becomes weaker as the proportion of species from both pools already present in the community increases. The relationship between alien and native species richness becomes positive when the number of individuals is allowed to vary, because the richness depends on number of individuals and/or area sampled. The meta-analysis showed that scale-dependence of the relationship between alien and native species richness is universal and compatible with the neutral model. Unless more evidence is available to refute it, the relationship between native and alien species richness should be considered a result of a neutral processes due to constraints on the number of individuals in the community.

Vegetative regeneration of individual genotypes of Asian Reynoutria taxa, which are invasive in the Czech Republic, was studied in R. sachalinensis (five genotypes), R. japonica (a single genotype present in the country), and their hybrid R. ×bohemica (nine genotypes). Identity of genotypes was confirmed by isozyme analysis. Ten rhizome segments of each genotype were planted in a randomized block design. After 30 d, the regeneration rate of each genotype was measured as the proportion of rhizomes that produced shoots. Emergence time and final mass of each shoot were recorded. The regeneration rate and final shoot mass were significantly affected by genotype in R. ×bohemica but not in R. sachalinensis. In R. ×bohemica, easily regenerating genotypes grew faster. Regeneration characteristics that crucially contribute to the fitness of these vegetatively spreading plants are closely related to each other. In genotypes with a low regeneration rate, early-emerging shoots produced more biomass, while in those with a high regeneration rate, shoot mass was independent of emergence time. Mean clone size recorded in the field was marginally significantly related to emergence time during regeneration; regeneration characteristics might thus affect the extent of R. ×bohemica invasion at a regional scale. Hybrids genetically intermediate between the parents regenerated better than those closely related to parents. Novel hybrid invasive genotypes may be produced by rare sexual reproduction, fixed by clonal growth, and present a previously unknown threat to native vegetation.

Abstract. The genus Reynoutria is represented by four taxa in the Czech Republic -R. japonica var. japonica and compacta, R. sachalinensis and R. · bohemica. Using isoenzyme analysis, we determined the degree of genotype variability in all taxa and compared clones of R. japonica var. japonica from the Czech Republic with those from Great Britain. While the rarely occurring tetraploid variety R. japonica var. compacta possesses low variability, the octoploid female clone of R. japonica var. japonica is genetically uniform in the 93 clones sampled and belongs to the same genotype that is present in the whole Europe. R. japonica var. japonica can be fertilized by the pollen of tetraploid R. sachalinensis and a hexaploid hybrid R. · bohemica is produced. In R. sachalinensis, 16 genotypes were found in the 50 clones sampled. R. · bohemica is genetically the most diverse taxon in the study area, with 33 genotypes recorded among 88 clones sampled.