The effect of trophic cascade on process chain in detrital food webs

本研究旨在利用蜘蛛-弹尾目-微生物-凋落物-土壤动物组成的系统来阐明营养级联对碎屑食物链中过程链的影响。我通过建立有和没有这种蜘蛛的田间围栏，研究了捕食者Neriene brongersmai对土壤生态系统的直接和间接影响。Neriene brongersmai是一种盛产于日本雪松林底的网络蜘蛛。我提出的四个先验假设如下：1)蜘蛛控制着弹尾目动物的丰度吗？2)蜘蛛是否间接地增加了土壤微生物的生物量？3)蜘蛛是否间接地增加了凋落物的分解速度？4)蜘蛛是否间接增加了不被蜘蛛直接吃掉的土壤动物？一项长达一年半的土地实验表明，假设1得到了支持，即蜘蛛抑制了弹尾目动物的丰度。与我的预期相反，假设2到4没有得到证实。我提出了这些负面影响的几个原因，包括1)蜘蛛的自上而下效应不足以引起营养级联，2)蜘蛛捕食在短期内减少了微生物的生物量，但快速的周转速度使长期生物量保持不变。这项研究未能证明营养级联和过程链之间的联系，但研究中使用的方法似乎是有用的。在未来，有必要进行操纵实验，以研究官能团水平的效应是如何运作的，以及官能团中物种或行会的多样性如何影响连接食物链和过程链的过程。

This study aimed at clarifying the effect of trophic cascade on the process chain in detrital food webs by using the system consisting spider-colembola-microorganism-litter-soil animals. I investigated direct and indirect effects of the predator Neriene brongersmai, a web spider abundant in the forest-floor of the Japanese cedar, on soil ecosystems by establishing field enclosures with and without this spider. Four a priori hypotheses that I had made are as follows,1)Do spiders control the abundance of Colembola?2)Do spiders indirectly increase the biomass of soil microorganisms?3)Do spiders indirectly increase the rate of decomposition of litter?4)Do spiders indirectly increase soil animals that are not directly eaten by spiders?A land 1/2 year long experiment showed that the hypothesis 1 was supported, i.e., spiders suppressed collembolan abundance. Contrary to my expectation, the hypotheses 2 to 4 were not verified. I proposed several reasons for these negative effects, including 1)top-down effect by the spiders was not strong enough to cause trophic cascade, 2)biomass of micoorganisms had decreased in a short term by spider predation, but the rapid turnover rate made the long term biomass invariable. This study failed to demonstrate the linkage between trophic cascade and process chain, but the approaches used in this study appeared to be useful. In the future, it is necessary to conduct manipulation experiments to examine how functional group level effect is operating, and how diversity of species or guilds within a functional group influences processes linking food web and process chain.