What is a sensitive species? A dictionary definition of sensitive is “quick to detect or respond to slight changes, signals, or influences.” According to Merriam-Webster, the definition of tolerant/tolerance is “the ability to accept, endure, experience, or survive something harmful or unpleasant”. So what makes a species or group of species such as amphibians, which are now currently considered “sensitive”, actually sensitive? Amphibians are listed as sensitive in many documents, for example, the US Forest Service defines sensitive species as “species that need special management to maintain and improve their status on National Forests and Grasslands, and prevent a need to list them under the Endangered Species Act (ESA)”. But the question should be, what are these organisms “sensitive” to? Amphibians have been around a long time (hundreds of millions of years!) and are surprisingly tolerant and resilient; some species of amphibians can freeze solid!
To really get a sense of this, let’s talk about extremes. Amphibians were of particular interest to early physiologists because of their unusual respiratory and metabolic adaptations. This led to some rather remarkable experiments. For example to determine if respiration could occur via the skin, physiologist W. F. Edwards removed lungs from frogs and found they could survive in cold water saturated with air for over a month (Edwards 1824). In another experiment a geologist (William Buckland) encased toads in sandstone and limestone chambers and buried them for a year to see how long they would survive*. Those in the more porous limestone survived the first year; the others did not (Buckland 1832). Other experiments involved coating frogs in paint or oil to see how it affected respiration (Edwards 1824, Wells 2007). We now have a better understanding of many amphibian adaptations, including their very low metabolism, and the ability to respire and hydrate through their skin, which makes them both resilient and sensitive (according to the dictionary definition) in a wide range of environments.
Macabre examples aside, the point is amphibians possess amazing adaptations that make them quite hardy organisms. We tend to think of them as “sensitive” without understanding they represent 360 million years of successful adaptation. Diversifying around the Devonian extinction event, amphibians successfully weathered the next three major global extinction events (ending the Permian, Triassic and Cretaceous). They are finely tuned to the ecosystems in which they persist, often with amazing resilience to very dynamic environments; from deserts to tropical rainforests to artic conditions where they can freeze solid (I am certainly sensitive to freezing solid). This close link means amphibians are both sensitive and tolerant to changes in their environment (according to the dictionary definition).
So that is why it is stunning and disconcerting that we are seeing these animals disappear in what is emerging as earth’s 6th major extinction crisis. What does it mean that over 1/3 of all species of amphibians being driven to extinction globally at an alarming rate? Yes, they are attuned to the environs in which they live, and it is true many species are tightly tied to specific terrestrial or aquatic habitats in which they evolved. However, it is foolhardy to think these species are simply fragile and that is why they are going extinct at greater rates than any other vertebrate taxa. Generally, many amphibian species are disappearing because habitats are being completely altered or eliminated, globally. Additionally, the global fungal disease called Chytridiomycosis, a lethal disease of amphibians caused by an aquatic pathogen Batrachochytrium dendrobatidis (a.k.a. “chytrid fungus” or “Bd”) is responsible for the worst case of disease-caused vertebrate biodiversity decline in recorded history (though there are recent developments which provide a glimmer of hope). Concerns about the impact of phenomena like climate change may be mute, regardless of a species’ sensitivity if there is no functioning ecosystem, habitat, or individuals remaining.
Why care if a species is sensitive? We have policies involved that have legal ramifications depending on the category or label attributed to a species. In the United States, sensitive has a very specific meaning (and I would suggest different than the dictionary definition) that is largely based on a species’ potential risk of extinction. So should we categorize species or taxa as “sensitive”? It is important to define how organisms are tied to the environments they live in as these connections will help us better quantify if a species is truly “sensitive”. Even when we can document shifts in a species’ population size or distribution, the hard part is often teasing out the important drivers of change. Often these drivers are numerous and all can interact or correlate confounding potential solutions. Ultimately, I think ecology is basically learning how little we really know while still trying to understand what has relevant meaning for better management of our environment. Being able to focus on a few sensitive species instead of all species certainly makes conservation objectives more manageable, but how we categorize species as sensitive or at risk of extinction seems to be far from unanimous.
So are amphibians “sensitive”? Perhaps we think of species which are becoming locally rare as sensitive, and we are conflating rare and sensitive…but that’s a topic for another blog post. In the case of amphibians, they are both tolerant and sensitive, and they have persisted for over 360 million years. Human occupancy of earth represents a mere fraction of this time, thus, how big a change can we be making that we are single handedly witnessing (and inflicting) the extinction of an entire taxa? We are still trying to figure that out, but in the meantime, organisms closely tied to environments they have evolved in over eons of biologic and climatic change, certainly appear “sensitive” to anthropogenic change.
* Certainly this was before Institutional Animal Care and Use Committee (IACUC) protocols were implemented, now it would take years and hundreds of pages of paper to proceed, if at all.
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