Pattern and scale in latitude–production relationships for freshwater fishes

Abstract

AbstractEmpirically understanding spatial variation in secondary production rates is central to ecology. Yet for most taxa, such patterns are rarely examined, especially at different levels of ecological organization (e.g., species‐ vs. community‐level patterns). We compiled data on biomass, production, and P/B rates of freshwater fish communities and species across latitudes and contrast patterns observed at the community level with those observed for species. At the community level, and at two distinct spatial scales (global vs. continental‐North American), negative or neutral relationships were apparent between biomass, production, and P/B with latitude; however, there was substantial scatter in these data. Yet at the species level in North America, production was often closely linked to latitude, but in the opposite direction: Many species showed improved production with latitude. Latitudinal increases in species‐level production rates were strongest for cool‐ and cold‐water species, and species rarely showed the opposite trend. Species‐level increases in production with latitude strengthened when production rates were normalized by the thermal opportunity for production, suggesting potential adaptations of individuals and populations to shorter growing seasons (i.e., “countergradient” production) at high latitudes. At the global scale, there were apparent unimodal relationships between community fish production measures and species richness; however, these patterns became linearized or non‐significant at the continental scale. Decreased interspecific competition at northern latitudes combined with genetic adaptations (e.g., countergradient growth) could explain a tendency for increased species production in northern populations, while total community production remains reduced. Latitude has contrasting effects on fish production at different spatial scales and levels of biological organization. Thus while freshwater fish communities are somewhat more productive and diverse at lower latitudes, species production in northern populations is often surprisingly high.