Interspecific competition among plants is not associated with patterns of phylogenetic dispersion: Results from the field

Background/Question/Methods

In the Darwinian, Gausian, and MacArthurian eras, a standard expectation of interspecific competition was exclusion of inferior competitors.  This idea led to many big theories, including limiting similarity and niche differentiation.  Here we focus on a modern extension: competition, through exclusion of similar species, should lead to phylogenetic overdispersion.  Despite the historical importance of the competitive exclusion principle, modern understanding of plant competition is that many processes promote coexistence, even when competition is intense. As a consequence, exclusion is not a necessary consequence of competition, and thus an a priori prediction of phylogenetic overdispersion is questioned.  To address this with data, we used a diverse native and mature grassland community to experimentally test whether there was a relationship between the strength of competition and phylogenetic dispersion.  We established 120 pairs of plots (fertilized/unfertilized), measuring competition (neighbor removal around 12 focal species) and plant community structure in each plot.  To measure phylogenetic dispersion, we used R (Picante) to merge species abundances in each plot with a species-level molecular phylogeny previously constructed for this field site, calculating several metrics of dispersion.  We conducted a GLMM to determine the effects of fertilization and competitive intensity on degree of phylogenetic dispersion in each plot.

Results/Conclusions

Plant interactions were highly variable among plots and species, ranging from facilitative to strongly competitive – though invariant with respect to fertilization treatment.  Phylogenetic dispersion within plots ranged from significantly clumped to significantly overdispersed – though this too was invariant with respect to fertilization treatment.  Regardless of the dispersion metric used, there was no relationship between the strength of competition and phylogenetic dispersion – in either fertilization treatment.  We conclude that the assumption that one can infer outcomes of competitive encounters through measuring dispersion patterns is not universally true.  To determine whether our results are typical or aberrant can be done by (1) comparisons to other studies, and (2) understanding the mechanisms that drive competition and species assembly in this system.  Actual data linking competition and dispersion patters are sparse, particularly in natural communities where the process of competitive exclusion (if it occurs) should have had time to play out. Thus, until more studies are conducted in more systems, we urge caution in inferring mechanisms from patterns.  We suggest that if one is interested in understanding if competition structures communities, it be measured directly; only this will keep at bay the infamous ‘ghost of competition past’.