Increasing donor ecosystem productivity decreases terrestrial consumer reliance on a stream resource subsidy

Abstract

Because nutrient enrichment can increase ecosystem productivity, it may enhance resource flows to adjacent ecosystems as organisms cross ecosystem boundaries and subsidize predators in recipient ecosystems. Here, we quantified the biomass and abundance of aquatic emergence and terrestrial spiders in a reference and treatment stream that had been continuously enriched with nitrogen and phosphorus for 5 years. Because we previously showed that enrichment increased secondary production of stream consumers, we predicted that aquatic emergence flux would be higher in the treatment stream, subsequently increasing the biomass and abundance of terrestrial spiders. Those increases were predicted to be greatest for spiders specializing on aquatic emergence subsidies (e.g., Tetragnathidae). By adding a (15)N stable isotope tracer to both streams, we also quantified nitrogen flow from the stream into the riparian community. Emergence biomass, but not abundance, was higher in the treatment stream. The average body size of emerging adult insects and the relative dominance of Trichoptera adults were also greater in the treatment stream. However, spider biomass did not differ between streams. Spiders also exhibited substantially lower reliance on aquatic emergence nitrogen in the treatment stream. This reduced reliance likely resulted from shifts in the body size distributions and community composition of insect emergence that may have altered predator consumption efficiency in the treatment stream. Despite nutrient enrichment approximately doubling stream productivity and associated cross-ecosystem resource flows, the response of terrestrial predators depended more on the resource subsidy's characteristics that affected the predator's ability to capitalize on such increases.

Bottom‐up nutrient and top‐down fish impacts on insect‐mediated mercury flux from aquatic ecosystems

“…We found that nutrient addition led to an increase in MeHg flux from aquatic ecosystems, primarily through enhancing the biomass of emerging insects whose tissues were contaminated with MeHg. The bottom-up effect of nutrients on the flux of resource subsidies (i.e., emerging insects) to terrestrial ecosystems has previously been demonstrated across productivity gradients in field studies [29] and in systems that have been experimentally manipulated [10][11][12]30]. The present study suggests that increased nutrient inputs into aquatic ecosystems increases the risk that terrestrial consumers will be exposed to greater fluxes of aquatically-derived MeHg.…”

“…The bottom-up effects of nutrients potentially influence the flux of MeHg from aquatic systems by regulating productivity and emergent insect biomass [10][11][12]. Nutrients may also alter the flux of MeHg by affecting the concentrations of MeHg of insects as has been shown for other consumers [13][14][15][16].…”

Influence of hydrological regime and land cover on traits and potential export capacity of adult aquatic insects from river channels

“…Changes to the aquatic environment that alter the habitat and food resources available for aquatic larvae also alter the abundance and identity of adult aquatic insects emerging from a river (Compson et al 2013), with flow-on effects to terrestrial consumers (Epanchin et al 2010). However, relatively little is known about how anthropogenic factors may affect the supply and consumption of adult aquatic insects by terrestrial consumers (but see Davis et al 2011;Greig et al 2012;Jonsson et al 2013;Paetzold et al 2007).…”

“…Such taxa are often molluscus and worms (Harding and Winterbourn 1995), which do not have winged adult stages. However, eutrophic conditions or nutrient additions can also increase emergence rates of Diptera (Davis et al 2011;Greig et al 2012;Welch et al 1988), which may increase the number of terrestrial consumers that a waterway can support (Racey et al 1998). Anthropogenic alterations to river flow regimes through water abstraction, river channelisation, dams or changes to the climate can alter the intensity, frequency and seasonal predictability of high and low river flows (Poff et al 1997), which may lead to changes in aquatic invertebrate communities (Meyer et al 1999).…”

“…However, few studies have investigated how anthropogenic changes to aquatic ecosystems may alter the transfer of resources between aquatic and terrestrial ecosystems (but see Davis et al 2011;Greig et al 2012;Jonsson et al 2013;Paetzold et al 2007).…”