Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.
PLoS One. 2013 Apr 23;8(4):e61772. doi: 10.1371/journal.pone.0061772. Print 2013.
Seabirds deposit large amounts of nutrient rich guano on their nesting islands. The increased nutrient availability strongly affects plants and consumers. Consumer response differs among taxonomic groups, but mechanisms causing these differences are poorly understood. Ecological stoichiometry might provide tools to understand these mechanisms. ES suggests that nutrient rich taxa are more likely to be nutrient limited than nutrient poorer taxa and are more favored under nutrient enrichment. Here, we quantified differences in the elemental composition of soil, plants, and consumers between islands with and without nesting cormorant colonies and tested predictions made based on ES by relating the elemental composition and the eventual mismatch between consumer and resource stoichiometry to observed density differences among the island categories. We found that nesting cormorants radically changed the soil nutrient content and thereby indirectly plant nutrient content and resource quality to herbivores. In contrast, consumers showed only small differences in their elemental composition among the island categories. While we cannot evaluate the cause of the apparent homeostasis of invertebrates without additional data, we can conclude that from the perspective of the next trophic level, there is no difference in diet quality (in terms of N and P content) between island categories. Thus, bottom-up effects seemed mainly be mediated via changes in resource quantity not quality. Despite a large potential trophic mismatch we were unable to observe any relation between the invertebrate stoichiometry and their density response to nesting cormorant colonies. We conclude that in our system stoichiometry is not a useful predictor of arthropod responses to variation in resource nutrient content. Furthermore, we found no strong evidence that resource quality was a prime determinant of invertebrate densities. Other factors like resource quantity, habitat structure and species interactions might be more important or masked stoichiometric effects.
海鸟在其筑巢的岛屿上大量排泄富含营养的粪便。增加的养分供应强烈影响着植物和消费者。消费者的反应因分类群而异，但造成这些差异的机制尚不清楚。生态化学计量学可能为理解这些机制提供工具。ES 表明，富营养类群比贫营养类群更有可能受到营养限制，并且在养分富集下更受青睐。在这里，我们量化了有无鸬鹚筑巢岛之间土壤、植物和消费者的元素组成差异，并通过将消费者的元素组成和最终与资源化学计量学的不匹配与观察到的岛屿类别之间的密度差异相关联，测试了基于 ES 的预测。我们发现，鸬鹚筑巢会极大地改变土壤养分含量，从而间接地改变植物养分含量和资源质量，进而影响食草动物。相比之下，消费者在岛屿类别之间的元素组成上只表现出很小的差异。虽然我们无法在没有更多数据的情况下评估无脊椎动物明显的体内平衡的原因，但我们可以得出结论，从下一个营养级的角度来看，岛屿类别之间的饮食质量（以 N 和 P 含量衡量）没有差异。因此，底向上的影响似乎主要通过资源数量的变化而不是质量的变化来介导。尽管存在很大的潜在营养不匹配，但我们无法观察到无脊椎动物化学计量学与其对鸬鹚筑巢反应之间的任何关系。我们得出的结论是，在我们的系统中，化学计量学不是预测节肢动物对资源养分含量变化反应的有用指标。此外，我们没有发现强有力的证据表明资源质量是无脊椎动物密度的主要决定因素。其他因素，如资源数量、栖息地结构和物种相互作用，可能更为重要或掩盖了化学计量学效应。
