Department of River Ecology, Helmholtz Centre for Environmental Research - UFZ, Brückstraße 3a, 39114, Magdeburg, Germany.
Workgroup Aquatic Chemical Ecology, University of Cologne, Zülpicher Straße 47B, 50674, Cologne, Germany.
Oecologia. 2023 Feb;201(2):479-488. doi: 10.1007/s00442-022-05308-3. Epub 2023 Jan 6.
Both the quantity and nutritional quality of food resources can strongly influence the foraging movements of herbivores, which in turn determine the strength of top-down control on primary producer biomass. Nutrient enrichment can alter the biomass and nutritional quality of primary producers, but the consequences for the foraging of herbivores and hence for top-down control are still poorly understood. In this study, we combined a two-factorial experiment (two nutrient levels × grazing by the freshwater gastropod Ancylus fluviatilis) with video analyses tracking grazers' movements to investigate nutrient enrichment effects on spatial ranges of grazing activity and algal biomass removal. Natural stream biofilms were grown in phosphorus-enriched (P+) and phosphorus-poor flumes (P-) for two weeks before A. fluviatilis were added to the flumes and allowed to graze on biofilm for an additional 2 weeks. Total periphyton biomass was enhanced by P+ and reduced by grazer presence. However, the total grazer effect depended on the nutrient level: at the end of the experiment, on average 95% of algal cover were removed by grazing in the P- flumes versus 26% in the P+ flumes. Fast movements of A. fluviatilis were detected significantly more often in the P- treatment, whereas grazers were detected resting more often in the P+ treatment. Our results demonstrate that nutrient enrichment can increase primary producer biomass both directly and indirectly by limiting the foraging ranges of herbivores. The resulting feedback loop between reduced grazing activity and increased plant biomass might in turn exacerbate eutrophication effects on habitat structure.
食物资源的数量和营养质量都可以强烈影响食草动物的觅食活动，而食草动物的觅食活动反过来又决定了上层控制对初级生产者生物量的强度。营养富集可以改变初级生产者的生物量和营养质量，但对于食草动物的觅食以及因此对上层控制的后果仍知之甚少。在这项研究中，我们结合了一个双因素实验（两个营养水平×淡水腹足类Ancylus fluviatilis 的放牧）和视频分析跟踪食草动物的运动，以调查营养富集对放牧活动的空间范围和藻类生物量去除的影响。在向水槽中添加Ancylus fluviatilis 并允许其在生物膜上放牧两周之前，在富磷（P+）和贫磷水槽（P-）中生长了两周的天然溪流生物膜。总周丛生物量被 P+增强，并被食草动物的存在减少。然而，总食草动物的影响取决于营养水平：在实验结束时，在 P-水槽中，藻类覆盖率的 95%被放牧平均去除，而在 P+水槽中则去除了 26%。在 P-处理中，Ancylus fluviatilis 的快速运动被检测到的频率明显更高，而在 P+处理中，食草动物被检测到休息的频率更高。我们的结果表明，营养富集可以通过限制食草动物的觅食范围直接和间接地增加初级生产者的生物量。减少的放牧活动和增加的植物生物量之间的这种反馈循环可能反过来加剧富营养化对栖息地结构的影响。
