The effects of intraspecific diversity and resource mutualisms on community dynamics

Background/Question/Methods

Dominant early successional species can have pronounced effects on community assembly during succession by altering biotic and abiotic conditions.  The source identity of establishing populations could also influence community dynamics if they vary in key traits related to direct and indirect species interactions.  Mutualistic symbioses between legumes and nitrogen-fixing rhizobia may be especially important and result in a competitive advantage for legumes early in succession, increase nitrogen availability to the rest of the community, and increase species turnover rates during succession. Therefore, the presence or absence of focal legumes and their rhizobium mutualists may help determine both the potential success of that species and community-level effects because of the large role nutrient availability plays in succession.  I used a greenhouse mesocosm experiment to investigate how variation among legume populations and the presence of rhizobium mutualists influence community composition and structure across a nitrogen gradient.  I planted six different populations of a focal dominant species (Chamaecrista fasciculata) into individual mesocosms planted with early successional plant communities (seven native prairie species) and manipulated the presence or absence of rhizobia and nitrogen availability (0g, 10g, or 20g per m²).

Results/Conclusions

I found that population effects on community assembly depend on the presence of rhizobium mutualists, indicated by a significant interaction between C. fasciculata population and rhizobia treatment on community composition, initially measured as percent cover (p<0.01).  When rhizobia are present, different populations of C. fasciculata promote communities that differ in composition (p=0.02); however, this effect does not occur when rhizobia are absent (p>0.05).  These results suggest that intraspecific variation in an early colonizing species can influence community assembly.  Because the patterns were only observed in the presence of rhizobia, population differences may be driven by among population variation in symbiotic interactions with rhizobia.  This study indicates that initial stages of community assembly following disturbance are influenced by both abiotic and biotic factors, including the colonizing genotypes of key early successional species and the presence of mutualists.