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Habitats and SpeciesIntroduction
With a catchment area of some 5000 square kilometres the RiverTweed is one of the six largest rivers in the country. The combination of its size, relatively low population base and essentially rural character has meant that it has remained relatively unaffected by land use changes and other developments. Consequently the water chemistry and species composition is largely intact and close to a natural state.
The Tweed Habitat Transition
The Tweed shows a transition through three distinct habitat types:-
- the fast-flowing, nutrient poor upper reaches above Peebles
- the more moderately flowing, somewhat richer in nutrients middle sections between Peebles and Kelso.
- the deep, slow flowing, more nutrient rich stretches found below the confluence of the Teviot Water and River Tweed at Kelso.
The Upper Reaches - Tweedsmuir to Peebles
The upper stretches of the Tweed and its tributaries drain rocks with a relatively low content of base minerals, mainly Ordovician and Silurian greywackes and shales. The catchment at this stage is characterised by low-intensity extensive land uses dominated by sheep, heather moorland and significant coniferous forestry. Only in small areas of land adjacent to the river, are soils of sufficient depth and fertility to support arable crops. From little more than an upland stream the Tweed rapidly gains in size but retains the characteristics of an upland stream of predominantly fast current and shallow depth. Consequently these physical factors are reflected in the types of vegetation that it can support. In the upper reaches, Mosses and Liverworts are the dominant plant types both within the channel and growing on rocks, and also the adjacent bank sides. Over 90% of the Moss species occurring in the river are restricted to this section. Larger plants also occur but are scarce as most species, with the exception of Water Crowfoot (Ranunculus species), are confined to areas of slower water downstream. The number of groups of invertebrate species is restricted compared with lower sections of the river with Mayflies and Stoneflies amongst the most abundant.
The Middle Reaches - Peebles to Kelso
The river by this stage has grown in stature. The underlying geology is subject to change with a transition from the base poor Ordovician and Silurian shales to richer, less acid sandstones and volcanic rocks below St Boswells. This shift in geology along with more intensive pastoral land use results in comparatively richer water chemistry within the river, although forestry remains a significant land use in the upper part of this section. Also the river becomes generally deeper and its rate of flow has reduced considerably from the upper stretches. This is reflected in the plant and animal communities, which are dominant in this stretch of the river. Emergent vegetation on the river banks is now more apparent, characteristic species include Reed Canary Grass and Bur-reed. Downstream from Peebles larger aquatic plants such as Pondweeds, Milfoil and Water Crowfoot which are rooted in the river bed become more abundant. The alien Canadian Pondweed is also found. The invertebrate fauna is increasingly diverse with Leeches, Molluscs, Aquatic Beetles and Shrimps becoming more significant. Stoneflies and Mayflies are still plentiful whilst Blackflies and Caseless Caddis flies are also relatively abundant.
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The Lower Reaches - Kelso to Berwick
Below Kelso, following its confluence with the Teviot, the river becomes much deeper and slower flowing. Consequently the bottom of the river changes from a predominantly gravelly substrate, to one including more silty material typical of a depositing stretch within a river. The Tweed now lies in the bottom of a broad low-lying intensively farmed river valley. Consequently, this type of land use adds to the already naturally high nutrient status of the river within this section. Larger plants are found in a greater variety but not necessarily in such abundance as upstream. This may be due to the limiting effect of light penetration. Typical plant species including those associated with slow-flowing, nutrient rich waters eg Shining Pondweed and Spiked Water Milfoil although Water Crowfoot is still plentiful. Invertebrates such as Mayfly and Stonefly species become less dominant in this stretch of the Tweed with Snails, Aquatic Beetles, Flatworms and Shrimps becoming more abundant. As the river reaches the sea, certain species of invertebrates characteristic of estuarine and brackish water become apparent.
The Tweed Estuary is of national importance for birds especially the flock of mute swans which, at around 600 strong, represent about 4% of the British population. The estuary is also important for Goldeneye with some 200-300 present during the winter months. A variety of other ducks and wader species occur in smaller numbers. There is a large Gull roost at Yarrow Slake.
Widespread Species
Some wildlife species including invertebrates are not constrained by the overall physical and chemical changes in the river between source and mouth and are widespread or found throughout the Tweed system. A good example is the otter, which is one of the most important of these species. Otters enjoy statutory protection and appear to be spread throughout the Tweed system. Given the apparent suitable habitat, the overall population density on some sections of the river is lower than that which might be expected. Nonetheless the Tweed remains an important site for otters in a UK context. The river is also home to a diverse range of breeding and overwintering birds. Characteristic waterside birds include Dipper, Grey Wagtail, Common Sandpiper, Oystercatcher, Black-headed Gull and Mallard. At certain times of the year the Tweed can support nationally important populations of Goosander and Goldeneye.
The Riparian Strip
The riparian strip is the zone that immediately adjoins the river and is influenced by it. It is of considerable importance as one of the major sources of energy for the river's organisms. This energy is in the form of organic debris of plant or animal remains washed or fallen into the stream. The riparian corridor, besides providing this hugely important energy source, is also often the only fragment of semi-natural vegetation in a highly modified landscape.
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