1. Introduction

Congratulations! You’ve reached the last tutorial of Learn-Biology.com’s AP Biology v.2.0 curriculum.

Since biology is the study of life, we should end by looking at the impact that our species is having on the living world.

2. Welcome to the Anthropocene Age and the Sixth Extinction

For as long as we’ve been a species, we humans have caused significant changes to the environment.

Even before agriculture, when humans lived as nomadic hunter-gatherers, we never lived in peaceful harmony with the environment. As humans spread from Africa to populate the rest of the world, waves of extinctions of megafauna (large animals including woolly mammoths, mastodons, and giant sloths) quickly followed. When we spread to Europe, most European megafauna became extinct. When we spread to North America, most North American megafauna became extinct. Why? Almost certainly because of overhunting.

As our population and technological prowess have grown, so has our impact. According to many geologists, we’re now living in the Anthropocene Age.

• The root “anthropo-” refers to humans, as in “anthropology.”
• The suffix “-cene” denotes a geological age).

To say that we’re living in the Anthropocene is to say that humans have become the dominant biological force on the planet. If you include our effects on the atmosphere (such as emitting gases like carbon dioxide, methane, and a variety of pollutants) and the water cycle (damming and otherwise controlling rivers), we’ve become a geological force, too.

As we’ve discussed in previous tutorials about biodiversity and extinction, human activity is resulting in the Sixth Mass Extinction. Let’s see what we’ve done.

3. Destruction of habitat

All of the following human activities take natural habitats that other species live in and convert them into artificial, non-natural habitats where humans live, play, and work.

1. Development of housing and urban areas, commercial and industrial areas, and developed recreation areas (golf courses, skiing areas, etc. )
2. Agriculture (farming) and aquaculture (farming the seas).
3. Energy production and mining. Mining for coal, which can involve the removal of entire mountain tops, is particularly destructive.
4. Fishing methods such as bottom trawling for shellfish or dredging. These types of fishing drag nets along the ocean floor, damaging or destroying whatever they encounter.
5. Forestry methods such as clear-cutting.
6. Natural system modification that changes habitats. This includes
   1. Fire suppression and/or Increasing fire frequency
   2. Damming rivers

Once converted into artificial habitats, the species that used to inhabit that area have to flee or perish.

If an animal or plant is endangered or threatened, habitat loss or degradation is the most likely cause. 80% of all threatened birds are experiencing habitat loss or degradation. The same is true for 83% of all threatened mammals, and 91% of all threatened plants.

Note that none of these activities are bad in and of themselves. People need houses, food, materials, energy, and so on. It’s the extent to which and the manner in which we’ve utilized the surface of our planet and the waters of our planet to support ourselves.

How much of the Earth’s surface have humans developed? In a 2012 report by the Geological Society of America, Roger Hooke and Jose Martin Duque estimated that humans have modified more than 50% of the Earth’s land surface. 

4. Habitat Fragmentation

4a. What is Fragmentation?

In addition to destroying habitat, one of the main ways that humans have affected the environment has been to fragment it into discontinuous, unconnected pieces. For any non-flying species, any difficult-to-cross highway will cut their territory into separate fragments. Even flying species like bats and birds might refrain from flying from an enclosed habitat (like a forest) through open territory to get to the next bit of forest.

Fragmentation causes several problems for the species living in these fragments.

4b. Fragmentation results in contiguous areas that are too small for populations to maintain themselves

Depending on its ecological niche, any population of a species needs a certain amount of area to maintain itself. For top-level carnivores, a sustainable amount of area can be difficult to fit within any sized reserve, much less a fragment of a reserve. A pride of lions, for example, needs 20 square kilometers to maintain itself when prey is abundant. If the prey is scarce, that same pride might need 400 square km. (source: Britannica).  A wolf pack needs 130 square kilometers to sustain itself when prey is abundant. That number increases to 2600 square kilometers when prey is scarce. (source: US Fish and Wildlife Service).

