Effects of competition and predation on zooplankton community composition: Evidence from the field and the laboratory
Predation on and competition among zooplankton species interact to structure the species composition and size structure of zooplankton communities in lakes. When levels of visually oriented zooplanktivorous fish are high, these predators selectively remove large-bodied species and smaller-bodied species (e.g., small copepods, small cladocerans, rotifers) predominate. Conversely, when levels of zooplanktivory are low, large-bodied species (e.g., large Daphnia) typically replace the smaller-bodied species, but there is not a consensus on the ecological mechanism responsible for this replacement. One possibility is that invertebrate predators achieve high densities when zooplanktivorous fish are rare, and these invertebrate predators selectively consume smaller-bodied zooplankton prey, enabling large-bodied zooplankton less vulnerable to invertebrate predation to flourish. Another possibility is that larger-bodied zooplankton grazers are competitively superior to smaller-bodied species and they exclude the smaller-bodied forms when levels of zooplanktivorous fish are low. This study examines monitoring data from Square Lake (Minnesota, USA) between two years (2010 & 2012) when the lake was stocked with rainbow trout (a zooplanktivore), and two years (2013 & 2014) when it was not to evaluate whether the absence of trout caused the expected changes in the population dynamics of two Daphnia species (the large-bodied D. pulicaria, and the smaller-bodied D. mendotae) in the lake’s zooplankton community. In addition, we report on the results of a competition experiment between the Daphnia species performed to determine whether there is evidence that the larger-bodied D. pulicaria is competitively superior to D. mendotae.
Consistent with expectations, monitoring data show that water column concentrations of the large-bodied Daphnia species (D. pulicaria) were significantly (p < 0.001) higher, while concentrations of a smaller-bodied Daphnia species (D. mendotae) were significantly lower (p <0.001) in years that trout were not stocked to the lake compared to years when they were stocked. The competition experiment between the two Daphnia species consistently showed that the larger-bodied D. pulicaria outcompeted D. mendotae when they were grown in combination and that D. pulicaria outperformed D. mendotae with respect to their nutritional status (lipid index) and fecundity when grown separately. The results of the laboratory experiment support the idea that competitive superiority enables large-bodied zooplankton grazers to predominate when predation by zooplanktivorous fish is low.