Testing coexistence mechanisms with cryptic species of Hyalella amphipods

Background/Question/Methods

The extreme ecological similarity of some coexisting species presents an important challenge to niche-based explanations of coexistence. Ecological theory predicts that when species compete for limited resources whichever species has even a slight advantage will deterministically overtake the other. Therefore, ecological communities should comprise species with non-overlapping niches. Hyalella amphipods were once thought to comprise a single species, but are now known to encompass over 30, many of which are morphologically indistinguishable. At least three of these cryptic Hyalella are found in Michigan. We surveyed 16 lakes in central Michigan to determine how commonly these Hyalella coexist, and if they differ in their patterns of habitat use within lakes. We performed transect surveys and used species-specific PCR primers to genotype all individuals collected. To test whether differences in habitat use were necessary to maintain the diversity observed in natural lakes, we set up replicate artificial lake mesocosms in which we provided combinations of habitat types (deep and shallow macrophyte beds, and shallow near shore waters). If these habitat differences matter for coexistence, local extinctions should be predictable based on the available habitats.

Results/Conclusions

We found that coexistence of all three species was widespread but that the three species had different distribution patterns across the three habitat types, suggesting a role for fine-scale habitat partitioning within lakes. Preliminary results indicate that extinctions in our mesocosms were not deterministic, as one species (which had the highest initial abundance) tended to dominate nearly all the tanks at the end of the experiment, regardless of whether its preferred habitat was present. Two of the three species were maintained in nearly all the habitat combinations, while the third, which is slightly larger, was usually lost without the very shallow near shore habitat. This effect was more pronounced in the presence of fish. These results provide good evidence that a combination of niche and neutral forces may jointly work to structure some communities and maintain diversity.