Allochthony in stream food webs decreases with temperature across a subcontinental scale

Riverine and riparian food webs are interconnected through allochthonous resource flows originating from adjacent ecosystems. These resources, like terrestrial leaf litter, form a significant part of benthic macroinvertebrate diets, thereby supporting higher trophic levels. Although allochthony is observed in streams worldwide, the extent to which it is influenced by various environmental gradients remains unclear.

Researchers studied the factors affecting allochthony across six river basins within the contiguous United States, taking into account climate, stream size, and flow permanence. Bayesian mixing models were employed to gauge macroinvertebrate dietary proportions in four functional feeding groups. Air temperature emerged as the primary determinant of allochthony, with consumers relying less on allochthonous resources as temperatures rise.

The models suggest that macroinvertebrate diets become predominantly autochthonous (originating from the stream itself) when mean annual air temperature reaches around 15°C. Shredders consuming leaf litter demonstrated a slightly higher threshold for this dietary shift. The study also revealed that smaller, non-perennial streams exhibit increased allochthony.

These findings underscore the intricate nature of stream allochthony and its response to a range of environmental factors occurring across multiple scales.