Compensatory growth responses of Potamogeton pectinatus to foraging by migrating trumpeter swans in spring stop over areas

We examined spring pond use by migrating trumpeter swans (Cygnus buccinator) to assess their short-term impact on tuber and rhizome density and biomass, and to evaluate the impact of spring foraging on summer macrophyte biomass and species composition. Trumpeter swans in the Canadian subpopulation of the Rocky Mountain population select ponds that are dominated by Potamogeton pectinatus, a macrophyte favoured for its energy-rich tubers and rhizomes. Swans significantly reduced the biomass of tubers and rhizomes present in the study ponds in spring by 24%, but there was no significant impact on overall P. pectinatus shoot density and biomass the summer following herbivory. However, there were significantly fewer small P. pectinatus shoots (<1.0 g DW) in used areas, and other macrophyte species (Myriophyllum exalbescens, P. zosteriformis) were present in areas where foraging had occurred. R pectinatus may compensate for the effects of herbivory, as the number of larger shoots (range 1.0-5.0 g DW) were similar in areas where trumpeter swans had foraged and been excluded. These larger plants likely produce the tubers that will be consumed by trumpeter swans the next spring. Pond areas out of the reach of foraging swans provide a refuge for the tubers and rhizomes, also enabling the persistence of P. pectinatus. (C) 2003 Elsevier Science B.V. All rights reserved.