A better predictor for intermediate trophic level invasion success: Propagule pressure or competitive ability?

Background/Question/Methods

Propagule pressure and competitive ability have both been found to allow for successful invasion by an introduced species.  However, many studies have found that propagule pressure is more important for invasion than competitive ability. Yet due to the complexity of natural systems, it has been extremely difficult to test these characteristics in more than one trophic level and to fully examine entire community dynamics.  Because of this difficulty, I used the natural, model microbial system found inside the leaves of the pitcher plant Sarracenia purpurea, to test the relative importance of propagule pressure versus high competitive ability for the invasion success of intermediate trophic level species, with and without predation pressure.  Species at intermediate trophic levels are especially interesting to examine. Within the pitcher plant food web they are predators on the bacteria, compete with other protozoans, and must also escape predation from mosquito larvae.  I used two intermediate trophic level species as my invaders: the competitively-dominant species (Colpidium sp.) and a second species (Bodo sp.) that is known to resist predation by surviving in high abundance but is not a good competitor.  This species has been found to be a successful invader in this system when at high abundance.  I tested the effect of initial density (1000, 500, 50 individuals per replicate) on the invasion success of each of these two species, with and without the top predator present.

Results/Conclusions

If propagule pressure is an important characteristic, a species would only be a successful invader if it entered a community in high initial densities.  If competitive ability is important, a species with that characteristic would successfully invade irrespective of initial inoculation densities.  If there is a trade-off between competitive ability and predation resistance, a species would not be a successful invader in the presence of a predator.  The results from my experiments showed that both species were able to establish, however the competitively-dominant species (Colpidium sp.) was more successful than Bodo sp. at all initial densities.  The abundance of the competitively-dominant Colpidium sp. continued to increase throughout the course of the experiment for all initial densities, with treatments containing the higher inoculations of this species resulting in a higher final density in each invaded community at the end of the experiment.  This was true even with the predator present.  Bodo sp. never performed as well as Colpidium sp., even when initial densities were high, both with and without predators.