Philpott Stacy M, Greenberg Russell, Bichier Peter, Perfecto Ivette
Department of Ecology and Evolutionary Biology, University of Michigan, 830 N. University, Ann Arbor, MI 48109, USA.
Oecologia. 2004 Jun;140(1):140-9. doi: 10.1007/s00442-004-1561-z. Epub 2004 Apr 17.
In food web studies, taxonomically unrelated predators are often grouped into trophic levels regardless of their relative importance on prey assemblages, multiple predator effects, or interactions such as omnivory. Ants and birds are important predators likely to differentially shape arthropod assemblages, but no studies have compared their effects on a shared prey base. In two separate studies, we excluded birds and ants from branches of a canopy tree ( Inga micheliana) in a coffee farm in Mexico for 2 months in the dry and wet seasons of 2002. We investigated changes in arthropod densities with and without predation pressure from (1) birds and (2) ant assemblages dominated by one of two ant species ( Azteca instabilis and Camponotus senex). We first analyzed individual effects of each predator (birds, Azteca instabilis, and C. senex) then used a per day effect metric to compare differences in effects across (birds vs ants) and within predator taxa (the two ant species). Individually, birds reduced densities of total and large arthropods and some arthropod orders (e.g., spiders, beetles, roaches) in both seasons. Azteca instabilis did not significantly affect arthropods (total, small, large or specific orders). Camponotus senex, however, tended to remove arthropods (total, small), especially in the dry season, and affected arthropod densities of some orders both positively and negatively. Predators greatly differed in their effects on Inga arthropods (for all, small, large, and individual orders of arthropods) both in sign (+/-) and magnitudes of effects. Birds had stronger negative effects on arthropods than ants and the two dominant ant species had stronger effects on arthropods in different seasons. Our results show that aggregating taxonomically related and unrelated predators into trophic levels without prior experimental data quantifying the sign and strengths of effects may lead to a misrepresentation of food web interactions.
在食物网研究中，分类学上不相关的捕食者通常被归入营养级，而不考虑它们对猎物组合、多种捕食者效应或诸如杂食性等相互作用的相对重要性。蚂蚁和鸟类是重要的捕食者，它们可能以不同方式塑造节肢动物群落，但尚无研究比较它们对共同猎物基础的影响。在两项独立研究中，我们于2002年的旱季和雨季，在墨西哥一个咖啡农场的一棵树冠层树木（米氏印加树）的树枝上，将鸟类和蚂蚁排除在外达2个月之久。我们调查了在有和没有来自以下两种捕食压力时节肢动物密度的变化：（1）鸟类，以及（2）由两种蚂蚁（不稳定阿兹特克蚁和老年弓背蚁）之一主导的蚂蚁群落。我们首先分析了每种捕食者（鸟类、不稳定阿兹特克蚁和老年弓背蚁）的个体效应，然后使用每日效应指标来比较不同捕食者（鸟类与蚂蚁）之间以及捕食者类群内部（两种蚂蚁）效应的差异。单独来看，鸟类在两个季节中都降低了总节肢动物和大型节肢动物以及一些节肢动物目（如蜘蛛、甲虫、蟑螂）的密度。不稳定阿兹特克蚁对节肢动物（总数、小型、大型或特定目）没有显著影响。然而，老年弓背蚁倾向于捕食节肢动物（总数、小型），尤其是在旱季，并且对一些节肢动物目的密度有正向和负向影响。捕食者对印加树节肢动物（所有节肢动物、小型节肢动物、大型节肢动物以及各个节肢动物目）的影响在效应的正负（+/-）和大小方面都有很大差异。鸟类对节肢动物的负面影响比蚂蚁更强，并且两种优势蚂蚁物种在不同季节对节肢动物的影响更强。我们的结果表明，在没有事先通过实验数据量化效应的正负和强度的情况下，将分类学上相关和不相关的捕食者归为营养级可能会导致对食物网相互作用的错误表述。
