Movement enables mobile organisms to locate ideal conditions and avoid harmful conditions and enemies. Movement therefore has important population and community level consequences. Organismal movement is multi-scale, driven by a combination of external and internal factors. For example, resource distribution and individual hunger levels can potentially interact to drive movement patterns while foraging. Using automated image-based tracking, we quantified individual snail movements on experimental landscapes where resource (algal biofilm) distribution varied from 0 to 100% coverage. We predicted snail trajectories, as measured with the power-law exponent, would be more tortuous (i.e., Brownian) in resource patches, as snails are feeding, and straighter in bare patches (Lévy and ballistic). We predicted that average speed would be lower and proportion of time spent moving would be greater in resource patches. Finally, we predicted that patterns of movements would change throughout the course of the experiment, as hunger levels diminish and then increase again towards the end of the experiment.
Results/Conclusions
Patterns of resource distribution strongly affected snail movement. Within resource patches, movement was Brownian, but outside of them trajectories were Lévy and ballistic in character. Average snail speed was slow inside biofilm patches, where snails spent most of their time, and fast outside of them, maximizing the time snails spent in resource patches. These local-scale patterns at the patch level explained patterns of movement at the landscape scale: snail movement was ballistic in resource-free landscapes, Lévy in landscapes with intermediate resource coverage, and Brownian in landscapes completely covered. Finally, movement patterns changed as hunger was satiated, and snails performed other behaviors that utilized different types of movement. Our results demonstrate that organismal movement is driven by both external (resource distribution) and internal (hunger) conditions, and that organismal movement is therefore flexible, changing as external and internal factors change, and is likely driven by a simple set of rules.