4c. Fragmentation causes population bottlenecks

To avoid a population bottleneck that leads to loss of genetic diversity, populations need to 1) maintain themselves above a certain minimum level, and 2) exchange genes with neighboring populations. To stick with the example of lions, an area with only one or two prides is prone to a population bottleneck where random genetic drift will cause each small population to lose genetic diversity. To prevent that, each pride needs its territory to adjoin with an adjacent pride’s territory. This allows for genes to flow between prides, and keeps each pride from suffering from inbreeding. In an isolated fragment, that’s exactly what happens: populations experience a bottleneck that leads to inbreeding and loss of genetic diversity through genetic drift.

4d. Fragmentation causes the Edge Effect

The boundary of any natural area has an edge, and the environment at the edge is often quite different from the environment in the interior. As you can see on the left, fragmented areas have much more edge habitat. This habitat experiences much more disturbance, which creates stress for the organisms living at the edge.

One of the largest studies of the edge effect is the Biological Dynamics of Forest Fragments Project. This project, set in Brazil’s Amazon rainforest, involves dozens of study fragments of different sizes, along with undisturbed areas of equal size deep within undisturbed tracts of forest. These fragments and the control areas have been studied for over 40 years.

Here’s an example of what researchers found. (source: Brazil’s Fragmented Forests).

The open flank of a newly created clearing leaves the remaining forest more vulnerable to wind and weather. Wind shear and turbulence forces on the forest edge lead to higher tree mortality. Desiccation caused by the inferior water retention capacity of the clearing adds to the damage…Research has shown that the drier air above a clearing can then suck the moisture out of the adjoining forest and cause desiccation penetrating hundreds of meters deep into the forest. Recent satellite measurements of moisture in the canopy even indicate that the effects can still be felt 2.7 kilometers away from the nearest forest edge.

4e. Fragmentation and the Extinction Vortex

When a population is stuck in a fragment of insufficient size, it can get sucked down into a death spiral called the extinction vortex. A whirlpool above a drain is an example of a vortex. In the same way that a vortex pulls water down a drain, an extinction vortex pulls a population toward extinction. The extinction vortex works like this:

1. Ecological disturbances make it difficult for a species to successfully exploit its niche. As a result, there’s a decrease in the population size. Fragmentation increases the chance that the population will be isolated
2. This lowered size leads to genetic drift, which decreases genetic variability.
3. Decreased variability makes the population less fertile and less able to adapt to change.
4. Additional disturbances lead to further population decline.

Because the system is a positive feedback loop, species that enter the vortex often quickly decline toward extinction.

5. Disruption of Ecosystems Quiz 1

6. Overharvest/Hunting to Extinction

As omnivores, humans have always harvested animals for food (or other animal parts, such as skins, bones, guts, or horns). In terms of biodiversity, this is only a problem when harvest becomes overharvest. As discussed above, humans have an ancient legacy of hunting their prey to extinction. This goes back at least as far as the extinction of animals like woolly mammoths 10,000 years ago.

With modern hunting methods, this process has accelerated. In the 1800s, the American Bison was nearly hunted to extinction. Their numbers were reduced from 60,000,000 to just a few hundred animals by 1900. (source: Wikipedia). Under subsequent protection, they now number about 500,000.

A North American bird called the passenger pigeon was once thought to be the most abundant bird in the world. Witnesses from the 1800s reported how it could take hours for a single flock to pass overhead. Overhunting led this species to become extinct in 1914 when the last surviving Passenger Pigeon died in the Cincinnati Zoo.

Other animals that have become extinct because of overharvesting include:

• The Stellar’s Sea Cow (a northern relative of the manatee, extinct since 1768)
• The Bubal Hartebeest (an antelope that lived in Northern Africa and the Middle East, extinct since the mid-1900s).
• The Pyrenean Ibex (a wild goat that lived in the Pyrenees Mountains between Spain and France, extinct since 2000).
• The Western Black Rhino. This rhinoceros population was hunted to extinction within the last decade. Rhinos are killed for their horns, which are used in traditional Chinese and Vietnamese medicine. There are five existing rhinoceros species, and all are endangered.

Animals that have been killed for predator control include:

• The Grizzly Bear: The last Grizzly in California was killed in 1922. Ironically, California’s official animal is the Grizzly bear.
• The Gray Wolf: The gray wolf was wiped out in the lower 48 states in 1926. However, wolves were reintroduced into Yellowstone National Park in 1995.
• The Tasmanian Wolf: The Tasmanian wolf, which lived in New Guinea, Australia, and Tasmania, became extinct in the 1930s.

