At an individual level wildlife can be affected by oil via:
- external contamination, in which the oil causes skin burns or changes the condition and physical properties of feathers or fur
- internal contamination (after inhalation or swallowing), in which the oil may cause internal burns or disturb physiological processes
At the population level wildlife can be affected if the oil spill causes a significant mortality within the breeding stock of a population, i.e. the adults that contribute to the reproduction process. This is often the concern when threatened or endangered species are affected by oil spills.
The external effects of oil are the most noticeable and are the most immediately debilitating. Oil, by disrupting the interlocking structure of feathers, destroys the waterproofing properties of the external feathers and soaks the downy insulating layer. This in turn can lead to:
- Hypothermia by reducing or removing the insulation and waterproofing properties of feathers
- Sinking or drowning as oiled feathers weigh more and cannot trap enough air to keep birds buoyant
- Increased risk of predation, as feathers matted by oil decrease a bird’s ability to fly away
- Dehydration and starvation as chilled birds drink less water intake and spend less time diving and swimming for food.
The internal effects of oil on birds, while not as visually apparent as the external effects, are equally life-threatening. Birds can ingest or inhale oil as they try to preen oil from their feathers or feed on a contaminated food source. Depending on the type of petroleum product, its weathering stage and its toxicity, poisoning through ingestion can range from sub-lethal to acute.
- Ingestion of oil frequently results in injury to the gastrointestinal tract, preventing the animal's digestive system from processing food or water and causing the animal to become progressively weaker in a very short period of time.
- Irritation of other mucosal surfaces can be seen, such as ulceration of eyes and the moist areas inside the mouth. Red blood cells may be damaged or destroyed resulting in anaemia.
- Kidney damage is believed to occur both as a direct effect of the toxins in the oil and as a secondary effect of severe dehydration. As an oiled bird becomes more debilitated, its immune system is compromised and the bird becomes susceptible to secondary bacterial and fungal infections.
- The oil may also have an effect on the ability of those birds that survive the oil spill to reproduce. Breeding and incubating behaviour, number of eggs laid, the fertility of those eggs and shell thickness (thin shells are more likely to break before hatching) may all be affected.
The external effects of oil on marine mammals including sea otters, sea lions, seals, walruses, sea cows or manatees, polar bears, dolphins, porpoises and whales, will vary, depending on the species but may include:
- Hypothermia in polar bears, sea otters and fur seals pups, by reducing or destroying the insulation of their thick fur. Adult fur seals have blubber and would be less likely to suffer from severe hypothermia if oiled
- Skin lesions are a problem for dolphins and whales (who do not have fur so oil will not easily stick to them) who swim through oiled areas
- Eye irritation is a problem for all marine mammal species exposed to oil
- Marine mammals, particularly seal and sea lion pups, become easy prey when oil sticks their flippers to their bodies, making it hard for them to escape predators; fur seal pups can drown in this situation
- Marine mammals lose body weight when they cannot feed due to contamination of their environment by oil
- The scent that seal and sea lion pups and mothers rely on to identify each other is disguised, leading to rejection, abandonment and starvation of the pups. In some species, mothers and pups rely on vocalisation rather than scent, making them less vulnerable to this risk.
- Inflammation or infection in manatees and difficulty eating due to oil sticking to the sensory hairs around their mouths
- Reduced ability to forage due to fouling of the baleen of surface feeding whale species
Internal effects, also vary by species but may include:
- Congestion of lungs and damaged airways from inhalation of oil vapours and droplets
- Emphysema and pneumonia are possible in most marine mammal species but are a particular concern for sea otters who spend much of their time on the surface where volatile chemicals from petroleum are strongest and cetaceans who come to the surface to breathe.
- Kidney, liver and brain damage, as well as anaemia and immune suppression, are potential side effects of ingestion and inhalation of oil
- Gastrointestinal ulceration and haemorrhage
- Anaemia from damaged red blood cells
- Damage to mucous membranes
As impact assessments come in from the Deepwater Horizon spill in the US, evidence is emerging for significant health effects in dolphins, including liver and lung damage, suggesting that, in some cases, oil can have a much greater impact on cetacean health than previously realised.
Sea turtles can become contaminated when they rise to the surface to breathe and find themselves in the midst of an oil slick.
In the breeding season females may become oiled when they arrive in the contaminated area or when they come ashore to lay eggs. Juveniles may become trapped in oil when they head to sea after hatching. Offshore, the tendency of sea turtles to spend time in surface convergence areas which provide both food and sheltering habitat-particularly for young animals- but where oil also collects- puts them at greater risk of inhaling and ingesting oil.
Although there is little statistical data on the effects of oil pollution on sea turtles, they are subject to the following impacts:
- Poisoning by absorption of toxic components through the skin or ingestion of contaminated food, leading to damage to the digestive tract and other organs
- Damage or irritation to airways, lungs, and eyes
- Contamination of eggs, which may inhibit their development
Impact assessment results from the Deepwater Horizon spill in the US, which occurred during sea turtle nesting and hatching season, will likely provide further information on the effects of oil on sea turtles.