In a multifaceted exercise in palaeoecology, Michael Graham and colleagues have taken the subject underwater. Their study is an investigation of how the distribution and productivity of giant kelp have changed since the end of the Last Glacial Maximum, about 20,000 years ago, in one of its present-day strongholds, the Southern California Bight (M. H. Graham et al. Proc. R. Soc. B doi:10.1098/rspb.2009.1664; 2009).
The significance of giant kelp (Macrocystis pyrifera; pictured) is that it is a foundation species — a dominant primary producer — that creates vast submarine forests in coastal areas of temperate regions. These kelp forests constitute ecosystems on which a multitude of other species depend.
Delving into the past of marine algae such as kelp poses particular challenges. Unlike the terrestrial flora, for example, there is no pollen record to track a shift in species abundance. Graham et al. had to find less direct approaches.
The authors worked on the assumption that the environmental needs of kelp have not varied over the past 20 millennia. They created 'niche-based' reconstructions through time of factors that primarily included the availability of a rocky sea floor in adequate light conditions, at depths of up to 25 metres. These reconstructions were complemented by palaeo-oceanographic records that allowed estimates of nutrient availability, and so of kelp productivity.
The result is a millennial-scale account of the dynamics of southern Californian kelp forests. The patterns are swiftly changing and complex. But the broad picture is of an apparent increase in biomass of three times or so from glacial-maximum levels until about 7,500 years ago, followed by a rapid fall of as much as 70% to today's levels.
An archaeological twist comes from a link between the food resources furnished by kelp forests and early human settlements. There is documentation of this for California, and Graham et al. suggest that past human migrations might also have had a kelp connection.