While there’s a growing movement for sustainable fishing, many fish stocks have been overexploited. According to the Food and Agriculture Organization of the United Nations, “the state of marine fishery resources…has continued to decline.” Whereas the proportion of sustainable fish stocks was 90% in 1974, that number fell to 65.8% in 2017.

7. Introduced/Invasive Species

Introduced species (also known as non-native or exotic species)  are species that were moved intentionally or accidentally from one location to another. In that new area, freed from the controls that kept their population in check, they often grow exponentially. If that’s the case, they become biological invaders: invasive species.

A few characteristics predispose species to becoming invaders. See if you can complete the following table.

You don’t have to look far to find invasive species. They’re everywhere. If you have a house cat, then you own a particularly destructive invasive species. House cats kill between  1.3–4.0 billion birds and 6.3–22.3 billion mammals annually. These kills are both by pets and feral cats (cats without an owner that live on their own). Source: Nature Communications, The American Bird Conservancy.

Rats are another invasive species. They’re responsible for 40 to 60 percent of all bird and reptile extinctions on islands. (source) The famously extinct Dodo bird of the island of Mauritius, once thought to have been hunted to death by Dutch sailors who first reached the island in the 1600s, was probably done in by rats, which raided Dodo nests and destroyed their eggs.

While birds are often the victims of invasive species, certain bird species are also invasive. The European Starling was released into New York’s Central Park by Shakespeare enthusiasts in the 1890s (because starlings are mentioned in Shakespeare’s plays). Now there are 200 million starlings in North America. They compete with native birds for nest sites, and with other insect-eating birds for food.

The House Sparrow, one of the most commonly seen birds in North America, is another invasive bird species. These birds were introduced into Brooklyn, New York, in 1851. Additional introductions occurred in San Francisco and Salt Lake City in the 1870s. Since 1900, they’ve been established in all of the lower 48 states.

Amphibians are the vertebrate clade that’s experiencing the highest rate of extinction, with over 40% of the world’s 8000 species listed as threatened or endangered. Why? The main cause is an invasive fungal pathogen called Batrachochytrium dendrobatidis (Bd). According to Australia’s Department of Sustainability,

The fungus invades the surface layers of the frog’s skin, causing damage to the outer keratin layer. Amphibian skin is unique because it is physiologically active, allowing the skin to tightly regulate respiration, water, and electrolytes. It is not yet known exactly how the fungus kills amphibians but it is thought that it may cause mortality by disrupting the normal function of the skin resulting in electrolyte depletion and osmotic imbalance.

Bd has caused declines in 500 amphibian species worldwide and has driven dozens to extinction. The map above shows the extent of amphibian decline worldwide.

Plants can be invasive, too. As they spread, they can overrun entire ecosystems, out-competing native plants and disrupting food webs, causing biodiversity losses. The image on the left shows one invasive plant called kudzu. Kudzu, which originated in Asia, was promoted as an ornamental plant and later grown for forage (food for animals) in the southeastern United States. Millions of acres were deliberately planted to reduce soil erosion in deforested areas. But with an ability to grow up to a foot each day, kudzu has overgrown entire forests.

• The Asian Citrus Psyllid is a gnat-sized insect. It carries a bacterium that causes citrus greening, a disease that kills citrus trees.
• The European Gypsy Moth is originally from France. It was introduced into the United States in the 1890s. Its caterpillars devour budding leaves, which can weaken and kill a variety of trees such as oaks, maples, apples, aspens, willow, birch, and pine. Its relative, the Asian Gypsy moth, has recently been detected in Washington State, Oregon, Oklahoma, and South Carolina. With a wider host range than its European cousin, it’s feared that the Asian Gypsy moth could cause significant damage throughout the United States and Canada.
• Cryphonectria parasitica is a parasitic fungus that attacks chestnut trees, causing a disease called chestnut blight. Introduced into the United States in the 1900s, it killed an estimated 4 billion chestnut trees. These trees were habitats for many native species of birds, insects, etc, so the loss of chestnuts had cascading ecological effects.

8. Invasive Species Case Study: The Japanese Barberry

Let’s look in a bit more detail at how an invasive plant species, the Japanese Barberry, can have effects that cascade throughout an ecosystem.

