Qualitative and quantitative account of indirect effects is (albeit often implicitly) becoming a common part of environmental management, and is indispensable for successful application of, for example, landscape engineering, biomanipulation, biogeochemical manipulation, strategic environmental assessment (SEA), and environmental impact assessment (EIA). In particular, mathematical methods can be helpful in this respect. For example, Ortiz and Wolff used Ecopath with Ecosim software to study benthic communities in Chile. They found that a simulated harvest of the clam Mulinia generated a complex interplay involving direct and indirect effects, and drastically changed the properties of the whole system.
McClanahan and Sala used a simulation model of the Mediterranean infralittoral rocky bottom to study possible effects of various management options. Running a number of 'what-if' scenarios they concluded that many of potential changes are likely to be indirect effects caused by changes in trophic composition. For example, if inver-tivorous fish were removed as part of a management scenario, sea urchins would reduce algal abundance and primary production, leading to competitive exclusion of herbivorous fish. Although similar interactions were known from tropical seas, these results were not anticipated by previous field studies in the Mediterranean.
In some cases appreciation and reliance on indirect effects may form the basis of environmental management measures. To date, there are numerous relevant examples, including, for example, biocontrol, bio- and biogeochemical manipulation or application of chemicals to reduce sediment P release for subsequent control of blue-green algae, infection of grass cultivars by endophy-tic fungi in turf industry, etc. It should be reiterated, however, that a sound knowledge of the system's natural history is absolutely indispensable for an application of any environmental control measure. The problems related to the detection and investigations of indirect effects (with the major ones listed above) are likely to provide challenging, and often unexpected complications, frequently after a considerable time period. Hence, a combination of empirical and theoretical work should precede any practical steps, and any desk study should be backed up by a thorough monitoring plus (where necessary) experimental program.
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