For most aquatic insects, emergence to adulthood means two things: sex and death. As a result, emergence is often synchronous (sex requires other individuals, of course), resulting in mating swarms. These swarms also make good meals for terrestrial predators, subsidizing their diet. More food can feed more predators, so streams that produce more emerging insects also tend to support more birds, spiders, lizards, etc. From a bottom-up perspective, the importance of this subsidy is clear. However, not all insects are limited to sex and death. Some, like dragonflies, feed as adults, meaning they’re both prey (bottom-up) and consumers (top-down) in terrestrial food webs, with potentially important consequences.
Dragonflies are also more diverse and abundant in warm climates. In contrast, stoneflies, some of which are predators in the stream but never as adults, are more abundant in cold climates. This means that predation by adult aquatic insects may vary along a temperature gradient. In this paper, I asked how a latitudinal changes in average stream water temperature might affect both the magnitude and trophic structure (proportion of predators) of insects emerging from streams. I used published benthic insect datasets (i.e. larval stream datasets) as a proxy for emergence, because emergence collections are relatively rare. Collections ranged from Alaska to Brazil, spanning average mean stream temperature from 4 to 25 degrees Celsius.
The trophic structure, but not the magnitude, of potential emerging insects varied across the temperature gradient. Warm streams had proportionally more adult predators than cold streams (range: 0-12% by abundance, and 0-86% by biomass), but not necessarily more overall prey. Thus, the “bottom-up” potential of emerging subsidies was consistent, while the “top-down” potential varied widely. I argue that this top-down potential should be considered more widely in studies of aquatic-terrestrial linkages, particularly in warm climates.
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