MEPS 518:85-94 (2015)  -  DOI: https://doi.org/10.3354/meps11016

ABSTRACT: The structure of rocky shore assemblages is determined to a large extent by the interaction between abiotic and biotic processes. Understanding of the strength and interactive nature of these processes over different spatial and temporal scales provides insight into key structuring mechanisms and clarifies what sets species’ distributions. Here we examine the potential interactive effect of physical environment (insolation stress) and grazing pressure on recruitment success of fucoid species (Fucus spp.) in North Wales and Portugal, in order to understand patterns of distribution at the centre and equatorward edge of their range. Contrary to expectations, amelioration of the physical environment through reduction of insolation stress did not improve recruitment in either geographical region. Grazing activity of patellid limpets was found to be an important process regulating fucoid recruitment only on northern European shores. The top-down control of fucoid recruitment at the northern latitude was apparent, even at half the normal densities of patellid limpets. The use of reduced light levels and reduced grazing pressure in southern regions was expected to improve fucoid recruitment, but results indicate amelioration of physical and biological pressures cannot compensate for an inherently low supply of propagules. Our study confirms that strong top-down control by grazers (Patella spp.) can directly determine the distribution and abundance of fucoid algae in core areas of its range, but toward the range edge factors other than those examined are responsible for low recruitment and, ultimately, the observed decline in fucoid abundance in southern European regions. 

KEY WORDS: Reproductive success · Geographical differentiation · Biotic stress · Grazing pressure · Abiotic stress · Solar radiation · Fucus spiralis · Fucus vesiculosus · Patella spp.