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.

针对生态群落鲁棒性的研究表明，单个物种的丢失或丰度下降可能引发次级灭绝与级联灭绝事件。然而，此类研究大多基于模拟分析，探究初级灭绝对食物网结构的影响。

本研究通过野外实验，探究降低常见物种丰度所产生的直接与间接效应，研究对象为依附于巴西常见灌木Baccharis dracunculifolia D.C.（菊科Asteraceae）的多样且独立的节肢动物类群。

在为期5个月的实验周期内，我们在15组重复样地与15组对照样地中，人为降低与B. dracunculifolia伴生的最常见造瘿物种Baccharopelma dracunculifoliae（胸喙亚目Sternorrhyncha：木虱科Psyllidae）的丰度。本研究探究了该处理对寄生B. dracunculifoliae的寄生性天敌的直接效应，以及对其余10种造瘿物种与50种伴生寄生性天敌的间接效应（间接效应通过第三物种介导或由环境途径传递）。

实验处理显著提升了处理组样地中B. dracunculifoliae的寄生率，但并未显著改变其余造瘿物种的物种丰富度与丰度。与对照组样地相比，处理组样地的食物网加权连接度、交互均匀度与鲁棒性（以模拟次级灭绝耐受度表征）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。

寄生性天敌物种几乎完全专一于特定的造瘿物种，因此实验处理对食物网结构产生的效应无法通过已记录的营养级联传递。反之，该效应必然通过两类途径传播：一是未被纳入食物网的营养级联（如共享天敌），二是非营养途径（如栖息地可利用性改变）。本研究结果表明，即便对于看似独立的生态群落，同时纳入营养与非营养性的直接、间接相互作用，对于理解其结构与动态均至关重要。