Page 153 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

The Mystery
of St. Kilda’s Shrinking Sheep
On the remote, windswept island of Hirta in Scotland’s Outer Hebrides, something odd is happening to the animals. The wild Soay sheep have shrunk in body size over the past quarter century, a trend that puzzled scientists until recently.
The Islands of St. Kilda Some 180 km west of the Scottish mainland, the St. kilda archipelago comprises sev- eral islands centred at 57.75° n latitude (Figure gn 5.1). These islands have a mild climate with moderate summer and winter temperatures, rain all year long (an average of 1400 mm), and snow occurring only rarely. The mild climate at this latitude is caused in part by the gulf Stream, a warm ocean current that flows in a clockwise direction around the north atlantic to northern europe (see Figure 5.8).
St. kilda’s principal island, Hirta, was continuously occupied for over 3000 years, with people living in stone shelters with sod roofs, and later in stone-walled houses. Domestic sheep were introduced to these islands about 2000 years ago and evolved in isolation as a small, primitive breed now known as Soay sheep. in 1930, the last residents left Hirta as their small society failed. The Soay sheep remained, unmanaged to this day (Figure gn 5.2).
The Problem of the Shrinking Sheep
Hirta’s sheep provide scientists an ideal opportunity to study an isolated popula- tion with no significant competitors or predators. according to evolutionary the- ory, wild sheep should gradually increase in size over many generations because larger, stronger sheep are more likely to survive winter and reproduce in the spring. This expected trend follows the principle of natural selection. Scientists began stud- ying Hirta’s Soay sheep in 1985. Surpris- ingly, they found that these sheep are not getting larger in size but instead are fol- lowing the opposite trend: The sheep are getting smaller . . . but why?
The answer, apparently, relates to tem- perature. recent temperature shifts have caused summers on Hirta to become longer, with spring arriving a few weeks earlier and fall a few weeks later. Winters have become milder and shorter in length.
Research Provides an Answer in a 2009 publication, scientists reported, first, that female sheep are giving birth at younger ages, when they can only pro- duce offspring that are smaller than they themselves were at birth. This “young mum effect” explains why sheep size is not increasing. But why are sheep shrink- ing? Since 1985, female Soay sheep of all ages dropped 5% in
ATLANTIC OCEAN
body mass, with leg
length and body
weight decreasing.
in addition, lambs
are not growing as
quickly. The expla-
nation ties to rising
temperatures: longer
summers increase the
availability of grass
feed, and milder win-
ters mean that lambs
do not need to put
on as much weight
in their first months,
allowing even slower-
growing individuals
to survive. With smaller lambs surviving winter and reaching breeding age, smaller individuals are becoming more common in the population.
This study indicates that local factors, such as temperature, are significant in the interaction between a species’ genetic makeup and its environment. Such fac- tors, even over a short time period, can
Boreray Soay St Kilda
UNITED KINGDOM
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6°W
Chapter 5 g now
0
50
100 KILOMETRES
▲Figure GN 5.1 Hirta island, part of the St. Kilda Archipelago, Scotland. [Colin Wilson/alamy photo.]
▲Figure GN 5.2 Wild Soay sheep on Hirta Island. [Bobbé Christopherson.]
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58°N
56°N
Hirta Dun
Stac Levenish
St Kilda
Hebrides
8°W
override the pressures of natural selection and evolution. in this chapter, we discuss temperature concepts, examine global temperature patterns, and explore some of the effects of temperature change on earth systems. animal body size is only one of many changes now occurring as a result of climate change.