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 copepod's 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.

The effect of mating behavior and temperature variation on the critical population density of a freshwater copepod

“…Observations of mating behavior were conducted at 5uC, 12uC, and 16uC, given that mean temperatures for the icefree period of seven alpine lakes in the Sierra Nevada (3358-3583-m elevation) ranged from 6.4uC to 16.5uC between 1996 and 2002 (Sarnelle and Knapp 2004). Two days prior to experiments, randomly selected males and gravid females were separated and acclimated to experimental temperatures.…”

“…In the Sierra Nevada, the diaptomid copepod Hesperodiaptomus shoshone typically suffers local extinction following the stocking of non-native trout (Knapp et al 2001). If stocking is halted and the fish die out or are removed, H. shoshone sometimes fails to reestablish (Sarnelle and Knapp 2004;Knapp and Sarnelle 2008), a pattern that has also been observed in a closely related species in the Rocky Mountains (Parker et al 1996). Lack of recovery is surprising given the widespread distribution of these species (Knapp et al 2001) and the persistence of long-lived diapausing eggs in lake sediments (Hairston 1996;Parker et al 1996).…”

“…Using Gerritsen's (1980) simplistic diffusion model of encounter rate and limited information about swimming speed, detection distance, and life history parameters, the critical density for H. shoshone recovery was initially estimated to be between 0.5 and 5 m 23 (Sarnelle and Knapp 2004). Results of a recent multi-lake reintroduction experiment indicated that the critical density was somewhat lower, between 0.2 and 3 m 23 (Kramer et al 2008).…”

“…Below this threshold, mate limitation is expected to result in a negative population growth rate. Mate limitation has been hypothesized to explain observed minimum population densities in other copepod species (Kiørboe 2006;Choi and Kimmerer 2008) and has been shown to contribute to the failure of H. shoshone populations to reestablish after fish disappearance (Sarnelle and Knapp 2004;Kramer et al 2008).…”

How do freshwater organisms cross the “dry ocean”? A review on passive dispersal and colonization processes with a special focus on temporary ponds

“…Species-rich communities show stronger and more complex biotic interactions that can impair the colonization success of new immigrants (Shurin, 2000;Cadotte et al, 2006). Moreover, immigrants can undergo Allee effects (Sarnelle & Knapp, 2004) which would further slow down their population growth rates.…”