A burrowing ecosystem engineer positively affects its microbial prey under stressful conditions.

Species that facilitate others under stressful conditions are often ecosystem engineers: organisms that modify or create physical habitat.However, the net effect of an engineering species on another depends on both the magnitude of the direct interactions (e.g., competition or predation) and the specific environmental context.We used a laboratory system to isolate the trophic and engineering impacts of a predator, the nematode , on its prey, under different levels of environmental stress. We predicted that under stressful surface conditions the nematodes would positively impact their prey by creating burrows which protected the bacteria.Colony plate counts of indicated that there was a stress-induced change in the net impact of nematodes on bacteria from neutral to positive. Predator engineering in the form of burrowing allowed larger bacteria populations to survive.We conclude that even in a simple two-species system a predator can positively impact prey via ecosystem engineering.