The invertebrate fauna of grassland and its influence on productivity. III. Effects on soil fertility and plant growth

The effects of grassland invertebrates on organic matter decomposition, soil fertility and plant growth are reviewed. Their role as primary decomposers is limited because of relatively low rates of metabolic and enzymatic activity compared with microorganisms. However, they greatly facilitate decomposition by fragmenting and incorporating litter into the soil and by stimulating microbial activity. Earthworms are the main litter consumers in productive temperate grasslands and termites in the tropics, while the activities of dung beetles, dipterous larvae and earthworms are essential for efficient dung decomposition. Earthworms and microbial‐feeding invertebrates facilitate mineralization and release into the soil solution of plant nutrients. Earthworms have a major influence on soil structure through burrowing, soil ingestion and mixing and they significantly increase plant growth. Herbivorous invertebrates at non‐outbreak densities normally consume less than 10% of net primary production in grassland, but consumption bears little relationship to overall effects on productivity. These depend on factors such as the nature of the damage caused and regrowth potential, tolerance for damage and effects on animal production at the time of attack. Sap feeding insects may cause wilting and reduced growth and selective feeding on preferred species can alter botanical composition, while aphid‐transmitted virus disease can depress yield. On the positive side, low levels of herbivory can stimulate growth, weed feeding insects may limit the distribution of pasture weeds, while the role of flower‐visiting insects can be important in the pollination of legume seed crops. Pest management approaches incorporating biological control, alteration of botanical composition, adjustment of stocking densities and accurately timed insecticidal applications offer prospects for the control of recurring pests such as grass grub and porina in New Zealand.