Functional Redundancy Patterns Reveal Non-Random Assembly Rules in a Species-Rich Marine Assemblage

The relationship between species and the functional diversity of assemblages is fundamental in ecology because it contains key information on functional redundancy, and functionally redundant ecosystems are thought to be more resilient, resistant and stable. However, this relationship is poorly understood and undocumented for species-rich coastal marine ecosystems. Here, we used underwater visual censuses to examine the patterns of functional redundancy for one of the most diverse vertebrate assemblages, the coral reef fishes of New Caledonia, South Pacific. First, we found that the relationship between functional and species diversity displayed a non-asymptotic power-shaped curve, implying that rare functions and species mainly occur in highly diverse assemblages. Second, we showed that the distribution of species amongst possible functions was significantly different from a random distribution up to a threshold of ∼90 species/transect. Redundancy patterns for each function further revealed that some functions displayed fast rates of increase in redundancy at low species diversity, whereas others were only becoming redundant past a certain threshold. This suggested non-random assembly rules and the existence of some primordial functions that would need to be fulfilled in priority so that coral reef fish assemblages can gain a basic ecological structure. Last, we found little effect of habitat on the shape of the functional-species diversity relationship and on the redundancy of functions, although habitat is known to largely determine assemblage characteristics such as species composition, biomass, and abundance. Our study shows that low functional redundancy is characteristic of this highly diverse fish assemblage, and, therefore, that even species-rich ecosystems such as coral reefs may be vulnerable to the removal of a few keystone species.

物种组合的功能多样性与物种丰富度之间的关系是生态学的核心基础命题，因其蕴含了功能冗余（functional redundancy）的关键信息——而具备功能冗余的生态系统被认为具有更强的恢复力、抵抗力与稳定性。然而，针对物种丰富的近岸海洋生态系统，这一关系的认知仍较为匮乏且缺乏相关实证记录。本研究采用水下视觉普查（underwater visual censuses）技术，针对南太平洋新喀里多尼亚海域的珊瑚礁鱼类——这是全球物种多样性最高的脊椎动物组合之一——的功能冗余模式展开分析。其一，研究发现功能多样性与物种丰富度之间的关系呈现非渐近幂函数曲线形态，这表明稀有功能与稀有物种主要分布于物种高度丰富的组合中。其二，研究证实，在样带物种数约低于90的阈值范围内，物种在各类潜在功能中的分配模式与随机分布存在显著差异。进一步对各功能的冗余模式展开分析后发现，部分功能在物种多样性较低时便会快速出现冗余增长，而另一些功能则需达到特定物种丰富度阈值后才会产生冗余。这一结果暗示存在非随机的群落组装规则，且存在若干基础功能需优先满足，才能使珊瑚礁鱼类组合形成基本的生态结构。最后，尽管已有研究表明栖息地在很大程度上决定了物种组成、生物量与丰度等群落特征，但本研究发现栖息地对功能-物种多样性关系的形态及功能冗余度几乎无影响。本研究表明，这类物种高度丰富的鱼类组合普遍存在较低的功能冗余度，因此即便像珊瑚礁这样物种丰富的生态系统，也可能因少量关键物种（keystone species）的消失而面临更高的脆弱性风险。