Ceulemans Ruben, Guill Christian, Gaedke Ursula
Institute of Biochemistry and Biology, University of Potsdam, Am Neuen Palais 10, Potsdam, 14469, Germany.
Ecology. 2021 Jul;102(7):e03379. doi: 10.1002/ecy.3379. Epub 2021 Jun 16.
It is well known that functional diversity strongly affects ecosystem functioning. However, even in rather simple model communities consisting of only two or, at best, three trophic levels, the relationship between multitrophic functional diversity and ecosystem functioning appears difficult to generalize, because of its high contextuality. In this study, we considered several differently structured tritrophic food webs, in which the amount of functional diversity was varied independently on each trophic level. To achieve generalizable results, largely independent of parametrization, we examined the outcomes of 128,000 parameter combinations sampled from ecologically plausible intervals, with each tested for 200 randomly sampled initial conditions. Analysis of our data was done by training a random forest model. This method enables the identification of complex patterns in the data through partial dependence graphs, and the comparison of the relative influence of model parameters, including the degree of diversity, on food-web properties. We found that bottom-up and top-down effects cascade simultaneously throughout the food web, intimately linking the effects of functional diversity of any trophic level to the amount of diversity of other trophic levels, which may explain the difficulty in unifying results from previous studies. Strikingly, only with high diversity throughout the whole food web, different interactions synergize to ensure efficient exploitation of the available nutrients and efficient biomass transfer to higher trophic levels, ultimately leading to a high biomass and production on the top level. The temporal variation of biomass showed a more complex pattern with increasing multitrophic diversity: while the system initially became less variable, eventually the temporal variation rose again because of the increasingly complex dynamical patterns. Importantly, top predator diversity and food-web parameters affecting the top trophic level were of highest importance to determine the biomass and temporal variability of any trophic level. Overall, our study reveals that the mechanisms by which diversity influences ecosystem functioning are affected by every part of the food web, hampering the extrapolation of insights from simple monotrophic or bitrophic systems to complex natural food webs.
众所周知，功能多样性对生态系统功能有强烈影响。然而，即使在仅由两个或至多三个营养级组成的相当简单的模型群落中，多营养级功能多样性与生态系统功能之间的关系似乎也难以一概而论，因为其具有高度的情境依赖性。在本研究中，我们考虑了几种结构不同的三营养级食物网，其中功能多样性的数量在每个营养级上独立变化。为了获得在很大程度上独立于参数化的可推广结果，我们检查了从生态合理区间采样的128,000个参数组合的结果，每个组合针对200个随机采样的初始条件进行测试。我们通过训练随机森林模型对数据进行分析。这种方法能够通过部分依赖图识别数据中的复杂模式，并比较包括多样性程度在内的模型参数对食物网属性的相对影响。我们发现，自上而下和自下而上的效应在整个食物网中同时级联，将任何营养级的功能多样性效应与其他营养级的多样性数量紧密联系起来，这可能解释了以往研究结果难以统一的原因。引人注目的是，只有在整个食物网具有高度多样性的情况下，不同的相互作用才会协同作用，以确保对可用养分的有效利用以及向更高营养级的有效生物量转移，最终导致顶级营养级的高生物量和高产量。随着多营养级多样性的增加，生物量的时间变化呈现出更复杂的模式：虽然系统最初变得不那么多变，但最终由于动态模式日益复杂，时间变化又再次上升。重要的是，顶级捕食者的多样性以及影响顶级营养级的食物网参数对于确定任何营养级的生物量和时间变异性最为重要。总体而言，我们的研究表明，多样性影响生态系统功能的机制受到食物网各个部分的影响，阻碍了从简单的单营养级或双营养级系统到复杂自然食物网的见解外推。
