Over the millennia, many peoples worldwide developed intimate knowledge of, and relationships with, particularly valued species of plants and animals. Tracking the development of these long-term human–species relationships requires temporally grounded records that provide insights into both the cultural and ecological sides of this equation. For instance, the archaeological faunal record can provide detailed information on the ecological and cultural effects of human–species interactions, whereas the palaeoecological record can provide insights into species ecology in the absence of significant human intervention. Taken together, these two records can offer a powerful lens through which to assess coupled social–ecological systems over broad spatial and temporal scales and can help establish ecological baselines for modern management. On the Northwest Coast of North America, clams are a valued cultural species with widespread importance that is reflected in origin stories, rituals language, and in the kilometers of deep and ancient shell middens that line the coastline. Detailed archaeological and ethnographic research indicates that Clams, especially Butter Clams, Saxidomus gigantea, and Littleneck Clam, Leukoma staminea, were eaten in abundance both seasonally and year round and both fresh and preserved. These species were a reliable, abundant, and easily harvested source of food that could be tended to increase abundance by applying various traditional cultivation techniques. One such technique, the building of rock-walled intertidal terraces called 'Clam gardens', expanded and enhanced Clam habitat and thus, Clam production.
In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 14 October 2019, Ginevra Toniello of the Department of Archaeology at Simon Fraser University, the Hakai Institute, and the Treaty, Lands and Resources Department of the Tsleil-Waututh Nation, Dana Lepofsky, also of the Department of Archaeology at Simon Fraser University, and the Hakai Institute, Gavia Lertzman-Lepofsky of the Department of Biological Sciences at Simon Fraser University, Anne Salomon, again of the Hakai Institute, and of the School of Resource and Environmental Management at Simon Fraser University, and Kirsten Rowell, of the Department of Biology at the University of Washington, and the Environmental Studies Program at the University of Colorado, compare Butter Clam size and growth patterns from different temporal, environmental, and cultural contexts spanning 11 500 years ago to the present, with the aim of understanding Clam use and mariculture (the cultivation of marine organisms for food) by Human populatins over this time period.
Complimenting the archaeological and ethnographic records, studies of subfossil and fossil Bivalves on the Northwest Coast have provided significant insights into the region’s palaeoecology. Such data have been used for reconstructing both preHuman and recent historical ecological conditions. To the best of Toniello et al.'s knowledge, no studies have combined both the archaeological and palaeoecological marine Bivalve records to fully explore the long-term relationships among Humans and Clams.
Toniello et al. investigate the historical ecology of Butter Clams throughout the Holocene along the northern coast of Quadra Island, Salish Sea, British Columbia through analyses of the palaeoecological, archaeological, and contemporary ecological records. Together, these records encompass 11 500 years of history, a period that spans the time before extensive human settlement to today. In their study sites in Kanish and Waiatt Bays, the coevolved history of Humans and Clams is reflected in the expansive archaeological shell middens with deposits dating to as old as 9000 years ago and the plethora of Clam gardens dating from sometime after 3500 years ago.
(A) Study sites on northern Quadra Island, British Columbia, showing Clam garden sites (blue dots), large midden settlement sites (yellow diamonds), and sampling sites (red stars). Toniello et al. (2019). (B) Clam gardens, Quadra Island. Clam garden built on soft sediment showing wall, Clam garden terrace, and 9000 year old midden. Living and dead (palaeo) Butter Clams were collected from the Clam garden terrace and the shell midden. Mark Wunsch/Greencoast Media in Toniello et al. (2019).
Based on their understanding of Clam life history, coastal ecology, and local cultural attributes, Toniello et al. predicted that Butter Clam sizes and lifespan increased over the last 11 500 years as environmental conditions became more favorable and as Humans altered the intensity and strategy with which they harvested and cultivated clams. Thet also predicted that today’s Butter Clams are not as productive as those in past environments, likely due to a combination of less favorable ocean conditions, habitats modified by modern development, and the absence of traditional, Indigenous mariculture practices. To evaluate these hypotheses, they estimated Butter Clam sizes at various ages with sclerochronological analyses (analysis of chemical variations in accretionary hard tissues) of Clam shells from 5 beach sites and from 3 contexts at these sites: (1) palaeobeaches below Clam gardens that contain layers of Clams that died in situ (death assemblages) before Clam gardens were constructed, (2) terrestrial archaeological shell middens composed of Clams originally harvested from active Clam gardens, and (3) now largely defunct Clam garden beaches containing Clams that both are living and died relatively recently. Based on radiocarbon dating and stratigraphic context, Toniello et al. grouped these samples into seven temporal periods, each characterized by particular cultural and environmental attributes which they predicted would differentially influence lam growth, and asked what variables best predict Clam size across a spectrum of ages and age at death over the past 11 500 years.
