Persistent border: an analysis of the geographic boundary of an intertidal species

Abstract

The biological performance of species close to their biogeographic boundaries is of
critical interest in a period of rapid climate change and can inform predictions of future patterns of
distribution. The classic view is that performance attributes (reproduction, growth, survival) will
gradually decline from the centre towards the edge of a species range. A persistent discontinuity in
the distribution of the intertidal barnacle Chthamalus montagui on the central south coast of England
has enabled us to test hypotheses about its performance and recruitment as the range edge is
approached. Although adult density was reduced by over 5 orders of magnitude along a 200 km distance,
there was little evidence of impaired performance at the range edge. There have been fluctuations
in abundance over the last 50 yr at shores approaching the border, which are associated with
changes in temperature and suggest thermal sensitivities. A study of recruitment in C. montagui and
in other intertidal barnacles revealed a region of very low recruitment for all species close to the border
of C. montagui. We propose that reductions in larval supply caused by complex regional hydrography
and suboptimal habitat quality, not adult performance, is most likely responsible for a steep
gradient in recruitment as the border is approached, although possible reductions in larval performance
cannot be totally discounted. The location of ‘low recruitment cells’ caused by oceanographic
processes that obstruct the dispersal of propagules needs to be identified when modelling the rate of
change of biological assemblages and the location and spacing of reserves.

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