Background/Question/Methods
How important is enemy escape in facilitating invasions? Many studies of invaders provide evidence for enemy release, without testing the assumption that natural enemies have important fitness effects. When fitness is been measured, the perspective seldom spans the entire life cycle. Ragweed (Ambrosia artemisiifolia) is attacked by both specialist and generalist consumers in its native North America, but enjoys enemy release in Eurasia.
I tested the effects of competition, insect attack and conspecific density on ragweed fitness in the field during Summer 2007, using a factorial design (3 densities x competitors present/absent x enemies present/absent). Competitors were removed by clearing plots before planting while insects were excluded chemically. I measured plant survival, insect damage, above-ground biomass and seed production for 2000 ragweed plants grown at Joker's Hill, Ontario, Canada. In a second experiment, seed mortality is being examined in the context of two nested factors: control, rodents excluded, rodents and fungi excluded. A third experiment will examine the effect of density, competitors and natural enemies on the survival of very young seedlings.
Results/Conclusions
Preliminary results suggest that high-density and competitor-free plots plots had the largest proportion of damaged leaves, while low-density, competitor-filled plots were safest. However, ragweed is tolerant; this damage did not translate into a loss of fecundity. In contrast, competition had a significant fitness cost: more plants died compared to those planted in open plots, and those surviving were much smaller and produced few to no seeds. Current work includes both the extension of this study to very young seedlings, where most mortality occurs, and to winter seedbank survival. With this more complete understanding of the impacts of natural enemies, the fitness costs of enemy attack can be evaluated in context. ANOVA will be used to compare treatment means, and a matrix population model will summarize the results into a complete survey of the life cycle. However, it can already be seen that aboveground insect damage does not signifigantly reduce fitness, making it unlikely that release from these consumers has caused ragweed’s invasiveness in Europe.