Professor Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
Abstract: The relative importance of allochthonous and autochthonous food resources to the diet of freshwater consumers, and the factors underpinning their availability, are still questioned. Freshwater consumers are predicted to display an allochthonous-based diet rather than autochthonous during periods of rainfall and flood events given the increased availability of those resources. The local environmental structure, such as surrounding vegetation and channel width, may also determine the availability of allochthonous resources and, in turn, the diet of consumers. Thus, both seasonal changes in environmental conditions and spatial factors could impact the diet of consumers, making dietary responses locally dependent. We conducted a meta-analysis using fish from tropical lotic ecosystems to investigate local spatial environmental determinants of the seasonal relative consumption of allochthonous and autochthonous resources by consumers. We gathered 656 observations of the diet of fish populations (individuals from a sampling event) from 83 localities across the tropics. For each locality, we retrieved the latitude, channel width, terrestrial biomass, and elevation from the articles and open databases. To standardize sampling events, we categorized them into two hydrological periods: high-water and low-water periods. Our main metric was the proportion of allochthonous resources in the diet of each observation (%Allo). We employed meta-regression models to analyze each hydrological period, with %Allo as the response variable, retrieved variables as fixed effects, and localities as the random factor. Contrary to what is expected, the diet of fish in tropical lotic ecosystems is mainly autochthonous, and dominated by aquatic invertebrates, regardless of the hydrological period. Terrestrial biomass and elevation were the primary drivers of increasing allochthony for both hydrological periods, while latitude and channel width did not affect allochthony. This suggests that spatial factors, rather than seasonality, play a crucial role in determining allochthony. Our results highlight the importance of considering scale dependency in Ecology, as our large-scale synthesis revealed a different pattern than what was previously predicted. We attribute the lack of clear seasonality effects on allochthony to the high variability and stochasticity of seasonality across the tropics, which may obscure potential trends. Overall, our big-picture approach provides novel insights into the diet of freshwater consumers in tropical lotic ecosystems, challenging previous assumptions about their relationship with seasonality.