Ancient clam gardens, traditional management portfolios, and the resilience of coupled human-ocean systems

Abstract

ABSTRACT. Indigenous communities have actively managed their environments for millennia using a diversity of resource use and conservation strategies. Clam gardens, ancient rock-walled intertidal beach terraces, represent one example of an early mariculture technology that may have been used to improve food security and confer resilience to coupled human-ocean systems. We surveyed a coastal landscape for evidence of past resource use and management to gain insight into ancient resource stewardship practices on the central coast of British Columbia, Canada. We found that clam gardens are embedded within a diverse portfolio of resource use and management strategies and were likely one component of a larger, complex resource management system. We compared clam diversity, density, recruitment, and biomass in three clam gardens and three unmodified nonwalled beaches. Evidence suggests that butter clams (Saxidomus gigantea) had 1.96 times the biomass and 2.44 times the density in clam gardens relative to unmodified beaches. This was due to a reduction in beach slope and thus an increase in the optimal tidal range where clams grow and survive best. The most pronounced differences in butter clam density between nonwalled beaches and clam gardens were found at high tidal elevations at the top of the beach. Finally, clam recruits (0.5-2 mm in length) tended to be greater in clam gardens compared to nonwalled beaches and may be attributed to the addition of shell hash by ancient people, which remains on the landscape today. As part of a broader social-ecological system, clam garden sites were among several modifications made by humans that collectively may have conferred resilience to past communities by providing reliable and diverse access to food resources.

Ancient Shellfish Mariculture on the Northwest Coast of North America

“…Moreover, transplant experiments showed that juvenile little neck clams grew 1.7 times faster and were more likely to survive in clam gardens than non-walled beaches (Groesbeck et al 2014). Similarly, recent research on British Columbia's Central Coast, 250 km north of Quadra Island, revealed that clam gardens encompassed twice the biomass and density of butter clams than unmodified beaches (Jackley et al 2015). In both cases, we found that differences in clam biomass between the two beach types were most pronounced at the upper tidal elevation of ideal clam habitat.…”

Breaking Resilience for a Sustainable Future: Thoughts for the Anthropocene

“…Walker et al (2006) argued over a decade ago that "some regimes that are considered undesirable can also be very resilient, e.g., harsh dictatorships and desertified regions of the Sahel, " and static lock-ins and traps have been identified (Cinner, 2011;Olsson et al, 2014). Nonetheless, the debate on resilience dynamics has remained oddly one-sided, focusing almost exclusively on enhancing sustainability-supporting forms of resilience (e.g., Davidson-Hunt and Berkes, 2002, p. 77;Cutter et al, 2008;Barnosky et al, 2012;Jackley et al, 2016). That this entails the danger of depoliticizing our understanding and decision-making relating to human-nature dynamics has been pointed out recently (Olsson et al, 2015).…”

Shellfish subsidies along the Pacific coast of North America

“…To ensure a consistent, easily-accessible supply of shellfish, some coastal communities engineered extensive clam gardens near habitation sites ( Fig. 2i, Lepofsky et al 2015, Jackley et al 2016. Constructed by building rock walls in the mid to low intertidal, clam gardens increase sedimentation rates by at least four-fold, which alters the beach slope at tidal heights optimal for clam production (Groesbeck et al 2014, Deur et al 2015, Neudorf et al 2017).…”