鳥類掠食對水柳(Salix warburgii) 跨食物鍊階層之影響

Abstract

食物網中，掠食者對植物造成的跨食物鏈階層(trophic cascades)影響受到許多重視，本研究檢測水柳食物網中，鳥類掠食對水柳和樹上的節肢動物（分植食性、真菌食性與掠食者三群）豐度的影響。實驗樣地位於北臺灣新店溪岸，水柳以枝條為單位，使用細網隔絕鳥類20個月，並與有鳥類掠食的枝條比較其生長、繁殖、防禦性化學物質的表現以及節肢動物的豐度。無鳥類掠食的枝條生長與繁殖較差，但節肢動物群並未受到影響。防禦性化學物質以酚類配醣(phenolic glycosides)作為代表，無鳥類掠食的枝條其含量較少；葉子所受的植食損傷，無鳥類掠食的枝條在實驗開始約一年後損傷較高，但在第二年則與有鳥類掠食的枝條無異。實驗結果顯示，有鳥類掠食的水柳枝條生長與繁殖表現較好，其植食性損害也較低，證實了掠食者對植物有正向的跨食物鏈階層之影響。Trophic cascades, defined as indirect effects of predators on plants via herbivores, play a crucial role in food web functioning. In this study I tested top-down trophic effects of avian predation on plants and arboreal arthropods in a willow (Salix warburgii) food web along a riparian zone of Xindian river in northern Taiwan. Bird predation was excluded by nylon netting around the branches for 20 months. The growth, reproduction and level of defensive phytochemicals of these branches were compared to that of control branches on the same trees; the abundance of arboreal arthropods and level of herbivory were also compared. The bird exclusion caused lower growth and reproduction in the willows but did not affect the abundances of 3 arthropod groups (i.e. herbivores, fungivores and predators) on them. The plant defensive phytochemicals, measured as the amount of phenolic glycosides, were lower for the bird exclusion branches. The level of herbivory was higher in the bird exclusion branches approximately one year after the treatment, but returned to a similar level as the control branches in the second year. This study demonstrated that avian predation has positive cascading effects on willows at branch level by improving their growth and reproduction, as well as reducing their herbivory.