Seeking water while avoiding predators: moisture gradients can affect predator–prey interactions

• How water scarcity affects predator–prey interactions is largely unknown.
• We experimentally found that both soil predators and prey aggregate in wet patches.
• Prey showed antipredator behaviour, tending to avoid wet patches with predators.
• We found evidence for a trade-off between prey water demand and predator avoidance.

Water is an essential resource that can affect the distribution and abundance of species across ecosystems. However, how water availability and its spatial heterogeneity affect animal movement, and associated predator–prey encounter and predation rates, is still poorly understood. Using predatory centipedes and springtails (prey) from the leaf litter of a beech forest, we conducted two laboratory experiments in which we manipulated water availability to test (1) whether centipedes and springtails decrease their mobility when moisture is limiting, (2) whether centipedes and springtails aggregate in patches were moisture is high, and, if so, (3) whether springtails tend to avoid these otherwise suitable areas when predators are present. Overall, average water availability did not affect levels of mobility, and both animal taxa were equally attracted to the moistest spots. Springtails, however, switched to slightly drier spots when predators were present, and as expected from physiological constraints in water loss, smaller springtails spent more time in wet spots, incurring higher predation risk. Larger animals were more active regardless of taxonomic affiliation, although springtails switched to a much higher mobility under predation risk. This size-dependent antipredator behaviour could explain why predation rates were similar across treatments. Since spatial heterogeneity in moisture is widespread in terrestrial ecosystems, our findings may be relevant to understanding predator–prey dynamics, not only in arid environments, but also in temperate ecosystems, such as beech forests.