Highly unsaturated fatty acids in nature: what we know and what we need to learn

The supply and demand of omega-3 highly unsaturated fatty acids (omega-3 HUFA) in natural ecosystems may lead to resource limitation in a diverse array of animal taxa. Here, we review why food quality in terms of omega-3 HUFAs is important, particularly for neural tissue, across a diversity of animal taxa ranging from invertebrate zooplankton to vertebrates (including humans). Our review is focused on omega-3 HUFAs rather than other unsaturated fatty acids because these compounds are especially important biochemically, but scarce in nature. We discuss the dichotomy between omega-3 HUFA availability between aquatic primary producers, which are often rich in these compounds, and terrestrial primary producers, which are contain little to none of them. We describe the use of fatty acids as qualitative and quantitative tracers for reconstructing animal diets in natural ecosystems. Next, we discuss both direct and indirect ecological implications of omega-3 HUFA limitation at the individual, population, food web, and ecosystem scales, which include: changes in behavior, species composition, secondary production rates, trophic transfer efficiency and cross-ecosystem subsidies. We finish by highlighting future research priorities including a need for more research on omega-3 HUFAs in terrestrial systems, more research their importance for higher order consumers, and more research on the food web and ecosystem-scale effects of omega-3 HUFA limitation.