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.