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Submitted on: 28 November 1999.
Revised on: 19 January 2000.
Accepted on: 26 February 2000.
___________________ECOLOGICAL APPLICATIONS---WILEY
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Submitted on: 02 July 1998.
Revised on: 20 October 1998.
Accepted on: 07 November 1998.
___________________ECOLOGICAL APPLICATIONS---WILEY