Species richness and evenness respond in a different manner to propagule density in developing prairie microcosm communities
Species richness and evenness respond in a different manner to propagule density in developing prairie microcosm communities
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DOI：
10.1007/s11258-006-9206-4
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发表时间：
2007-06-01
期刊：
影响因子：
1.7
通讯作者：
Stirling, Gray
中科院分区：
文献类型：
--
作者：
Wilsey, Brian;Stirling, Gray
多样性有两个基本组成部分：丰富度，即给定区域内的物种数量，以及均匀度，即相对丰度或生物量在物种之间的分布情况。此前，我们发现丰富度和均匀度在植物群落中可能呈负相关，并且均匀度可以比丰富度解释更多的香农多样性指数（H'）变化，这表明多样性组成之间的关系可能很复杂。由于迁移率或当地物种相互作用的影响，均匀度和丰富度之间可能会出现非正相关关系，并且关系可能会根据这两个过程如何构建当地群落而变化。在这里，我们测试了在群落建立的早期阶段，温室群落中种子密度的变化（以及对已建立的植物密度和竞争的相关影响）随着时间的推移，多样性成分是否受到相同或不同的影响。在我们的温室实验中，我们在充满裸田土壤的草原微观世界中以三种密度播种，并从 22 种草和杂草物种中提取混合物，以测试竞争强度或幼苗密度的增加是否会影响早期群落建立过程中多样性成分之间的关系。增加种子密度的处理导致多样性成分以不同的方式响应并与时间有不同的关系。在实验早期，当出苗率较高时，丰富度随种子密度线性增加，但在实验后期与密度无关。由于密度较高的金光菊占主导地位，所有采样日期的均匀度均随种子密度呈对数线性下降。实验初期，多样性指数较弱地反映了丰富度的差异，但随着金光菊竞争效应的增强，多样性指数更强烈地反映了均匀度的差异。这表明物种均匀度和多样性指数并不总是与丰富度呈正相关。基于这些结果，我们认为均匀度和丰富度可能受到不同过程的影响，丰富度更多地受到新苗数量的影响，而均匀度更多地受到竞争等物种相互作用的影响。这些结果表明，只要有可能，就应该在植物多样性研究中测量这两个多样性组成部分。
Diversity has two basic components: richness, or number of species in a given area, and evenness, or how relative abundance or biomass is distributed among species. Previously, we found that richness and evenness can be negatively related across plant communities and that evenness can account for more variation in Shannon's diversity index (H') than richness, which suggests that relationships among diversity components can be complex. Non-positive relationships between evenness and richness could arise due to the effects of migration rate or local species interactions, and relationships could vary depending on how these two processes structure local communities. Here we test whether diversity components are equally or differentially affected over time by changes in seed density (and associated effects on established plant density and competition) in greenhouse communities during the very early stages of community establishment. In our greenhouse experiment, we seeded prairie microcosms filled with bare field soil at three densities with draws from a mix of 22 grass and forb species to test if increased competition intensity or seedling density would affect the relationships among diversity components during early community establishment. Increased seed density treatments caused diversity components to respond in a different manner and to have different relationships with time. Richness increased linearly with seed density early in the experiment when seedling emergence was high, but was unrelated to density later in the experiment. Evenness decreased log-linearly with seed densities on all sampling dates due to a greater dominance by Rudbeckia hirta with higher densities. Early in the experiment, diversity indices weakly reflected differences in richness, but later, after the competitive effects of Rudbeckia hirta became more intense, diversity indices more strongly reflected differences in evenness. This suggests that species evenness and diversity indices do not always positively covary with richness. Based on these results, we suggest that evenness and richness can be influenced by different processes, with richness being more influenced by the number of emerging seedlings and evenness more by species interactions like competition. These results suggest that both diversity components should be measured in plant diversity studies whenever it is possible.