|ADLER, PETER - Utah State University|
|KLEINHESSELINK, ANDREW - Utah State University|
|HOOKER, GILES - Cornell University|
|Taylor, Joshua - Bret|
|ELLNER, STEPHEN - Cornell University|
Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2018
Publication Date: 4/28/2018
Citation: Adler, P.B., Kleinhesselink, A., Hooker, G., Taylor, J.B., Ellner, S.P. 2018. Weak interspecific interactions in a sagebrush steppe? Conflicting evidence from observations and experiments. Journal of Ecology. 99:1621-1632.
Interpretive Summary: Interspecific competition is arguably the primary obsession of plant community ecologists. However, in a series of papers based on long-term demographic data and multispecies population models for semiarid plant communities, we found surprisingly weak interspecific interactions. These models predicted minimal competitive release. Specifically, removal of a dominant species would have had limited impact on the abundances of remaining species. Therefore, we conducted a five-year removal experiment in a sagebrush steppe plant community to test this prediction. We found that models based on long-term observational data adequately predicted vital rate and population-level responses-to-removals for three of our four study species. For the fourth species, the model based on observational data underestimated population growth in the removal treatment, but results of additional analyses question whether this mismatch actually reflected competitive release. Overall, the results of our experiment increase our confidence that the structure and dynamics of this plant community are determined more by variation in intraspecific interactions than by strong interspecific competition for resources. We expect this conclusion will provide food for thought for a broad audience of plant ecologists raised on a steady diet of studies emphasizing the importance of interspecific competition.
Technical Abstract: (1) A previous study of four dominant species from a sagebrush steppe community used historical data sets and multispecies population models to demonstrate weak interspecific interactions. The models predicted little competitive release following species removals. (2) We conducted a removal experiment to test this prediction. We established new quadrats in the same location where the historical data were collected. We assigned half the new quadrats to a perennial grass removal treatment and half to a treatment in which we removed the dominant shrub, Artemisia tripartita. We modeled survival, growth and recruitment as functions of local neighborhood species composition along with an indicator variable for removal treatment. If our baseline model, which accounts for local plant-plant interactions, explains responses to removal, then the fitted removal indicator effect should be non-significant. (3) For survival and recruitment, the removal treatment effects were never significantly positive, indicating that competitive release was not underestimated by the interspecific effects included in the baseline model. For Poa secunda recruitment, the removal treatment was actually negative, indicating that our baseline model overestimated competitive release. (4) For growth, the removal treatment effect was significant and positive for two species, Poa secunda and Pseudoroegneria spicata, indicating that the baseline model underestimated competitive release. However, including information about the location of these grass individuals with respect to removed A. tripartita failed to improve the growth regressions, raising questions about the mechanisms driving the positive response to removal. (5) For three species, individual based models and integral projections models based on the vital rate regressions showed that removal treatment effects had little impact on population growth rates or simulated equilibrium cover following a species removal. For P. spicata, the population models that included effects of A. tripartita removal projected greater competitive release than the baseline models. (6) Synthesis: Although models based on observational data did not perfectly predict species responses to removal, the experimental results increase our confidence that competitive release is in fact small for at least three of our four study species. The implication is that interspecific resource competition is not the primary factor determining the dynamics of this community.