The role of laboratory terrestrial model ecosystems in the testing of potentially harmful substances.

: A classification of terrestrial model ecosystems (TMEs) was introduced which is based upon the physical properties of intactness of the physical medium and openness to the atmosphere. This gave rise to four types of system, namely open and closed intact systems and open and closed homogeneous' systems. These systems have different capabilities with respect to fate and effect end-points with various substances. The large closed TMEs are generally very complex, require a high degree of operator skill, expensive and therefore not replicable. Whilst these can provide estimates of losses due to volatility, they are not useful for determining effect end-points because of low replicability; high replicability being necessary because of natural variation in organism response. Open systems, especially those having intact soil-cores, are usually smaller, less complex and therefore more replicable. These have provided useful information on integrative functional effect end-points, but can only produce mass balances with non-volatile substances. Homogenization of the medium has also helped elucidate ecotoxicological effects by increasing replicability, but may introduce artifacts because of the disruption to soil organisms.A major limitation of TME studies would seem to be that few effect end-points can be non-destructively sampled. Further investigations into these may provide information on recovery of terrestrial ecosystems over time after substance application, perhaps using multivariate statistical techniques. Other problems concerning TMEs are related to complexity and scale. In this respect ecosystem functions in which microorganisms play a major role, such as nutrient cycling, provide the greatest similarity when compared to field evaluations of the same substances, especially where the TME is intact. However, effects upon structural aspects of biological communities have in general not been well researched in TMEs. Once these have been added to the more complete set of functional end-points, TMEs will provide a very useful tool in hazard assessments of potentially harmful substances.