Data from: Experimentally reducing species abundance indirectly affects food web structure and robustness

Studies on the robustness of ecological communities suggest that the loss or reduction in abundance of individual species can lead to secondary and cascading extinctions. However, most such studies have been simulation-based analyses of the effect of primary extinction on food web structure.
In a field experiment we tested the direct and indirect effects of reducing the abundance of a common species, focusing on the diverse and self-contained assemblage of arthropods associated with an abundant Brazilian shrub, Baccharis dracunculifolia D.C. (Asteraceae).
Over a 5-month period we experimentally reduced the abundance of Baccharopelma dracunculifoliae (Sternorrhyncha: Psyllidae), the commonest galling species associated with B. dracunculifolia, in 15 replicate plots paired with 15 control plots. We investigated direct effects of the manipulation on parasitoids attacking B. dracunculifoliae, as well as indirect effects (mediated via a third species or through the environment) on 10 other galler species and 50 associated parasitoid species.
The experimental manipulation significantly increased parasitism on B. dracunculifoliae in the treatment plots, but did not significantly alter either the species richness or abundance of other galler species. Compared to control plots, food webs in manipulated plots had significantly lower values of weighted connectance, interaction evenness and robustness (measured as simulated tolerance to secondary extinction), even when B. dracunculifoliae was excluded from calculations.
Parasitoid species were almost entirely specialized to individual galler species, so the observed effects of the manipulation on food web structure could not have propagated via the documented trophic links. Instead, they must have spread either through trophic links not included in the webs (e.g. shared predators) or non-trophically (e.g. through changes in habitat availability). Our results highlight that the inclusion of both trophic and non-trophic direct and indirect interactions is essential to understand the structure and dynamics of even apparently discrete ecological communities.

有关生态群落鲁棒性（robustness）的研究表明，单个物种的丧失或丰度降低可能引发次级灭绝和级联灭绝。然而，此类研究大多基于模拟分析，探讨初级灭绝对食物网结构的影响。
本研究通过野外实验，探究了常见物种丰度降低的直接与间接效应，研究对象为与一种广布巴西灌木——白叶阔苞菊（Baccharis dracunculifolia D.C.，菊科Asteraceae）相关联的多样且独立的节肢动物类群。
在为期5个月的实验中，我们在15个重复样地中人为降低了与B. dracunculifolia关联的最常见致瘿昆虫——巴查拉木虱（Baccharopelma dracunculifoliae，胸喙亚目Sternorrhyncha：木虱科Psyllidae）的丰度，并设置15个对照样地进行配对。本研究同时探究了该处理对寄生B. dracunculifoliae的寄生蜂的直接效应，以及对另外10种致瘿昆虫和50种相关寄生蜂的间接效应（间接效应可通过第三物种介导或经由环境介导）。
实验处理显著提高了处理样地中B. dracunculifoliae的寄生率，但并未显著改变其他致瘿昆虫的物种丰富度与丰度。与对照样地相比，人为处理后的样地内食物网的加权连接度（weighted connectance）、交互均匀度（interaction evenness）以及鲁棒性（robustness）均显著降低，即便在计算过程中剔除B. dracunculifoliae的数据后依然如此。
寄生蜂物种几乎完全专一性地对应单一致瘿昆虫物种，因此实验处理对食物网结构产生的观测效应无法通过已记录的营养联系（trophic links）传播。由此推测，效应的传播途径应为两类：一是未被纳入食物网的营养联系（例如共享天敌），二是非营养联系（例如栖息地可利用性的改变）。本研究结果表明，即便是看似独立的生态群落，要充分理解其结构与动态，必须同时纳入营养与非营养的直接和间接相互作用。