Counting sheep in a changing world

Soay sheep, St Kilda

Soay sheep have been grazing on the remote Scottish archipelago of St Kilda for more than three thousand years

Studies of the wild sheep on St Kilda show that population size is increasing while the sheep themselves are getting smaller. To find out why, researchers are exploring the ecological links between the sheep and the vegetation on which they feed, and are asking how climate affects the system as a whole, and those relationships in particular.

Soay sheep, survivors of the earliest sheep to be domesticated in Europe, have been grazing on the remote Scottish archipelago of St Kilda for more than three thousand years. Nobody is quite sure how they got to the islands in the first place, but in 1932, two years after the last remaining people and livestock were evacuated, a small group was moved from Soay, the island after which they are named, on to neighbouring Hirta.

Free from predators

The sheep are one of the most intensively studied animal populations in the world. They interact with a complex mosaic of vegetation but are free from predation and interspecific competition. For over 25 years researchers from across the UK and further afield have collaborated to catch, measure, and sample DNA from over 8,500 animals. By tracking animals from birth to death we have obtained almost unprecedented detail about the lives of individual sheep.

Ecological and evolutionary change

The onset of plant growth has advanced and the season lengthened, meaning higher food availability at key junctures in the sheep breeding cycle

A strong signal to emerge is that things are changing on St Kilda. Population size fluctuates from year to year, but shows a clear upward trend, and the average size of sheep is decreasing. So what is driving these trends? Is it some form of environmental effect or could we be seeing evolutionary responses to natural selection?

Actually, disentangling these two mechanisms is not straightforward since ecological and evolutionary processes are not independent. For instance, while many traits are under selection, the strength of that selection and the amount of genetic variation on which it can act depends on environmental conditions. But while some traits may indeed be responding to selection, the weight of evidence so far suggests environmental change is more important.

What role does climate play?

St Kilda, Scotland

St Kilda has an oceanic climate, with extra rainfall caused by its elevation

Weather is a plausible driver of the changes we see in Soay sheep. St Kilda has an oceanic climate, with extra rainfall caused by its elevation. Gales are common, coming from all directions throughout the year. But to identify which specific variables drive the system’s dynamics we need to understand how local weather impacts the sheep.

Climatic variation certainly has direct impacts, increasing thermoregulation costs in winter months and imposing behavioural constraints (eg, limiting foraging activity). However, probably more important are indirect effects through plant quantity and quality.

Plant growth imposes constraints on all herbivores and life history tactics must be adjusted to fit seasonal variation. On Hirta, production exceeds offtake for about six months of the year making life relatively easy for the sheep.

Winter’s survival challenge

But from November to April times get progressively harder and starvation becomes a real threat. Life in the winter months is about survival, growth being limited to the period of food abundance.

Since 2005, plant productivity for the March to August period has been increasing significantly, presumably due to rising annual temperatures. The onset of plant growth has advanced and the season lengthened, meaning higher food availability at key junctures in the sheep breeding cycle.

The longer vegetation period means sheep should have a longer period for growth, but it also means increased overwinter survival – so potentially more animals competing for the food. It is still unclear what the net effect of this will be on population dynamics in the long-term.

Written by

  • , Department of Biological Science, Imperial College at Silwood Park
  • , Institute of Evolutionary Biology, University of Edinburgh
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