A robust test of the competition-defense tradeoff in a nitrogen-limited grassland

Background/Question/Methods

Consumers can maintain prey diversity if consumption is most detrimental to the dominant resource competitor. Various mechanisms can lead to this outcome; however, one well-described mechanism for plants is a tradeoff between allocation of resources to competitive ability and to defense against herbivores. The tradeoff predicts that plant species that are the most effective resource competitors should exhibit the greatest relative benefit when herbivores are removed. Although this competition-defense tradeoff has been invoked to explain the top-down regulation of plant communities, it has rarely been tested experimentally in the field, owing to the difficulty in quantifying competitive ability in mixed natural communities. At Cedar Creek, a nitrogen-limited grassland in Minnesota, common prairie plants have been grown in monoculture and mixture (16-species) plots since 1994.  Each species’ competitive ability has been quantified in terms of R_N*, a measure of nitrogen resource use efficiency known to predict dominance in nitrogen-limited grasslands. We treated a subset of these monoculture and mixture plots with insecticide, foliar fungicide, soil fungicide and all pesticides to establish consumer removal subplots paired with untreated controls. Aboveground biomass of each plant species in all plots has been measured annually to determine the relative change in primary productivity in response to consumer removal in different community contexts.

Results/Conclusions

We did not find the expected positive correlation between species’ competitive ability (1-R_N*) and response to consumer removal in either mixture or monoculture. In monoculture, there was no difference in response to consumer removal among treatments.  However, there was an overall negative correlation between competitive ability and response to consumer removal.  These data suggest that plants that are the poorest resource competitors are those that experience greatest relative benefit in the absence of consumers, a conclusion opposing the competition-defense tradeoff hypothesis.  In mixture, there appeared to be no relationship between competitive ability and response to removal.  Additionally, there was no correlation between response to removal in monoculture and response to removal in mixture.  These findings suggest that allocation of resources to competition and to defense is apparently not the most important tradeoff axis for maintaining plant biodiversity in nutrient-limited grasslands.