CONSUMPTION OF AQUATIC BRYOPHYTES BY ALPINE STREAM INVERTEBRATES IN NEW-ZEALAND

Gut contents of 23 invertebrate taxa that dwell among bryophytes in two small New Zealand alpine streams with different riparian vegetation were examined. Bryophyte fragments were found in guts of 14 taxa, but only the tipulid Limonia hudsoni and the oeconesid caddisflies Zelandopsyche ingens and Oeconesus similis commonly consumed bryophytes. Bryophyte-dwelling animals consumed more periphyton at the unshaded site or riparian vegetation at the shaded site than did animals dwelling amongst gravels. Stable carbon isotope ratios were compared between selected animals and plants, and indicated that animals in the unshaded site relied primarily on autochthonous detritus, whereas animals from the shaded site relied on allochthonous detritus. An indication of the food value of five bryophytes (Fissidens rigidulus, Cratoneuropsis relaxa, Bryum blandum, Plagiochila retrospectans, and Hepatostolonophora paucistipula) and of seven common riparian plants (Chionochloa pallens, C. flavescens, Hebe subalpina, H. odora, Nothofagus solandri var. cliffortioides, Blechnum capense, and Marsippospermum gracile) was obtained by comparing their proximate biochemical composition. This revealed that bryophytes contained more refractory and indigestible compounds than riparian plants and represented a less nutritious food source for animals. Avoidance of bryophyte tissues thus reflects a combination of the presence of alternative foods and their low nutritional value; bryophytes may also contain antifeedant compounds.