Time course of plant diversity effects on Centaurea jacea establishment and the role of competition and herbivory

Aims Invasion resistance in experimental plant communities is known to increase with increasing diversity and further to depend on the presence of particular functional groups.To test whether these effects also hold true for the invader establishment phase beyond the seedling stage,we studied survival and performance of Centaurea jacea L.(brown knapweed)planted into experimental grassland communities of varying plant biodiversity over three consecutive years.Moreover,we analysed the role of insect herbivory and biomass of the recipient community for mediating diversity effects.Methods In 2005,seedlings of Centaurea were transplanted into experimental grassland communities(the Jena Experiment)covering a species richness(1–60)and functional group richness(1–4)gradient.Half of these transplants and the community surrounding them in each plot were sprayed with insecticide while the other half served as control.In 2006 and 2007(during the second and third year after transplantation),we recorded survival,growth-related(e.g.transplant biomass,height)and reproduction-related traits(e.g.number of flower heads).Annual data on community aboveground biomass served as covariate to investigate mediating effects of aboveground competition with the recipient community.Important Findings Species richness was the most important factor responsible for Centaurea limitation.Higher levels of diversity decreased survival and all performance traits in both years.These diversity effects were partly driven by community biomass,but not fully explained by that covariate,suggesting the importance also of further processes.The influence of functional group richnesswas strong in the second year after transplantation and weaker in the third year.Among the particular functional groups,only the presence of legumes showed strong negative effects on Centaurea survival and weak negative effects on growth and reproduction,the latter two being mediated by biomass.Insect herbivore reduction considerably benefited Centaurea in sprayed monocultures,where it grew significantly larger than in all other diversity levels and than in the control subplots.We conclude that effects of plant community properties on invading individuals change in the course of establishment,that plant species richness effects are also important during later stages of establishment,and that biomass(especially at high diversity)and herbivory(especially at low diversity)of the recipient community are important in mediating community effects on invaders.

Biotic resistance limits the invasiveness of the western flower thrips, Frankliniella occidentalis （Thysanoptera： Thripidae）, in Florida

The spread of the western flower thrips, Frankliniella occidentalis （Pergande）, has resulted in the world-wide destabilization of established integrated pest management programs for many crops. It is hypothesized that frequent exposure to insecticides in intensive agriculture selected for resistant populations, which allowed invasive populations in the eastern USA to overcome biotic resistance from the native community of species. Research conducted in Florida to understand the role of biotic factors in limiting the abundance of the western flower thrips is reviewed. Orius spp. （Hemiptera： Anthocoridae） are effective predators that suppress populations of thrips on crop and non-crop hosts in southern and northern Florida. Orius are more effective predators of the western flower thrips than the native flower thrips, E tritici （Fitch） and E bispinosa （Morgan）. The native species are competitors of the western flower thrips. Excessive fertilization and the use of broad-spectrum insecticides in crop fields further enhances populations of the western flower thrips. Interactions with native species clearly limit the abundance of western flower thrips in Florida, but populations are abundant in fertilized crop fields where application of insecticides excludes predators and competitor species.