Predator-enhanced parasite transmission: When one threat unleashes another
The compounded effects of multiple natural enemies on a focal species may not be equal to the sum of each enemy acting on its own. Predators may decrease parasitism by removing infectious hosts from their susceptible compatriots (Duffy et al. 2005, Packer et al. 2003), or they may increase parasitism directly or by altering host traits (Cáceres et al. 2009). Furthermore, predators and parasites might interact with each other, thus modifying risk for the host/prey (Janssens & Stoks 2014). Chaoborus predators are associated with epidemics of the fungal parasite, Metschnikowia bicuspidata, in Daphnia populations (Cáceres et al. 2009). Daphnia face a tradeoff in susceptibility to these two enemies: larger Daphnia eat more and are more likely to ingest spores while grazing, while small Daphnia are more susceptible to the gape-limited predator (Duffy et al. in prep). We performed a mesocosm experiment where we tested the effects of both Metschnikowia and Chaoborus on infection levels, Daphnia densities, and evolution in Daphnia populations in 15L buckets. Mesocosms were stocked with 11 genotypes of Daphnia that differed in susceptibility to predation and parasitism. We saved Daphniasubsamples at five dates for microsatellite analysis to assess host evolution in response to these natural enemies.
Parasitism was not significantly different in the presence of Chaoborus, however interesting host density effects were apparent among the treatments. Daphnia densities were highest in control mesocosms exposed to neither parasite nor predators (parasite: F=8.61, p=0.006; predator: F=6.38, p=0.02; parasite x predator: F=5.31, p=0.03). Interestingly, Daphnia densities were significantly lower but approximately equal in all treatments with natural enemies; that is, the densities in the treatment with two natural enemies were not significantly lower than those in the treatments with just one natural enemy. This may indicate cryptic population dynamics where Daphnia respond to selection from predators, parasites, and/or both enemies on fast time scales, masking population differences that might have been apparent in the absence of host evolution. This possibility is currently being explored by using microsatellites to determine the genetic composition in the different treatments.