Trophic cascades in the bryosphere: the impact of global change factors on top‐down control of cyanobacterial N₂‐fixation

Abstract

Trophic cascades in which predators regulate densities of organisms at lower trophic levels are important drivers of population dynamics, but effects of trophic cascades on ecosystem-level fluxes and processes, and the conditions under which top-down control is important, remain unresolved. We manipulated the structure of a food web in boreal feather mosses and found that moss-inhabiting microfauna exerted top-down control of N2 -fixation by moss-associated cyanobacteria. However, the presence of higher trophic levels alleviated this top-down control, likely through feeding on bacterivorous microfauna. These effects of food-web structure on cyanobacterial N2 -fixation were dependent on global change factors and strongly suppressed under N fertilisation. Our findings illustrate how food web interactions and trophic cascades can regulate N cycling in boreal ecosystems, where carbon uptake is generally strongly N-limited, and shifting trophic control of N cycling under global change is therefore likely to impact ecosystem functioning.

Novel bacterial lineages associated with boreal moss species

“…) do, in fact, appear to associate with bacteria capable of N 2 ‐fixation at rates within the range of feather mosses, which are well‐known to host bacteria capable of sustaining relatively high rates of N 2 ‐fixation (DeLuca et al ., ; Turetsky et al ., ). Dicranum , for example, had an average fixation rate of 10.7 µg N • g dry weight moss −1 day −1 which is comparable to rates documented previously for feather mosses (Kardol et al ., ; Warshan et al ., ). Consistent with other studies, Pleurozium schreberi and Sanionia uncinata had the highest average rates of N 2 ‐fixation at 46.1 and 52.4 µg N • g dry weight moss −1 day −1 respectively.…”

Priority effects are interactively regulated by top‐down and bottom‐up forces: evidence from wood decomposer communities

“…The exact mechanisms remain uncertain, but increased nitrogen availability may have altered the effects of fungivores by altering the composition of the fungal community that the Collembola were interacting with (Kardol et al . ; Morrison et al . ); by facilitating defensive responses to fungivore grazing, such as the production of nitrogen‐rich secondary metabolites (Crowther et al .…”

Soil fertility shapes belowground food webs across a regional climate gradient

“…; Kardol et al . ), especially during stages of ecosystem development where soil fertility is high, that is in the absence of bottom‐up control (Crowther et al . ).…”