Video Transcript
In this video, we will learn how to define extinction and investigate some key
reasons why species may go extinct. We will also learn how to explain the impact extinction can have on ecosystems and
biodiversity and some ways it may be prevented.
Perhaps you have heard of dinosaurs, the wide range of reptiles that once dominated
the Earth. But why do we not see any dinosaurs today? One of the proposed theories is that an asteroid impact in what is now Mexico caused
many species of plants and animals, including dinosaurs, to disappear. Other than those that developed into modern-day birds, most species of dinosaurs
became extinct.
Extinction means that a species no longer exists. All of the existing members of that species have died, leaving no offspring. Some other examples of extinct animals you may have heard of include the dodo bird,
the saber-toothed cat, and the woolly mammoth.
How does extinction happen? All species rely on a relatively stable environment that provides enough resources,
such as water, food, and shelter, in order to survive. Sometimes an environment can become unsuitable for a species or can fail to provide
the necessary resources. If all the members of the species die before having time to reproduce, the species
will ultimately become extinct.
Species’ population sizes and survivability depend on a few key factors. Some of these include the availability of food and water, their capacity to
reproduce, and population sizes of their predators. All the organisms living in a specific environment are interconnected with the
environment and with each other. A demonstration of the feeding relationships between some of the different organisms
in the African savanna are shown in this simplified food web, with pink arrows
demonstrating which organism feeds on another organism. Together, these connections form an interdependent system with the organisms’
environment and each other called an ecosystem.
The African savanna has long, dry seasons, but there is usually a lot of sunlight
available. In this environment, various species of grasses can grow. These grasses are food sources for zebras and gazelles. As long as there is enough grass, they have enough food to survive and reproduce. Zebras and gazelles in turn are food sources for lions, allowing lions to survive and
reproduce. Lions prevent the population of zebras and gazelles from overgrowing. With too many grazing animals, the grass would not have time to regenerate.
Now, let’s imagine how a drought might affect the fine balance of this ecosystem. If there is a prolonged drought, there will not be enough water for much grass to
grow. There will be more competition between all animals for drinking water and between the
animals that rely on grass for food. Many zebras and gazelles will die, and those that remain may produce fewer offspring,
so the numbers of grazing animals will decrease. Ultimately, there will be no more animals available for the lions to eat, and they
will die too.
The collection of all the various species living in an ecosystem constitutes what is
called its biodiversity. Biodiversity refers to the variety of different species living in a given area and
can be measured by the number of species. When the biodiversity is low in an ecosystem, this means that there are only a few
organisms. And these organisms may rely heavily on each other for food. Such an ecosystem, for example, a desert, is said to be a simple ecosystem. Simple ecosystems are rather fragile because any disturbances can directly affect the
survival of other species through a domino effect.
When an ecosystem contains many species with multiple connections between them, like,
for example, in a tropical rainforest, this is called a complex ecosystem. Complex ecosystems are more stable than simple ecosystems, because in a complex
ecosystem, if one species disappears, another one can compensate for its
absence. Thus, a large biodiversity of species makes a complex ecosystem more resistant to
changes.
There are many other relationships between species beyond food webs. Some species rely on other species for shelter, like this squirrel monkey relying on
this tree to nest in. Some species rely on each other even for reproduction. Some flowering plants would no longer be able to reproduce at all without the help of
insects like bees and butterflies pollinating their flowers.
Let’s see how disturbances in ecosystems may lead to species’ extinction. When an ecosystem is disturbed, an individual might not be able to find a partner to
reproduce with. Eventually, there may be too few individuals to produce offspring at all. When all those individuals themselves die, the species will become extinct. This process may happen very quickly or over multiple generations depending on how
fast the environment changes.
When a species has very few members left and struggles for survival because of its
changing environment but has not yet become extinct, it is referred to as an
“endangered” species. Endangered species are those that are at risk of becoming extinct.
There are thousands of examples of endangered species on Earth today. A particularly charismatic example of an endangered species is the Tapanuli orangutan
found on the island of Sumatra in Indonesia. In 2017, it was estimated that there are no more than 800 of these individuals left,
making them critically endangered and the species that is most endangered of all
great apes. The stresses of climate change, ongoing clearing of their protected forest habitat
for mining, power generation and logging, hunting, and illegal trade of young
orangutans is quickly leading to their decline and could result in this species’
extinction in the near future.
As we’ve just seen, changes in an environment that can lead to a species becoming
extinct vary. They include anything that can make survival and/or reproduction difficult. Some notable changes that threaten species’ survival are natural disasters, disease,
climate changes, and man-made causes.
Let’s look at how natural disasters can cause extinction. Natural disasters refer to any naturally occurring situation that results in great
damage to an area or loss of life in that area. Some examples of potential natural disasters include floods, hurricanes, earthquakes,
volcanic eruptions, or forest or grassland fires. Organisms can immediately die because of a natural disaster. For those that survive, the environment may be changed drastically by the disastrous
event so that it is difficult or impossible for the species to maintain its survival
in the long term.
