Impact Type: Deposition of pollutant
Acidification affects birds primarily through their trophic dependence on prey. The dipper is largely dependent for its food on aquatic species such as mayfly, caddis larvae and fish which it finds in small, relatively fast flowing rivers. In habitats where Dipper territory coincides with freshwater acidification, there is a potential for adverse effects (Ormerod et al 1986, Ormerod 1991, Reynolds and Ormerod, 1991). These include:
- Reduced population numbers and breeding success.
- Deterioration in habitat quality.
- Delayed laying.
- Reduced clutch size, egg mass, brood size and nestling growth.
- Reduced mass and condition in adults.
- Reduced food sources.
These observations have been linked to decline in available prey and calcium intake in the diet of affected birds (Reynolds and Ormerod 1991). Later research by Ormerod & Rundle (1998) provided no support for effects of upland acidification on calcium availability to insectivorous terrestrial vertebrates. However, from the patterns found in eastern Scotland and elsewhere, it appears that the effects of acidity on Dipper populations are important generally, during both the breeding and non-breeding seasons (Logie 1995).
Dipper habitats have been studied in Wales and also in Scotland. It would be difficult to set a critical load for this habitat as the dose response linkage is between acid inputs, water chemistry and the abundance of invertebrates. However, work by Logie et al. (1996) suggests that critical load models may be generally applicable. Dipper populations can fluctuate greatly; studies of the population on the River Esk (Lothian, Scotland) have shown fluctuations of up to six fold over the last 60 years (Wilson 1996).
|Critical Load/ Level|
No estimate available