Nematodes as a biological indicator
Nematodes are used as biological indicators of soil health because the number and types present in a soil reflect changes in the microbes they consume, and the soil’s physical and chemical environment.
The ratio of bacterial- to fungal-feeding nematodes indicates the rate of nutrient cycling. In addition, populations of omnivorous and predatory nematodes indicate whether the soil biology is affected by pollutants or disturbances, and whether the soil has some capacity to suppress pathogenic organisms.
This fact sheet from Soil Quality provides a good overview of why nematodes are good biological indicators.