The Japanese Barberry, a shrub with green leaves and bright red fruit, was introduced to North America as an ornamental plant in 1975. It soon began to spread, growing in dense, prickly thickets.

Unlike other shrubs, the barberry is avoided by deer, enabling it to grow without any top-down control.

In a recent study, Japanese barberry was found to reduce the number of arthropod herbivores (such as insect larvae or caterpillars) and predators (such as spiders). Species richness was “significantly lower in the leaf litter around the Japanese barberry and on the … plants themselves.” Compared to otherwise similar landscapes that had not been invaded, the barberry-infested community structure is much less complex, a process known as “trophic downgrading.”

Use the interactive diagram below and the questions that follow it to demonstrate your understanding of the Japanese Barberry’s effect on biodiversity. The term detritivore refers to an organism that eats detritus — leaves and other organic material that falls to the ground.

9. There are many other threats to biodiversity

There are entire courses devoted to learning about threats to biodiversity. The ones listed above are most relevant to an AP Biology course. But there are others, including pollution and climate change. One effect of pollutants in the environment is biological magnification, which you can learn about by following the preceding link to our module on Trophic Levels and Ecological pyramids. If you’re interested in learning more about climate change, you can go to this page for a series of music videos and a tutorial about the Greenhouse effect and Global warming.

10. Threats to Biodiversity Quiz 2

11. What can be done?

Stemming the tide of the Sixth Extinction will require changes in the way we humans interact with one another and with the natural world. Some of these changes include

• Controlling human population growth. The Earth is finite. To a large degree, more people on the planet means more resources (especially land) that get allocated to meet human needs. The more land that gets devoted to human needs, the less undeveloped habitat there is for wildlife.
  A hopeful trend already underway is that the growth rate for the human population is dropping. This process is called the demographic transition.  Elevating the economic and educational status of women —a good thing in and of itself— is the most effective way to limit human population growth. Why? Because women with more economic and educational opportunities tend to have smaller families, simply because they have their first child later in life. So promoting women’s equality worldwide helps humanity, promotes biodiversity, and enhances equality and social justice.
• Shifting from fossil fuels to renewable energy to reduce climate change. Anthropocentric climate change is creating habitat disturbances at a rate that wildlife can’t adapt to. If we want to maintain habitat, especially in tropical areas that are the highest in biodiversity, we need to slow the rate of human-caused climate change (or stop it altogether). To do that, we need to stop releasing carbon dioxide and other greenhouse gases into the atmosphere.  And to do that, we need to shift from fossil fuel combustion in cars, factories, and power plants to non-carbon-based energy sources like wind and solar.

On a smaller scale, we can take some actions to rescue and preserve species that are in (or on the verge of entering) the extinction vortex. So to start, let’s review how the extinction vortex works.

11a. Protecting endangered species

Species are disappearing at such a high rate that many are being lost before we can take any action. However, legislation like the US’s Endangered Species act, passed in 1973, has made it a legal obligation to “protect and recover imperiled species and the ecosystems upon which they depend” (US Fish and Wildlife Service). Because of laws like the Endangered Species Act, many species are recovering from direct or indirect threats. This includes the California condor, the Black Footed Ferret, the Florida Manatee, the American Alligator, and many others (see this article in the Washington Post).

11b. Renewing Genetic Variation in Declining Populations

In some cases, it’s possible to introduce genetic diversity into a declining population. This was what was done to prevent the Illinois population of the Greater Prairie Chicken from becoming extinct.

Greater Prairie Chickens are known for their incredible mating displays. Here’s how this is described in All About Birds from Cornell University.

Few performances in the bird world are more memorable than the dawn display of Greater Prairie-Chickens at their booming ground, or lek—the traditional spot where males dance, call, and try to impress females with their vigor. When displaying, the males erect ear-like plumes on the head and blow up bright orange air sacs on the neck, transforming themselves from brownish chicken-like birds into brightly colored performers, all the while drumming with their feet and producing whooping and cackling calls.

Here’s a short video of these birds in action.

Based on historical records, Greater Prairie chickens in Illinois had experienced a steep decline in numbers and distribution, as shown below.

Based on everything you’ve learned above, you could expect that by 1994, the two isolated Illinois populations were experiencing genetic drift and reduced fertility.