Is competition among plants like a boxing match or a demolition derby? Why competitor suppression may not matter in plant communities

Background/Question/Methods

Ecologists generally evaluate the competitive ability of plant species based on the overall outcome of competition, measured as the size, survival, or fitness of a target individual growing with neighbors versus without neighbors.  But there are two distinct components of competitive ability that influence this overall outcome – the suppression of neighbors and the tolerance of the competitive effects of neighbors.  In a manner akin to a boxing match, both strategies are thought to improve competitive performance.  We used simulation models informed by experimental competition trials to ask whether tolerance of competitive effects of an invader or ability to competitively suppress an invader is more important for the survival of native plant ecotypes following exotic invasion.  These models were built using experimentally assessed tolerance and suppression abilities of 23 ecotypes of the native grass Pseudoroegneria spicata (bluebunch wheatgrass) competing against the invasive forb Centaurea stoebe var macranthos (spotted knapweed). 

Results/Conclusions

Ability to tolerate competition from Centaurea was 100 times more important for the success of Pseudoroegneria than ability to suppress Centaurea.  This occurred because benefits of suppressing the invader were shared with other Pseudoroegneria ecotypes, whereas benefits of tolerance were exclusive to individual ecotypes.  Thus, competition may be more like the multiplayer chaos of a demolition derby than a one-on-one boxing match.  In a demolition derby, in which multiple cars crash into one another until only one remains, there offensive driving is not advantageous because drivers who damage a competitor benefit not only themselves, but every other driver on the track.  As a result, the best strategy is to avoid contact entirely until only two cars remain. The advantages of a this strategy are not limited to demolition derbies; in any event involving combat between three or more competitors it is far more important to avoid taking damage than it is to cause damage to other competitors.  In plant communities, as in demolition derbies, there may be minimal advantage to offensive tactics.  This finding has implications for our understanding of how the selective forces caused by competition influence plant communities, the coexistence of plant species, and the outcome of invasions.