Non-consumptive and resource-availability effects in a ciliate predator-prey pair

Uriah Daugaard and Owen L. Petchey, Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland, Reinhard Furrer, Institute of Mathematics, University of Zurich, Zurich, Switzerland

Background/Question/Methods
Non-consumptive effects (NCE) of predators on prey, such as induced defensive strategies, are frequently neglected in the analysis of predator-prey interactions. Yet, these effects can have demographic impacts as strong as the ones caused by direct consumption. As a counterpart to NCE, resource-availability effects (RAE) can prompt changes in predators as well, e.g., in its foraging behavior.
We investigated non-consumptive and resource-availability effects in the ciliate predator-prey pair Didinium nasutum and Paramecium caudatum. We tested for NCE on prey swimming speed and spatial grouping behavior. For RAE, we investigated predator swimming speed and diffusion in absence and presence of prey, and the dependence of predator movement on local prey scarcity.
We collected individual movement and location data through videography of laboratory-based microbial populations. To assess variability in swimming speeds among individuals, we characterized the distribution of these speeds using maximum likelihood fits of suitable distributions. To measure predator diffusion, we calculated mean square displacements. We used a linear model to assess the relation between local prey abundance on predator displacements. To test for changes in prey grouping, we employed a mixed effects model with prey grouping as the response variable.
Results/Conclusions
The predator showed two distinct swimming speeds (fast and slow). In the absence of prey, fast and slow predator individuals swam respectively 20.1% and 6.7% faster. The predator made larger displacements in the monoculture as the mean square displacement after one second was 14.3% (fast) and 36.9% (slow) larger when there was no prey. Moreover, predator displacements per second increased by 2.3% (fast) and 3.7% (slow) for each decrease in number of local prey neighbors. Similarly, the prey showed two swimming speed regimes as well. The proportion of prey individuals showing the fast movement behavior increased by 12.6% from 65.3% to 77.9% in predator presence. We found week evidence for a 7.8% increase in prey grouping in the presence of predators.
Changes in behaviors, such as caused by NCE and RAE, entail changes in energy expenditure. In the case of NCE it has been shown that this can greatly impact prey populations, while less is known about the relation between RAE and demography. These effects might scale up to influence the community and ecosystem stability. Yet, this link has remained largely unexplored and should be a focus of future community stability studies.