Archaeological evidence for long-term human impacts on sea turtle foraging behaviour

Abstract

Early conservation efforts to prevent the loss of green sea turtles ( Chelonia mydas ) from the Caribbean Sea jumpstarted marine habitat and biodiversity protection. However, even there, limitations on historical observations of turtle ecology have hampered efforts to contextualize foraging behaviours for conservation management. We integrate isotopic and zooarchaeological evidence from green sea turtles harvested at the Miskito Cays (Nicaragua) to assess foraging behaviour before and after a step change in harvesting intensity. Highly structured isotopic evidence shows greater foraging adaptability in earlier populations. This provides a counterpoint to recent synthesis, suggesting the ecological non-exchangeability of sea turtles, which complicates conservation planning focused on genetic-stock-based repopulation. In contrast, our results suggest future populations would have a capacity for higher degrees of ecological exchangeability than current perspectives allow. This highlights a need to consider the kinds of longer term perspectives, such as those offered by archaeological materials, when planning for future sea turtle recovery.

Deep antiquity of seagrasses supporting European eel fisheries in the western Baltic

“…For instance, recent studies focusing on herbivores specializing in seagrass habitats observed consumer δ 13 C values that extend the higher-end boundaries for the known range for bone collagen of archaeological marine animals. Specifically, sea turtle bone collagen from the tropics had δ 13 C values up to ca –3.0‰ ([ 47 ] and up to –5.0‰ from the Mediterranean [ 48 ]), though latitudinal impacts on the δ 13 C of marine DIC [ 49 ] mean that an endpoint in temperate locations, such as our study region, would be 2–3‰ lower. In that context, while we therefore expect a degree of overlap in the potential δ 13 C range for bone collagen from consumers living in marine, estuarine and freshwater habitats (from the bottom end of the marine δ 13 C range up to about –9.0‰), values falling well above this threshold result from intensive use of seagrass habitats.…”

“…Stable sulfur isotope values from two samples support the interpretation that extreme eel δ 13 C values reflect a long-term, intensive focus on eelgrass habitats. While marine environments are thought to have a globally homogeneous sulfate isotopic composition, with a high δ 34 S of approximately +20.0‰ [ 52 ], there are contexts in which this broad-scale observation does not hold [ 47 , 53 , 54 ], and seagrass habitat use offers one important exception (for review see [ 55 , 56 ]). While most marine primary producers (and their consumers) have δ 34 S values close to those of marine sulfates, eelgrasses can incorporate sulfide-cycled sulfur [ 57 , 58 ].…”

“…Reduction of sulfate to sulfide in anoxic sediments is associated with a strong discrimination against 34 S [ 59 , 60 ], meaning that when associated sulfur is eventually incorporated into marine food webs it imparts distinctively low δ 34 S values in local consumers (e.g. [ 47 , 53 , 61 , 62 ]). Havnø eel δ 34 S values are much lower than would be expected for typical marine consumers, which is consistent with the interpretation that these eels relied on primary production derived from eelgrass meadows.…”

“…Archaeological literature often assumes that humans with marine-intensive diets will have higher bone collagen δ 34 S values, reflecting the isotopic composition of seawater sulfate (which is typically expected to be higher than terrestrial foods; [ 63 ]). However, recent studies exploring isotopic compositions of marine fauna, including turtles [ 47 ], fish [ 53 ] and mammals [ 54 ], have highlighted how the intensive use of seagrass- and benthic microalgae-rich habitats can create isotopic patterns that would not be consistent with prevailing interpretive frameworks. Our data linking low δ 34 S values with extremely high δ 13 C values in eels confirm that these wider patterns, showing lower δ 34 S in other archaeological marine consumers elsewhere, occur in Northern Europe.…”

“…Factors contributing to these shifts could involve a wide range of variables, including changes in bottom-up (nutrient availability) and top-down (mesograzer intensity and species composition) pressures, which are, in turn, mediated by a range of biotic and abiotic factors, including structural complexity, epiphytic community composition and potential human impacts [80,[91][92][93][94]. However, the fact this level of subsidy from seagrass primary production (as evidenced by low δ 34 S and extremely high δ 13 C values) has not frequently been observed in contemporary studies of seagrass food webs (outside of those focusing on herbivorous megafauna specializing in direct consumption of seagrass; for review, see [47]) suggests that something fundamental was different about eelgrass habitats in the distant past. While acknowledging the many potential factors at play, to the extent that anthropogenic eutrophication has been credited with promoting the presence and abundance of epiphytic algae in eelgrass habitats in the region over the past century [18,95], it is possible that the stronger input from eelgrasses to vertebrate food webs in our data reflects eelgrass habitats in an unaltered oligotrophic state, one in which epiphytic algae had a more limited presence than known for the western Baltic today.…”