Between 1990 and 2021, there was a notable decrease in the utilisation of plant-based "green" food webs by small mammals in boreal forests, and a rise in the use of microbially mediated "brown" food webs, interspersed with a rise in fungal carbon absorption of over 30%.

Climate warming is altering food webs and carbon flow in high-latitude areas: Study

A new study finds clear evidence that climate warming is altering carbon flow and food-web dynamics among above-ground consumers in Arctic tundra and boreal forest ecosystems

Ecosystems in high latitudes are warming at unprecedented rates, which has broad implications for biological processes. Thawing permafrost and the release of long-sequestered carbon from northern soils are significant concerns as that can lead to carbon-cycle feedbacks, thereby accelerating rising temperatures.

About 25% of Earth’s surface is covered in permafrost, which stores almost twice as much carbon as the atmosphere does now. However, as a result of increased microbial breakdown of this previously frozen soil organic matter, greenhouse gas emissions have increased. While we know that warming climate and extreme climate events are a big threat to food security, a recent study looked at the impact of climate-mediated permafrost decay on high-latitude terrestrial food webs in Arctic tundra and boreal forest.

Although terrestrial food webs have historically been categorised as either above-ground ‘green’ webs supported by plants or below-ground ‘brown’ webs fuelled by detritus, the study said that green and brown food webs are interlinked and that microbially mediated energy supports higher-order consumers in both above- and below-ground systems.

Increased microbial activity, changing food webs 

Researchers used stable isotope analysis of amino acids to trace carbon flow through food webs exposed to warming and quantifed changes in the assimilation of microbial carbon by Arctic tundra and boreal forest consumers.

They found that rapid warming of high-latitude ecosystems is increasing microbial activity and accelerating the decomposition of permafrost soils. Between 1990 and 2021, there was a notable decrease in the utilisation of plant-based “green” food webs by small mammals in boreal forests, and a rise in the use of microbially mediated “brown” food webs, interspersed with a rise in fungal carbon absorption of over 30%.

Similarly, the study said, fungal carbon assimilation rose 27% in wolf spiders under experimental warming in Arctic tundra. These findings revealed a climate-mediated ‘browning’ of high-latitude food webs and pointed to an understudied pathway by which animals can impact carbon cycling under climate warming.

The study found that climate-mediated permafrost decay and the rapid proliferation of microbial energy channels in below-ground ecosystems may subsidise above-ground consumers and restructure high-latitude food webs, with potential consequences for ecosystem processes such as nutrient cycling.

Carbon assimilation above and below the ground

The coupling of green and brown food webs by animal consumers represents a direct channel between above- and below-ground carbon pools that can alter nutrient cycling. Animals play important but often underappreciated roles in carbon cycling through numerous direct and indirect processes, including herbivory, predation and waste production.

The researchers detected widespread changes in carbon assimilation by small mammals exposed to long-term warming in a boreal forest, highlighted by a significant shift from plant- to fungal-based food webs. The observed food-web shifts and functional responses were consistent across species, ecosystems, and long- and short-term warming scenarios, added the study. 

The study found clear evidence that climate warming is altering carbon flow and food-web dynamics among above-ground consumers in Arctic tundra and boreal forest ecosystems, and also showed that these changes are the consequence of a state change from predominantly green, plant-based food webs to brown, microbe-based food webs. 

Fungi are primary decomposers of plant biomass and soil organic matter in both boreal forests and recently thawed permafrost, where they transform recalcitrant biomass (for example, cellulose and lignin) into labile biomolecules such as amino acids that are more readily assimilated by both plants and animals. This fungal energy channel represents a substantial portion of the assimilable energy in terrestrial ecosystems.

According to the study, an increased use of fungal energy channels by above-ground consumers drove the food-web shifts in our Arctic tundra and boreal forest ecosystems. This reliance on fungal energy is consistent with other studies showing that fungi regulate the coupling of green and brown food webs in boreal forest and Arctic tundra ecosystems.
This study illustrated that animals link green and brown food webs and showed that climate warming consistently alters the coupling of these energy channels across species and ecosystems.