When complex movement yields simple dispersal: behavioural heterogeneity, spatial spread and parasitism in groups of micro-wasps

Abstract

Understanding how behavioural dynamics, inter-individual variability and individual interactions scale-up to shape the spatial spread and dispersal of animal populations is a major challenge in ecology. For biocontrol agents, such as the microscopic Trichogramma parasitic wasps, an understanding of movement strategies is also critical to predict pest-suppression performance in the field.We experimentally studied the spatial propagation of groups of parasitoids and their patterns of parasitism. We investigated whether population spread is density-dependent, how it is affected by the presence of hosts, and whether the spatial distribution of parasitism (dispersal kernel) can be predicted from the observed spread of individuals.Using a novel experimental device and high-throughput imaging techniques, we continuously tracked the spatial spread of groups of parasitoids over large temporal and spatial scales (eight hours; and six metres, ca. 12,000 body lengths). We could thus study how population density, the presence of hosts and their spatial distribution impacted the rate of population spread, the spatial distribution of individuals during population expansion, the overall rate of parasitism and the dispersal kernel (position of parasitism events).Higher population density accelerated population spread, but only transiently: the rate of spread reverted to low values after four hours, in a “tortoise-hare” effect. Interestingly, the presence of hosts suppressed this transiency and permitted a sustained high rate of population spread. Importantly, we found that population spread did not obey classical diffusion, but involved dynamical switches between resident and explorer movement modes. Population distribution was therefore not Gaussian, though surprisingly the distribution of parasitism (dispersal kernel) was.Even homogenous asexual groups of animals were shown to develop behavioral heterogeneties over a few hours. Explorer individuals were responsible for most parasitism and dispersal, and determined spatial spread and density-dependent dispersal. We showed that simple Gaussian dispersal did not emerge from simple diffusion, but rather from the interplay of several non-linearities at individual level. This suggests expectations from classical diffusion theory may not hold generally to active dispersers. These results highlight the need to take into account behaviour and inter-individual heterogeneity to understand population spread in animals.

A double-spiral maze and hi-resolution tracking pipeline to study dispersal by groups of minute insects

“…This has also been highlighted on T. cacoeciae. Indeed, higher population density promotes population spread 33 . Since our objective here was more technical and methodological than biological, we chose to set our working densities first.…”

Study methodology impacts density-dependent dispersal observations: a systematic review

“…Numerous species of migrating animals, such as salmon [42] and salamanders [122], will alternate between movement away from a breeding site and aggregation as they return to it, with correlated changes in their density. Alternatively, density may have no perceivable effect on dispersive movements (e.g., [51]), or its effect on dispersal may vary according to a density 'threshold' (e.g., [7,33,78]) or even appear to be temporary (e.g., [17]). Individual assemblages may therefore be linked to different types of density-dependent dispersal: positive, negative, neutral, and even non-linear [41].…”