Abstract:
Global warming is widely predicted to reduce the biomass production of top predators, or even result in species loss.
Several exceptions to this expectation have been identified, however, and it is vital that we understand the underlying
mechanisms if we are to improve our ability to predict future trends. Here, we used a natural warming experiment in
Iceland and quantitative theoretical predictions to investigate the success of brown trout as top predators across a
stream temperature gradient (4–25 °C). Brown trout are at the northern limit of their geographic distribution in this
system, with ambient stream temperatures below their optimum for maximal growth, and above it in the warmest
streams. A five-month mark-recapture study revealed that population abundance, biomass, growth rate, and production
of trout all increased with stream temperature. We identified two mechanisms that contributed to these
responses: (1) trout became more selective in their diet as stream temperature increased, feeding higher in the food
web and increasing in trophic position; and (2) trophic transfer through the food web was more efficient in the warmer
streams. We found little evidence to support a third potential mechanism: that external subsidies would play a
more important role in the diet of trout with increasing stream temperature. Resource availability was also amplified
through the trophic levels with warming, as predicted by metabolic theory in nutrient-replete systems. These results
highlight circumstances in which top predators can thrive in warmer environments and contribute to our knowledge
of warming impacts on natural communities and ecosystem functioning.
