The effect of open drainage ditches on the plant and invertebrate communities of moorland and on the decomposition of peat.
A model of the function of open drainage ditches (grips) on peat moorland in northern England is described. The model makes the following predictions: (a) ditches will be ineffective on high altitude blanket bog, where rainfall is in excess of 1200 mm, but will function to some extent on lower moors; (b) where ditches run parallel to the contours and are functioning effectively, the water-table will be lowest immediately down-gradient from the ditch and highest just above the ditch. The predictions have been tested and confirmed in the field on high and low altitude moors by: (a) measuring the position of the water-table across a transect between two ditches; (b) measuring the rate of cellulose decomposition at different distances above and below ditches; (c) recording the vegetation across transects between ditches; (d) sampling the invertebrate fauna at a different distances above and below ditches. There has been little advantage in the extensive use of open drainage ditches in the uplands. The small changes that occur at lower altitudes appear to have contributed little to improve the moors for sheep or grouse. Any improvement in the nutritional value of Calluna vulgaris is offset by decline in cover and the spread of unpalatable grasses. Adverse effects arising from the loss of heterogeneity on the drained areas may well produce a net disadvantage for grouse populations.