Biogeographical Boundaries, Functional Group Structure and Diversity of Rocky Shore Communities along the Argentinean Coast

We investigate the extent to which functional structure and spatial variability of intertidal communities coincide with major biogeographical boundaries, areas where extensive compositional changes in the biota are observed over a limited geographic extension. We then investigate whether spatial variation in the biomass of functional groups, over geographic (10′s km) and local (10′s m) scales, could be associated to species diversity within and among these groups. Functional community structure expressed as abundance (density, cover and biomass) and composition of major functional groups was quantified through field surveys at 20 rocky intertidal shores spanning six degrees of latitude along the southwest Atlantic coast of Argentina and extending across the boundaries between the Argentinean and Magellanic Provinces. Patterns of abundance of individual functional groups were not uniformly matched with biogeographical regions. Only ephemeral algae showed an abrupt geographical discontinuity coincident with changes in biogeographic boundaries, and this was limited to the mid intertidal zone. We identified 3–4 main ‘groups’ of sites in terms of the total and relative abundance of the major functional groups, but these did not coincide with biogeographical boundaries, nor did they follow latitudinal arrangement. Thus, processes that determine the functional structure of these intertidal communities are insensitive to biogeographical boundaries. Over both geographical and local spatial scales, and for most functional groups and tidal levels, increases in species richness within the functional group was significantly associated to increased total biomass and reduced spatial variability of the group. These results suggest that species belonging to the same functional group are sufficiently uncorrelated over space (i.e. metres and site-to-site ) to stabilize patterns of biomass variability and, in this manner, provide a buffer, or “insurance”, against spatial variability in environmental conditions.

本研究首先探究潮间带群落（intertidal communities）的功能结构（functional structure）与空间变异（spatial variability）与主要生物地理边界（biogeographical boundaries）的契合程度——生物地理边界指在有限地理范围内出现生物区系（biota）组成发生显著变化的区域。随后，本研究进一步探讨在地理尺度（数十公里）与局地尺度（数十米）下，功能群（functional groups）生物量的空间变异是否与各功能群内部及功能群间的物种多样性存在关联。研究以丰度（abundance，包括密度（density）、盖度（cover）与生物量（biomass））及主要功能群组成来表征潮间带群落的功能结构，并通过野外调查（field surveys）对阿根廷西南大西洋沿岸、横跨6个纬度、跨越阿根廷生物地理省与麦哲伦生物地理省之间边界的20个岩质潮间带岸线（rocky intertidal shores）进行了量化采样。单一功能群的丰度分布模式并未与生物地理区域完全匹配。仅短命藻类（ephemeral algae）呈现出与生物地理边界变化相契合的显著地理间断，且该现象仅存在于潮间带中带（mid intertidal zone）区域。基于主要功能群的总丰度与相对丰度，本研究划分出3~4个主要的样地集群，但这些集群既未与生物地理边界相契合，也未遵循纬度分布规律。由此可见，决定此类潮间带群落功能结构的生态过程，不受生物地理边界的影响。在地理尺度与局地空间尺度下，针对多数功能群与潮间带各潮带而言，功能群内部的物种丰富度（species richness）提升，均与该功能群总生物量的增加及空间变异的降低呈显著相关。上述结果表明，隶属于同一功能群的物种在空间尺度（即米级尺度与样地间尺度）上的空间相关性足够低，能够稳定生物量变异的分布模式，以此为环境条件的空间变异提供缓冲（buffer）或‘保险’（insurance）。