Planet earth thrives with life. Thousands of creatures, along with a bewildering array of bacteria, algae, and single-celled organisms call our mound of the earth their home. In reality, however, our thriving ecosystem pales in comparison to the diversity of life during ages past. Did you know that around 99.9% of all species have now gone extinct? Let’s take a deep dive into the causes of extinction.
What exactly is extinction?
Extinction, or the dying out of a species, is a completely natural phenomenon. Species die out due to a variety of environmental forces which include natural disasters, global change, habitat fragmentation, and exploitation, but they can also occur due to evolutionary changes observed in their members (e.g. decline in population, poor reproduction, genetic inbreeding).
Dozens of new species are wiped out from the face of the earth every day, and the numbers may be surprising. More than 20,000 animals and plants are currently on the brink of disappearing forever, and around a quarter of known mammal species are at great risk of extinction. The earth experiences loss daily. This is, even if it sounds tragic, a completely natural process.
Extinction is viewed as neither positive nor negative, but humans have inadvertently accelerated the process due to their role in disease, climate change, habitat loss, hunting, and overfishing. Here are the 9 main causes of extinction.
The 9 Causes of Extinction
1. Asteroids
How it could happen
When people hear the word extinction, they immediately associate it with a gargantuan meteor cannonballing towards the earth. A meteorite impact is, after all, one of the most popular causes of extinction — an established reason why dinosaurs were wiped out 65 million years ago.
Asteroids are giant, rocky masses that orbit the sun and can reach hundreds of meters in diameter. When an asteroid survives the trip through the earth’s atmosphere and reaches the surface, it is then called a meteorite. Meteorites of giant proportions that smash into the earth can have several cataclysmic effects.
What history has taught us
It was Nobel Prize-winning physicist Luis Walter Alvarez who theorized that a layer of iridium-rich clay found within the earth was caused by a massive asteroid collision. This collision was determined to cause the widespread death of dinosaurs millions of years ago.
We know that a meteorite can cause devastation in the place of impact, but how could destruction in a localized area cause a global mass extinction? The secondary effects of the collision were pinpointed to be one of the reasons why dinosaurs, and around 75% of the earth’s animals, were wiped out so quickly.
On impact, the meteor created a massive blast wave that threw large amounts of soot and debris into the atmosphere. The soot traveled around the world and effectively blocked out the sun, reducing the amount of light that could reach the earth. This had a massive impact on the growth of plants, leading to a domino effect of destruction and causing the ecosystem to collapse.
It’s also highly likely that many more of Earth’s mass extinctions were caused by asteroid collisions, including the more severe Permian-Triassic extinction.
See Related: What is Overfishing? Examples & Solutions to Prevent
2. Climate Change
How it could happen
Our climate can change drastically, even without the sun-blocking effects of a giant meteorite impact. Climate change is defined as a lasting change in the usual weather of a place. Among the many causes of extinction, climate change is one we consistently face daily. It’s observed in a variety of ways – when a place gets much more rain than it does in a year or when temperatures get unusually hotter or colder for a month or season.
Climate change poses a significant threat to terrestrial animals as plants, animals, and organisms require delicate amounts of heat and water to survive, and the tiniest change can leave them unable to adapt.
Lawrence Krauss’ Book, The Physics of Climate Change, defines climate change in layman’s terms while offering the perspective we need to understand our role in the process and the steps we can take to change our course.
What history has taught us
To see the cataclysmic effects of climate change, one has to look no further than 11,000 years ago, during the end of the last ice age. With global heat rising, various megafauna mammals could not adapt while simultaneously succumbing to predation by early humans.
Today, climate change is occurring at an alarming rate — so much so that many scientists believe is leading to the Earth’s sixth major extinction. A massive fraction of both freshwater and terrestrial species are facing increased extinction risk under climate change during and after the 21st century as climate change interacts with other devastating stressors, such as pollution, habitat modification, and invasive species.
With global temperatures rising, scientists have predicted a few effects on wildlife: the Amazon could lose 69 percent of its plant species while 60% of Madagascar’s species run the risk of localized extinction, among others.
See Related: Don’t Read This If You Think Climate Change is Fake
3. Disease
How it could happen
For years, scientists believed that it is highly unusual for a single disease to spur the extinction of an entire species, but several incidents have proved them wrong.
For a disease to wreak total havoc, it needs to coincide with several other stressors which include loss of habitat, lack of genetic diversity, or starvation. The introduction of a lethal bacterium or virus at a species’ most vulnerable moment may lead to widespread and uncontrollable death.
