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Three Decades of Satellite and Field Data Showed Greener Red Deer Forage on Rum

Wild red deer stag in Glen Etive, Scotland
Research · Ecology

Three decades of satellite data and field measurements show a long-term increase in peak vegetation greenness on the Isle of Rum in Scotland, where wild red deer (Cervus elaphus) have been studied for more than half a century. The researchers also found that a commonly used satellite vegetation index tracked living plant biomass in one of the grassland habitats selectively grazed by the deer.

The study assembled Landsat imagery from 1991 to 2023 at 30-metre resolution and calculated annual maximum Normalised Difference Vegetation Index, or NDVI, across the long-term deer study area. NDVI uses the way vegetation reflects red and near-infrared light to measure greenness; higher values generally indicate greener, more actively growing vegetation. To test whether this satellite measure reflected food available on the ground, the researchers compared it with vegetation biomass collected directly from calcareous grassland, a habitat preferred by the deer, and with an independent MODIS satellite vegetation index.

Satellite greenness tracked live biomass in preferred grassland

Landsat NDVI increased with the amount of live vegetation biomass measured in June and July. The relationship was modest: even relatively large increases in live biomass corresponded to much smaller increases in annual maximum NDVI. Landsat NDVI also agreed with the independent MODIS measure, supporting its use as an indicator of annual peak vegetation greenness and forage availability in this system.

Peak vegetation greenness increased over three decades

All three measures – Landsat NDVI, MODIS vegetation greenness and field-measured live biomass – increased over the long term. The Landsat data showed that the rate of greening differed among vegetation types, with some grasslands used by deer among the habitats showing marked increases. The authors conclude that annual peak vegetation greenness has increased on Rum and that this suggests improving forage conditions for the deer over time.

The direct comparison between satellite greenness and biomass was restricted to calcareous grassland, so the authors caution that the same NDVI–biomass relationship should not automatically be assumed for every vegetation type on the island. They nevertheless identify the long field record as an unusually strong test of whether remotely sensed greenness corresponds to biologically meaningful changes in a key red deer feeding habitat.

Sources and image credit

  1. Butt, S., Macphie, K., Turner, R. S., Morrison, S. J., Morris, A., Pakeman, R. J., Kruuk, L. E. B., Pemberton, J. M., & Froy, H. (2026). Satellite-derived NDVI predicts forage availability in a wild ungulate system: Ground-truthing using field-collected vegetation biomass. Ecology and Evolution, 16(3), e73258. https://doi.org/10.1002/ece3.73258Primary peer-reviewed open-access article first published 12 March 2026 under CC BY 4.0. The study combined Landsat imagery from 1991–2023 with long-term field measurements of live vegetation biomass on the Isle of Rum and found a positive NDVI–biomass relationship in calcareous grassland together with long-term increases in Landsat NDVI, MODIS EVI and field-measured live biomass.

Image: Andrewmckie (Andrew McKie) — wild red deer stag (Cervus elaphus) in Glen Etive, Scotland, photographed 20 January 2019; photographer's own work; Wikimedia Commons; CC BY-SA 4.0; representative Scottish species image, not a deer from the Isle of Rum study · CC BY-SA 4.0 · Source

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