Quantifying impacts of plastic debris on marine wildlife identifies ecological breakpoints

Quantifying sublethal effects of plastics ingestion on marine wildlife is difficult, but key to understanding the ontogeny and population dynamics of affected species. We developed a method that overcomes the difficulties by modelling individual ontogeny under reduced energy intake and expenditure caused by debris ingestion. The predicted ontogeny is combined with a population dynamics model to identify ecological breakpoints: cessation of reproduction or negative population growth. Exemplifying this approach on loggerhead turtles, we find that between 3% and 25% of plastics in digestive contents causes a 2.5–20% reduction in perceived food abundance and total available energy, resulting in a 10–15% lower condition index and 10% to 88% lower total seasonal reproductive output compared to unaffected turtles. The reported plastics ingestion is insufficient to impede sexual maturation, but population declines are possible. The method is readily applicable to other species impacted by debris ingestion. Quantifying harm caused to marine wildlife by plastic pollution is crucial for identifying ecologically relevant indicators and conservation strategies, with sublethal impacts being especially insidious because of their latent nature. Using loggerhead turtles as the target species, we present a novel methodology to quantify sublethal impacts of plastics ingestion, and show that reported plastics ingestion is unlikely to seriously impede sexual maturation and egg production of the turtles, but could still be causing population decline. The method is modular and readily applicable to other groups of species.