School of Marine and Tropical Biology, and ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Queensland 4811, Australia.
Ecology. 2012 Apr;93(4):891-901. doi: 10.1890/11-1753.1.
Biodiversity may provide insurance against ecosystem collapse by stabilizing assemblages that perform particular ecological functions (the "portfolio effect"). However, the extent to which this occurs in nature and the importance of different mechanisms that generate portfolio effects remain controversial. On coral reefs, herbivory helps maintain coral dominated states, so volatility in levels of herbivory has important implications for reef ecosystems. Here, we used an extensive time series of abundances on 35 reefs of the Great Barrier Reef of Australia to quantify the strength of the portfolio effect for herbivorous fishes. Then, we disentangled the contributions of two mechanisms that underlie it (compensatory interactions and differential responses to environmental fluctuations ["response diversity"]) by fitting a community-dynamic model that explicitly includes terms for both mechanisms. We found that portfolio effects operate strongly in herbivorous fishes, as shown by nearly independent fluctuations in abundances over time. Moreover, we found strong evidence for high response diversity, with nearly independent responses to environmental fluctuations. In contrast, we found little evidence that the portfolio effect in this system was enhanced by compensatory ecological interactions. Our results show that portfolio effects are driven principally by response diversity for herbivorous fishes on coral reefs. We conclude that portfolio effects can be very strong in nature and that, for coral reefs in particular, response diversity may help maintain herbivory above the threshold levels that trigger regime shifts.
生物多样性可以通过稳定执行特定生态功能的组合体来为生态系统崩溃提供保险（“投资组合效应”）。然而，这种情况在自然界中发生的程度以及产生投资组合效应的不同机制的重要性仍然存在争议。在珊瑚礁中，食草动物有助于维持珊瑚为主的状态，因此食草动物水平的波动对珊瑚礁生态系统有重要影响。在这里，我们使用澳大利亚大堡礁 35 个珊瑚礁的广泛时间序列丰度数据，来量化食草鱼类投资组合效应的强度。然后，我们通过拟合一个明确包含这两种机制的群落动态模型，来分解其背后的两种机制（补偿相互作用和对环境波动的差异响应[“响应多样性”]）的贡献。我们发现，投资组合效应在食草鱼类中作用很强，表现为丰度随时间几乎独立波动。此外，我们发现高响应多样性的有力证据，对环境波动的响应几乎独立。相比之下，我们发现几乎没有证据表明系统中的投资组合效应是通过补偿性生态相互作用来增强的。我们的研究结果表明，对于珊瑚礁上的食草鱼类来说，投资组合效应主要是由响应多样性驱动的。我们的结论是，投资组合效应在自然界中可能非常强大，特别是对于珊瑚礁而言，响应多样性可能有助于维持食草动物的数量高于引发状态转变的阈值水平。
