Large fish forage lower in the food web and food webs are more truncated in warmer climates

Body size is usually considered a good indicator of trophic position in fish communities. Indeed, the proverbial wisdom that "Big Fish Eat Little Fish'' is consistent with observations from marine systems where systematic removal of the largest individuals has occurred, with cascading effect in the ecosystems. Trophic cascades are also well documented in temperate lakes but may not be as pronounced in (sub)tropical lakes due to higher degree of fish omnivory. We analysed fish communities along a climatic gradient and showed the classical correlation between body size and relative trophic position disappears in warmer climates where large fish appear to be feeding systematically on the lowest trophic levels. This concurs with experimental findings demonstrating that omnivorous fish tend to include more plant and less animal matter in their diet at higher temperatures. Accordingly, the community-wide trophic web indicators, calculated based on stable isotopes (delta C-13 and delta N-15), showed that the average degree of trophic diversity declined from cold to warm lakes and that the trophic webs become more truncated towards warmer climates. This has implications for lake restoration approaches in warmer climates and in temperate lakes within the context of global warming.