The effects of pesticide exclusion strips on faunal populations in Great Britain.
In lowland Britain, the grey partridge Perdix perdix has suffered a severe decline in numbers over the past thirty years. Spring pair densities have been surveyed by the Game Conservancy since 1933 and the national decline since 1952 compared with the mid 1980s has been about 80%. Similar declines have been observed in North America and Eastern Europe (Potts 1986).
The reason for this decline in Britain is considered to be the increased chick mortality rates observed during this decline phase (Potts 1986). For the first two to three weeks of life, grey partridge chicks rely almost entirely on insects before switching to a plant diet at about three weeks of age (Green 1984). Decreased chick survival rates are considered to be caused by the lack of sufficient insect food in the diet of chicks during these crucial early weeks (Potts 1986).
In lowland Britain, broods of grey partridges forage on arable land that has become more and more intensively farmed since the 1950s. Intensification and the associated increased use of pesticides during this time has been shown to be responsible for some of these observed increases in chick mortality. Indeed, many groups of non-target insects, especially those known to be preferred food items of partridge chicks (Green 1984), have been shown to have declined since the late 1960s from data accumulated in the Game Conservancy's Sussex Monitoring Data Bank (Potts 1986). The pesticides currently used on cereals are not directly toxic to grey partridge chicks. Their effects on partridge populations are via a removal of preferred insect food. Many of these chick food items have been shown to be reduced directly by the use of broad-spectrum compounds, especially insecticides (Vickerman and Sunderland 1977) but also fungicides (Vickerman 1977, Vickerman and Sotherton 1983) and herbicides (Sotherton 1982). Many of these preferred cereal dwelling arthropods are weed feeders. Therefore, their numbers can also be reduced by the removal of their host plants by herbicide use (Southwood and Cross, 1969, Vickerman 1974, Sotherton 1982).