5 Disappearing ecosystems: the example of the sub-Arctic willow

You have probably noted that the UK is like a bridge between species associated with mainland Europe and species you might associate with cooler northern climes.

Have a look at the map of world biomes.

The UK is classified as temperate forest. However, it is clear from our earlier discussions that northern and upland areas in the UK (in particular in Scotland) have a great deal in common with Scandinavian countries. Many of the species we share with northern European countries exist at fairly high altitudes in Scotland.

Sub-Arctic willow scrub is an assemblage of species found above the tree line at high altitudes. It contains a mix of Salix (willow) species, some normally associated with Arctic, sub-Arctic, or alpine areas and others with a northern distribution. In the UK its distribution is limited to a few high mountainous areas in the Scottish Highlands. In the UK it is a relic of the last glaciation; the habitat also exists in mountainous areas in Sweden and Finland.

The Scottish Crop Research Unit has published a report entitled ‘Population genetics of sub-Arctic willow’. What we learn from this report is that the willow species within this particular community can reproduce in two different ways: asexually (simply spreading) and sexually through the dispersal of viable seed. One of the issues for these communities is that the chances of sexual reproduction are limited because the plants are either male or female; many of the habitats contain very few genetically different plants, and some only one. This, coupled with the low level of pollen and seed dispersal, leads to populations with low genetic diversity.

This is a problem, because genetic diversity is important in relation to resistance and resilience. If a species within a particular ecosystem is genetically diverse, then the likelihood that there are individuals who are adapted to slightly different environmental conditions is higher.

These plant communities and the numerous invertebrates that depend on them are vulnerable to environmental change because their genetic diversity appears to be decreasing. They occupy habitats that are at the very southern edge of their range; those habitats are highly fragmented. These plant communities are essentially stuck on islands, with the lower and warmer areas between these isolated pockets acting as a barrier to dispersal and migration.

You can learn more about the research into these plant communities at:

• the Joint Nature Conservation Committee website, where you can find more information about the distribution and exact composition of this plant community

• the UK Biodiversity Action Plan website, which has information on wooly willow (Salix lanata)

• the ARKive website, which has more general information.