Historically, habitat loss has caused a huge decline in red squirrel populations in the UK. Woodland clearances in the 19th century for fuel, agriculture and industry resulted in a widespread loss of tree cover, leaving populations fragmented and vulnerable. Red squirrel fortunes then suffered another major blow with the introduction of grey squirrels and squirrel pox, and these remain the major threats today:
Grey squirrels were introduced to the UK in 1876 when a pair were released in Henbury Park, Cheshire. Further releases occurred over the next 50 years until it became illegal to release them. They quickly spread throughout the UK due to their ability to out-compete our native red squirrels, causing local red squirrel populations to go extinct at an alarming rate. There were once about 3.5 million red squirrels in the UK but today only around 161,000 remain. There are, however, 3.5 million greys.
How do grey squirrels cause reds to go extinct?
Grey squirrels are naturally found in oak and hickory forests in north east America, so they are particularly well adapted to deciduous woodland. Although red squirrels can also happily live in deciduous woods, greys have a competitive edge. They're able to break down polyphenols, natural defence compounds that are found in seeds such as acorns, earlier in the year than reds and extract more nutrients from them. Deciduous woods are commonplace in England, Wales and northern Ireland, and this is one reason why reds have disappeared from these areas. As a general rule of thumb, once greys arrive in an area, red squirrels will go extinct within 15 years.
Grey squirrels are able to live at much higher densities in deciduous and some mixed woodland than are reds. Greys will live at up to 9 squirrels per hectare, whereas reds are normally only found at densities of up to 1 per hectare. This enables greys to establish strong populations which are able to better withstand harsh conditions and times of low numbers than reds can.
An important food source for red squirrels in the winter is food stores that they've built up during the autumn. Grey squirrels will pilfer these caches which causes red squirrels to lose weight and have reduced breeding success the following spring. Juvenile survival is also reduced.
A major threat to red squirrels is squirrel pox (SqPV), a virus that is carried by grey squirrels and which doesn't affect them, but is fatal to reds. Red squirrels infected with squirrel pox develop lesions around the face and paws, and die within 2 weeks. It can spread through red squirrel populations at an incredible rate.
If you see a squirrel which appears to be infected with squirrel pox it is very important that you report it - please inform your local authority and if the squirrel is at a feeder in your garden, stop feeding immediately to reduce the risk of spread.
Adenovirus is another emerging threat, which causes lesions in the digestive system, intestinal bleeding and diarrhoea. Leprosy is present in some areas, e.g. Brownsea Island, but it is not known what the long-term impacts of this disease are on populations.
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