Spatial structure creates community-level selection for nutrient recycling

Nutrient cycling is a ubiquitous feature of ecosystems at all scales, allowing productivity to rise beyond the limits set by external nutrient inputs. Nutrient cycling occurs as a side-effect of the metabolism of a diverse set of species that each performs a step in the recycling loop. Recycling loops can be large and involve many steps. At each step the possibility exists for ‘side-reactions’ in the form of species with metabolisms that consume an intermediate metabolite but do not create the product needed to complete the recycling loop. Also, at least some of the biochemical reactions in any closed recycling loop must be endergonic (energyconsuming) and thus recycling loops may be vulnerable to invasion or parasitism by species that consume intermediates but do not produce costly products needed to close the loop. The possibility of such destabilising side-reactions appears to conflict with the apparent stability and ubiquity of nutrient recycling in nature.