9780521861724htl 1..2

after it has passed. In other words, both multiple stressors and taxon-specific

solutions can conspire to reduce the likelihood of detecting clear patterns.

We believe that our understanding of the structure and functioning of stream

ecosystems has increased because of size-focused studies, but can this understand-

ing be used to develop biomonitoring tools? Robson, Barmuta and Fairweather

(2005 ) note that body size has appeal as a taxon-free and easy-to-measure metric. In

addition, it bears relationships, either directly or indirectly, to a variety of human-

induced stressors. Size alone will certainly not furnish sufficient predictive power,

however, when prioritizing rivers for rehabilitation or measuring river improve-

ment. On the other hand, body size, in combination with other traits related to

trophic role, survival and reproduction, may be used to create river health metrics

of more general utility than those that depend simply on taxonomic composition

or species diversity (e.g. Charvet, Kosmala & Statzner,1998 ;Gayraudet al., 2003 ;

Statzneret al., 2004 ;Dole ́decet al., 2006 ). The potential advantages of trait-based

metrics are two-fold. First, a multiple-trait index is likely to be applicable across

large geographic areas despite taxonomic differences. Second, the representation

of particular traits can be expected to throw light on the mechanisms underlying

stressor impacts, providing focus for management efforts.

Acknowledgements

Thanks to Sylvain Dole ́dec for supplying Fig.5.1. RT was funded by the

Biodiversity Research Centre at the University of British Columbia.

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