How do differences in species hatching phenology and the presence of predators affect population and ecosystem level properties of aquatic food webs?

Background/Question/Methods A growing amount of evidence indicates that both the order in which species enter into a community and the process of predation can play an important role in controlling community dynamics. Seldom considered is how differences in species hatching phenology and predation interact to affect algal resources and growth rates of later arriving species. Large branchiopods (fairy shrimp and clam shrimp) and cladocerans produce dormant eggs, which hatch at different times. Fairy shrimp hatch quickly after pond filling followed by clam shrimp and then cladocerans. Consequently, fairy shrimp may monopolize algal resources immediately that could negatively affect clam shrimp and cladocerans. Flying insect predators, particularly Notonecta, increase in abundance as time after pond filling increases, and they can readily consume branchiopods and cladocerans. Thus, the impact of hatching phenology may be reduced in the presence of predation if the Notonecta preferentially consume branchiopods. To investigate these effects we used artificial mesocosms, where we independently manipulated the occurrence of fairy shrimp (Eubranchipus vernalis), clam shrimp (Eulimnadia texana), cladocerans (Daphnia laevis) and Notonecta irrorata.

Results/Conclusions We found phytoplankton abundance often declined through time and the identities of the zooplankton species present at a particular time also had an important effect on phytoplankton abundance. When single species were present, clam shrimp enhanced phytoplankton abundance relative to mesocosms with no zooplankton, while fairy shrimp and cladocerans had no impact on phytoplankton abundance. The ability of clam shrimp to enhance phytoplankton abundance depended on what other species were present. When clam shrimp were present with cladocerans the abundance of phytoplankton was less than that observed when clam shrimp were alone or when clam shrimp only occurred with fairy shrimp. Though periphyton abundance increased through time, none of the zooplankton species had an important effect on periphyton abundance. The occurrence of Notonecta had no effect on algal abundance. The presence of clam shrimp slowed the per capita rate of cladoceran population growth when Notonecta were present but otherwise rates of growth did not vary among treatments. Together these results indicate that species differing in hatching phenology have little influence on ecological systems when alone, but interactions between particular pairs of co-occurring species can have important effects on both ecosystem level and population level responses.