A goal of ecology is to identify the stabilizing mechanisms that maintain species diversity in the face of competitive exclusion and drift. For tropical forest tree communities, it has been hypothesized that high diversity is maintained via Janzen-Connell effects, whereby host-specific natural enemies prevent any one species from becoming too abundant. Here we explore the plausibility of this hypothesis with theoretical models. We confirm a previous result that when added to a model with drift but no competitive exclusion—that is, a neutral model where intrinsic fitnesses are perfectly equalized across species—Janzen-Connell effects maintain very high species richness that scales strongly with community size. However, when competitive exclusion is introduced—that is, when intrinsic fitnesses vary across species—the number of species maintained by Janzen-Connell effects is substantially reduced and scales much less strongly with community size. Because fitness variation is pervasive in nature, we conclude that the potential of Janzen-Connell effects to maintain diversity is probably weak and that the mechanism does not yet provide a sufficient explanation for the observed high diversity of tropical forest tree communities. We also show that, surprisingly, dispersal limitation can further reduce the ability of Janzen-Connell effects to maintain diversity.

中文翻译：

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Janzen-Connell效应是竞争排斥的弱障碍

生态学的目标是确定面对竞争性排斥和漂移而维持物种多样性的稳定机制。对于热带林木群落，据推测，通过Janzen-Connell效应可以保持高度多样性，从而使寄主特定的天敌阻止任何一种物种变得过于丰富。在这里，我们用理论模型探索该假设的合理性。我们证实了先前的结果，当将其添加到具有漂移但没有竞争排斥的模型（即，内部模型在各个物种之间完全均等的中性模型）中，Janzen-Connell效应保持了很高的物种丰富度，并随着社区规模的扩大而强烈地扩展。但是，当引入竞争排斥时，也就是说，当内在适应性因物种而异时，通过Janzen-Connell效应维持的物种数量将大大减少，并且随着社区规模的扩大而扩展的强度将大大降低。由于适应性变化本质上是普遍存在的，因此我们得出结论，Janzen-Connell效应维持多样性的潜力可能很弱，并且该机制尚未为观察到的热带林木群落的高度多样性提供充分的解释。我们还表明，令人惊讶的是，分散限制会进一步降低Janzen-Connell效应维持多样性的能力。我们得出的结论是，Janzen-Connell效应维持多样性的潜力可能很弱，并且该机制尚未为观察到的热带林木群落的高度多样性提供充分的解释。我们还表明，令人惊讶的是，分散限制会进一步降低Janzen-Connell效应维持多样性的能力。我们得出的结论是，Janzen-Connell效应维持多样性的潜力可能很弱，并且该机制尚未为观察到的热带林木群落的高度多样性提供充分的解释。我们还表明，令人惊讶的是，分散限制会进一步降低Janzen-Connell效应维持多样性的能力。