Minimally disturbed? Assemblage prediction through the concept of environmental filters helps to define the status of reference sites used in freshwater bioassessment
Bruce C. Chessman, Ph.D., Department of Natural Resources, 10 Valentine Avenue, Parramatta, NSW, Australia
Data collected for bioassessment of fresh waters usually are interpreted by comparison with reference data obtained from designated reference sites. Reference sites often are described as ‘minimally disturbed’, which implies that the reference data obtained from these sites provide a benchmark that is negligibly tainted by human impact. However, objective evidence of minimally disturbed status is often weak or lacking. The capacity to predict the natural composition of freshwater assemblages from a knowledge of the preferences or tolerances of individual taxa for multiple environmental filters provides an objective way to test the hypothesis of minimal disturbance. The application of this test to macroinvertebrate assemblages at potential reference sites on rivers in New South Wales, Australia, suggests that while minimally disturbed sites may still exist in the least developed regions of the State, such sites no longer occur in the highly modified landscapes that prevail in many regions. This inference is supported by historical evidence of regional extinctions. Hence a ‘reference condition’ for macroinvertebrates that is based on reference sites cannot universally represent minimal disturbance.