Department of Ecology and Evolutionary Biology, Brown University, 80 Waterman Street, Box G-W, Providence, Rhode Island 02912, USA.
Ecology. 2012 Apr;93(4):868-78. doi: 10.1890/10-1785.1.
Theory and experiments indicate that changes in consumer diversity affect benthic community structure and ecosystem functioning. Although the effects of consumer diversity have been tested in the laboratory and the field, little is known about effects of consumer diversity in the subtidal zone, one of the largest marine habitats. We investigated the grazing effects of sea urchins on algal abundance and benthic community structure in a natural subtidal habitat of the Galápagos Islands. Three species of urchins (Eucidaris, Lytechinus, and Tripneustes) were manipulated in inclusion cages following a replacement design with three levels of species richness (one, two, and three species) with all possible two-species urchin combinations. Identity was the main factor accounting for changes in the percentage of substrate grazed and benthic community structure. Two out of the three two-species assemblages grazed more than expected, suggesting a richness effect, but analyses revealed that this increased grazing was due to a sampling effect of the largest and commercially valued urchin species, Tripneustes. Benthic community structure in treatments with Eucidaris, Lytechinus, and Tripneustes alone was significantly different at the end of the experiment, suggesting that resource use differentiation occurred. Communities in Tripneustes enclosures were characterized by abundant crustose coralline algae and grazed substrate, while those without it contained abundant green foliose algae (Ulva sp.). An unexpected emergent property of the system was that the most species-rich urchin assemblage underyielded, grazing less than any other assemblage with Tripneustes, effectively reversing its dominant influence observed in the two-species treatments. While further experiments are needed to discern the mechanisms of underyielding, it may be related to changing interspecific interactions as richness increases from two to three species or to density-dependent Tripneustes grazing. This study highlights the general importance of evaluating consumer richness effects across the entire range of species richness considered, as the performance of the most species-rich consumer assemblage could not be predicted by manipulations of intermediate levels of consumer species richness.
理论和实验表明，消费者多样性的变化会影响底栖群落结构和生态系统功能。尽管已经在实验室和野外测试了消费者多样性的影响，但对于其中最大的海洋栖息地之一——潮下带，人们对消费者多样性的影响知之甚少。我们调查了加拉帕戈斯群岛潮下带自然生境中海胆对藻类丰度和底栖群落结构的摄食作用。我们采用包含笼的替代设计，用三个物种丰富度水平（一个、两个和三个物种）操纵了三种海胆（Eucidaris、Lytechinus 和 Tripneustes），并包含了所有可能的两种海胆组合。身份是解释基质被摄食的百分比和底栖群落结构变化的主要因素。三种海胆中的两种的摄食量超过了预期，表明存在丰富度效应，但分析表明，这种摄食量的增加是由于最大和最具商业价值的海胆物种 Tripneustes 的采样效应。在实验结束时，仅含有 Eucidaris、Lytechinus 和 Tripneustes 的处理之间的底栖群落结构有显著差异，表明发生了资源利用分化。Tripneustes 围栏内的群落以丰富的壳状珊瑚藻和被摄食的基质为特征，而没有壳状珊瑚藻的群落则含有丰富的绿色叶状藻类（Ulva sp.）。该系统出现了一个意想不到的新特性，即物种最丰富的海胆组合的摄食量低于其他任何含有 Tripneustes 的组合，实际上扭转了在两种海胆处理中观察到的其主导影响。虽然需要进一步的实验来辨别未收获的机制，但它可能与随着物种丰富度从两个增加到三个，种间相互作用发生变化，或者与密度依赖的 Tripneustes 摄食有关。本研究强调了在考虑的整个物种丰富度范围内评估消费者丰富度效应的重要性，因为最丰富的消费者组合的表现不能通过对中间水平的消费者物种丰富度的操纵来预测。
