Weis Jerome J, Cardinale Bradley J, Forshay Kenneth J, Ives Anthony R
Department of Ecology, Evolution & Marine Biology, University of California-Santa Barbara, Santa Barbara, California 93106, USA.
Ecology. 2007 Apr;88(4):929-39. doi: 10.1890/06-0943.
Over the past decade an increasing number of studies have experimentally manipulated the number of species in a community and examined how this alters the aggregate production of species biomass. Many of these studies have shown that the effects of richness on biomass change through time, but we have limited understanding of the mechanisms that produce these dynamic trends. Here we report the results of an experiment in which we manipulated the richness of freshwater algae in laboratory microcosms. We used two experimental designs (additive and substitutive) that make different assumptions about how patches are initially colonized, and then tracked the development of community biomass from the point of initial colonization through a period of 6-12 generations of the focal species. We found that the effect of initial species richness on biomass production qualitatively shifted twice over the course of the experiment. The first shift occurred as species transitioned from density-independent to dependent phases of population growth. At this time, intraspecific competition caused monocultures to approach their respective carrying capacities more slowly than polycultures. As a consequence, species tended to over-yield for a brief time, generating a positive, but transient effect of diversity on community biomass. The second shift occurred as communities approached carrying capacity. At this time, strong interspecific interactions caused biomass to be dominated by the competitively superior species in polycultures. As this species had the lowest carrying capacity, a negative effect of diversity on biomass resulted in late succession. Although these two shifts produced dynamics that appeared complex, we show that the patterns can be fit to a simple Lotka-Volterra model of competition. Our results suggest that the effects of algal diversity on primary production change in a predictable sequence through successional time.
在过去十年中，越来越多的研究通过实验操纵群落中的物种数量，并研究这如何改变物种生物量的总生产量。这些研究中的许多都表明，丰富度对生物量的影响会随时间变化，但我们对产生这些动态趋势的机制了解有限。在此，我们报告一项实验的结果，在该实验中我们在实验室微观环境中操纵了淡水藻类的丰富度。我们使用了两种实验设计（加法设计和替代设计），这两种设计对斑块最初如何被定殖做出了不同假设，然后从初始定殖点开始跟踪群落生物量的发展，历经焦点物种的6 - 12代。我们发现，在实验过程中，初始物种丰富度对生物量生产的影响在质量上发生了两次转变。第一次转变发生在物种从种群增长的密度独立阶段过渡到密度依赖阶段时。此时，种内竞争导致单培养物比多培养物更缓慢地接近其各自的承载能力。因此，物种在短时间内往往会超产，从而产生多样性对群落生物量的正向但短暂的影响。第二次转变发生在群落接近承载能力时。此时，强烈的种间相互作用导致生物量在多培养物中由竞争优势物种主导。由于该物种的承载能力最低，多样性对生物量的负面影响导致了后期演替。尽管这两次转变产生的动态看起来很复杂，但我们表明这些模式可以拟合到一个简单的竞争洛特卡 - 沃尔泰拉模型。我们的结果表明，藻类多样性对初级生产的影响在演替过程中按可预测的顺序变化。