As predicted, nonharvested clams (i.e., Clams that died in situ from natural mortality) show a steady increase in size at death and age at death from 11 500 to 11 000 years ago until the early-Late Holocene (4200 to 2900 years ago), corresponding to improving environmental conditions. In the midden (2800 to 2300 and 500 to 200 years ago) and living samples, Clams were harvested from active and inactive Clam gardens, respectively, and did not live out their full lifespans. Thus, the measurements do not evaluate natural mortality but rather, in the case of the middens, show the preferred size and age at which clams were harvested. However, in the Early Historic Period, where Clams died of natural mortality in Clam gardens, we can compare their age at death and size at death with the early-Late Holocene Clams. We find that the median age at death and size at death of the Early Historic Clams drop significantly and are 16 to 40% smaller than those in the early-Late Holocene samples (4200 to 2900 years old). In addition, the Early Historic samples are statistically indistinguishable from the Early Holocene time periods (11 500 to 11 000 and 10 900 to 9500 thousand years ago) when there was minimal Human presence on the Pacific Northwest coastal landscapes. Notably, Early Historic Period Clams grew in the years following 1782 AD when the Indigenous populations declined dramatically as a result of introduced diseases.
Butter Clams from 11 5 to 11 000 years ago (Left) and from 10 900 to 9500 years ago (Right), illustrating the differences in butter clam shell size in some samples. Toniello et al. (2019).
To further explore butter Clam growth rates Toniello et al. fitted growth curves of Clam growth increments in each of the seven time periods and compared estimated theoretical maximum Clam sizes. Visual inspection of the early life history growth in the growth curves suggests that the Clam growth trajectories across time periods broadly fit Toniello et al.'s predictions that Clams grow faster as environmental conditions become more favorable. Additionally, Clams from 11 500 to 11 000 years ago, Clams from 10 900 to 9500 years ago, and those from today appear to follow the same growth pattern and grow relatively slowly in their early years. In contrast, Clams living 4200 to 2900 years ago, the midden Clams harvested from Clam gardens (2800 to 2300 years ago and 500 to 200 years ago), and the Early Historic Clams all have a relatively faster growth trajectory in their early life history.
These growth curves allow Toniello et al. to compare the theoretical maximum length for the midden and living samples as if they were not harvested, assuming that earlier rates of growth in an individual are predictive of later growth rates. The estimated infered lengths of the 11 500 to 11 000 year old Clams are significantly smaller than most of the other time periods, except for the 10 900 to 9500 year old, Early Historic, and living specimens. This again suggests that Clams growing under modern conditions (in the last 200 years) reach sizes similar to Clams that lived in the Earliest Holocene. The infered length estimates further suggest that Clams from the middens, which were harvested from active Clam garden beaches, would have reached roughly the same maximum length as the nonharvested Clams in the early-Late Holocene. This suggests that Clams under intense human management have the potential to grow as large as the largest Clams living in environments with relatively few Humans. Toniello et al.'s estimates of potential length, combined with their empirical data together indicate that, as environmental conditions improve and as traditional management intensifies, Butter Clams have the capacity to grow to older ages and larger sizes than before.
For each individual Clam, Toniello et al. compared how size at age for the first 5 years changed across temporal category. Each size at age follows a roughly similar pattern and mostly parallels that of age at death, size at death, and infered maximum length. Toniello et al. found that, contrary to thei predictions, age 1 Clams in the Early Historic Period are bigger than all other times. By ages 4 and 5, Clams in all time categories except living and those from 10 900 to 9500 years agp are larger than those in the oldest period (11 500 to 11 000 years ago).
