Zooplankton recovery after fish removal: Limitations of the egg bank

Abstract

The Allee effect, operating via mate limitation, theoretically reduces the probability of recovery from local extinction of dioecious zooplankton relative to species that can reproduce asexually. We removed fish from four alpine lakes in the Sierra Nevada in which both Hesperodiaptomus shoshone (a calanoid copepod) and Daphnia middendorffiana were historically present but had become locally extinct after fish stocking. After complete fish removal, D. middendorffiana returned to all four lakes, whereas H. shoshone failed to return in any of the lakes, despite diapausing eggs observed in the sediments of two lakes and a few individuals in the water column of one lake during the first summer after fish removal. We estimated the potential magnitude of the Allee effect, as it may have affected the recovery of H. shoshone, by comparing estimates of minimum founding population size of H. shoshone with estimates of actual founding population sizes of D. middendorffiana. It took 4 yr for D. middendorffiana to recover to detectable levels in one of the four lakes, which we suggest was the result of a very small founding population. The latter was three to four orders of magnitude smaller than the minimum founding population size we calculated for H. shoshone, indicating the potential for a large effect of mate limitation on the copepodÂs ability to recover. H. shoshone might never return to these lakes without human intervention because of the combined effects of mate limitation and low rates of overland dispersal.