Measuring levels of habitat degradation is a key task for climate scientists, but, ironically for this data-driven field, nature is hard to quantify. By the time an ecosystem has changed enough to be noticeable to laymen, it has likely already suffered serious damage. To identify habitats under stress, researchers have relied on chemical testing of samples and measurements of physical quantities such as temperature and humidity. These methods give exact answers about how much the environment has changed. Therefore, researchers have previously relied on them to give an accurate picture of how species were affected by climate change and pollution.
However, imagine the case of a researcher trying to monitor the state of a river with a polluting factory in its upper reaches. They will have to make measurements and collect samples at points along the length of the river—perhaps hundreds of kilometres. To track changes, they will have to repeat this regularly. Even if we ignore the cost and time needed to conduct chemical tests on the samples, this will still require significant funding. In a developing country with political instability, sample collection may be impossible. Traditional quantitative methods may also fail to detect short-term peaks or dips in conditions, if measurements are only taken occasionally.
Furthermore, the researcher cannot determine the actual effects a chemical or a combination of chemicals could have on a species. Chemicals interact in complex ways inside organisms, leading to different effects at the same concentration. With hundreds of physical and chemical factors, relying solely on instrumental measurements risks underestimating the importance of interactions between what can seem like unrelated figures.[1]
Bioindicators serve as a complementary method. They are species, or groups of species, that respond to environmental changes. A simple example is the cliched canary in coal mines. Their small size and fast respiration rate mean they succumb to carbon monoxide poisoning before humans do; therefore, miners carried them to detect dangerous levels of the gas. Of course, organisms respond to altered conditions in different ways, such as anatomical or behavioural changes, population decrease, and even explosive growth in the case of algal blooms. These are all monitored by climate scientists to track the effects of climate change, pollution, and habitat loss in a cost-effective way.[2]
In theory, any species can be a bioindicator, but certain factors make some more suitable than others. Because they show the state of a habitat to human scientists, they, or their effects on the habitat in response to a change, should be easy to spot. They also tend to be less mobile species; otherwise, organisms found in one area may not represent the environmental conditions there, compromising their usefulness as bioindicators.
Anurans—frogs and toads—are used as bioindicators by many scientists. Their skin absorbs water and water-soluble chemicals, and they cannot excrete toxins effectively. Furthermore, their amphibian lifestyle means they are vulnerable to changes affecting either land or water. For example, in one study in Ethiopia, researchers Saber et al. recorded the prevalence of different species of frogs and analysed the rate of deformities in collected specimens.[3] This strategy allows researchers to collect more data from the same population.
Bioindicators aren’t limited to being animals. Lichens—colonies of bacteria and algae growing symbiotically with certain species of fungi—are also used as bioindicators. They grow so slowly that they can be used to determine the age of exposed rock surfaces over 1000 years old. This property makes them ideal bioindicators. In one study, researchers Conti et al. recorded the prevalence of different species of lichen in an area at regular intervals.[4] This was a cost-effective way to measure the concentration of polluting sulphur and nitrogen oxides in the area.
Of course, no extant species is sensitive to every type of environmental change, as they would have died out long ago if they were. Therefore, scientists must investigate many potential bioindicator species to learn about the impact of climate change on a particular habitat. As active participants in the ecosystem, bioindicators give a more relevant view of how organisms react to the combination of physical and chemical factors in their environment. While they are not perfect indications of how an ecosystem reacts to change, they certainly address many shortcomings of direct measurements.
1. Holt, E.A. and Miller, S.W. (2011). Bioindicators: Using Organisms to Measure Environmental Impacts. Nature Education Knowledge.
2. Parmar, T.K., Rawtani, D. and Agrawal, Y.K. (2016). Bioindicators: the Natural Indicator of Environmental Pollution. Frontiers in Life Science, 9(2), pp.110–118. doi:https://doi.org/10.1080/21553769.2016.1162753.
3. Saber, S., Tito, W., Said, R., Mengistou, S. and Alqahtani, A. (2017). Amphibians as Bioindicators of the Health of Some Wetlands in Ethiopia. The Egyptian Journal of Hospital Medicine, 66, pp.66–73. doi:https://doi.org/10.12816/0034635.
4. Conti, M. E., & Cecchetti, G. (2001). Biological monitoring: lichens as bioindicators of air pollution assessment — a review. Environmental Pollution, 114(3), 471–492.https://doi.org/10.1016/s0269-7491(00)00224-4
