In situ warming strengthens trophic cascades in a coastal food web

Abstract

Global warming may affect most organisms and their interactions. Theory and simple mesocosm experiments suggest that consumer top–down control over primary producer biomass should strengthen with warming, since consumer respiration increases faster with warming than plant photosynthesis. However, these predictions have so far not been tested on natural communities that have experienced warming over many generations. Natural systems display a higher diversity, heterogeneity and complexity than mesocosms, which could alter predicted effects of warming. Here we used an artificially heated part of the northern Baltic Sea (the Forsmark Biotest basin) to test how warming influences trophic interactions in a shallow coastal food web with four trophic levels: omnivorous fish, invertivorous fish, herbivorous invertebrates, and filamentous macroalgae. Monitoring of fish assemblages over six years showed that small invertivorous fish (gobiids, sticklebacks and minnows) were much less abundant in the heated basin than in unheated references areas. Stomach content analyses of the dominating omnivorous fish – Eurasian perch Perca fluviatilis – revealed a strikingly different diet within and outside the Biotest basin; gammarid crustaceans were the dominating prey at heated sites, whereas invertivorous fish (e.g. gobiids) dominated at unheated sites. A 45‐day cage experiment showed that fish exclusion did not affect the biomass of algal herbivores (gastropods and gammarids), but reduced algal biomass in heated sites (but not unheated). This suggests that warming induced a trophic cascade from fish to algae, and that this effect was mediated by predator‐induced changes in herbivore behavior, rather than number. Overall, our study suggests that warming has effectively compressed the food chain from four to three trophic levels (algae, gammarids and perch), which have benefitted the primary producers by reducing grazing pressure. Consequently, warming appears to have restructured this coastal food web through a combination of direct (physiological) and indirect (species interactions) effects.

Trophic niche segregation allows range‐extending coral reef fishes to co‐exist with temperate species under climate change

“…Stomach content analysis is used to provide direct evidence of prey items consumed in the hours leading up to the fish's collection (Rybczynski, Walters, Fritz, & Johnson, ). Stable isotope signatures provide a measure of where in δ 15 N and δ 13 C isospace fishes are situated, but stomach content analysis reveals on what taxa, and in what quantities, tropical and temperate fish are feeding (Svensson et al, ). Only stomachs collected in 2018 were used for stomach content analysis, due to disintegration of the 2017 stomachs that were stored in ethanol.…”

Simultaneous exposure to a pulsed and a prolonged anthropogenic stressor can alter consumer multifunctionality

“…Ongoing global environmental change is dramatically altering the Earth's ecosystems (Sala et al 2000, Boyd et al 2014. Anthropogenic perturbations, such as global change and chemical pollution, can, for example, disrupt trophic links (Winder andSchindler 2004, Visser andBoth 2005) and food-web structure, thereby modifying energy transfer rates and efficiencies , Svensson et al 2017, Zarco-Perello et al 2017, Nordström and Bonsdorff 2017. Predicting the occurrence and magnitude of such ecosystemlevel affects is, however, difficult.…”

Temperature effects on prey and basal resources exceed that of predators in an experimental community

“…Previous studies have outlined that the strength of trophic cascades becomes weaker when predators suffer from greater negative interactions (Finke & Denno, 2004), such as in the form of interference competition or intraguild predation (Schmitz, 2007). In contrast, the strength of trophic cascades may magnify if greater consumption rates of predators at higher temperature remain unconstrained in relation to their negative interactions (Kratina, Greig, Thompson, Carvalho-Pereira, & Shurin, 2012;Svensson et al, 2017).…”