Microcosms metacommunities in river networks: Niche effects and biodiversity

Background/Question/Methods

Many highly diverse landscapes exhibit hierarchical spatial structures that are shaped by geomorphological processes. Riverine ecosystems, among the most diverse habitats on Earth, represent an outstanding example of such mechanisms. A previous study has highlighted the effect of connectivity on species distribution at local and regional scales, but habitat capacity was kept uniform.

We studied the interaction of connectivity and habitat capacity in an aquatic microcosm experiment, in which microbial communities were grown in 36-well culture plates connected by dispersal. Dispersal occurred by periodic transfer of culture medium among connected local communities, following river network topology.  We compared three different spatial configurations of local community volumes: i) power law distributed volumes, according to drainage area;  ii) spatial random permutation of the volumes in the above configuration;  iii) equal distribution of volumes (preserving the total volume with respect to the above configurations). The net effect of habitat capacity on community composition was measured in a control treatment in which communities were kept isolated for the whole duration of the experiment.

Size distribution measurements for each community were taken with a CASY cell counter, and species abundances data on log scale precision were collected by direct microscope observation.

Results/Conclusions

In the treatments in which community volumes were varying along the landscapes, we observed niche effects: some protozoan species preferentially occupied larger nodes, an effect observed systematically in the isolation treatment. Nevertheless, there is evidence that position along the network played a significant role in shaping biodiversity patterns. Specifically, more connected and/or more downstream nodes generally had higher species richness than more isolated nodes.

We found a lower evenness in species richness in uniform landscapes compared to the heterogeneous ones, the latter showing higher levels of beta-diversity. The average species richness and the regional diversity were constant over the three treatments, whereas the level of biomass was higher in the heterogeneous landscapes, in particular in the random configuration of local community volumes. These results suggest that a combination of connectivity and habitat heterogeneity contributes to metacommunity processes and ecosystem functioning.