What history has taught us
The flu of 1918 killed millions of people while Ebola outbreaks caused the gorilla population to teeter toward extinction. While there were countless losses, none completely wiped out an entire mammal species until the incident on Australia’s Christmas Island.
In 2008, a pathogen led to the extinction of an entire rodent species in Australia – a topic of a study published in the academic journal PLoS One. This study presented the first evidence that mammals can be driven extinct entirely because of a disease. Before the study, only snails and amphibians were recorded to succumb to a disease so lethal that it could cause extinction.
According to the study, a merchant ship inadvertently carried flea-ridden black rats to Christmas Island more than a century ago, and within a decade, both of the island’s native rat species became extinct.
4. Loss of habitat
How it could happen
Among the many causes of extinction, habitat loss is possibly the biggest threat to planet life today. In the United States, it is the primary threat to wildlife survival.
Intensive timber harvesting, expansion of agricultural land, overgrazing, and the harvesting of wood for fuel and various other forest products have led to widespread forest loss and degradation. When wildlife habitats are abused, they may no longer provide the water, food, and cover that animals and plants need to survive.
Another kind of habitat loss is called ‘habitat fragmentation’ – a phenomenon that occurs when wildlife habitat is cut up into fragments by various roads and development. The fragments that remain may not be large enough to support the species that require larger places to live. This means that creatures have fewer places to find food, rest, feed, and find their mates.
What history has taught us
Today, tropical rainforests receive the brunt of habitat destruction – of the 16 million square kilometers of rainforest that originally existed worldwide, only 9 million square kilometers remain today.
Habitat loss is the Javan tiger cause of extinction between the 1950s and 1980s through the expansion of coffee and hevea plantations, rice fields, and teak forests. In the 1970s, habitat loss and hunting were also the Caspian tiger cause of extinction when human settlement ate away at its habitat during the Industrial Revolution.
The alteration of habitat due to the impact of exotic herbivores and changed fire regimes was also the pig-footed bandicoot cause of extinction.
5. Lack of Genetic Diversity
How it could happen
Among the many causes of extinction biodiversity is one of the most complex. It’s easier to understand genetic variation in light of human relationships. It’s much healthier to marry and breed with a complete stranger as opposed to your first cousin, lest you run the risk of generating undesirable genetic traits through inbreeding.
Lack of genetic diversity happens among a species when they start dwindling in numbers and available mates become harder and harder to come by.
According to evolutionary theory, we should not only focus on conserving the individual numbers of species but also their capacity to evolve in the face of shifting environmental variables. For example, genetic variation is essential in a species’ ability to withstand diseases. When a population with a lack of genetic variation is exposed to a new disease, the population will not be able to evolve and will therefore die off.
What history has taught us
Cheetahs have survived a population collapse that occurred more than 12,000 years ago due to loss of genetic diversity and inbreeding. Therefore, today’s modern cheetahs are incredibly prone to diseases and have low sperm quality.
According to a study from the Smithsonian Conservation Biology Institute, wild cheetahs are still losing genetic diversity at an alarming rate. As such, they may not have the resiliency to survive another major environmental disruption.
In 1983, a deadly virus swept through the entire population of large cats in an Oregon breeding facility, killing about 50% of the captive cheetahs, while none of the lions even developed symptoms. Scientists attributed this incident to the cheetah’s low genetic variation that caused a poor immune system that impaired their ability to fend off disease.
See Related: Conservation vs Preservation: The Differences
6. Invasive Species
How it could happen
Species are in eternal competition with one another – and this survival transpires over eons. Other times, the contest is quicker and more one-sided, with an aggressive species completely overcoming the other in a fight for resources.
When an animal or plant is transplanted into another (voluntarily or inadvertently), the organism can wildly reproduce to the point of exterminating the native population and becoming one of the most stubborn causes of extinction.
An invasive species is not indigenous to a particular location, but not all non-native species can be invasive. Most of the food crops in the United States, including varieties of tomatoes, wheat, and rice, are not indigenous to the region. A species needs to adapt to a new area easily, reproduce fast, and harm the economy, property, and wildlife of a region in order to be considered invasive.
When an invasive species cause extinction, it competes with native organisms for resources, reduces biodiversity, and alters habitats. Since 1500, invasive species are the primary cause of animal and plant extinctions for over 500 years.
What history has taught us
A study by the Frontiers in Ecology and the Environment discovered that around 13% of the 953 extinctions since 1500 were caused by an invasive species.
