Cornell Howard V, Karlson Ronald H, Hughes Terence P
Department of Environmental Science and Policy, University of California, One Shields Avenue, Davis, California 95616, USA.
Ecology. 2007 Jul;88(7):1707-15. doi: 10.1890/06-0742.1.
Community similarity is the proportion of species richness in a region that is shared on average among communities within that region. The slope of local richness (alpha diversity) regressed on regional richness (gamma diversity) can serve as an index of community similarity across regions with different regional richness. We examined community similarity in corals at three spatial scales (among transects at a site, sites on an island, and islands within an island group) across a 10 000-km longitudinal diversity gradient in the west-central Pacific Ocean. When alpha diversity was regressed on gamma diversity, the slopes, and thus community similarity, increased with scale (0.085, 0.261, and 0.407, respectively) because a greater proportion of gamma diversity was subsumed within alpha diversity as scale increased. Using standard randomization methods, we also examined how community similarity differed between observed and randomized assemblages and how this difference was affected by spatial separation of species within habitat types and specialization of species to three habitat types (reef flats, crests, and slopes). If spatial separation within habitat types and/or habitat specialization (i.e., underdispersion) occurs, fewer species are shared among assemblages than the random expectation. When the locations of individual coral colonies were randomized within and among habitat types, community similarity was 46-47% higher than that for observed assemblages at all three scales. We predicted that spatial separation of coral species within habitat types should increase with scale due to dispersal/extinction dynamics in this insular system, but that specialization of species to different habitat types should not change because habitat differences do not change with scale. However, neither habitat specialization nor spatial separation within habitat types differed among scales. At the two larger scales, each accounted for 22-24% of the difference in community similarity between observed and randomized assemblages. At the smallest scale (transect-site), neither spatial separation within habitat types nor habitat specialization had significant effects on community similarity, probably due to the small size of transect samples. The results suggest that coral species can disperse among islands in an island group as easily as they can among sites on an island over time scales that are relevant to their establishment and persistence on reefs.
群落相似性是指一个区域内物种丰富度中，该区域内各群落平均共享的比例。将局部丰富度（α多样性）对区域丰富度（γ多样性）进行回归分析得到的斜率，可以作为不同区域丰富度地区间群落相似性的指标。我们在西太平洋中部10000公里的纵向多样性梯度上，研究了三个空间尺度（一个地点的样带间、一个岛屿上的不同地点以及一个岛屿群内的不同岛屿）的珊瑚群落相似性。当α多样性对γ多样性进行回归分析时，斜率以及群落相似性随尺度增加（分别为0.085、0.261和0.407），这是因为随着尺度增加，γ多样性中有更大比例被纳入α多样性之中。我们还使用标准随机化方法，研究了观察到的组合与随机组合之间的群落相似性差异，以及这种差异如何受到栖息地类型内物种的空间分离和物种对三种栖息地类型（礁坪、礁顶和礁坡）的特化影响。如果栖息地类型内存在空间分离和/或栖息地特化（即分布不均匀），那么各组合间共享的物种数量将少于随机预期。当在栖息地类型内和之间对单个珊瑚群体的位置进行随机化处理时，在所有三个尺度上，群落相似性都比观察到的组合高46 - 47%。我们预测，由于这个岛屿系统中的扩散/灭绝动态，栖息地类型内珊瑚物种的空间分离应随尺度增加，但物种对不同栖息地类型的特化不应改变，因为栖息地差异不会随尺度变化。然而，在不同尺度间，栖息地特化和栖息地类型内的空间分离均无差异。在两个较大尺度上，它们各自占观察到的组合与随机组合之间群落相似性差异的22 - 24%。在最小尺度（样带 - 地点），栖息地类型内的空间分离和栖息地特化对群落相似性均无显著影响，这可能是由于样带样本规模较小所致。结果表明，在与它们在珊瑚礁上的建立和持续存在相关的时间尺度上，珊瑚物种在一个岛屿群内的不同岛屿间扩散的难易程度，与在一个岛屿上的不同地点间扩散的难易程度相同。