One example of such a natural disaster is the asteroid impact in what is now the
Yucatan region of Mexico that is generally credited with having caused the
extinction of the dinosaurs. The asteroid measured about 10 kilometers across, and it left a crater that is
approximately 150 kilometers across. Many organisms would have been killed at the moment of the impact but also by the
long-term consequences of the impact.
The fossil records suggest that the asteroid impact raised a cloud of dust into the
atmosphere that blotted out sunlight over the Earth for a long time. This climate change is called an “impact winter.” As the sunlight was blocked out by the atmospheric dust, plants couldn’t produce
their own food by photosynthesis, a process that requires light, over an extended
period and therefore died. Other species that relied on those plants for food died as well. The extinction of the many different species of dinosaurs within a relatively short
period of time is an example of a mass extinction.
A mass extinction is the extinction of many species, usually about 75 percent of all
species living on Earth, in a relatively short period of geological time. This amount of time doesn’t give time for species to adapt their physical features to
the new conditions since the process of evolution of a species can take millions of
years to occur.
Today, most scientists agree that we are currently living in a period of time in
which the sixth mass extinction event is ongoing. This is because environments all over the world are currently changing drastically,
mostly due to human activities causing climate and habitat changes. Some of the human activities causing loss of biodiversity all around the world
include exploiting forests, expanding agriculture fields, or building new towns. This leaves less natural habitat and resources available for the native species in
that area.
Humans also may bring exotic, otherwise known as nonnative or invasive, species into
an area. Those species may then compete with native species for the same food or may be
predators of the native species, possibly causing the native species to eventually
go extinct. Humans can also push species to extinction by hunting them for food, for sport, or
because they are seen as pests. Uncontrolled hunting by humans has resulted in the extinction of several species
within recent history, such as dodos, passenger pigeons, and quaggas.
Let’s take a look at how the different organisms within ecosystems can be protected
from extinction.
Preserving biodiversity is one way to protect an entire ecosystem. The more species present and the more relationships there are among these species,
the less likely it is that the absence of one species will trigger the extinction of
all the species in that ecosystem. Maintaining biodiversity is therefore a high priority in environmental
conservation. Conservation is the maintenance of biodiversity by various means, for example,
scientific studies, education, and making laws to protect the environment. One very straightforward way to prevent the loss of biodiversity and thus extinction
is to create protected areas in which human activity is reduced to as little as
possible.
In addition, many countries have laws that forbid the hunting of endangered species
and poaching is heavily punished. Some countries aim at using renewable sources of energy, such as solar or wind
energy, to limit the effects of human activities on the environment. Education and raising awareness concerning these issues is also very important
because we can all contribute to protecting the environment.
For species that are highly endangered, some zoological parks have breeding programs
to help increase that species’ population size. Gene banks, which are locations where reproductive material including seeds and sperm
and eggs of animals is stored, are also used. They can help introduce more genetic diversity to endangered species, which can
promote their chance of survival. By using breeding programs and gene banks, we may be able to prevent the extinction
of some species and therefore reduce the environmental changes that both lead to and
result from species becoming extinct.
Let’s apply what we’ve learned about extinction to a couple of practice
questions.
The diagram shows a simple aquatic food chain. Based on this diagram, what is likely to happen to the killer whales if the sea lions
go extinct? (A) Their population will increase. (B) Their population will decrease. Or (C) there will be no change in the killer whale population.
The diagram shows us that algae are eaten by fish. The fish are eaten by sea lions. The sea lions in turn are eaten by killer whales, which are otherwise known as
orcas. Since sea lions are a source of food for killer whales, the loss of sea lions would
result in less available food for the killer whales. As a result, the killer whale population may die out as well, if they are not able to
find other sources of food. Therefore, as some killer whales in this situation may starve or simply not produce
as many offspring due to reduced energy intake, what is likely to happen to the
killer whales is described in answer option (B): their population will decrease.
Let’s have a go at another question together.
Some information on quaggas is provided in the following passage. The quagga was a species, closely related to the zebra, that lived in South
Africa. When Dutch colonists settled there, they thought that the quaggas would compete with
their domestic animals like goats and sheep for food. They also desired the skin of quaggas for decoration and would eat their meat. Using information from the passage, what was the reason quaggas went extinct? (A) Quaggas were hunted to extinction by Dutch settlers. (B) Quaggas were outcompeted by goats and sheep. (C) Quaggas died from diseases introduced by Dutch settlers and their livestock. Or (D) quaggas died due to a sudden change in the environment.
The passage in the question tells us two things that happened related to the quagga
when Dutch settlers arrived in South Africa. First, the settlers were concerned the quagga would compete with their livestock,
which suggests they may have wanted to get rid of the quagga. Second, the settlers ate the quaggas and used their skins. So, the extinction of the quagga was related to the Dutch settlers hunting and
killing them. Therefore, the reason why quaggas went extinct is best described in answer option
(A): quaggas were hunted to extinction by Dutch settlers.
Let’s recap some of the key points we have covered in this video about
extinction. Extinction is when a species ceases to exist. Extinction can be caused by natural disasters, diseases, a changing environment, or
man-made causes. Extinction of species decreases biodiversity, which makes an ecosystem less
stable. Protected areas, conservation laws, and gene banks are some methods that can be used
to prevent extinction.
