Predator-prey interaction in soil food web: functional response, size-dependent foraging efficiency, and the influence of soil texture

In a series of laboratory experiments, we presented carnivorous Macrobiotus richtersi (Tardigrada, Macrobiotidae) with nematode prey to assess their importance as predator. We investigated consumption rate for (a) different prey densities (10-400 prey individuals), (b) different prey biomasses (22-80 ng), (c) different prey species (Pelodera teres, Rhabditidae, versus Acrobeloides nanus, Cephalobidae) and (d) different environments (2-D agar surface versus 3-D sand fractions of three different textures). M. richtersi consumed up to 4.6 mu g nematode prey in 4 h, that is, 43% of the tardigrade's body mass. Predation rate was positively correlated with prey density. The optimal prey in the present investigation was the biggest prey because it yielded the highest biomass uptake per time. In addition, the size of M. richtersi played an important role in consumption rate. Bacterivorous nematodes reacted differently to attack. Even in a water film on stiff agar where nematode agility was limited, a vigorous undulation reaction of P. teres led to a measurable reduction in consumption rate. A. nanus, in contrast, showed little response to attack. Microcosm experiments with sands of different particle size demonstrated that M. richtersi is able to chase and consume small bacterivorous nematodes in a 3-D soil matrix. However, consumption rate in sand microcosms was significantly reduced compared with pure agar. The sand matrix improved nematode agility and possibly provided small pores as refuge for the nematodes. The lowest consumption rate was observed in fine sand. Effects of predatory tardigrades on nematode numbers in the field are discussed.