Changes in feeding selectivity of freshwater invertebrates across a natural thermal gradient

Abstract

Environmental warming places physiological constraints on organisms, which may be mitigated by their feeding behavior. Theory predicts that consumers should increase their feeding selectivity for more energetically valuable resources in warmer environments to offset the disproportionate increase in metabolic demand relative to ingestion rate. This may also result in a change in feeding strategy or a shift towards a more specialist diet. This study used a natural warming experiment to investigate temperature effects on the feeding selectivity of three freshwater invertebrate grazers: the snail Radix balthica, the blackfly larva Simulium aureum, and the midgefly larva Eukiefferiella minor. Chesson’s Selectivity Index was used to compare the proportional abundance of diatom species in the guts of each invertebrate species with corresponding rock biofilms sampled from streams of different temperature. The snails became more selective in warmer streams, choosing high profile epilithic diatoms over other guilds and feeding on a lower diversity of diatom species. The blackfly larvae appeared to switch from active collector gathering of sessile high profile diatoms to more passive filter feeding of motile diatoms in warmer streams. No changes in selectivity were observed for the midgefly larvae, whose diet was representative of resource availability in the environment. These results suggest that key primary consumers in freshwater streams, which constitute a major portion of invertebrate biomass, can change their feeding behavior in warmer waters in a range of different ways. These patterns could potentially lead to fundamental changes in the flow of energy through freshwater food webs.

A simple model predicts how warming simplifies wild food webs

“…We documented 49,324 feeding interactions from 1,128 individual consumers collected from the Hengill streams using gut content analysis. We employed three different Immersion of macroinvertebrates in 62% nitric acid at 65 °C for 18 hours removes all organic matter except for silicate diatom frustules, enabling accurate identification of diatoms 36 , the major primary producers in the streams 13 . A 1 ml sub-sample of the resulting suspension of diatom frustules was pipetted onto a glass coverslip and allowed to dry before fixing to glass slides by adding a drop of naphrax on a 60 °C hotplate.…”

“…A 1 ml sub-sample of the resulting suspension of diatom frustules was pipetted onto a glass coverslip and allowed to dry before fixing to glass slides by adding a drop of naphrax on a 60 °C hotplate. We identified the first 100 diatoms (where possible) encountered in a continuous, non-overlapping 100 µm-wide transect following a fixed route across the slide, which was found to be sufficient for accurately characterising the species present on each slide 36 . Dissection of gut contents allowed us to quantify predation on other macroinvertebrates and feeding interactions with basal resources other than diatoms, i.e.…”

Thermal regime drives a latitudinal gradient in morphology and life history in a livebearing fish

“…Evidence for adaptive diversification along environmental gradients stems from many different taxa (e.g. plants: Abbott & Brennan, 2014;invertebrates: Diamond & Chick, 2018;Gordon et al, 2018;reptiles: Álvarez-Ruiz et al, 2018;birds and mammals: Weir & Schluter, 2007), but is particularly varied for fishes, which have, for example, been studied along elevational gradients (Rahel & Hubert, 1991;Olinger et al, 2016), salinity gradients (Mück & Heubel, 2018), temperature gradients (Ohlberger et al, 2008) and predation gradients (Walsh & Reznick, 2009). However, these studies are often constrained by focusing on a single environmental variable, when it is more likely that across the studied populations additional gradients exist that might select for similar or different responses compared to the focal gradient (Reznick et al, 2001;Riesch et al, 2015).…”

Long‐term exposure to higher temperature increases the thermal sensitivity of grazer metabolism and movement

“…Snail population densities are highest in the warmer streams in our study system (Nelson et al., ; O'Gorman et al., ), indicating they are capable of meeting their elevated energetic demands at higher temperatures, despite a lower standing stock of algal resources (O'Gorman et al., ). This may occur if snails select more energetically profitable resources, such as more readily accessible diatom patches (Gordon, Neto‐Cerejeira, Furey, O'Gorman, & Bierbach, ), or if they move faster and farther to find and consume resources (Shipley et al., ; Visser & Kiørboe, ). Faster snails that cover more ground in the warmer streams are likely to increase their encounters with, and thus consumption of, resource patches.…”