Effects of tidal amplitude on intertidal resource availability and dispersal pressure in prehistoric human coastal populations: the Mediterranean–Atlantic transition

Abstract

In this paper I argue that there is a growing body of evidence supporting an increasingly central position of coastal environments in human evolution and dispersals, rather than as merely peripheral habitats. Eustatic fluctuations during glacial cycles have meant that most prehistoric coastlines are now underwater, and lack of evidence to date of a close relationship between people and the coast can be most plausibly ascribed to the limited studies so far on submerged sites. Coastal environments provide high diversity in food resources, consisting of multiple ecotones in close proximity, which reduces the need to forage widely. One of the richest and most easily exploited coastal resources by human populations living on the coast are molluscs from marine rocky intertidal communities, which recent evidence has highlighted as important as far back as the Middle Palaeolithic. However, the density of these resources is limited by a number of factors, and this varies geographically. One of the main large-scale factors limiting rocky intertidal mollusc densities is tidal amplitude, beyond which smaller-scale local factors such as exposure to wave action and shore aspect, further affect species distributions. The area around the Strait of Gibraltar is used as a case study of an area, which is affected by large variations in tidal amplitudes thus allowing for quantitative comparisons between taxonomically and climatically similar regions. Shorelines along the Mediterranean coast, with reduced tidal amplitudes, exhibit compressed zonations and harbour fewer macro-mollusc individuals, with the reverse being the case along the Atlantic coast, which has significantly larger tides. Data from Middle and Upper Palaeolithic sites along the Strait are used to establish harvested species and present-day data are used to model the potential distributions and associated variables such as calorific returns of key food species. An optimal foraging model is used to explore the effects of differential levels of resource availability along these coastlines. Results suggest that the nature and variability of the coastline has a direct influence on the tempo and mode of subsistence strategy followed and whether certain strategies can be adopted at all. The possible implications of these biogeographical distributions for foraging economics, dispersals and social structures in Homo are discussed.