Purpose of Review

Natural enemies are an important component for forest functioning. By consuming herbivores, they can be effective top-down regulators of potential pest species. Tree mixtures are generally expected to have larger predator and parasitoid populations compared to monocultures. This assumption is based on the “enemies” hypothesis, a classical ecological concept predicting a positive relationship between plant diversity (and complexity) and natural enemies, which, in turn, should increase top-down control in more diverse environments. However, the “enemies” hypothesis has mostly been tested and supported in relatively simple agricultural ecosystems. Until recently, research in forests was sparse. We summarize the upcoming knowledge-base for forests and identify forest characteristics likely shaping relationships between tree diversity, natural enemies (abundance, species richness, diversity), and top-down control. We further identify possible implications for mixed species forestry and key knowledge gaps.

Recent Findings

Tree diversity (almost exclusively quantified as tree species richness) does not consistently increase enemy abundance, diversity, or result in herbivore control. Tests of the “enemies” hypothesis are largely based on aboveground natural enemies (mainly generalists) and have highly variable outcomes across taxa and study systems, sometimes even finding a decrease in predator diversity with increasing tree diversity. Recurrent effects of tree species identity and composition indicate that a closer focus on tree functional and phylogenetic diversity might help to foster a mechanistic understanding of the specific circumstances under which tree diversity can promote top-down control.

Summary

Our review suggests that the “enemies” hypothesis may not unambiguously apply to forests. With trees as structurally complex organisms, even low-diversity forests can maintain a high degree of habitat heterogeneity and may provide niches for many predator and parasitoid species, possibly blurring correlations between tree and natural enemy diversity. Several further factors, such as latitude, identity effects, intraguild predation, or functional and phylogenetic components of biodiversity, may confound the predictions of the “enemies” hypothesis. We identify topics needing more research to fully understand under which conditions tree diversity increases natural enemy diversity and top-down control—knowledge that will be crucial for forest management.

中文翻译：

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树木多样性对自然敌人的影响-森林“敌人”假说的评述

审查目的

天敌是森林功能的重要组成部分。通过食用草食动物，它们可以成为潜在有害生物自上而下的有效调节剂。与单一栽培相比，通常期望树木混合物具有更大的捕食者和类寄生虫种群。该假设基于“敌人”假说，这是一个经典的生态概念，预测植物多样性（和复杂性）与天敌之间的正相关关系，这反过来又应在更多样化的环境中增强自上而下的控制。但是，“敌人”假说大部分已经在相对简单的农业生态系统中得到了检验和支持。直到最近，对森林的研究还很少。我们总结了即将到来的森林知识基础，并确定了可能塑造树木多样性之间关系的森林特征，天敌（丰度，物种丰富度，多样性）和自上而下的控制。我们进一步确定了对混合物种林业和关键知识缺口的可能影响。

最近的发现

树木的多样性（几乎完全量化为树木的丰富性）不会持续增加敌人的数量，多样性或导致食草动物的控制。对“敌人”假设的检验主要基于地上的自然敌人（主要是通才），并且在整个分类单元和研究系统中具有高度可变的结果，有时甚至发现捕食者多样性随着树木多样性的增加而减少。树种身份和组成的反复影响表明，更加关注树的功能和系统发育多样性可能有助于促进对树种多样性可以促进自上而下控制的特定情况的机械理解。

概要

我们的评论表明，“敌人”假设可能不会明确适用于森林。由于树木是结构复杂的有机体，即使是低多样性的森林也可以保持高度的生境异质性，并可能为许多捕食者和寄生性物种提供利基，这可能会模糊树木与天敌多样性之间的相关性。诸如纬度，身份影响，行会内捕食或生物多样性的功能和系统发育组成等其他一些因素可能会混淆“敌人”假说的预测。我们确定需要进一步研究的主题，以完全了解在什么情况下树种多样性会增加天敌多样性和自上而下的控制-这些知识对于森林管理至关重要。