Effects of Species Composition and Environmental Context on Redundancy Within a Functional Group: A Test With Freshwater Mussels

被认为在生态系统中发挥相同功能作用的物种通常被置于功能组、类型或行会中，并且预计在这些组中生态功能的冗余度最高。由可互换的冗余物种组成的功能群的概念很有吸引力，因为它简化了生态系统的研究和管理。然而，有一些问题限制了这一概念的适用性。被分配到许多功能群或行会的物种之间的冗余程度是未知的，我们不了解物种的功能角色随环境背景变化的程度。我们对物种的作用以及它们如何受环境影响的知识绝大部分来自于对短命的陆生植物的研究。我们现在需要扩大对物种在其他同样重要的系统中所起作用的理解。淡水系统，特别是溪流，正在以比陆地或海洋系统更高的速度失去生物多样性，但很少有研究检查这些系统中的功能冗余。淡水贻贝是一种长寿的底栖穴居滤食性动物，被认为在河流生态系统功能中起着重要作用。在这个功能群中，生态角色的潜在冗余应该很高，因为物种具有相似的生活史，通常以物种组合的形式发生，并且几乎没有证据表明物种之间在微栖息地或资源偏好方面存在差异。然而，我们几乎没有关于贻贝组合对流功能的总体重要性或单个物种的作用的定量信息。近年来，许多贻贝的数量急剧下降。为了预测物种和整体贻贝生物量的减少将如何影响河流生态系统，我们必须量化贻贝总体丰度和单个物种生态系统过程的影响，我们必须了解这些关系如何随环境条件而变化。建议的研究将解决以下问题：(1)贻贝功能群对河流生态系统的总体重要性是什么？贻贝在溪流中进行什么生态过程？这些过程与整体贻贝丰度/生物量有何关系？(2)贻贝功能群内的物种是否存在生态冗余？贻贝物种是否改变了它们在生态过程中的表现？贻贝物种的生态效应是否因物种组成和/或丰度而异？(3)贻贝功能类群的总体重要性和类群内物种角色的影响如何受到环境背景的影响？整个贻贝群和单个物种的生态效应是恒定的还是随着环境条件的变化而变化？一系列互补的实验室溪流和野外封闭实验将测量不同丰度和环境条件下贻贝的单一和多物种组合的生态功能。实验室溪流实验将集中于贻贝(如：过滤率（营养物质排泄）和野外封闭实验将研究贻贝对其他底栖生物群落的影响。小规模的实验室实验和分析将量化潜在的贡献(例如。不同贻贝物种在不同条件下向河流的营养物排泄和生物沉积速率，以及物种之间生态功能差异的机制(例如：吉尔形态、行为)。最后，将实验结果与美国国家科学基金会资助的贻贝生物多样性调查的定量信息相结合，预测贻贝在不同河流系统中的功能作用和冗余。

Species that are believed to play the same functional role in ecosystems are often placed infunctional groups, types or guilds, and redundancy of ecological function is predicted to be highestwithin such groups. The concept of functional groups composed of interchangeable, redundantspecies is appealing because it simplifies the study and management of ecological systems.However, there are problems limiting the applicability of this concept. The degree of redundancyamong species assigned to many functional groups or guilds is unknown and we do not understandthe degree to which the functional roles of species change with environmental context. The vastmajority of our knowledge of species roles and how they are influenced by the environment comesfrom studies of short-lived, terrestrial plants. We now need to broaden our understanding ofspecies roles in other equally important systems.Freshwater systems, and streams in particular, are losing biodiversity at a higher rate thanterrestrial or marine systems, but few studies have examined functional redundancy in thesesystems. Freshwater mussels are a functional group of long-lived, benthic, burrowing filter-feedersthat are thought to play an important role in stream ecosystem function. Within this functionalgroup, the potential for redundancy in ecological roles should be high because species have similarlife histories, typically occur as speciose assemblages, and there is little evidence for differencesin microhabitat or resource preferences between species. However, we have almost no quantitativeinformation on either the overall importance of mussel assemblages to stream function or the rolesof individual species. In recent years, many mussel populations have undergone a drastic decline.In order to predict how this loss of both species and overall mussel biomass will impact streamecosystems, we must quantify the effects of both overall mussel abundance and individual specieson ecosystem processes, and we must understand how these relationships may change withenvironmental conditions.The proposed research will address the following questions: (1) What is the overall importance ofthe mussel functional group to stream ecosystems? What ecological processes do musselsperform in streams? How do these processes relate to overall mussel abundance/biomass? (2)Are species within the mussel functional group ecologically redundant? Do mussel species varyin their performance of ecological processes? Do ecological effects of mussel species within anassemblage vary based on species composition and/or abundance? (3) How are the overallimportance of the mussel functional group and effects of species roles within the group influencedby environmental context? Are the ecological effects of the overall mussel assemblage and ofindividual species constant or do they change with environmental conditions? A series ofcomplementary laboratory stream and field enclosure experiments will measure the ecologicalfunction of single and multi-species assemblages of mussels under varying abundance andenvironmental conditions. Laboratory stream experiments will focus on ecological processesperformed by mussels (eg. filtering rates, nutrient excretion) and field enclosure experiments willexamine the effects of mussels on the rest of the benthic community. Small-scale laboratoryexperiments and analyses will quantify the potential contributions (eg. nutrient excretion andbiodeposition rates) of different mussel species to streams under differing conditions, andmechanisms underlying differences in ecological function between species (eg. gill morphology,behavior). Finally, experimental results will be combined with quantitative information from an NSF-funded mussel biodiversity inventory to make predictions about mussel functional role andredundancy in different stream systems.