| Fluctuations in the density or the traits of one or a few species can radiate throughout an ecological community affecting patterns of biodiversity. The temporal and spatial consequences of shifts in phenotypically plastic species traits were investigated in Glen Elder Reservoir, Kansas. Daphnia species were most abundant in the lacustrine zone, with D. pulicaria numerically dominant and positively correlated to algal abundance in early spring and D. mendotae dominant later in 1999 and 2000 and positively correlated to water temperature. Coexistence of Daphnia species was facilitated by life history plasticity under different levels of fish predation risk and water temperature. D. pulicaria abundance and size at first reproduction declined, while resting egg (ephippia) formation more than doubled at high temperatures. However, D. mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues reflecting trade-offs in thermal tolerance and ability to escape predation. In a mesocosm experiment, the trophic effects of Daphnia trait shifts and coexistence included decoupling a trophic cascade through diversity maintenance within the herbivore trophic level. Larval walleye (Stizostedion vitreum) had no effect on phytoplankton biomass or overall Daphnia density. However, D. mendotae density increased over time with fish presence. The shift in Daphnia spp. dominance explains the overall benign effect of fish on total Daphnia abundance and supports the hypothesis that species and trait diversity can decouple a trophic cascade. Thus, phenotypic plasticity that depends on environmental context also contributes to species and trophic interactions as well as patterns of biodiversity. |