Toniello et al. compared the relative strength of evidence for several environmental and cultural attributes affecting Clam growth with a multimodel inference approach, allowing us to explore alternative mechanisms driving variation in size at age. Depending on the age of the individual, different terms are included in the model average. For young clams (ages 1 to 2), they found evidence that beach slope and sea surface temperature inversely affect size at age but are less important for clams ages 3 to 5, suggesting that young Clams are relatively more sensitive to changes in these factors. Coarse substrate has a strong positive influence on size at age for clams ages 1 to 5, with the strength of the effect being stronger as a Clam gets older compared with fine substrate. Similar to Fish, slight growth differences are more difficult to detect as Clams age and growth slows, and therefore, the effects that Toniello et al. detect in older clams are actually conservative. Taken together, the inclusion of more terms in the model average (slope, sea surface temperature, and substrate) for Clams ages 1 to 2 indicates that relatively more environmental factors influence the growth of younger Clams than older ones. As predicted, for Clams at all ages (1 to 5), the presence or absence of a Clam garden wall is an important factor affecting Clam growth, and the presence of a Clam garden results in a strong positive effect on size at age.
Similar factors affect both age at death and size at death, with Clam garden wall, slope, and substrate having the most influence. Not surprisingly, the presence of a Clam garden has a strong negative influence on both size at death and age at death, since most of the Clams from Clam gardens in the sample were harvested (midden samples from 2800 to 2300 years old and from 500 to 200 years ago and living), thus truncating their size and age at death. While substrate does not appear to affect age at death, size at death is affected by substrate, where coarse substrates result in larger size at death compared with fine substrates. This indicates that coarser substrates may be better for Clam growth. Flat and moderate intertidal beach slopes, typical of Clam gardens, result in older and larger age at death and size at death compared with steep beach slopes.
Toniello et al. also compared environmental and cultural factors affecting the growth curves and potential size. Substrate is the most important variable governing maximum infered potential size, where coarser substrates are associated with larger potential sizes, compared with fine substrates. Flat and moderate beach slopes exert a positive influence on potential size as compared with steep slopes.
Taken together, Toniello et al. show that not only are there several factors that influence size of a clam throughout its life but also, that the effects of these factors vary throughout the life of the clam. For instance, sea surface temperature appears to be negatively correlated with the size of Clams at age 1, but Toniello et al. did not detect an effect of sea surface temperature by age 2. The absence of effect in age 2 is at least in part due to the incongruity of comparing the millennia scale of sea surface temperature data with yearly-scale growth patterns of Clams that are individual snapshots of different years within that period of time. Similarly, flat and moderate beach slopes are negatively correlated with the size of Clams at ages 1 and/or 2 but do not appear in the model average for individuals ages 3 to 5. Clam gardens, however, have a positive effect on the growth of young clams as suggested by previous experimental data. Finally, coarse substrate, characteristic of the unwalled early-Late Holocene beaches as well as cultivated Clam gardens, appears to be beneficial to growth in all age categories, but the strength of this relationship increases with age.
With the retreat of the Cordilleran ice sheets roughly 13 500 years ago, coastal areas provided increased habitat for many species, including Humans and Bivalves. Toniello et al.'s earliest Butter Clam samples consist of subfossil death assemblages dating to 11 500 to 11 000 years. When alive, these Clams burrowed into silts, fine sands, and poorly drained glacial-marine clays; on steep beach slopes; and in relatively cold sea surface temperatures. These environmental conditions are not ideal habitats for this species of Clam, and not surprisingly, this contributed to the small size, young age at death, and slow growth of these Clams.
Butter clam environmental conditions began improving after about 11 000 years ago. These improvements include a transition to coarse (gravel–sand) substrate due to paraglacial deposits and hydrodynamic erosion, an increase in sea surface temperatures, and stabilizing sea levels. These factors likely contributed to increased phytoplankton productivity and more stable substrates for Bivalve settlement. These improved conditions are reflected in Toniello et al.'s measured and modeled data by relatively larger size and older age at death of the 4200 to 2900 year old intertidal subfossils compared with those from 11 500 to 11 000 years ago. However, the growth trajectories for young Clams are similar in these two time periods, which suggests similar ability to grow. Possibly, a decrease in mortality in the 4200 to 2900 years ago period allowed for more larger and older individuals in that period’s death assemblage.
The abundance of Early Holocene subfossil shells within the intertidal sediments suggests that these early Clam populations were not under significant predatory pressure by Humans. Based on the Early Holocene archaeological record elsewhere on Quadra Island, Toniello et al. suspect that Humans were visiting their study area at least by 10 000 years ago, but there is no known record of sustained settlement. However, even if early campsites were found in the study area, it would be difficult to explore the role of Clams in the Human diet, because the region’s acidic soils limit preservation of Clam shells in older archaeological deposits.
