Food web structure including grazing and detritus food chains: case studies in forests and farm ponds.

本研究的目的是阐明碎屑注入对放牧食物网的重要性，这一研究迄今尚未得到充分研究。我们估计了在两个不同的系统，森林和农场池塘，从碎屑到放牧链的生物或能量的输入。森林：我们估计了网蜘蛛这种主要的陆生多面手捕食者依赖碎屑食物链中的生物的程度。在草地生境中，夏季对放牧链的依赖程度较高，而在森林生境中，对腐质链的依赖程度一直较高，且随季节变化不大。然而，即使在相同的栖息地，这种依赖也会随着网的结构和位置而变化。通过野外实验考察了地下猎物的输入是否能维持网蜘蛛的种群密度和物种丰富度。我们在地面上建立了有和没有床单的蜘蛛围栏，以观察拦截从地面出现的昆虫的效果。结果表明，在没有地面覆盖的围场中，个体数量和种类数量都更丰富。由于在没有地面覆盖的围栏中，飞虫的丰度更高，因此碎屑注入似乎对地面上的多面手捕食者的维持有重大影响。我们利用稳定同位素分析方法估计了农场池塘的食物网结构。我们从农场池塘收集了各种生物和碎屑，包括小龙虾、昆虫、鱼类、浮游生物和垃圾。在森林环绕的池塘中，小龙虾对凋落物和DOM的依赖程度较高，而在没有森林环绕的池塘中，初级生产者对小龙虾至关重要。此外，低同位素比率凋落物的池塘在食物网水平上的总体同位素比率较低。这些结果表明，凋落物似乎是能量和养分循环的基础。

The aim of this study is to clarify the importance of detrital infusion into grazing food webs, which was not well studied so far. We estimated input of organisms or energy from detritus to grazing chains in two different systems, forests and farm ponds.1. Forests:We estimated the extent to which web spiders, major terrestrial generalist predators, depend on organisms from detritus food chain. In a grassland habitat, dependency on grazing chain became higher in summer, but in a forest, the dependency on detritus chain was consistently high and did not change greatly with seasons. However, the dependency varied with web structure and web placement even in the same habitat. We also examined whether population density and species richness of web spiders are sustained by input of prey from underground by field experiments. We established spider enclosures with and without a sheet on the ground to see the effects of interception of insects emerging from the ground. As a result, both number of individuals and number of species were more abundant in enclosures without a sheet on the ground. Because abundance of flying insects was higher in enclosures without a sheet on the ground, it appears that detrital infusion has a significant impact on the maintenance of generalist predators aboveground.2. Farm pondsWe estimated food web structure in farm ponds by using stable isotope analysis. We collected various organisms and detritus from farm ponds, including crayfish, insects, fish, plankton, and litter. In ponds surrounded mostly by forests, crayfish depended heavily on litter and DOM, but in ponds with no forests nearby, primary producers were important for crayfish. Moreover, ponds having litter with a low isotope ratio showed a low overall isotope ratio for the food web level. These results suggest that litter seems to be the basis for energy and nutrient cycling.