What do the geometric and stochastic properties of swimming behaviour have to teach us about zooplankton behavioural ecology?

Abstract As quoted by Rudi Strickler in his early seminal work, zooplankton exhibit a vast behavioural repertoire: they do not move following straight lines, but along pathways that can be more or less convoluted, they alternate periods of activity with periods of relative stasis, and when they move their speed often fluctuates erratically. In this context, based on a generalization of the first application of fractals in the early 90s to quantify the swimming patterns of fish larvae and copepods, this paper introduces a new conceptual framework allowing to describe how the explicit consideration of both the geometric and stochastic components of copepod movements can be used (i) to define their fundamental and realized behavioural niches, respectively, both in the absence of extrinsic stimulus and in response to abiotic and biotic stressors and (ii) to quantify response strength across stimuli and species. The applicability of this new approach to zooplankton behavioural ecology is illustrated on the basis of the analysis of the behavioural complexity of three species of cladocerans and two species of copepods in response to a wide range of natural and anthropogenic stressors.