In my previous article, I mentioned a sixth mass extinction that is already among us. Considering that this is a pretty daunting piece of information to be briefly mentioned but not explained, I figured I would describe what is meant by this sixth mass extinction. This biological catastrophe is also referred to as the Holocene Extinction or the Anthropocene Extinction. These two additional names to describe the sixth mass extinction are very interesting – and explain why this mass extinction is taking place to begin with.
The Holocene is the current geological epoch, but many scientists now propose that we are in the Anthropocene, indicating the significant human impact on the Earth’s geology and ecosystems. The weight of the idea that humans have altered and damaged the Earth so much that it grants an entirely new geological epoch nearly forces you to take a seat.
Thousands of populations of critically endangered vertebrate animal species have been lost in a century, indicating that the sixth mass extinction is human-caused and accelerating. The exponential growth of the human population and consumption rates inevitably accelerates the extinction crisis.
A mass extinction event caused by humans threatens human health. The loss of entire populations and species have severe consequences for society through the degradation of valuable ecosystem services. Each species plays a key role in ecosystems. As they die out, the species they interact with are also certain to follow their fate. Species disappearance in a geographical region leads to regional collapses of biodiversity. As species go extinct, humans lose access to critical ecosystem services provided by these organisms whether they are aware of it or not.
Extrinsic forces are increasing the susceptibility of high-risk species. Extrinsic forces include global warming and increased climatic variability. High-risk species are those with small geographic ranges, low fecundity, and specialized habitats. Among the vertebrate animal species that are suffering most amidst this sixth great mass extinction, amphibians are likely the only major group that is at risk globally. Before looking at the world of amphibians during this mass extinction event, here is a brief overview of the first five mass extinctions:
The first mass extinction occurred at the Ordovician-Silurian boundary about 440 million years ago. During this time, life was dominated by benthic marine organisms. As a result of this extinction event, approximately half of all genera, about 100 families, were wiped out. Scientists claim that the cause of this event was related to the sudden shift in climate from greenhouse conditions to icehouse conditions. Gondwanaland, a supercontinent, passed over the pole and initiated glaciation. Glaciation lowered the sea level and sea temperatures. This eliminated many shallow seas and drastically changed habitats located near the shore. The sea levels rebounding in the aftermath may have caused additional extinctions.
The second mass extinction event occurred at the End-Devonian time period about 365 million years ago. At this point, land plants, sharks, and bony fishes existed. This mass extinction again targeted marine organisms, particularly reef-building corals and other marine invertebrates. Amphibians evolved in the Late Devonian and survived this extinction event. The third mass extinction event occurred about 250 million years ago and is referred to as the Permian-Triassic or End-Permian Event. This is noted to be the worst mass extinction with a 90% loss of all species. Greater than half of all land species, mostly plants, were lost. While terrestrial vertebrates perished, early representatives of the three orders of amphibians that we still see to this day, survived.
The End-Triassic event, occurring over 200 million years ago, is the fourth mass extinction event. This event affected large terrestrial animals (mostly terrestrial amphibians), plants, and marine species. In this way, the End-Triassic extinction event paved the way for the evolution of the dinosaurs by wiping out large terrestrial species. Once again, representatives of the three living orders of amphibians survived.
The fifth mass extinction occurred 65 million years ago at the Cretaceous-Tertiary time period, also known as the K-T boundary event. This event eliminated all dinosaurs and is the best understood of all five mass extinction events. After an asteroid collision with the Earth near the Yucatan Peninsula of Mexico, debris shot up into the air. Upon return to the atmosphere, the debris generated intense radiation while some stayed in the atmosphere for decades and blocked sunlight. Consequences of the asteroid coupled with diverse climatic changes are leading causes for this event. Not only did all three orders of amphibians again escape extinction, but a great number of families and extant amphibian genera also survived.
All five previous mass extinction events exhibited a profound loss of biodiversity during a relatively short period and while amphibians survived these events, they are in the exact opposite position this time around. This is what makes this situation so compelling. As mentioned in my previous article, amphibians are a great indicator species. The unique lifecycle and physiology of amphibians put them in close contact with land, water, and air. Amphibians are exposed to a wide variety of environments, making them vulnerable to poor environmental conditions. Amphibian eggs and larvae develop in an aquatic environment. As adults, amphibians take part in terrestrial activity. Frog larvae are typically herbivorous whereas adults are typically carnivorous. So, not only do amphibians live in various environments, they are exposed to a wide variety of food, predators, and parasites.
Amphibians also have permeable, moist, well-vascularized skin which makes them very susceptible to any pollutants they may come in contact with. For amphibians, cutaneous respiration is more important than respiration by the lungs. As thermal-conformers, amphibians are extremely sensitive to environmental temperature changes. This is especially important for tropical species that have experienced little temperature variation as such species may have little acclimation ability in rapidly changing thermal regimes. Most amphibians occur in the tropics and have small geographic ranges which makes them more susceptible to extinction.
As a result of this human-induced biological massacre, the amphibian extinction rate is two hundred and eleven times greater than the background rate. The background extinction rate refers to the rate of extinction in geological and biological history during the periods in between major extinction events before humans became the primary contributor to extinctions. Amphibians are facing synergistic impacts, a multitude of threats, as the reason why these species are suffering so much.
There are three main reasons as to why amphibians are facing extinction so heavily: (1) Habitat destruction, (2) Global warming, and (3) Epidemics of infectious disease. Habitat destruction exacerbates the fact that amphibians exist in already small ranges. Amphibians at greatest risk of extinction are those with few populations in areas undergoing rapid habitat conversion due to human activity. Additionally, decreasing range sizes place amphibians in increasingly close contact with each other, making it easier for disease to spread. Sensitive to climatic variability, global warming is a major stressor on many vertebrate species.
Climate changes such as increased temperatures and precipitation also serve as catalysts for disease outbreaks such as chytridiomycosis. Chytridiomycosis is a globally distributed virulent emerging infectious disease that poses a significant threat to amphibians. This disease is caused by a pathogenic fungus called Batrachocytrium dendrobatidis (Bd). This fungus lives on keratin in the outer layer of the skin, blocking the ability for cutaneous respiration. Outbreaks of chytridiomycosis are responsible for collapses of entire amphibian populations and may be the leading threat to amphibian survival.
Amphibian or not, the human-caused sixth mass extinction is accelerating. Scientists note that those species that have been driven to the brink of extinction are likely to become extinct sooner rather than later. The distribution of these species overlaps with other endangered species. This calls for ongoing regional biodiversity collapses. Similarly, the extinction of one species is often cause for the extinction of other species through debilitated ecological interactions. Pressures that the human race places on the biosphere are rapidly growing.
If a brief look into a sixth mass extinction that is underway was not enough, the current situation of the Covid-19 pandemic due to the human link to wildlife trade should be enough to highlight that there is an accelerating existential threat right in front of our faces. It is urgent to talk about and push this environmental crisis to the forefront of a worldwide agenda. The loss of biodiversity is also the loss of humanity’s crucial life support systems.
