Short-term compensatory response to the loss of a dominant C4 grass is not determined by abundance of functionally redundant species

Francis A. Chaves, Biology, Colorado State University, Fort Collins, CO and Melinda Smith, Department of Biology, Colorado State University, Fort Collins, CO

Background/Question/Methods
Dominant species, by definition, the most abundant species in ecological communities, are not exempt from being lost due to anthropogenic changes or natural disasters, with serious consequences to ecosystem function. When dominants are lost, the expectation is that newly available resources can be used by the remaining species to compensate for the function lost. However, compensation, or the degree of functional recovery in ecosystem processes in response to species loss, does not occur in every natural ecosystem. One of the main constraints proposed to limit compensation is the presence of redundant species that can perform the same function as the species lost. In this study we evaluated the biomass compensation response in a tallgrass prairie plant community following the loss of the dominant C4 tall grass species, Andropogon gerardii. We asked whether biomass compensation would be influenced by the presence and initial abundance of functionally redundant species. To address this question, we experimentally removed A. gerardii from intact plant communities codominated by C4 perennial tall grasses, Sorghastrum nutans and Panicum virgatum, and evaluated their biomass compensatory response during two growing seasons (2015-2016). During the second growing season, we also added nitrogen to alleviate nitrogen limitation. We predicted that higher abundance of functionally redundant species would produce a higher compensatory response in the remaining community.
Results/Conclusions
Removal plots exhibited on average 43.6% less biomass that control plots (no removal), even when N was added, suggesting that the presence of functionally redundant species in high abundance together with nutrient addition did not promote complete biomass compensation in the short term, following the removal of the dominant species A. gerardii. Of the two codominants, P. virgatum showed a positive response to the removal of the dominant species, while S. nutans had a negative to neutral response, indicating that these two species considered functionally equivalent to A. gerardii in terms of effect (i.e. high aboveground biomass production) are not necessarily similar in their response to the loss of a dominant species. Our study suggests that a major constraint on compensation is the ability of a functionally redundant species to rapidly respond demographically. Thus, we contend that identifying response traits of functionally redundant species related to plant reproduction and successful establishment of new individuals, is key to understanding variation in compensatory responses to the loss of dominant species.