Herbicides are chemicals designed to kill plant pests. Therefore, when they enter the environment, they can be considered a chemical pollutant. An early one was 2,4-dichlorophenoxyacetic acid or 2,4-D. It was inexpensive and effective as a pesticide. It controls broadleaf weeds and is the most widely used herbicide in the world. It is a synthetic auxin which is a plant hormone. This chemical was used in the 1940's. Later, atrazine was introduced but it can cause groundwater contamination. It doesn't break down easily and can be transported down the soil profile to the water table in alkaline soils. Glyphosphate has been used since 1974 and controls weeds by pairing it with crop plants resistant to the chemical. Each herbicide has different toxicities. Ecologically, bird populations suffer as herbicides remove plants, which is food as well as habitats that the birds and their prey, rely on. It is also likely that pesticide use may lead to cancer and other diseases in humans. Sometimes fish and invertebrates may be sensitive to herbicides. Amphibians are particularly affected by these toxic chemicals.