Eco-evolutionary feedbacks in the functional role of a polymorphic colonizer: Arctic charr in subarctic lakes of Alaska and Iceland

Background: The Arctic charr, Salvelinus alpinus, is likely to be the first fish to colonize northernmost freshwater systems. It often exhibits sympatric morphotypes. Question: How do ecosystem characteristics and the life history of Arctic charr interact to generate food webs in subarctic lakes? Organisms: We focus on allometric dietary patterns of Arctic charr. We compare them with selected, co-occurring fish species, including threespine stickleback (Gasterosteus aculeatus) among others, and both benthic and limnetic invertebrates. Field sites: Subarctic lakes in two regions that differ greatly in colonization history (ten in Iceland and four in Alaska). Methods: We used natural variation in the stable isotope ratios δ¹³C and δ¹⁵N to estimate trends in trophic position and relative use of benthic and limnetic carbon sources with body size among lakes and among species × morph × body size brackets within lakes. Results: (1) In subarctic lakes with low levels of fish diversity, food chains lengthened via insertion of intermediate prey. (2) Ecosystem attributes, including the presence of competitors and competitor resource use, were associated with allometric shifts in Arctic charr resource use. (3) Species attributes, including the presence and nature of polymorphism exhibited by the Arctic charr, likewise shifted allometric trends in resource use. (4) Both the expression of resource polymorphism and interspecific interactions (i.e. predation, competition, or both) affected food web structure.