Ecology of nematophagous fungi: Effect of soil moisture, organic matter, pH and nematode density on distribution

Abstract

The effect of organic matter, soil moisture, pH and the indigenous soil nematode density on the distribution of nematophagous fungi has been examined. Using 206 samples of soil and organic material, the presence and absence of groups and individual species of nematophagous fungi have been compared statistically to indicate which of these major factors are important in their distribution. The presence of predatory fungi was more influenced by pH and moisture than the other soil factors, while the presence of the conidia-forming endoparasites was influenced by organic matter. The presence of obligate parasites was associated with high soil nematode densities while facultative predators were independent of nematode density. Endoparasites with conidia that do not attract nematodes were found in soils with a higher nematode density than those species producing nematode-attracting conidia. This would increase the chance of infection by the random ingestion of the conidia by a suitable host. Non-spontaneous trap forming predators were isolated from soils with low organic matter and low moisture. These species are able to compete well saprophytically under such conditions, and when nutrients or moisture temporarily improve, they are able to maintain their competitive advantage by utilizing the expanding nematode population. Spontaneous trap-forming predators were found in soils with relatively high organic matter and moisture. In such soils there is a rich microbial flora and fauna, and the ability to form traps spontaneously offers these predators a direct competitive advantage over saprophytes and non-spontaneous trap-forming predatory fungi. Attempts to model the data were generally unsuccessful due to the qualitative-quantitative nature of the data, the small data base and the resulting high number of distinct co-variate patterns formed by the independent variables. The methods of isolation used are compared with earlier investigations where isolations were made from extracts taken from large weights of sample material.