脊椎動物與節肢動物掠食者對亞熱帶森林常綠喬木—紅楠（Machilus thunbergii）之跨營養階效應

Abstract

掠食者透過對植食者的捕食影響植物，亦即掠食者之跨營養階效應（indirect trophic effect），往往會因掠食者間的獵物區隔或同功群內捕食而產生不同的方向與強度。因此，在評估掠食者群聚之跨營養階效應時，需要考量掠食者的種間互動。本研究在亞熱帶闊葉林透過掠食者移除實驗來檢測掠食者對植食性節肢動物（啃食性、吸食性）豐度與群聚的影響，以及對紅楠（Machilus thunbergii）葉片（葉面積受損比例、葉變異發生機率）的影響。掠食者移除處理分為四組，包括控制組、脊椎動物掠食者移除組（以圍網移除鳥與蝙蝠）、節肢動物掠食者移除組（手動移除蜘蛛）、脊椎動物掠食者與節肢動物掠食者都移除組（鳥、蝙蝠、蜘蛛皆移除）。結果顯示，當鳥、蝙蝠與蜘蛛都被移除時，食葉性節肢動物的豐度上升且紅楠葉面積受損比例增加；同時，任一掠食者的存在即可抑制植食性節肢動物之數量，並減少紅楠的葉片受損，且掠食者並不影響植食性節肢動物的群聚組成，據此可推測脊椎動物掠食者與節肢動物掠食者在獵物物種上有高度重疊。此外，食葉動物數量與葉片受損量之間不具相關性，這可能是因為造成嚴重葉受損的是少數幾個物種（如楠六點天蛾Marumba cristata bukaiana）。鳥與蝙蝠移除組較控制組有更高的蜘蛛豐度，顯示脊椎動物掠食者會捕食節肢動物掠食者並導致其數量下降（同功群內捕食），但這個現象並未削弱掠食者群聚之跨營養階效應。本研究證實掠食者群聚對植物產生正向的跨營養階效應，且脊椎動物與節肢動物所提供的掠食功能具冗餘現象（functional redundancy），導致掠食功能不隨掠食者多樣性發生變化。Predators can affect plants through preying on herbivores (i.e. indirect trophic effect), but the direction and strength of such effects may vary depending on prey partitioning and intraguild predation among different predators. Therefore, it is important to consider species interactions while evaluating indirect trophic effects of predator community. In this study, I experimentally excluded different predators at a subtropical broad-leaf forest to examine their effects on the abundance and community composition of herbivorous arthropods (folivores, sap-suckers), as well as on the leave damage (proportion leaf area chewed, occurrence of leaf discoloration) of the Japanese bay tree (Machilus thunbergia). Predator removal included four treatments: control, vertebrate predators exclusion (birds and bats removed by netting), arthropod predators exclusion (spiders manually removed), and both predators exclusion (birds, bats and spiders all removed). The results showed that when birds, bats and spiders were all excluded, the folivores increased in abundance and M. thunbergia had a greater proportion leaf area chewed. At the same time, vertebrate predators or arthropod predators alone could suppress herbivore abundance and leaf damage, and neither influenced herbivore community compositions, suggesting extensive prey overlaps between vertebrate predators and arthropod predators. Interestingly, the folivore abundance was not correlated with proportion leaf area chewed, suggesting that a few species (e.g. Marumba cristata bukaiana) might be causing most of the leaf damage. The exclusion of birds and bats led to an increase in spider abundance, indicating intraguild predation where vertebrate predators were preying on arthropod predators. However, this intraguild predation did not dampen the trophic effects of the predator community. This study demonstrated that the predator community had positive indirect effects on the trees, in which vertebrate predators and arthropod predators exhibited functional redundancy, decoupling predatory function and predator diversity.