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

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 LIS 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 steel) 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.