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Why Are Animal Populations Decreasing? (5 Reasons)

In the mid-1960s, scientists studying preserved and living moss samples discovered that levels of phytochemicals meant to protect these samples from UV radiation were significantly lower in living samples than in preserved samples. By the end of the 1970s, scientists were issuing a dire warning about the depletion of the ozone layer due to human-caused pollution.

They predicted that unless global leaders began reducing the emission of greenhouse gases, the ozone layer would be unable to protect animal and plant life from genetic damage. In addition, the continuing accumulation of greenhouse gases would ultimately lead to catastrophic global warming.

Fast forward to 2022. Although the ozone layer is no longer decreasing, it is not increasing. Consequently, we are experiencing the devastating repercussions of global warming and climate change.

Droughts, floods, hurricanes, incredibly powerful typhoons, deadly tornado outbreaks, and the disappearance of water resources due to drought are impacting every continent on Earth.

Why are Animal Populations Decreasing? (5 Reasons)

1. The Holocene Extinction

Scientists estimate the continuing Holocene Extinction began around 200 years ago with the onset of the Industrial Revolution, the destruction of ecosystems, and the unrestricted hunting of already endangered animals. Animals hunted to extinction by humans include the Dodo bird, Stellar’s sea cow, the Tasmanian tiger, the Javan tiger, and the Zanzibar leopard.

While most of these animals were killed for their meat and hides, the Zanzibar leopard was deliberately exterminated by island people who believed the leopards were sent by witches to harm villagers.

The Holocene extinction event is still ongoing and severe. However, the reason for the decline of animal populations today has changed from aggressive hunting practices to global warming, climate change, and the annihilation of land and water ecosystems.

2. Climate Change

Photo of drought and green grass
Cracked earth and green grass | Image by Tumisu from Pixabay

Human-caused climate change is also causing an extinction event not seen on Earth since the Permian-Triassic extinction over 250 million years ago, when 96 percent of all animal species were wiped out by a massive volcanic eruption.

Filling the atmosphere with huge amounts of carbon dioxide and dust that blocked sunlight from reaching the earth, this eruption led to the development of vast populations of bacteria that emitted pure methane.

It also killed off enormous populations of animals and plants that needed oxygen, plant life, warmth, and other animals to survive. Today, instead of volcanoes erupting, we now have an extinction event that is primarily caused by humans.

An example of climate change impacting an animal population involves the endangered South African penguin. Inhabiting the waters surrounding southern Africa, this penguin species feeds mostly on squid and small fish.

However, the warming of southern Africa’s coastal waters has displaced fish and squid by producing abnormal currents that push the penguin’s food sources away from the coast.

Since all penguin species rarely swim more than 20 miles away from where they nest, the displacement of their food and habitat destruction has led to a rapidly decreasing population of African penguins. Scientists estimate there are less than 50,000 African penguins alive today.

3. Depletion of Finite Resources

Large scale quarry
Large scale quarry | Image by keesstes from Pixabay

Scientists estimate that around 100 AD, Western Europe (countries that are now in the European Union) consisted of 80 percent forest and 20 percent grassland. Fifty percent of forests in eastern North America were eradicated for agriculture and timber purposes between 1600 and 1900.

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Today, the percentage of forested areas in the EU is less than 35 percent. The percentage of forested areas in eastern North America is less than that.

The region impacted the most by deforestation is the Amazon rainforest. From ore mining, farming, and livestock grazing to wildfires and the encroachment of urban areas, rainforests are suffering the worst human-initiated exploitative practices.

Unfortunately, 70 to 80 percent of plant and animal species live in rainforests and other tropical woodlands. Destroying trees and plants not only deprives animals of food and shelter but also eliminates a primary source of oxygen replenishment and removal of carbon dioxide from the atmosphere. Some estimates put the extinction rate of rainforest animals as eight percent per decade, with that percentage increasing as long as deforestation continues.

4. Air, Water, and Land Pollution

Landfill | Image by prvideotv from Pixabay

Just like humans, animals can die from exposure to toxic substances in the air, water, and soil. For example, acid rain is the result of rain picking up nitrogen oxides and sulfur dioxide molecules contained in the atmosphere.

The reaction between oxygen, water, and these toxic chemicals forms nitric and sulfuric acids before reaching the ground. Produced by fossil fuels, gas-powered vehicles, manufacturing facilities, and oil refineries, acid rain turns water and soil into deadly resources for animals.

Other ways air, land, and water pollution contribute to animal extinction include:

  • Millions of sea birds and animals are killed every year by ingesting plastic garbage floating in the oceans.
  • Chemical runoff from large farms has created hundreds of “dead zones” by polluting lakes and streams with toxic substances that deplete oxygen in water sources by increasing algae growth.
  • Cruise ships dump hundreds of thousands of gallons of wastewater and sewage into oceans because there are no environmental laws stopping them.

Additionally, lions, crocodiles, eagles, and other “apex” predators that are higher up in the food chain are more affected by air, land, and/or water pollution because of their larger bodies. Biomagnification is thought to be one underlying reason why so many apex predators are on the endangered species list.

The larger the animal, the more pollutants can find their way into the animal’s tissues. Another consequence of biomagnification is the fact that apex predators tend to eat animals further down the food chain.

The tissues of these animals may already contain chemicals they ingested via water or soil. Consequently, the apex predator is doubly harmed by the consumption of contaminated food.

5. Invasive Species

Sea walnut
Sea walnut – an invasive species | image by Holger Krisp via Wikimedia Commons | CC BY 3.0

Globalization has rapidly led to the introduction of invasive species to the U.S. Europe, and Asia. An invasive species is an animal or plant that is non-native to a region and causes damage to that area’s ecosystem.

Most invasive species are introduced into a specific ecosystem by the accidental release of a specific animal into that ecosystem, through the ballast water of ships, or by surviving in international shipments that are noncompliant with paper or wood material regulations.

An example of an invasive species is the sea walnut, an animal resembling a jellyfish that was native to the North and South American eastern coasts. In the early 1980s, researchers discovered the sea walnut living in the Caspian and Black Seas via ballast water. Rapidly reproducing and forming massive populations, sea walnuts caused the closing of many local fisheries due to their voracious appetite for zooplankton.

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Since the fish caught by fishermen in the area needed to consume zooplankton to survive, these fish died out, leaving the coastal economy devastated. A comprehensive list of invasive species that have destroyed ecosystems and the animals that live in them can be found here.

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