Variation in behaviour of native prey mediates the impact of an invasive species on plankton communities

Abstract

Trait variation in predator populations can influence the outcome of predator-prey dynamics, with consequences for trophic dynamics and ecosystem functioning. However, the influence of prey trait variation on the impacts of predators is not well understood, especially for introduced predators where variation in prey can shape invasion outcomes. In this study, we investigated if intra-specific differences in vertical position of Daphnia influenced the impacts of the invasive zooplankton predator, Bythotrephes cederströmii, on plankton communities. Our results show that vertical position of Daphnia influenced Bythotrephes predation on smaller cladoceran species and impacts on primary production. Larger reductions in small cladoceran density and greater algal biomass were observed in mesocosms with less spatial overlap between Daphnia and Bythotrephes. These results suggest that differences in vertical position of Daphnia can alter the type and magnitude of Bythotrephes impacts in invaded systems.

Keywords: Anti-predator response; Daphnia catawba; Daphnia mendotae; Intraspecific trait variation; Trophic cascade; Vertical distribution; Zooplankton.

Figure 1. Visualization of the structural equation model (full model) used to assess the impacts of proportion of hypolimnetic Daphnia and Bythotrephes cederströmii presence on per capita change in density of major zooplankton groups and total algae.

Model assumes that all paths (represented by arrows) between per capita change density of each zooplankton group, the proportion of hypolimnetic Daphnia and Bythotrephes presence are possible.

Figure 2. Visualization of the best structural equation model predicting per capita change in density (A) for major zooplankton groups and total algal biomass in week 3, (B) most common zooplankton species, (C) green algal biomass, and (D) diatom biomass.

Arrows represent standardized path coefficients that are statistically significant (p < 0.05) with standardized coefficient value. Arrow width is scaled with the size of the coefficient. Arrow colour signifies positive (green) and negative (red) path coefficients.

Figure 3. Effect of proportion of total hypolimnetic Daphnia in week 0 on the per capita change in (A) total Daphnia and (B) small cladoceran density, and (C) total algal biomass in week 3 in Bythotrephes absent and present mesocosms.

Values above the dashed line at 1 indicate increasing density between week 0 and 3. Shaded regions represent the 95% confidence interval estimated from the best fitting model.

Figure 4. Effect of proportion of total hypolimnetic Daphnia in week 0 on the per capita change in (A) D. mendotae, (B) B. freyi/leideri, (C) E. longispina, (D) C. scutifer density, and biomass of (E) green algae, and (F) diatoms in week 3.

For green algae and diatoms, no effect of Bythotrephes presence was detected. Values above the dashed line at 1 indicate increasing density between week 0 and 3. Shaded regions represent the 95% confidence interval estimated from the best fitting model.