There is a temporal gap in our analysis for the period 9500 to 4200 years ago due to the absence of both midden and intertidal Clam samples. Although there are archaeological sites from this time, they have yet to be sampled for faunal analyses. Within the beach sediments, taphonomic factors are likely responsible for the lack of intertidal subfossil shells. In particular, the persistent wave action caused by relatively stable sea levels during this 5000 year period coupled with reduced sedimentation would have resulted in increased sediment erosion and the displacement and deterioration of intertidal subfossil shells. Before and after this time, the steady decline in sea level meant less wave action and thus, a regular influx of sediment that buried dead Clams quickly, making them more likely to be preserved in the intertidal sediment column.
When we pick up the palaeorecord again about 4000 years ago, we see that environmental conditions affecting Butter Clam growth continued to improve. These improvements, which differentially affect different age classes, include the further accumulation of coarse (gravel–sand) substrates and stabilized sea surface temperatures. These beneficial environmental conditions are reflected in the trend of increased age at death, size at death, and maximum size of the 4200 to 2900 year old intertidal subfossil Butter Clams as compared with those from earlier in the Holocene. The large size of these Clams must have been appealing to Human harvesters, who began to harvest Clams in earnest by about 5000 years ago as indicated by the local preservation of shells in archaeological settlement sites. However, the prevalence of long-lived Clams in Toniello et al.'s intertidal Clam death assemblage, dating to 4200 to 2900 years ago and originating from one particular beach (EbSh-77) 0.8 km from the nearest village, suggests that this location was not under heavy predatory pressure by Humans. Toniello et al. suspect that this in turn reflects the fact that Humans were focusing their harvesting on beaches immediately adjacent to their settlements.
After about 3500 years ago, Human–Clam relationships in Toniello et al.'s study area intensify as indicated by the building of some of the first Clam gardens. At this time, large settlements increase in number in Kanish and Waiatt Bays, filling all inhabitable coastal landforms and reflecting an increase in local human populations. By about 2700 years ago, Clam harvesting was so intensive that subfossil Clams are virtually absent from intertidal deposits, and instead, there are large quantities of harvested Clam shells in the middens. Clam garden construction was likely iteratively related to increasing Human populations, both as an impetus for enhancing a reliable and productive food source and trade item and in turn, by allowing for the increasingly larger Human population and complex social relations.
Despite intensive harvesting, several lines of evidence suggest that, as a result of traditional management practices, Clam populations in the study area thrived throughout the Late Holocene. In general, building Clam gardens increased the accessibility of Clams to Human harvesters by decreasing beach slope, increasing the amount of beach exposed during low tide, and increasing the proximity of Clams to Human settlements. Such increased accessibility of existing Clam beaches could easily result in overharvesting. However, in our study area, this potential increase in harvesting pressure was offset by generations of Indigenous peoples building Clam gardens, thereby increasing the viable Clam habitat. Toniello et al. suggest that creation of this new Clam habitat combined with other cultivation methods (e.g., tilling, removal of nonHuman predators, altering substrate, rock removal) and spatially explicit designated access rights ensured an ongoing, sustainable harvest of Clams. Furthermore the creation of the coarse sediment garden terrace and associated rock wall had the added benefit of increasing the abundance and availability of other edible marine foods (e.g., Red Rock Crabs, Sea Cucumbers, Snails, a variety of Seaweeds).
In two different ways, the size of the Clams in the middens in Toniello et al.'s study supports their inferences about the development of sustainable traditional harvesting practices. The first is the comparable estimated maximum size of Clams from the nonwalled early-Late Holocene beaches (4200 to 2900 years old) and those from the heavily harvested Clam gardens in the middens (2800 to 2300 years old and 500 to 200 years old). Given the importance of substrate to the growth of young and old Clams, Toniello et al. surmise that the similarity in Clam size in these two time periods is in large part due to the coarse sediment substrate in both the naturally productive beaches and the created Clam garden habitats.
The second way in which Toniello et al.'s midden data support inferences of sustainable harvest practices is that there is no indication of fishing pressure selection, such as a gradual decline in harvested Clam size over time (i.e., compare 2800 to 2300 years ago and 500 to 200 years ago). Furthermore, the majority (62%) of clams within the midden samples are larger than today’s fisheries size limit of 63 mm, suggesting the possibility of a culturally prescribed set of harvesting size restrictions as well as preferences for particular Clam sizes.
