Decline of birds of prey, populations seriously depleted:
- Peregrine Falcon in UK with all known territories deserted by 1992
- Europe: Golden Eagle and Sparrow Hawk
- USA: Bald Eagle, Osprey, Cooper’s Hawk
Decline due to bird’s eggs fragility, resulting in many not being viable: Ratcliffe in 1967, demonstrated that shell egg thickness had declined sharply since the 1940s by observing egg collections, but tracking the source of the problem proved difficult.
It was noticed that the level of decline in Sparrow Hawk was relative to the intensity of the agriculture and the use pesticide, but the evidence was only circumstancial. Pesticides toxic impact had been previously denounced in Rachel Carson’s book, “Silent Spring” in 1963.
Three main groups of chlorinated hydrocarbons were thought responsible of the decline:
- Polychlorinated Byphenols (PCBs): manufactured from the 1920s by Monsanto for electrical insulation, heat transfer fluids, hydraulic systems or in plastics. When plastics are burnt PCBs are released in the atmosphere.
- Organochlorine pesticides (OCs): DDT (Dichloro-diphenyl-trichloroethane) from 1945 which was extremely performant against Malaria, now banned in most countries but still used in India and parts of Africa.
Other OCs are lindane and chlordane which were pesticides to control pests from cabbages (brassicas), top fruits and cereals.
- Cyclodiene pesticides (drins) from 1950 to control soil pests (cabbage root fly in brassicas, wireworms in potatoes). Example: Dieldrin, Aldrin, Heptachlor.
At first Organochlorine Pesticides (OCs) thought not responsible as were shown to be non-toxic to vertebrates: tests on humans showed no signs of toxicity, however they were administered a very small dose during trial (0.5 mg kg⁻¹) when toxic effects in humans are noted at 10 mg kg¯¹. Based on these tests the link between bird decline and OCs wasn’t established at first.
It was then established that three properties of OCs were a significant risk to birds:
- The breakdown products of OCs are very persistent
DDT (Dichloro-diphenyl-trichloroethane) metabolic pathway: breaks down to DDD (Dichloro-diphenyl-dichloroethane) then finally to DDE (Dichloro-diphenyl-ethane). Thus finding the metabolite DDE proves DDT is involved.
- Organochlorine pesticides are hydrophobic, so insoluble in water but highly soluble in lipids where they bioaccumulate. This can be shown with a liver biopsy.
- Organochlorine pesticides concentrations are magnified up the food chain: biomagnification thus higher animals have much higher concentrations of DDE.
However, even though OCs were linked to eggshell thinning of birds of prey, the cause was not yet determined. A study involving Ring Doves was conducted where 12 pairs of doves were fed 40ppm of DDE for 126 days, and the results measured against a set of controls again of 12 pairs of doves. It was observed that the tested pairs nested fewer times with longer periods in-between. They also laid less eggs which had thinner eggshells and a significantly higher mortality rate of the young.
The experiment established that the organochlorine pesticides affected the reproduction, and the cause was found when it was discovered in another unrelated experiment that chlordane, an OC, could induce the activation of the cytochrome P450 enzyme. Indeed, P450 can metabolise the natural chemical oestrogen, a hormone that controls both reproduction and calcium production. Ocs are therefore endocrine disruptors (= disrupt hormone balance).
So birds reproducing less and as need a lot of calcium for their eggs were greatly affected.
Dieldrin, PCBs and DDT stimulate P450 (so the three main groups of chlorinated hydrocarbons) P450 was then switched on and stated metabolising oestrogen inappropriately level of oestrogen went down causes a lower production of Calcium (Ca) so reduced ability to make eggshells; and reduces reproductive activities DDE inhibits the enzyme carbonic anhydrase which controls the uptake of Ca by the oviduct results in thin shells, broken eggs Dieldrin is toxic to embryos
DDT no very toxic to vertebrates and humans, but is to birds.
Lindane still in use today to treat roofs, not known to affect birds but bioaccumulates in bats – 30 years half-life.
100,000 tonnes of DDE estimated to be in ecosystems, will persist for another 100-200 years. Detectable in all animals, human milk.
This highlighted the difficulty of first realising there is an environmental problem; and second on how difficult it is to prove a causal relationship as even if have circumstancial evidence it might be impossible to establish the relationship. Here for example, awareness of the problem happened by observation that birds were declining and that it seemed odd enough for someone to investigate but it took another 15 years befor it was noticed that the eggshells were thinning; also the survey on eggshell thickness was only possible as some historical collections existed.
Chlorinated hydrocarbons: Organochlorine pesticides (OCs) thus DDT, DDD and DDE, Polychlorinated Byphenols (PCBs) linked to endocrine
