Quality, then quantity: determinants of autochthony in freshwater food webs.

Understanding how resource quantity and quality shape food webs is a central question in ecology. In freshwater ecosystems, autochthonous organic matter (OM) generally offers higher nutritional quality than allochthonous OM. Although freshwater consumers may preferentially forage on limited but high-quality autochthonous OM, its scarcity often necessitates reliance on more abundant, lower-quality allochthonous OM. Using tank bromeliads as model ecosystems, we conducted a five-month field experiment to test how the dietary contribution of autochthonous OM varies with its quantity. We manipulated incident light to generate a gradient in autochthonous OM availability, evaluating whether its contribution to freshwater consumer diets increased or plateaued due to foraging preferences. We combined Bayesian stable-isotope mixing models with precision-weighted beta regression to quantify diet sources and their drivers. Across the light gradient, autochthonous OM consistently dominated consumer diets (76-95%) despite being > 1,000× less abundant than allochthonous inputs. Median autochthony increased with autochthonous supply, driven by light incidence. Maximum water storage showed a weaker, marginal negative association with autochthony after accounting for light incidence, suggesting that larger tank bromeliad ecosystems exhibited lower autochthony. This pattern may reflect the fact that tank bromeliads are living plants rather than inert containers. Larger seedlings potentially take up more nutrients from the water tanks, which likely reduces nutrient availability to autochthonous primary producers. Overall, our results support a quality-then-quantity hierarchy, in which consumers preferentially consume high-quality autochthonous OM but their realized reliance on this resource is ultimately constrained by the ecosystem's capacity to produce and retain it.