Seed limitation, seed predation, and competition interact to impact grassland community structure in Montana, USA

Background/Question/Methods

Ecologists have historically focused on competition as an important process structuring communities. Within this framework, ecologists often assume that the dominant species is the strongest competitor because it has the highest biomass in the community. However many species that occur at lower abundances may have a greater cumulative impact on community structure than one dominant species. Furthermore, competition may not be the most important process structuring plant communities since dispersal limitation can constrain species richness and seed predation can shift species abundances. We examined the local impact of species removal, seed limitation, and seed predation on plant community structure in a native grassland in Montana, USA. To test for competitive impacts, we removed 40% of either the dominant or common species’ cover in experimental plots.  In our controls we either removed all the vegetation or nothing was removed. To test for seed limitation, we added seeds of 20 native species of low abundance to half the plots. To test for seed predation, we replicated this set of plots in and out of a rodent exclosure that covered an area of 10m x 10m. Thus we had a total of 128 plots that were each 0.5m x 0.5m (4 removals x 2 seeding x 2 exclosure x 8 sites).

Results/Conclusions

Preliminary results indicate that seed limitation has a strong impact on plant species richness, seedling abundance, and the identity of seedlings. Seed limitation also influences seed predation and competition in some interesting ways. For example, the removal treatments alone did not change the richness or abundance of seedlings. However, there was a significant difference between removal treatments in combination with seed additions and rodent exclosures. Interestingly, despite removing a high amount of biomass with the dominant species this treatment was not different from the control with no removals. However, when we removed several common species with a much lower biomass, seedling abundance was similar to the complete removals and significantly higher than the control with no removals. These results suggest that competition from several species may have greater impact on the establishment of rare species than competition from a single species with the highest abundance. However, these impacts of competition were only evident when we added seeds and prevented rodents from eating them. Contrary to the hypothesis that removed species will be replaced by functionally similar species, we saw that grass removals caused an increase in forb cover the following year.