If you’re familiar with history around mining and coal extraction, you’re familiar with the idea of the canary in the mine shaft. In case you aren’t, canaries were used in 19th century America- up until fairly recently- as an indicator of declining air quality in mine shafts, as canaries were more susceptible to carbon monoxide and other dangerous gases than humans were. It wasn’t a perfect system, and the canaries were probably not too thrilled with the situation, but they saved more than a few lives in an era before modern technology had figured out gas sensors. This concept of a sentinel or indicator species is still in use today, especially in ecology, where multiple indices have been developed to quickly assess habitat quality and changes in an ecosystem with the help of indicator species. Today’s indicator species are not situated carefully in areas where we think there might be a risk, but based on the assemblage of organisms in the wild, with the net effect being a large scale early warning system that we simply have to look at from time to time.
One of the groups that work quite well as indicators is the amphibians. They, like canaries, are more easily affected by environmental contaminants than many other organisms. Amphibians aren’t good at detecting carbon monoxide, but water pH, heavy metals, salts, water temperature, and other pollutants all have an impact on how amphibians grow, develop, and survive. In many cases, amphibians have two distinct life phases- the juveniles live in the water while the adults live on land- which means that they can monitor both of those habitats fairly easily. With the diversity of amphibians, there is also a diversity of possibly damaging situations for our friendly little salamanders and frogs. Because of this, we look at the entire amphibian community and not just one species. Some amphibians are like bullfrogs and very tolerant of changes to the environment; while others are like chorus frogs and far less tolerant, giving us not only an alarm bell like the canary did, but a gauge of how bad the problem is. For example, an environment that is losing one or two species is probably not as bad yet as one that has only bullfrogs left.
Coming into this whole mess are invasive plants, or plants that come from a different part of the world, but do really well in their new home, well enough to start pushing out the native plant neighbors. Plants are interesting in the environment because they form the structure of many habitats by providing a variety of different shelters and vertical spaces. Think of a forest, a meadow, and an open field with some shrubs; three very different physical spaces, thanks to the plants that grow there. The effect is similar to the structures that we build on the landscape- they can provide spread out low density homes, clusters of residences, or large sky-scrapers.
Unlike our structures, plants provide not just physical structure, but act as other living organisms do in a community. They take up and use resources, while releasing other chemicals, and try to avoid being eaten or getting sick through the use of other chemicals. They impact our environment, and they impact the environment that our neighborhood sentinels experience, sometimes in negative ways. We have a growing body of literature that links invasive plants with detrimental outcomes for amphibians. It’s still early in the study of these interactions, but there seems to be problems for amphibians that correlate with invasive plants. Obviously, the important factor determining whether or not an invasive plant is problematic for amphibians or other living things may well be particular traits that a plant has, and not so much the origin of the plant, but there’s still a lot that we’re learning right now about these complex interactions.
There’s the rub for today’s canaries in coal mines- the level of complexity. They aren’t monitoring for a single chemical or handful of them, but a wide array of possible toxins, and from a variety of sources. They also aren’t just responding to plants in their environment, but other organisms (invertebrates, microbes, etc.) who are also responding to the newly introduced specie. The more we learn about the world around us, the more we can put together the pieces in the puzzle. And there are quite a few pieces to ecological puzzles, as complex as these interactions can be.
For further information on invasive plant and native amphibian interactions, check out the following papers:
Brown CJ, Blossey B, Maerz JC, Joule SJ. 2006. Invasive plant and experimental venue affect tadpole performance. Biological Invasions 8(2):327–338.
Cohen JS, Mearz JC, Blossey B. 2012. Traits, not origin, explain impacts of plants on larval amphibians. Ecological Applications 22(1):218-228.
Kappes H, Lay R, Topp W. 2007. Changes in Different Trophic Levels of Litter-dwelling Macro fauna Associated with Giant Knotweed Invasion. Ecosystems 10(5):734-744.
Maerz JC. 2005. Can secondary compounds of an invasive plant affect larval amphibians? Functional Ecology 19(6):970-975.
Martin LJ, Blossey B. 2013. Intraspeciﬁc variation overrides origin effects in impacts of litter-derived secondary compounds on larval amphibians. Oecologia 173(2):449-59.
McEvoy NL, Durtsche RD. 2003. The effect of invasive plant species on the biodiversity of herpetofauna at the Cincinnati Nature Center. Norse Science 1:51–55.
McEvoy NL, Durtsche RD. Effect of the invasive shrub Lonicera maackii (Caprifoliaceae; Amur honeysuckle) on autumn herpetofauna biodiversity. J Kentucky Acad Sci 2004;65:27-32.
Watling J, I. 2011. Extracts of the invasive shrub Lonicera mackii increase mortality and alter behavior of amphibian larvae. Oecologia 165(1):153-159.