Symbiont competition: A classic perspective provides new insights to diversity in symbiont assemblages

Background/Question/Methods

Competition, perhaps more than any other ecological concept, has endured a long, rich, and at times tumultuous history in our quest to understand patterns of biodiversity and species evolution. Yet surprisingly, competition has received relatively little attention in studies of symbiotic assemblages. We argue that competition is not only relevant, but especially germane to understanding symbiont ecology and evolution because symbionts are often necessarily bound in close proximity and compete for limited host-based resources. For example, crayfish throughout the Northern Hemisphere serve as hosts to diverse assemblages of ectosymbiotic worms. Several hundred worms may compete for space and resources on a single host. We asked three questions: Q1) Does intra-specific competition affect individual symbiont fitness? Q2) Do fitness effects on competition modulate symbiont transmission? And, Q3) How does inter-specific competition affect regional symbiont assemblages? We related in situ reproductive success to intra-specific competition for host attachment sites in a field study. We conducted a laboratory experiment to test the effects of competition and host quality on symbiont transmission. Finally, we used a behavioral study to quantify the niche-space of several symbiont taxa and assessed inter-specific competition by relating niche similarity and patterns of co-occurrences drawn from literature. 

Results/Conclusions

Q1) Symbiont fitness was decreased by intra-specific competition for attachment sites. Field survey data revealed that symbiont reproduction is contingent on host attachment site and each site can support only a limited number of symbionts. Attachment site occupancy was highly nested, a pattern consistent with strong competition for preferred sites. Less preferable sites were typically not occupied unless preferred sites were unavailable. Q2) Competition for preferred attachment sites influenced symbiont transmission among hosts. During laboratory experiments, symbionts were more likely to disperse to new hosts when current host quality and intra-specific competition yielded low potential symbiont fitness suggesting that increased intra-specific competition leads to increased symbiont transmission. Q3) We found evidence that inter-specific competition shapes the composition symbiont assemblages. Behavioral study results revealed species-specific preferences for attachment sites. A re-analysis of regional symbiont species co-occurrences from the literature demonstrated significant checker-boarding and a negative relationship between niche similarity and local symbiont species co-occurrences. Our results demonstrate that competition among symbionts can be a considerable force that drives symbiont population dynamics and sculpts symbiont diversity.