Major advances in metacommunity theory have been made by assessing dynamics in the simple food webs of small freshwater pools on rocky outcrops. Appledore Island is a < 1 km2 island off the coast of Maine that is home to > 4000 freshwater rock pools. Two of the most abundant organisms in the Appledore rock-pools are the herbivorous cladoceran zooplankton Moina macrocopa (hereafter Moina) and Daphnia pulex/pulicaria (hereafter Daphnia). Although Moina exists in many more pools than Daphnia, it exhibits strong boom-bust population cycles, which may be caused by the presence of a newly discovered microsporidian parasite. Although Daphnia has not been observed to have this parasite in the field and does not show boom-bust cycles, a higher susceptibility of Daphnia to a shared parasite may be the reason that Moina occurs in more pools and that the two species rarely co-occur. I performed a lab experiment to test whether Moina and Daphnia where equally susceptible to the microsporidian parasite by crushing several infected, spore-filled individuals and feeding them to Moina and Daphnia. I monitored treatment and control the individuals for signs of infection, survival, and the number of individuals in the first clutch (Moina) or first three clutches (Daphnia).
Results/Conclusions
Moina survival was reduced in the treatment experiment, but clutch size for surviving individuals between the treatment and control was not significantly different. This result suggests that the observed boom-bust cycles of Moina may be driven by the microsporidian parasite.
In contrast, while there was no observed infection or increase in mortality in Daphnia, the individuals exposed to spores produced significantly more offspring in their first clutch compared with the control group. Furthermore, a greater number of Daphnia in the treatment group produced dormant eggs as their second clutch compared to the second clutch of control Daphnia. These results suggest that the microsporidian parasite does not typically drive Daphnia extinct in many pools, and that Daphnia is found in few pools for some other reason. The influence of the microsporidian parasite on Daphnia reproduction, however, may be important, especially given my previous finding that wind dispersal of dormant eggs is common on the island.