In ecological terms, the Insh Marshes are a floodplain fen of the poor fen type (acidity pH 5 or less). Fen is a rare habitat in the Cairngorms National Park, with otherwise only a small area at Dinnet, on the Dee floodplain. The Insh Marshes are the largest continuous area of poor fen in the UK. They cover over 300 ha, although the complete wetland complex of which it is part covers almost 600 ha.
The Insh Marshes are an important site not only because of their size, but also because they are relatively natural and unmodified. Such fen systems were once much more common but are now rare. Although the largest area of its type, the Insh Marshes are small (about one-tenth) compared with the calcareous rich fen of the Norfolk Broads in East Anglia.
The main factors influencing the aquatic plant communities of the Insh Marshes and succession are:
- hydrology of the adjacent River Spey and its floodplain – the frequency and extent of flooding.
- flow and depth of water
- substrate type – mud, gravel, stones
- water chemistry – nutrient rich or poor.
Generally, the Insh Marshes have a diverse flora and fauna. Previous attempts to drain and build flood-control banks have probably increased the diversity of natural habitats, and therefore of associated species.
The plants found are significant in that they do not exist elsewhere in the UK, owing to the altitude. The Insh Marshes are the most important site for string sedge and pillwort.
The Marshes contain a number of breeding birds, including goldeneye, wigeon, teal, snipe, redshank, curlew, water rail, sedge warbler and reed bunting. The spotted crake and wood sandpiper are rare nationally, and the numbers breeding at Insh Marshes constitute a significant proportion of the British population. Otters and osprey are common sights. A number of listed invertebrates occur, but the only endemic is the reed beetle found in the fen habitat.