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Top-Down or Bottom-Up? Herbivory, Hysteresis, and the Control of Arctic Shrubification
Arctic shrub expansion is widely interpreted as a bottom-up (abiotic-constrained) response to warming, yet large herbivores can exert strong top-down (consumer-regulated) controls on vegetation structure and Earth-system feedbacks. Through grazing, browsing, and trampling, Arctic herbivores promote graminoid-dominated communities with higher surface albedo, compact winter snow that enhances soil cooling, accelerated nutrient cycling via herbivory/egestion pathways, and rooting architectures that can enhance near-surface soil carbon storage. We synthesise evidence from a recent meta-analysis of herbivore exclusion experiments to assess the climatic relevance of these mechanisms and the viability of Arctic rewilding as a Nature-based Climate Solution.
Across studies, vegetation responses to herbivore removal are context-dependent and unevenly sampled across Arctic climates, with limited inference for colder, drier, and permafrost-dominated regions. Overall, herbivore exclusion increases shrub height and biomass and, in some systems, lichen abundance, with effect sizes strongest in warmer and wetter subarctic environments. Effect sizes also increase with exclosure duration and are sensitive to exclosure size, highlighting both time-lagged responses and potential experimental artefacts.
More fundamentally, tundra-herbivory interactions may exhibit hysteresis and alternative stable states, such that vegetation responses to herbivore exclusion (the only used experimental setup in studying herbivore/plant dynamics) do not necessarily predict responses to herbivore (re)introduction. Under such dynamics, exclosure experiments may only capture reversible within-state variation, rather than transitions between bottom-up and top-down controlled regimes. Discriminating among these possibilities requires large-scale, long-duration enclosure experiments that minimise fence artefacts and explicitly quantify abiotic feedbacks using remote sensing and in situ measurements, as well as emerging causal-inference frameworks that leverage observational, non-experimental data.
Date:
26 January 2026, 12:00
Venue:
Venue to be announced
Speaker:
Dr Marc Macias-Fauria (University of Oxford)
Organising department:
Department of Biology
Organiser contact email address:
ec@biology.ox.ac.uk
Host:
Claudio Sillero-Zubiri (University of Oxford)
Booking required?:
Not required
Booking email:
eb@biology.ox.ac.uk
Audience:
Members of the University only
Editor:
Aisha Campbell
