THE errant ways of humans have the most unexpected effects on ecosystems. Ecologists J Graveland and his colleagues at the Netherlands Institute of Ecology say that air pollution and the subsequent acid rain have had enormous impact on the reproduction of a common European bird -- the great tit (Nature, Vol 368, No 6470).

The scientists observed that the great tits (Parus major) in the Buunderkamp forest in The Netherlands, have been increasingly laying defective eggs. The proportion of the birds laying defective eggs increased from 10 per cent in 1983-84 to as much as 40 per cent by 1987-88. The defective shells were thin and porous, and had a rough surface. The scientists observed that these eggs failed to hatch because they tended either to dry up or break. And, interestingly, almost half the female great tits with damaged eggs deserted their nests.

Because all defects were linked to the shells of the birds' eggs, calcium deficiency appeared a likely cause. And sure enough, scientists found that most defective eggs were found in forest regions with poor soils, deficient in calcium. The calcium needs of female great tits are predominantly met by feeding on snails, whose shells have a high calcium content. But the scientists found that in forests with poor soils, the number of snails was meagre.

The scientists found that on poor soils the snail populations have decreased substantially over the years, whereas an attendant decrease in snail numbers on calcium-rich soils was not observed. A similar decline was also observed on poor soils in Sweden.

Snails require calcium obtained by eating soil and rock in addition to their regular food, and by absorption through their skin, to build their shells. Acid deposition, the scientists explain, has previously been implicated in a reduction in soil calcium -- through leaching -- in poor soils. This calcium deficiency, passed on through the snails, has an impact on the birds which are the next link in the food chain.

The scientists found that the calcium content of great tit food in forests on rich soils -- where the birds' eggs were usually normal -- was 50 per cent more than that of great tits living in poor soil regions. And on feeding caged birds snail shells and chicken egg shells, they found that there was a reduction in the number of empty nests, unhatched eggs and nest desertions.