Almost all mercury compounds are toxic and can be dangerous at very low levels in both aquatic and terrestrial ecosystems. Because mercury
is a persistent substance, it can build up, or bioaccumulate, in living organisms, inflicting increasing levels
of harm on higher order species such as predatory fish and fish eating birds and mammals through a process know as
"biomagnification". Although the long-term effects of mercury on whole ecosystems are unclear, the survival of some affected populations
and overall biodiversity are at risk.
In the environment, particularly lakes, waterways and wetlands, mercury can be converted to a highly toxic, organic compound called methylmercury
through biogeochemical interactions. Methylmercury, which is absorbed into the body about six times more easily than inorganic mercury, can
migrate through cells which normally form a barrier to toxins. It can cross the blood-brain and placental barriers, allowing it to react directly
with brain and fetal cells. Mercury contamination causes a wide range of symptons in organisms, and affects the kidneys and neurological systems
in particular. While low levels may not be directly lethal for individual organisms, toxicological effects
like impaired reproduction, growth, neuro-development, and learning ability, in addition to behavioral changes, can lead to increases in mortality
and the risk of predation for some wildlife.