Communities adjust their temperature optima by shifting producer-to-consumer ratio, shown in lichens as models: I. Hypothesis.

An apparent paradox exists in the ecology of Antarctic lichens: their net photosynthetic temperature optimum (around 0 degrees C) lies far below the temperature optima of their constituent algae and fungi (around 20 degrees C). To address this paradox, we consider lichens as microbial communities and propose the "community adaptation" hypothesis, which posits that in each thermal regime there is an equilibrium between photosynthetic primary producers (photobionts), and heterotrophic consumers (mycobiont and parasymbiont fungi). This equilibrium, expressed as the producer/consumer ratio (R(p/c)), maximizes the fitness of the community. As respiration increases with temperature, more rapidly than does photosynthesis, R(p/c )will shift accordingly in warm habitats, resulting in a high-growth temperature optimum for the community (the lichen). This lends lichens an adaptive flexibility that enables them to function optimally at any thermal regime within the tolerance limits of the constituent organisms. The variable equilibrium of producers and consumers may have a similar role in thermal adaptation of more complex communities and ecosystems.