Today's food chain length – nearly half the size it used to be? Prehistoric skeletal remains document ecosystem change in the upper Gulf of California

Background/Question/Methods

The ratio of 15N/14N (δ¹⁵N) from consumer and prey tissue is commonly used in ecological studies to determine trophic level, food web structure, and mean trophic level in aquatic ecosystems. Tracking food chain length or mean trophic level in a system over time provides insights about biodiversity and ecosystem resilience. Yet the absence of baseline information before anthropogenic disturbances makes comparative studies addressing ecosystem responses difficult. We test if organic matter encapsulated in ancient fish otoliths and clamshells can define the food web structure and test if changes in food chain length can be detected over the past 5,000 years (Anthropocene epoch).This approach uses skeletal remains (>1000 ybp) to establish the first baseline assessment of the food chain length from before human alterations to the system.

Results/Conclusions

Our calibration results indicate that otolith δ¹⁵N is on average 1.6 ‰ lower compared to muscle tissue. Our results from otoliths of three species of fish (top predator, secondary predator and bottom feeder) and shells of two species of clams demonstrate that nitrogen in aragonite remains can reconstruct food web structure from the past. Comparisons between prehistoric and modern δ¹⁵N values show a provocative downward shift in the top predator and secondary predator, suggesting a decrease in mean trophic level by at least one full trophic level. The over all results for this analysis indicate ~ 45% compression in food chain length since human alteration to the Colorado River Estuary.