Lead in the ecology of roadside verges.

The pattern of the deposition of lead (Pb) from motor exhausts on roadside vegetation from nine verges in rural Leicestershire is described. This is shown to be a function of proximity to traffic, reactions of airborne Pb with time and predominant wind characteristics. Splash from vehicles enhances Pb deposition next to the road. Most Pb is bound by grass from solution. Live and dead grass are shown to have markedly different capacities to bind Pb, and this is attributed to the condition of the cuticle and the ability of Pb-rich solutions to penetrate intercellular spaces. The Pb content of both states of grass are shown to undergo seasonal changes, with a winter peak and summer low. This is a function of precipitation and standing time of the grass. A mean estimate of 5.2 g of Pb/m2/year is added to the soil on a site with 7000vehicles/16h day, m from the road. Input increases with precipitation and splash. The invertebrate community of six verges were sampled with pitfall traps and Tullgren extractions, and their diversities compared. No decline in diversity could be attributed to Pb, though a greater diversity was apparent at a site which had been left unmown. Snails and Sminthurid collembolans may avoid areas of high Pb contamination, though woodlice numbers were low at all sites. The sexes of the woodlouse Porcellio scaber are shown to differ in their Pb and Ca content, and an interaction in the uptake of the two metals is shown in both P. scaber and juveniles of the snail Helix aspersa. It is suggested that the digestive diverticula may be the main site of Pb and Ca absorption in both species. Toxicological experiments dosing P. scaber with sub-lethal levels of Pb could show no effect on the rate of oxygen consumption or fecundity. No increase in mortality in either P. scaber or H. aspersa could be attributed to a high Pb diet. The distribution of Pb in the tissues of adult H. aspersa is found to correspond to that of marine molluscs. Few effects on the biotic components of roadside verges have been found, though it is suggested that Pb-tolerant plants may increasingly dominate the vegetation.