Below are photographs of two wetlands. The one on the right has obviously been damaged by human activities. The one on the left looks like it is healthier.
Some human activities can damage a wetland without leaving clearly visible signs.
For example, some herbicides and pesticides kill tadpoles, dragonflies, and other wetland creatures. To conduct a bioassessment, a scientist visits a wetland and collects information about the number of different kinds of organisms and what types of organisms are living there. The scientist also collects information about the habitat quality, water level, and chemistry to support the biological information. The scientist then analyzes the information and compares the wetland's biological information to reference conditions. This is similar to the way a human doctor would collect information about a patient, such as blood pressure and temperature, and compare it to a known range of condition.