Abstract
Maintaining food production while sustaining productive ecosystems is among the central challenges of our time, yet, it has been for millennia. Ancient clam gardens, intertidal rock-walled terraces constructed by humans during the late Holocene, are thought to have improved the growing conditions for clams. We tested this hypothesis by comparing the beach slope, intertidal height, and biomass and density of bivalves at replicate clam garden and non-walled clam beaches in British Columbia, Canada. We also quantified the variation in growth and survival rates of littleneck clams (Leukoma staminea) we experimentally transplanted across these two beach types. We found that clam gardens had significantly shallower slopes than non-walled beaches and greater densities of L. staminea and Saxidomus giganteus, particularly at smaller size classes. Overall, clam gardens contained 4 times as many butter clams and over twice as many littleneck clams relative to non-walled beaches. As predicted, this relationship varied as a function of intertidal height, whereby clam density and biomass tended to be greater in clam gardens compared to non-walled beaches at relatively higher intertidal heights. Transplanted juvenile L. staminea grew 1.7 times faster and smaller size classes were more likely to survive in clam gardens than non-walled beaches, specifically at the top and bottom of beaches. Consequently, we provide strong evidence that ancient clam gardens likely increased clam productivity by altering the slope of soft-sediment beaches, expanding optimal intertidal clam habitat, thereby enhancing growing conditions for clams. These results reveal how ancient shellfish aquaculture practices may have supported food security strategies in the past and provide insight into tools for the conservation, management, and governance of intertidal seascapes today.
Highlights
Sustaining global food production presents one of the greatest environmental and humanitarian challenges of the 21st century
To assess if and how clam garden engineering altered intertidal height and optimal growing conditions for clams, we modeled the relationship between intertidal height and a) density and biomass of surveyed L. staminea and b) survivorship and growth of transplanted L. staminea in clam gardens and non-walled beaches in both bays, by fitting Gaussian models (Eq.1) to each metric of clam productivity (y) as a function of intertidal height where: y~a à e{0:5(x{s m
Physical Characteristics of Beach Types On Quadra Island, British Columbia (BC), clam garden terrace heights varied between bays, but their slopes were consistently shallower than unaltered beaches (Fig 4, S2A, Table S1, F(1,19) = 6.914, p = 0.017)
Summary
Sustaining global food production presents one of the greatest environmental and humanitarian challenges of the 21st century. Society will need to develop clever ways to meet demands on terrestrial and marine resources and spaces efficiently, while maintaining ecosystem productivity and resilience. We provide empirical evidence of an ancient form of mariculture that magnified shellfish production in a limited space, providing practical insights into sustainable marine management techniques which may inform local food security strategies of today. Humans have been altering, exploiting, and managing marine and terrestrial ecosystems for millennia [5], [6], [7], [8]. Recent archaeological evidence and oral historical knowledge suggests that First Peoples around the world actively managed and enhanced nearshore ecosystems to maintain and increase productivity [15], [16], [17], [8]
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