The larger clams that characterized Kanish and Waiatt Bays beaches throughout much of the Late Holocene declined in numbers sometime in the Early Historic Period, which was instead characterized by slower-growing, smaller Clams. Toniello et al. attribute this decline in Clam productivity to the disease-related decimation of Indigenous human populations beginning 1782 AD and the consequent reduction in management of Clam gardens. Since fewer Clams were being harvested at this time, more nonharvested Clams died in situ of natural mortality and formed intertidal death assemblages. While there must have initially been some positive legacy effect on Clam growth from the engineered intertidal slope and years of cultivation, Toniello et al. suggest that eventually Clam habitats degraded as a result of the breakdown in the traditional Clam management practices that had been part and parcel of daily Human–Clam interactions for millennia.
Toniello et al.'s analyses of the Clams currently living in the now-defunct Clam garden beach suggest that growing conditions for Clams continued to worsen in the last century. It is striking that the growth patterns of Clams living in the beach today are most similar to the Clams that lived and died in the unstable and relatively unproductive habitats of the Early Holocene. As in the Early Historic Period, Toniello et al. propose that the current low productivity is due to the decline in traditional management, including ongoing tilling through harvesting. However, given the importance of substrate on Clam growth, Toniello et al. also attribute this recent pattern to the logging-induced deposition of silts on the Clam beaches, possibly compounded by recent changes in ocean temperatures and productivity. These fine sediments created a substrate less conducive to Clam growth than the coarse-grained substrates of the Clam gardens and early-Late Holocene nonwalled beaches. Ironically, the shallow slope of the Clam garden beaches and the lip of the Clam garden wall itself, initially created to enhance Clam production, now act as a sediment trap for logging silts in some Clam gardens.
Toniello et al.'s understanding of the historical ecology of Humans and Butter Clams on Quadra Island not only illustrates the long-term and intertwined relationships of these two species but also, serves as a model for studying the intricacies of other Human–species relationships. In the case of Butter Clams, a culturally valued species, there was a myriad of ecological and cultural factors that influenced population viability throughout the Holocene. Toniello et al. expect similarly complex interactions among Humans and other species through time—whether through direct and deliberate interaction or through more indirect processes. Such complex Human–Clam interactions highlight the value of deeper-time baselines for informing modern fisheries management.
On the Northwest Coast of North America, as in coastal communities worldwide, the Human–Clam relationship is age old and continues today. Tracing that history and situating these relationships in the context of modern management decisions take bringing together data from multiple sources and using diverse types of analyses. They also require recognizing the sometimes-active role of Humans in modifying coastal ecosystems of the past as well as the present and that not all long-term Human–ecological interactions have negative ecological consequences on biological diversity.
In Toniello et al.'s study area, their analyses of shells from intertidal death assemblages, archaeological shell middens, and modern Clams provide insights into how Clams, Clam habitats, and Human–Clam relationships changed through time in a specific place. More specifically, the analyses reveal how Clam life histories have responded to shifts in harvesting, habitat alterations, climate and environmental factors, and management practices. Taken together, the temporal and spatial variability that we document is another reminder of the need to gather site- and time-specific baselines for modern management. Toniello et al. have demonstrated that ocean temperatures and substrate play a role in Butter Clam life history. Thus, it is no surprise that there is considerable variation in estimates of Butter Clam size in the literature, just as there are in our modern data and paleodata. Management plans based on local, modern, and palaeoecological data are likely to be more robust than those based on more general spatiotemporal data from the literature. However, under future climate change scenarios, environmental variables are likely to resort in different combinations than those of recent history and perhaps, with few analogs in the past.
Previous research on Clam gardens in our study area demonstrated that Clam gardens today are at least twice as productive as nonwalled beaches. This has implications for the numbers of people who can be locally supported by this ancient innovation in mariculture. Toniello et al.'s data, however, show that Clams in Clam gardens today are far less productive than they were before European contact and industrial logging—that is, when traditional management systems were active and shell–sand–gravel vs. silt-rich beaches dominated clam habitats. This highlights the possibility that, if traditional mariculture methods were applied to Clam beaches today, they could produce even greater yields than those estimated based on current ecological conditions, assuming similar pelagic production and oceanic conditions. In fact, many Indigenous communities along the Pacific Northwest Coast are exercising their rights to access and collective choice by restoring Clam gardens and the traditional protocols associated with them.
Many coastal First Nations with whom Toniello et al. work observe that today’s Clam beaches are less productive, because they are no longer managed in traditional ways. In recognition of widespread marine environment degradation and the loss of coastal resources, local communities worldwide have spearheaded efforts to manage, restore, and conserve coastal resources and improve biodiversity and food security.
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