Parasites can stabilize consumer-resource dynamics, but do they? An evaluation using models, an experiment, and field data

Background/Question/Methods

Consumer resource theory presents a conundrum. The simplest, canonical models of consumer-resource interactions often predict that resource densities should fluctuate considerably through time. Yet, in nature resource dynamics do not fluctuate nearly so much. Although a variety of mechanisms have been proposed to resolve this theory-data dissonance, a new hypothesis has recently emerged: parasites can stabilize dynamics of consumer-hosts and their resources. Since parasites are ubiquitous, this mechanism promises a general, broadly applicable resolution to the conundrum. We further develop the ‘parasite hypothesis’ using a zooplankton (Daphnia) grazer-host – fungal (Metschnikowia) parasite – algal resource system. We first illustrated the stabilizing effects of parasitism using a mathematical model build generically around the system. The model explicitly tracks nutrient content of all species involved. We then looked for predicted stabilization in a mesocosm experiment. Finally, we searched for parasite-mediated resource stabilization in the field, using a before-after start comparison of epidemics in 16 lakes over three years.

Results/Conclusions

The model predicted that parasites can stabilize dynamics of the grazer-host and its resource through two pathways. First, the virulent parasite can elevate death rates of grazer-hosts, thereby reducing the probability of over-exploitation of the resource (a key factor exacerbating cycling). Second, the parasite locks up nutrients in spores, decreasing effective productivity that also fuels cycling. However, stabilizing effects should become more pronounced in systems with a more nutritious resource rather than a more defended, less nutritious one. Poor nutritional quality of the resource weakens control of hosts by parasites, thereby undermining the stabilization effect. As predicted, in the mesocosm experiment parasite outbreaks dampened fluctuations of a nutritious resource (the alga Scendesmus), not fluctuations of a more defended, lower quality resource (the alga Oocystis). Thus, the experiment confirmed the theoretical predictions. However, in the field, algal dynamics became less stable, not more stable, during autumnal epidemics of the fungus. Nutrient enrichment from lake turnover in autumn likely overwhelmed the stabilizing effect of disease (as illustrated by a variation on the model). Thus, the experiment confirmed the stabilizing effects of parasites. However, in this particular system, other factors undermined the "parasite hypothesis" in the field.