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

Evie A Wieters,Gabriela Palomo,Paula Pappalardo,Christopher Mcquaid,Sergio A Navarrete,Christopher Fulton

doi:10.1371/journal.pone.0049725

Abstract

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.

Highlights

Boundaries between geographically distinct biotas are shaped and promoted by historical and ecological processes that set limits to the distribution of a large fraction of species
We describe geographical variation in the functional structure of rocky intertidal communities along much of the coast of Argentina and evaluate the hypothesis that major changes in the absolute and relative abundance of functional groups match traditional, well-known biogeographic boundaries. Across this region we evaluate whether: a) increased species richness leads to increased functional group biomass, and b) increased species richness leads to reduced susceptibility to environmental fluctuations, as estimated by the level of variability in functional group biomass over scales of tens of metres within sites, and over geographical scales of hundreds of kilometres among sites
A large fraction of intertidal species in our database find their geographic endpoints within the reported biogeographic boundaries (Fig. 1b), supporting again the occurrence of significant compositional changes within the region

Summary

Introduction

Boundaries between geographically distinct biotas are shaped and promoted by historical and ecological processes that set limits to the distribution of a large fraction of species. Transition zones are areas of intense abiotic and biotic stresses modulated by limited dispersal, which limits colonization of habitat beyond a species range, and by vicariance processes that divide previously homogeneous genetic entities [1]. Beyond this broad understanding, we still have scarce information about whether geographic areas of extensive changes in species composition are the geographic points where major changes at the population and community levels can be expected. On the Pacific coast of North America, the well established biogeographical break at Point Conception does not correspond with phylogeographical breaks of most but a few species [6,7], whereas major genetic differentiation has been detected at Cape Mendocino, a place of little compositional changes [4]

Methods

Results

Discussion

Conclusion