The importance of soil biota in maintaining ecosytem integrity is examined by a review of studies of soil processes and soil biota in arid ecosystems. In decomposition and mineralization processes, there is a temporal succession of microarthropod and nematode species. Tydeid mites are keystone species in the early stages of decomposition. Soil pore neck size variation affects the spatial distribution of soil biota and requires that species differ in size to provide efficient processing of organic matter. In arid ecosystems, the sub-set of soil biota that is active at any point in time is determined by the soil water potential and soil temperature. The structure of soil microarthropod assemblages is similar in several arid regions of the world and abundance and diversity are directly related to quantity of litter accumulations and soil organic matter. The unique life histories and behavioural characteristics of desert soil macrofauna (termites and ants) determine the effects of these organisms on soil properties and soil formation. The soil biota, by affecting the spatial and temporal distribution of essential sesources (water and nutrients), are essential to the maintenance of the ecosystem integrity of arid ecosystems.

The importance of the biodiversity of soil biota in arid ecosystems

We sampled the soil microarthropod community monthly in the oak-mesquite sand hill ecosystem. Small fungiphagous prostigmated mites (pyemotids, lordalychids and tarsonemids) that dominated the soil fauna in winter were replaced by large predaceous mites (rhodacarids and laelapids) in summer and autumn. We compared organic matter loss and microarthropod and nematode populations in shinnery oak (Quercus harvardii) using insecticide and untreated litter in fiberglass litterbags. Microarthropods extracted from litterbags showed a seasonal pattern similar to the soil cores except that collembolans and psocopterans were abundant in the litter and not in the soil cores. Numbers of free living nematodes were consistently greater than from untreated litter. The ratio of non-stylet to stylet bearing nematodes extracted from litter decreased from 4:1 in one month bags to 0.8:1.0 in the one year bags. Laboratory experiments showed that rhodacarid mites fed voraciously on nematodes. Untreated litter exhibited higher rates of organic matter loss than the insecticide treated litter; 20% and 35% respectively. We suggest that the abundant mesostigmatid mites prey on free living nematodes and that eliminating the predators allows the nematodes to overgraze the fungi and bacteria. The soil modifies the microclimate in buried litter allowing for higher biological activity, hence higher rates of decomposition.

The role of microarthropods and nematodes in decomposition in a semi-arid ecosystem.

Changes in precipitation and soil water availability predicted to accompany global climate change would impact grasslands, where many ecosystem processes are influenced by water availability. Soil biota, including microarthropods, also are affected by soil water content, although little is known about how climate change might affect their abundance and distribution. The goal of this study was to examine soil microarthropod responses to altered soil water availability in tallgrass prairie ecosystems. Two separate experiments were done. The first utilized control and irrigated plots along a topographic gradient to examine the effects of soil water content on microarthropod densities. Microarthropods, mainly Acari, were significantly less abundant in irrigated plots and were generally less abundant at the wetter lowland sites. The second study utilized reciprocal core transplants across an east-west regional precipitation gradient. Large, intact cores were transplanted between a more mesic tallgrass site (Konza Prairie) and a more arid mixed-grass site (Hays) to determine the effects of different soil water regimes on microarthropod abundance and vertical distribution. Data from non-transplanted cores indicated greater total microarthropod densities at the drier Hays site, relative to the wetter Konza Prairie site. Data from the transplanted cores indicated significant effects of location on Acari densities in cores originating from Hays, with higher densities in cores remaining at Hays, relative to those transplanted to Konza. Acari densities in cores originating from Konza were not affected by location; however, oribatid mite densities generally were greater in cores remaining at Konza Prairie. These results confirm the importance of soil water content in affecting microarthropod densities and distributions in grasslands, and suggest complex, non-linear responses to changes in water availability.

Responses of soil microarthropods to changes in soil water availability in tallgrass prairie

Ecosystem research is in its infancy, its roots are varied, it has a reasonable theoretical basis but it lacks the extensive data so essential for the development of the subject. The very complexity and scale of ecosystems makes analysis and comprehension difficult. However, the stimulus for research lies in the recognition that the ecosystem is a level of organization at which many organisms and processes interact. It is a unit which man manages for agriculture and forestry, and the flow of energy, carbon and nutrients provides a common currency for comparison of components and ecosystems.

Nutrient Cycling and Decomposition in Natural Terrestrial Ecosystems

The main factors affecting the abundance and composition of the invertebrate fauna in grassland are reviewed. Old grasslands with species-rich swards and complex structure support an abundant and diverse fauna compared with uniform leys with few species. Seasonal and successional changes in the sward are accompanied by equally marked changes in the invertebrate community. Climate and weather have a major influence, but their effects may be modified locally by the availability of shelter. Soil physical and chemical characteristics influence the soil fauna directly and the above ground fauna indirectly through the vegetation. Food quality as determined by physical and chemical plant characteristics strongly influences invertebrate growth, reproduction and mortality. The main biotic factors affecting abundance are natural enemies and disease; these, in combination with weather and food quality, are often the main agents in determining population density in natural communities.

Management practices such as grazing, cutting, fertilizer application, burning, soil water control and pesticide use profoundly influence the sward and induce correspondingly great changes in the invertebrate community.

The invertebrate fauna of grassland and its influence on productivity. II. Factors affecting the abundance and composition of the fauna

The fauna associated with the decomposition of grassland herbage decaying in the soil and on the soil surface was studied. Herbage was confined in nylon bags of different mesh sizes, the size of animals admitted being controlled by mesh size. Quite marked changes occurred in the populations of the main invertebrate groups and of the main species of Acari and Collembola present during the course of decomposition, the changes occurring in buried material being quite different to those occurring in surface material.

J.P. Curry

Papers

The decomposition of organic matter in soil part II. The fauna of decaying grassland herbage