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的物种通常被归入功能组、类型或行会，预计生态功能的冗余度在这些组中最高。由可互换、冗余物种组成的官能团的概念因其简化了生态系统的研究和管理而备受关注，但也存在一些问题限制了这一概念的应用。被分配到许多官能团或行会的物种之间的冗余程度是未知的，我们也不了解物种的功能角色随着环境背景而变化的程度。我们关于物种角色以及它们如何受到环境影响的知识，绝大多数来自对短命的陆地植物的研究。我们现在需要扩大我们对物种在其他同样重要的系统中作用的理解。淡水系统，特别是溪流，正在以比陆地或海洋系统更快的速度丧失生物多样性，但很少有研究检验这些系统中的功能冗余。淡水贻贝是一类长寿、底栖、穴居的滤食性生物，被认为在河流生态系统功能中发挥着重要作用。在这一功能组内，生态角色冗余的可能性应该很高，因为物种具有相似的生活史，通常作为特殊的组合出现，而且几乎没有证据表明物种之间在微生境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，许多贻贝种群数量急剧下降。为了预测这种物种和整个贻贝生物量的损失将如何影响河流生态系统，我们必须量化整个贻贝丰度和单个物种对生态系统过程的影响，并且我们必须了解这些关系如何随着环境条件的变化而变化。拟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？它们在溪流中执行哪些生态过程？这些过程与整个贻贝丰度/生物量有何关系？(2)贻贝功能群中的物种在生态上是多余的吗？贻贝物种在其生态过程中的表现是否有所不同？根据物种组成和/或丰度的不同，聚集区内贻贝物种的生态效应是否有所不同？(3)环境背景如何影响贻贝功能群的总体重要性和群体内物种角色的影响？整个贻贝组合和个别物种的生态影响是恒定的，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围栏实验将测量不同丰度和环境条件下单种和多种贻贝组合的生态功能。实验室河流实验将重点研究贻贝的生态过程(例如。过滤速度、营养物质排泄)和野外围隔实验将考察贻贝对底栖动物群落其余部分的影响。小规模的实验室实验和分析将量化潜在的贡献(例如。不同贝类物种在不同条件下对溪流的营养排泄和生物沉积速率)，以及物种间生态功能差异的机制(例如。鳃形态、行为)。最后，实验结果将与NSF资助的贻贝生物多样性清单中的定量信息相结合，以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.