Indirect effects of global change: How biotic interactions influence species responses to long-term environmental change

Courtney G. Collins and Sarah Elmendorf, Institute of Arctic and Alpine Research, University of Colorado, Boulder, Boulder, CO, Lauren Shoemaker, Department of Botany, University of Wyoming, Laramie, WY, Megan Szojka, University of Wyoming, Katharine N. Suding, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO

Background/Question/Methods
Global change is altering patterns of community assembly and species coexistence, and these impacts can vary across multiple, interacting global change drivers. Responses to environmental change are often non-linear, and can be strongly influenced by biotic interactions. In addition, species responses can differ by their relative dominance within a community, and lead to increased dominance of certain species or reshuffling of competitive hierarchies. Thus, to accurately predict shifts in community dynamics under global change, we must simultaneously consider 1) interactive effects of multiple global change drivers, 2) competitve interactions among species, and 3) their interaction. Here, we present a 15 year fully-factorial Nitrogen (N) addition, warming and snow manipulation experiment and corresponding shifts in alpine plant community composition at Niwot Ridge, Colorado. We used gjamTime, a dynamic, biophysical competition model to jointly estimate the influence of multiple global change drivers on the density-independent growth rates of groups of dominant, subdominant, moderate and rare plant species within the community. Furthermore, we assess how species responses to the environment are influenced by density-dependent interactions with other species (i.e. environment-species interactions “ESIs”). We asked: “What density dependent and independent mechanisms lead to further favoring dominant species versus reordering species hierarchies under global change?

Results/Conclusions
For raw changes in abundance, the dominant species became more dominant and subdominant and moderate species declined in cover over time in all but one global change treatment. This pattern was driven by both direct (positive) responses of the dominant species to changing environmental conditions (especially added snow and N) and indirect changes in competitive interactions (weakening of competitive effects from moderate species and strengthening of competitive effects on subdominant and moderate species) under global change treatments. The warming only treatment was the single scenario where the dominant species declined and subdominant species increased over time. In these plots, the dominant species’ growth rates had a neutral response to temperature, and increased competitive effects of subdominant species negated the benefits of competitive release from moderate species. ESIs also showed that subdominant species had non-linear responses to ambient Nitrogen deposition in warming only plots, driven by strong competition with dominant species which had an opposing response to ambient N. Overall, these results suggest that different combinations of global change drivers can both reshuffle species’ hierarchies and/or lead to competitive exclusion by the dominant species. Predicting which outcomes will occur requires considering density dependent and independent mechanisms as well as environment-species interactions.