Division of Biology, Kansas State University, Manhattan, Kansas 66506, USA.
Ecology. 2012 May;93(5):1002-15. doi: 10.1890/11-1142.1.
The relationship between plant nutrient content and insect herbivore populations and community structure has long interested ecologists. Insect herbivores require multiple nutrients, but ecologists have focused mostly on nitrogen (an estimate of plant protein content), and more recently phosphorus (P); other nutrients have received little attention. Here we document nutrient variation in grass and forb samples from grassland habitats in central Nebraska using an elemental approach; in total we measured foliar concentrations of 12 elements (N and P, plus S, B, Ca, Mg, Na, K, Zn, Fe, Mn, and Cu). We detected significant variability among sites for N, P, Mg, Na, K, and Cu. We next used a model selection approach to explore how this nutritional variation and plant biomass correlate with grasshopper densities (collectively and at the feeding-guild level), and principal component analysis to explore nutrient correlations with grasshopper community species composition. When all grasshoppers were pooled, densities varied among sites, but only P was associated with abundance of the elements shown to vary between sites. Different responses occurred at the feeding-guild level. For grass specialists, densities were associated with N, plus P, Mg, and Na. For forb specialists, N and P were often associated with density, but associations with Na and K were also observed. Finally, mixed-feeder abundance was strongly associated with biomass, and to a lesser extent P, Mg, Na, and Cu. At the community level, B, Ca, Zn, and Cu, plus biomass, explained > 30% of species composition variation. Our results confirm the positive association of N and P with insect herbivore populations, while suggesting a potential role for Mg, Na, and K. They also demonstrate the importance of exploring effects at the feeding-guild level. We hope our data motivate ecologists to think beyond N and P when considering plant nutrient effects on insect herbivores, and make a call for studies to examine functional responses of insect herbivores to dietary manipulation of Mg, Na, and K. Finally, our results demonstrate correlations between variation in nutrients and species assemblages, but factors not linked to plant nutrient quality or biomass likely explain most of the observed variation.
植物养分含量与昆虫食草动物种群和群落结构之间的关系一直是生态学家感兴趣的话题。昆虫食草动物需要多种养分，但生态学家主要关注氮（估计植物蛋白质含量），最近则关注磷（P）；其他养分则很少受到关注。在这里，我们使用元素方法记录了内布拉斯加州中部草原栖息地的草和杂草样本中的养分变化；总共测量了 12 种元素（N 和 P，加上 S、B、Ca、Mg、Na、K、Zn、Fe、Mn 和 Cu）的叶浓度。我们检测到不同地点的 N、P、Mg、Na、K 和 Cu 存在显著差异。接下来，我们使用模型选择方法来探索这种营养变化和植物生物量如何与蚱蜢密度（总体和在取食群水平）相关联，并使用主成分分析来探索养分与蚱蜢群落物种组成的相关性。当所有的蚱蜢被汇集在一起时，密度在不同的地点有所不同，但只有 P 与显示出在不同地点之间变化的元素的丰度有关。在取食群水平上出现了不同的反应。对于草食性昆虫，密度与 N 以及 P、Mg 和 Na 有关。对于杂草食性昆虫，N 和 P 通常与密度有关，但也观察到与 Na 和 K 的关联。最后，混合食者的丰度与生物量密切相关，与 P、Mg、Na 和 Cu 的相关性较弱。在群落水平上，B、Ca、Zn 和 Cu 加上生物量，解释了 >30%的物种组成变化。我们的结果证实了 N 和 P 与昆虫食草动物种群的正相关关系，同时表明 Mg、Na 和 K 可能发挥作用。它们还表明，在考虑植物养分对昆虫食草动物的影响时，有必要探索取食群水平的影响。我们希望我们的数据能促使生态学家在考虑植物养分对昆虫食草动物的影响时，不仅要考虑 N 和 P，还要呼吁研究昆虫食草动物对 Mg、Na 和 K 的饮食处理的功能反应。最后，我们的结果表明养分变化与物种组合之间存在相关性，但与植物养分质量或生物量无关的因素可能解释了大部分观察到的变化。
