Many scientists believe that the Earth is in the throes of the sixth mass extinction of the past 500 million years. More species of plants and animals are dying out since the end of the Cretaceous period 65 million years ago.
On Normal And Abnormal Extinction Rates
While extinction is a normal phenomenon, it typically occurs at a “background” rate of one to five species per year. Researchers estimate that the Earth is currently losing species at 1000 to 10,000 times that. That means that dozens of species are going extinct every day. Scientists fear that 30 to 50 percent of the world’s species could be extinct by the end of this century.
The journal “Science Advances” published a study conducted by several organizations including the National Autonomous University of Mexico, Stanford University, and UC Berkeley that compared extinction rates in mammals. The researchers stated that the background extinction rate should be two animal species out of every 10,000 vertebrate species per 100 years. By that estimate, only nine vertebrate species should have died out since 1900; the actual number is 477, including 69 mammal species.
Previous mass extinctions had natural causes, like the asteroid impact that is believed to have wiped out the dinosaurs. By contrast, the sixth extinction is being caused by human activity. Habitat loss, invasive species, and climate change are all drivers of this destruction. While these phenomena do occur naturally, human activities are causing them to happen at an abnormally fast pace. Threatened plants and animals, therefore, do not have a chance to adapt.
Causes of the Sixth Extinction
The causes of species extinction can vary. Climate change is one cause, but it is not the sole reason. In 2015, John Alroy from the Macquarie University in Australia published a study about extinction rates in reptiles and amphibians in the journal “Proceedings of the National Academy of Sciences of the United States of America.” He found that roughly three percent of all frog species have disappeared since the 1970s. Many of those extinctions were caused by a chytrid fungus called Batrachochytrium dendrobatids. Alroy has therefore warned people not to focus exclusively on climate change, for mass extinction has multiple causes.
To make matters worse, the roots can feed on each other. Organisms of any type depend on specific habitats that fulfill their needs. Animals, for example, need habitats that will provide them with food and water sources and safe shelters for themselves and their young. Changes in climate can cause variations in an animal’s habitat that reduce its suitability.
The Case of the Polar Bear
For example, polar bears need ice from which to hunt seals. As temperatures increase in the Arctic, the ice melts earlier and earlier. Over the past 20 years, the ice-free period in the Hudson Bay of Northern Canada has grown by 20 days. That cuts the polar bears’ seal hunting season by almost three weeks. The Bears thus have fewer opportunities to hunt the seal pups they depend on for food.
Consequently, the Hudson Bay bear population has declined by over 20 percent, and the surviving bears have, on average, lost about 15 percent of their body weight. To make matters even worse, the melted ice increases the distances bears have to swim to get from place to place. As a result, more bears drown on the way.
Climate Change and Marine Life
Climate change also causes problems for marine and aquatic organisms. All of that melting Arctic and Antarctic ice is causing the sea levels to rise. At first glance, rising sea levels may seem beneficial to sea creatures, as it’s increasing their living space. But the various kinds of seaweeds, algae, and other plants still need sunlight to perform photosynthesis. The deeper the water gets, the less sunlight reaches the plants that need it. Those plants that are rooted to the floor can’t move upward to reach the light. They, therefore, die – as does anything that depends on them for food.
The increasing amounts of carbon in the atmosphere also cause ocean acidification. The ocean absorbs about a quarter of the carbon dioxide that human agricultures and industries have pumped into the air. All of that carbon has affected the ocean’s chemistry and made it more acidic. The increased acidity of the ocean harms the various organisms living in it.
For instance, ocean acidification coupled with climate change leads to coral bleaching. Many corals have a symbiotic relationship with algae. They provide the algae with protection and shelter, and the algae provide food and color. When a coral is stressed by environmental changes, it expels the algae. The coral turns white in the process and is, therefore “bleached.” While coral can survive bleaching for a time, if the bleaching and the conditions that caused it persist, the coral will die from starvation.
An invasive species is a species that damages an ecosystem to which it is not native. About 42 percent of endangered or threatened species are affected by invasive species. While an invasive species can sometimes travel to a new ecosystem on their own, they are often transported deliberately or accidentally by humans.
Invasive species can harm native species in the following ways:
- Outcompete them for food or habitat
- Expose them to new diseases
- Eat them
- Replace or destroy food sources
An example of the last would be an invasive plant that supplants a native plant that local herbivores feed on. If the invasive is inedible, those herbivores will either starve or have to go elsewhere for food.
Halting the Sixth Extinction
To prevent more species from going extinct, humans need to reduce their carbon footprint. On an individual level that can mean driving fuel-efficient cars, adjusting thermostats and eating less meat. The latter is important as it takes more land to grow animals for food than it does to raise crops.
Similarly, people should support corporations, organizations and political leaders that understand and accept the realities of the sixth extinction. They should buy from companies with good environmental practices and avoid businesses that follow bad practices, this being something we all should adopt to avoid species extinction.