Detritivores as indicators of landscape stress and soil degradation

Detritivores are small- to medium-sized invertebrates that comminute and break down organic materials such as leaves, twigs and roots, especially within or upon the soil surface, or nearby. Detritivores constitute the majority of the invertebrate biomass pyramid in most environments and provide a key role in organic matter turnover; they also provide alternative food for polyphagous predators that can be active in pest control on crops. Many arthropod taxa are detritivores in soil and litter layers. Here, we focus on the bioindicator potential of three key detritivore groups: slaters, millipedes and oribatid mites. There are possibly 300 species of slaters (terrestrial isopods or Oniscidea) in Australia with 13 of these being introduced, mostly from north-western Europe. These non-native species are the dominant species in disturbed environments such as intensively managed forests and agricultural fields. Slaters are promising indicators of landscape disturbance, soil contamination and tillage. Millipedes are potentially important indicators of stress in agricultural landscapes, given their sensitivity to litter and soil moisture gradients and to physical and chemical perturbations. However, because there is a close association between the millipede fauna and moist plant communities in Australia, they are generally absent from drier landscapes and, therefore, their use as bioindicators in agricultural environments here is problematic. An exception to this association is the increasingly ubiquitous introduced Black Portuguese millipede. This species is tolerant of much drier conditions than most natives, and is likely to change the nature of nutrient cycling processes in pastures and native grasslands in much of southern Australia. Oribatid mites are present in all Australian terrestrial ecosystems. The few studies that have examined their response to disturbance and land use in Australia are consistent with the body of work conducted outside Australia. This consistent response means that the oribatids may be developed as indicators in agricultural, pasture and forested environments. However, the paucity of information on oribatids over appropriate spatial scales in Australia makes the use of this group extremely difficult at this time.