How similar species assemble in communities? A regional-scale study involving cryptic species of Hyalella amphipods

Background/Question/Methods

Many species are presumed to coexist stably by occupying different ecological niches. However, the existence of ecological differences among species of a community is not always clear. This is especially the case of species that are morphologically similar but genetically different, also called cryptic species. From a regional perspective, processes linked to the metacommunity concept are proposed to explain the assemblage of such species. If cryptic species display ecological differences, different species could be associated with specific environmental conditions, and species with similar requirements could co-occur. If cryptic species are ecologically equivalent, local assemblages could be mainly explained by the random migration of individuals from regional sources: difference in species assemblages would be expected to be greater with distance among localities. The objective of our study was to assess if Hyalellaamphipod cryptic species are distributed among lakes according to specific environmental conditions or in agreement with neutral expectations. To do so, we sampled amphipods of the complex in 65 lakes of Bas-Saint-Laurent (Québec, Canada) and measured a suite of environmental parameters. We also compared observed species distribution with a neutral model expecting distance-decay of similarity. 

Results/Conclusions

Four Hyalella cryptic species were found in lakes of the study region based on CO1 sequence analyses (percentage of divergence varying from 20.0 to 26.0% among species). Most lakes had at least two or more cryptic species co-occuring. Constrained ordinations revealed a weak but significant association of these species with environmental conditions, especially with parameters linked to lake productivity (dissolved organic carbon and chlorophyll a concentrations and percentage of organic matter in the sediments). The distribution of these species did not seem to depend on distance among lakes, as the probability that two individuals were conspecifics was similar for most distances. These results suggest that cryptic species of the H. azteca complex might have different ecological preferences but with high overlap in their ecological niches, suggesting that a mix of neutral and niche-based processes could explain their assemblage in communities.