Species attributes and patterns of circulation determine the location and permeability of marine biogeographic boundaries

Background/Question/Methods

How species are distributed and connected is important for population dynamics, gene flow, and understanding of species richness gradients.  Classic biogeographic studies emphasized differences in species composition between regions to define biogeographic provinces and delimit biogeographic boundaries. Here we analyze the permeability of biogeographic boundaries to different species to gain mechanistic insight into the processes that maintain species boundaries in the coastal ocean. We identify sites with high frequency of range boundaries using more than 2000 benthic marine invertebrates in five phyla along the northwestern Atlantic coast and address whether their magnitude and location vary as a function of species’ taxonomy, pelagic larval duration and bathymetric range. We estimated ranges boundaries and bathymetric distribution for all species using GBIF occurrences data. All species were classified as shallow or deep based on depth distribution, and a subset of species was classified by the type of larval dispersal using published information. We compared the distribution of boundaries between taxonomic groups, separating between northern and southern boundaries and assessed whether bathymetric distribution and larval dispersal affect the distribution of boundaries.

Results/Conclusions

We observed clusters of species boundaries at Cape Hatteras, Cape Cod and the Bay of Fundy that are, for the most part, independent of taxonomic group. The boundaries were asymmetric; we found a larger fraction of northern than southern boundaries. Consequently, there was a higher percent of species shared going from the poles to the equator than in the reverse direction. Both shallow and deep species exhibited important peaks of northern boundaries, but peaks in southern boundaries were higher for deep species. Pelagic larval duration was more important to explain distributions of boundaries for deep species, where species with long larval dispersal had significantly higher frequency of boundaries than species with short larval dispersal. Species boundaries tend to be pinned to regions of reduced water transport, and cannot be solely explained by the alongshore temperature variation. This suggests that in species that are strongly structured by the currents we could see abrupt jumps from one retention zone to the other instead of a gradual change with a rise in ocean temperature.