Vegetation responses to Soay sheep grazing on St Kilda

1. The population of Soay sheep on the island of Hirta in the Outer
Hebrides has been the subject of continuous study for more than 35 years.
This paper focuses on the botanical aspects of the plant-herbivore
interaction, showing how the vegetation affects and is affected by the
sheep. 2. Grazing impacts on biomass and spatial structure varied across
plant communities, with Holcus/Agrostis grasslands affected most and Wet
Heath least, consistent with the hypothesis that herbivore impacts are
proportional to plant productivity. 3. Within plant communities, the
negative relationships between sheep numbers and plant abundance (sward
height, gap/tussock cover and biomass) were significant in March but not
significant in August, as expected if sheep numbers are limited by food
supply in winter. 4. In most species, flower stem density declined with
increasing sheep numbers. There were no examples where unpalatable plants
showed increased flowering (e.g. from competitor release under selective
grazing). 5. Plant production in temporary grazing exclosures (above
ground net primary production) was greatest in Holcus/Agrostis grassland
(12.6t/ha dry matter/year), lower in Nardus grassland (5.1t/ha/yr) and
least in Plantago sward (1.3t/ha/yr) associated with differences in
historical nutrient supply and microclimate.   6. The net effect
of grazing on plant species richness was positive: a few highly palatable
species were excluded, but small-scale coexistence of grazing tolerant
species was enhanced by defoliation. 7. The Soay sheep population
fluctuated from 908 (in 1988) to 2208 (in 2009), increasing by an average
of 39 extra animals per year over the period 1985-2011. Between 2011 and
2020, the population fluctuated less widely and showed no trend. 8.
Population change (ln(N(t+1)/N(t)) was inversely density dependent but
positively correlated with plant production in Holcus/Agrostis grassland
which increased during the study. 9. This plant-herbivore interaction is
highly resilient, and though some species (Festuca rubra and Ranunculus
acris) declined in the Holcus/Agrostis grassland, there was no significant
upward trend in the abundance of unpalatable plant species. 10.
Implications for future studies and analyses of plant-herbivore data.
Concentrating on estimating primary productivity and herbivore offtake,
rather than simply measuring change in plant biomass, is likely to provide
greatly improved explanatory power for understanding herbivore population
dynamics.