Populations of two prey nematodes and their interaction are controlled by a predatory nematode

Many nematode species are candidate predators of other nematodes. However, the impact of predation on prey population dynamics and assemblage structure is unknown. We performed microcosm experiments in which the effect of the predacious nematode Enoploides longi-spiculosus on the population development of 2 prey nematode species, Diplolaimelloides meyli and D. oschei, was investigated at different predator abundances. One experiment used monospecifc prey populations, another used a mixed assemblage with both prey species. With monospecific prey populations, abundances of both prey species decreased with increasing predator abundances. Size-selective predation released small juveniles (J1, J2) completely, and older juveniles (J3, J4) partly, from predation. Since previous studies had demonstrated that D. meyli partially inhibits population development of D. oschei in the absence of predators, and Enoploides prefer D. oschei over D. meyli, we expected predation to emphasize the dominance of D. meyli over D. oschei in mixed prey populations. However, our results showed the opposite, viz. strong inhibition of D. oschei by D. meyli in control microcosms without predators, and decreasing inhibitory effects at increasing predator abundance, resulting in more equitable abundances of both prey species. Predation thus alleviated the inhibitory effect of 1 prey species over the other. We conclude that predatory nematodes like Enoploides exert pronounced effects on their prey populations, but are unlikely to drive individual prey populations to extinction. Further, predator effects on prey assemblage structure depend as much on indirect effects as on direct predator-induced prey mortality. Experiments with more complex and natural species combinations are required to allow proper assessment of the importance of the present findings for natural assemblages