A metacommunity of one: dispersal structures intraspecific competition in headwater streams

Background/Question/Methods

Metacommunity ecology emphasizes the role of dispersal in linking local processes at regional scales.  Competition is one such local process that can be affected by dispersal.  Dispersal can influence intraspecific trait variation, thereby affecting the strength of intraspecific competition and, consequently, the relative importance of intra- vs. interspecific interactions.  Building on prior work in a headwater salamander system, we tested how dispersal influences the strength of intraspecific competition via one of two spatial mechanisms: (1) self-sorting of individuals by traits that affect competitive performance or (2) variation in gene flow resulting in evolutionary divergence of individuals and in traits affecting competitive performance.  To test these alternative mechanisms we conducted a mesocosm experiment in the Hubbard Brook watershed of New Hampshire with spring salamanders, Gyrinophilus porphyriticus, from streams with low and high rates of dispersal. 

Results/Conclusions

Spatial patterns of intraspecific competitive performance differed significantly in the low- and high-dispersal streams.  In the low-dispersal stream, downstream individuals were better intraspecific competitors than upstream individuals, whereas the opposite was true in the high-dispersal stream.  This difference in local competitive performance supports the self-sorting mechanism, where high-performance individuals are more likely to move upstream in the high dispersal stream, and to remain downstream in the low dispersal stream.  Our study underscores the importance of integrating natural rates of dispersal into mechanistic experiments to understand the strength of species interactions.  Spatial variability in the strength of intraspecific interactions could have profound implications on the outcome and strength of interspecific interactions.