PS 37-43 - Has insect decline led to a decrease in terrestrial prey input to riverine systems?

Abstract: The population of terrestrial insects is declining by 9% every decade, which has an adverse effect on trophic cascades, ecosystem services, and ecological processes. Many studies have examined the effects on the terrestrial environment, but less attention has been paid to how this can affect the stability of aquatic ecosystems. Drift-feeding fish in the river rely on those terrestrial insects that fall into the water (TIW). In Virginia, TIWs make up 13% of the total diet of the sunfish and bluegills and 51% for the rainbow trout in California. TIW and aerial insects contribute up to 79% of the salmon diet In Cairngorm (Scotland) and 90% in Hokkaido (Japan). Disrupting the cross-ecosystem energy flows can directly affect the fish population and the aquatic ecosystem. Therefore, the objective of our study is to test whether the biomass of TIWs has decreased over time.

We conducted a meta-analysis by searching the literature using keywords (e.g., terrestrial prey input, insects, biomass, etc.). For the preliminary analyses, we chose samples that were collected under the bank vegetation using pitfall traps (15 sites, n=211). We collected data on insect dry biomass (mg/m²/day), sampling year, sample country, reach width (m), riparian vegetation type (coniferous, broad-leaf, mixed, and open land), canopy cover (%), altitude (m), and weather conditions (monthly average air temperature (℃) and precipitation (cm)). We used gamma generalized linear mixed models to test the relationship between the TIW biomass and the sampling years. Sample locations and each literature were considered random effects.

Preliminary results (model with the lowest AIC) suggest that sampling year, riparian vegetation type, monthly air temperature, and rainfall have an effect on the biomass of TIW (R-squared 81%). Daily TIW biomass decreased from year between 1995 to 2013, at a rate of 0.13 g/m² per year.

Our study demonstrates a decrease in the biomass of TIW. This is concerning for the aquatic ecosystem, particularly in areas where anadromous salmonid species, as keystone species, play important roles in transporting nutrients between the ocean and the river. For future research, we intend to incorporate insect biomass samples that were gathered from an open canopy above the water's surface. We believe this is the first attempt to look at the changes of the TIW biomass in aquatic systems and may have implications for conservation practices.