The role of natural enemies, light, and nutrients in colonization of exotic-dominated old field communities by native species
Escape from natural enemies may help some exotic species to achieve dominance in previously native-dominated communities. The benefit of enemy release may be greatest among fast-growing, poorly defended species adapted to high-nutrient environments. These exotic species may receive the benefits of high nutrient availability without the costs of increasing enemy damage that their native competitors experience. In order for native species to colonize exotic-dominated habitats, they must overcome this enemy-related disadvantage. In an exotic-dominated old field (Durham, NC), we added seed of 11 locally common native species to plots that received factorial manipulations of soil nutrient availability (control, +10g/m2 NPK), and above-ground fungal and insect enemy pressure (ambient enemy pressure, excluded via pesticide application). Because litter both reduces light availability and is a source of inoculum for many fungal pathogens, we removed litter from half of each plot and left litter intact on the other half. We measured the abundance of germinating seedlings, as well as light availability, and foliar enemy damage to the resident community. We expected exotic species to have the greatest advantage over native colonizers in fertilized plots with intact litter and ambient enemy pressure, resulting in low colonization by native species in these plots.
Although fertilization increased enemy damage to the community and enemy exclusion successfully reduced damage, we found no effect of enemy exclusion or enemy damage on seedling establishment at the end of the growing season. Fertilization and litter removal interacted to significantly influence seedling establishment. Litter removal increased establishment, particularly in unfertilized plots. Conversely, litter removal had a much more muted effect in fertilized plots. Both fertilization, which increased biomass production of the resident community, and intact litter increased light attenuation, reducing the light available to seedlings. These results support the idea that fertilization can strongly reduce colonization by increasing the interception of light by the resident community, but that the presence of litter largely preempts this effect. Although enemies did not substantially affect seedling success in the first year, they may play a larger role later in establishment, for colonizers that successfully pass through the initial filter of light limitation. These results also have implications for establishment after disturbances such as fire, which remove litter and often increase nutrient availability.