Editor’s Choice: Volume 109 Issue 12

The editor’s choice for our December issue is “The dynamics of vegetation grazed by a food-limited population of Soay sheep on St Kilda” by Crawley et al. This research article provides an unprecedented record of vegetation change in response to and impacts on an unmanaged ungulate population on a temperate island over 35 years.

Here, Associate Editor Kyle Tomlinson explains the importance of this research, which was selected as this month’s Editor’s Choice article!

Studies of the dynamics of herbivore-vegetation interactions over long time scales are rare, though these are necessary to really understand these interactions in detail. The paper by Crawley et al. presents the results of over 35 years of cumulative research into the dynamics of plant-sheep interactions on Hirta Island in the Outer Hebrides in the UK, including 25 years of annually quantified plant species abundances (taken in summer and winter) and sheep abundances. In addition to the questions tackled by the authors, the quantity and quality of the underlying dataset will make it useful to test other theories of plant-herbivore interactions.

Figure 1: (Left panel) View of the research site on Hirta Island, Outer Hebrides, the United Kingdom. (Right panel) Detail of the close-cropped Holcus/Agrostis grazing lawns, with heath on the slopes behind, and a temporary grazing exclosure in the foreground.

The vegetation of Hirta varies from heavily grazed lawns to lightly grazed heathlands, providing an opportunity to study long-term impacts of differential grazing across these community types with an unmanaged herbivore population constrained only by the limits of the island and interannual climate variability. The paper focusses on year-to-year and seasonal changes in the impacts of grazing by the sheep on the native vegetation, in terms of several measures of structure (including biomass, sward height, gap and tussock structure, and flower number), species richness, and above-ground net primary production. The sheep population showed an increasing trend up to 2011, but with interannual fluctuations as it over- then undershot available forage resources. After 2011, the population flattened off, but over- and undershooting has persisted. Differential feeding rates across vegetation types caused flowering suppression on the most heavily utilized species, and there were differences in intra-seasonal impacts of grazing on plant structure, but further negative trends in vegetation structure over time were for the most part absent. The authors found no evidence of shifts in vegetation composition towards more unpalatable species in the heavily utilized swards over time, and no decline in plant or animal productivity that would indicate system degradation due to overgrazing. Indeed, the heavily grazed Holcus/Agrostis grassland lawns increased productivity during the study, probably reflecting nutrient inputs by the sheep.

This study provides clear evidence that temperate grass-grazer systems can shift to higher productivity when trending to ecological carrying capacity, likely through the expansion of highly productive grazing lawns and seasonal movements that access different vegetation elements. The implications are that overgrazing and system collapse caused by human-managed systems may be caused by supplementary feeding that sustains animal numbers beyond the carrying capacity of the system, or by constraining seasonal movements of animals attempting to access different forage resources.  

Figure 2: (Left Panel) Interannual variation in the sheep population on Hirta Island over 35 years and the long-term trend. (Right Panel) trends in annual productivity in temporary grazing exclosures of different vegetation types that are differentially grazed by the sheep over 20 years. Black – heavily utilized Holcus/Agrostis grassland; Red – less utilized Nardus grassland; Green –heavily utilized Plantago meadow.

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