LINKING THE HYDRAULIC WORLD OF INDIVIDUAL ORGANISMS TO ECOLOGICAL PROCESSES: PUTTING ECOLOGY INTO ECOHYDRAULICS

Ecohydraulics is a relatively new name for a long-standing field investigating the interactions between ecology and the physics of flowing water. In recent decades, a burgeoning literature has developed relating species' densities to environmental variables, particularly those associated with flow. These abundance-environment relations (AERs) are sometimes used in habitat-based association models (HAMs) to predict changes to species' abundances, especially human-caused changes to the environment. Unfortunately, some conclusions arising from AERs are based on untenable ecological assertions or faulty assumptions including: illogical inference of cause-and-effect, muddling of hypotheses such as 'habitat preference' with other causal mechanisms, outdated ideas about environmental tolerance and optimization. Furthermore, the effects of biological interactions in AERs are routinely ignored, when biological interactions form the core of most modern models in community ecology. HAMs underpinned by AERs are often used to predict changes in populations, but lack information on demographic rates (births, deaths, migration), which are essential for population-level predictions. These weaknesses are discussed at length and, we suggest, must be acknowledged and surmounted if ecohydraulics is to advance our understanding of stream ecosystems and our capacity to manage streams. We give two examples of research at the ecological-physical interface that are grounded in ecological principles: (1) how flow influences feeding rates of filter-feeding insects (Simuliidae and Hydropsychidae) and the consequences for larval growth rates, adult body size and fecundity, and (2) how the physical factors that determine the density of rocks emergent from the water's surface may affect population densities of insects where females lay their eggs on emergent rocks. Ecohydraulics is an interdisciplinary field that requires expertise in both ecology and hydraulics, and true interdisciplinary teams may have the best prospects of addressing the many unanswered and untackled questions. Copyright (C) 2009 John Wiley & Sons, Ltd.