Throughout the world there are certain species that are particularly sensitive to changes within the environment, especially pollutants - they are referred to as bioindicators. These species are often monitored and studied by scientists when assessing overall ecosystem health.
Frogs, lichen, and certain macroinvertebrates are only a few examples of bioindicator species.
There are a few different ways bioindicators can be studied. One method is looking at population numbers. If you live in an area that used to have a lot of frogs and toads, and now there are significantly less, there might be a problem. This also works in the reverse - there are some species that are more present in toxic conditions, so if you find more of them, there also might be a problem! Do any of you remember in high school science class going to a river or stream near you, using a net to catch creatures from the water, and then counting how many of each species you found? You were studying macroinvertebrate bioindicators! Sometimes bioindicators have the ability to store substances and the amount of a pollutant or toxin can be measured from the species directly. Other species can be used as an "early warning" system when looking at their life cycles. Amphibians have complex life cycles and permeable skin, so things like toxins and pollutants can create havoc on their systems that can manifest in early stages of life. This can help scientists identify and work to fix environmental issues before they become a much larger issue.
Different environments will have different bioindicators, so I challenge you to learn more about the ones near you. Scientists are not the only ones who can or should be aware of the overall health of the ecosystem - we share this planet with so many amazing creatures, lets help to take care of it and recognize it's warning signs.