The black rat, for example, spread around the world through boats and caused the extinction of mammals, plants, birds, and reptiles.
When the red fox was introduced in Australia by the British, it drove out several mammal species including the lesser bilby. As early as 1900, the introduction of feral cats to New Zealand wiped out the Stephens Island wren.
See Related: Are Humans Animals? 15 Things to Know
7. Lack of food
How it could happen
All creatures hold a special place in the food chain, arranged neatly through an ecosystem. A smooth transfer of energy is only possible when the complex web of our ecological system remains intact, so when a particular species dwindles in population or becomes extinct, this leads to a disastrous cascading effect on others. Without food to eat, a species dies off, leaving other creatures that rely on them to die of starvation as well.
What history has taught us
The extinction of one species can threaten the viability of another. For example, in Britain, the population of red ants dwindled as a result of fewer sheep grazing on pastures.
Since red ants thrive in short grass, the lack of grazing sheep proved to be detrimental to their habitat. As a result, the population of red ants plummeted which led to the extinction of a large butterfly species that ate red-ant eggs as part of its life cycle.
A disruption in the food chain caused by the death of a single species can cause disastrous ecosystem-wide effects. The decline of sea otters can cause the overpopulation of sea urchins that munch on kelp for food. The resulting reduction of kelp forests can affect the marine species that rely on this type of habitat.
See Related: Endangered Species in California You Need to Know
8. Pollution
How it could happen
The contamination of air, water, and soil by the byproducts of our daily life can have detrimental effects on both humans and wildlife. Among the many causes of extinction, pollution is one that we can attribute directly to humans.
Marine life, in particular, especially seals, fish, crustaceans, and corals, and notoriously sensitive to traces of toxic chemicals in oceans, rivers, and lakes. At the same time, changes in oxygen levels caused by industrial pollution can strangle entire land populations.
To this day, many species have experienced pollution events that threatened their habitat and caused widespread death. Some of these species have even been pushed to extinction.
The two types of pollution are direct and indirect. The latter, which involves the toxic pollutants directly impacting the habitats of animals, is more easily studied.
Indirect pollution, or the destruction of the ozone layer and the development of global warming, on the other hand, is harder to quantify.
Still, all hope is not lost. Richard Emory’s book, Fighting Pollution and Climate Change, offers great insights into how we can minimize our byproducts and save the planet.
What history has taught us
As of 2019, around 69% of the 494 critically endangered species in the United States are predicted to continuously decline. Around 48 of the mentioned number is expected to suffer because of pollutants, wastewater, and excess energy pollution.
Marine life receives the brunt of our unstoppable generation of waste. The statistics are concerning: around 100 million marine animals perish each year due to plastic waste, and around 1 in 3 sea creatures die from getting entangled in plastic waste materials.
Over 500 locations in the world are considered as dead zones where marine life cannot exist.
See Related: 4 Types of Water Pollution You Need to Know About
9. Human Predation
How it could happen
What is the most likely cause of human extinction? According to the Future of Humanity Institute, human extinction is more likely to result from our own activities. Regardless, it’s downright unfair to blame all of the world’s extinction on humans, as we’ve only occupied the world for the last 50,000 years.
It’s hard to deny, however, how much we’ve contributed to the many causes of extinction thanks to the ecological havoc we wreak. Elizabeth Colbert’s book, The Sixth Extinction, details this event comprehensively.
We’ve hunted animals to the point of completely wiping them out, depleted entire populations of marine mammals, and killed the passenger pigeon and dodo bird at lightning speed. According to a Landmark United Nations-backed report, human activities are driving up to one million animal and plant species to the brink of extinction.
What history has taught us
We are one of the main causes of animal extinction, and statistics have proven this. While extinctions are always multi-faceted, we can mention a few events that can be directly linked to human action. The dodo bird, for example, was an easy source of fresh meat and was hunted to extinction in 1681.
Discovered in 1741, the Steller’s sea cow was an easy target for Russian seal hunters, who considered them a source of meat on long sea journeys. Years of wasteful killing decimated the population until they were completely exterminated by 1768, less than 30 years when it was discovered.
The Western Black Rhinoceros was considered extinct in 2011 when traditional Chinese medicine drove high demand for their horns and led to extreme poaching.
The sea mink, which lived along the coasts of New Brunswick and Maine, was hunted to extinction for its prized fur during the second half of the 19th century. Ruthless hunting was also the quagga cause of extinction during the 1880s.
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