Studies of changes in the populations of invertebrates associated with cyclical processes in heathland
During its life cycle the Calluna vulgaris L. plant passes through four phases related to its age and physiognomy. These are the pioneer, building, mature and degenerate phases. Within each phase, the Calluna plant provides possible habitats for invertebrates within the Calluna-derived litter, on the surface of this litter and in the canopy of the Calluna plant; many invertebrates are also found in the atmosphere above the plant canopy. The invertebrate fauna occurring within the litter includes mites, springtails, false scorpions, harvestmen, spiders, beetles, millipedes, Hemiptera, flies and Hymenoptera. The most numerous and frequently occurring of these are the mites, being almost equally divided into carnivores and those that feed on detritus. The size and diversity of the invertebrate population found within the litter is greatest under pioneer Calluna in heaths that have not been managed by burning, and greatest under degenerate in heaths that have been managed by burning. Otherwise it varies little as the Calluna passes through the different phases. The invertebrate fauna is greatest in numbers and highest in diversity on the surface of the litter. All those found within, the litter also occur in this habitat, often in greater numbers. The numbers and frequency of the beetles and millipedes are greater in this habitat than within the litter, although very few millipedes are found on the litter surface under managed heather. The carnivores and detritus feeders constitute the largest feeding classes. The diversity is greatest under degenerate unmanaged plants and building managed plants. Many more differences in the number of invertebrates are found between phases of managed than unmanaged heather. The invertebrate fauna found within the Calluna canopy includes harvestmen, spiders, springtails, thrips, leafhoppers, spittle bugs, beetles, flies, Hymenoptera and mites. The diversity of their populations in the unmanaged heather is greatest in the canopy of pioneer plants, while in the managed heath it is greatest in the degenerate plants. What few numerical differences are found involve larger numbers in the pioneer than other phases of heather. Many more differences are found between phases of managed than unmanaged heather. The invertebrate fauna found above the Calluna canopy consists mainly of Diptera and Hymenoptera, although spiders, leafhoppers, aphids and beetles are also found. The largest feeding class is the carnivores, animal parasites are found in greater numbers here than elsewhere. The size and diversity of the populations do not differ above different phases of Calluna, both managed and unmanaged, although that above managed heather is lower than that above unmanaged heather. Management by fencing and fertilizing reduces the number of invertebrate families present and has a variable effect on the diversity of the invertebrate populations.