Ecosystem Disruption (HS Level) Student Learning Guide

1. Introduction: The Anthropocene Age and the Sixth Extinction

Wherever you live, a quick look at your surroundings will tell  you that we humans have an enormous impact on our surrounding environment.

Based on fossils and DNA, humans attained their current form about 300,000 years ago. About 60,000 years ago, humans started to develop more advanced technologies: bows and arrows, ceramics, sewing needles, fishhook, boats, and spear-throwers.

Even before humans invented agriculture — about 10,000 years ago — our impact upon the environment was significant. For example, 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. When humans reached New Zealand, a huge flightless bird called the Moa became extinct? Why? Almost certainly because of overhunting.

With the invention of agriculture and other technologies, and the vast increase in human population that has followed, our impact upon the environment and the other species with whom we share this planet has increased. In the past two centuries, with our population reaching the bilions and our civilization touching every corner of the globe, we have become a planet-altering force.

Human population is about to reach 8 billion people. It’s estimated that humans have developed over 50% of the Earth’s surface. According to many biologists and 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 age is to say that humans have become the dominant biological force on the planet. If you include our effects on the atmosphere (by releasing pollutants) and the water cycle (damming rivers), we’ve become a geological force, too.

Here are some of the effects our species is having on the living (and non-living) world:

1. The Sixth Extinction: Humans are causing a steep drop in biodiversity. Biodiversity is the diversity of living things. Human activities have accelerated the rate of extinction to somewhere between 100 and 1000 times its background rate. This increased rate of extinction qualifies as a mass extinction event, similar in scale to what happened when an asteroid impact wiped out the dinosaurs 65 million years ago. That was the fifth mass extinction. Human beings are the cause of the sixth extinction, which is happening right now.
2. Habitat destruction, modification, or degradation: Almost every animal or plant that is endangered or threatened is suffering from or destruction or modification of its habitat.
3. Invasive Species: As we’ve spread over the globe we’ve deliberately and inadvertently moved other species around the globe with us. Many of the species that have been introduced into new regions have caused significant changes in their new environments, often outcompeting or directly destroying native species. As we’ll see below, these invasive species have been a major driver of extinction.
4. Climate Disruption: Combustion of fossil fuels has changed the composition of our atmosphere. Carbon dioxide, which acts as a heat-trapping greenhouse gas, has increased from about 280 ppm (parts per million) before the Industrial Revolution to over 400 ppm today. This 30% increase has caused an increase in average global temperatures of about 1° C.  This has already led to significant melting of polar and glacial ice and melting of permafrost, both of which could accelerate warming trends. Global warming is also causing sea levels to rise,  changing rainfall patterns, and altering wildlife habitats.
5. Ocean Acidification: Absorption of carbon dioxide by the oceans has led to ocean acidification, which has affected marine life such as coral (which can’t form their exoskeletons if the pH is too low). As a result, coral reefs are in decline around the world.

If you want to learn more about climate change and ocean acidification, you can do so in our tutorial about the Greenhouse Effect and Climate Disruption. 

2. Flashcards: The Anthropocene Age

3. It’s not that humans are bad. What The Matrix Gets Right (and Wrong) about Humans and Biodiversity

One of my favorite movies is The Matrix. This science fiction movie is about a band of humans who are trying to free the rest of humanity from enslavement by an artificial intelligence. While human bodies provide energy to their robotic masters, humans minds are trapped inside a vast computer simulation. Within that simulation, “intelligent agents” keep humans under control.

Here’s how one of these intelligent agents — Agent Smith (played by Hugo Weaving)— explains himself to Morpheus, one of the leaders of the human resistance movement (played by Laurence Fishburne).

Note: My guess is that there might be copyright issues with this excerpt, and that YouTube won’t allow this video to last for very long,  Please email me if the video is missing. Just in case, here’s a transcript of what Agent Smith says to Morpheus.

Agent Smith: I’d like to share a revelation that I’ve had during my time here. It came to me when I tried to classify your species. I realized that you’re not actually mammals. Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment but you humans do not. You move to an area and you multiply and multiply until every natural resource is consumed. The only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus. Human beings are a disease, a cancer of this planet. You are a plague, and we are the cure.

Agent Smith gets a few things right: For tens of thousands of years, our pattern as a species was to move to an area, consume the resources there, and then move on. That’s what nomadic hunter gatherers do. This worked fairly well when all of Earth’s human population numbered no more than 10,00,000 or so (the estimated number before the dawn of agriculture, about 10,000 years ago). We might have hunted the woolly mammoth and the giant sloth to extinction (see below), but the Earth’s basic systems were relatively unharmed.

But it’s not true that “Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment.” Every mammal exploits its environment as effectively as it can. If an equilibrium is reached, it’s because of regulation from below (a lack of resources such as food) or from predators, parasites, or competitors that keep populations in check.

We humans have been doing what every animal does. We’ve tried to exploit our environment as effectively as we can, and to reproduce as much as is natural for our species. The problem is that we’re so smart that we figured out (for the most part) how to expand our food supply, to destroy or marginalize our potential predators, to outcompete our competitors, and cure ourselves (mostly) of parasites. With the advent of agriculture and then industrialization, this has allowed our population to expand into the billions, and allowed us to modify the environment to meet our ends. That, however, has been disastrous for most of the other species we share the planet with. Hence, the Sixth Extinction of the Anthropocene.

Let’s see what we’ve done.

4. If a species is endangered or threatened with extinction, it’s most likely because we’ve destroyed or degraded its habitat

If an animal or plant is endangered or threatened, habitat loss or degradation is the most likely cause.

• Habitat loss is when an animal or plant’s habitat is completely destroyed (usually because it’s been changed into something else). When a forest is cut down and becomes a farm, that’s habitat loss.
• Habitat degradation is when the quality of a habitat declines. When a bay becomes polluted and its native species are replaced by species from other areas, that’s habitat degradation.

All of the following activities involve destruction or degradation of natural habitats:

1. Development of housing and urban areas, commercial and industrial areas, recreational areas such as golf courses and skiing slopes, etc.
2. Agriculture (farming) and aquaculture (farming the seas).
3. Energy production and mining. Mining for coal, which can involve removal of entire mountain tops, is particularly destructive.
4. Fishing methods in which nets are dragged 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

How much of the Earth’s surface have humans developed? This point was mentioned above, but it’s worth repeating. 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. 

In terms of the Sixth Extinction, how significant is habitat loss or degradation?

• 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 building cities, or fishing or forestry, etc. can be done in ways that protect biodiversity. The problem is the extent to which (and sometimes the way in which) we humans have utilized the Earth’s land and water resources in order to support ourselves. And with human population approaching 8 billion, our ability to maintain our own species while providing habitat for other species is becoming increasingly challenging.

5. Habitat Fragmentation

5a. What is Habitat 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 .

5b. Fragmentation results in insufficient area for populations to maintain themselves

An ecological niche is the way a species makes its living. 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 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).

5c. Fragmentation causes population bottlenecks

An idea from population genetics is that a healthy population needs to have enough individuals to maintain a certain level of genetic diversity. If the level of genetic diversity becomes too low, a population will suffer in two ways.

1. It won’t be able to reproduce healthy offspring.
2. It won’t be able to adapt to changes in its environment.

A population bottleneck occurs when a population falls below a healthy level. To avoid a population bottleneck, populations need to either 1) maintain themselves above a certain minimum level, 2) exchange genes with neighboring populations, or both.

To stick with the example of lions, an fragment with only one or two prides is prone to a population bottleneck. The result would be inbreeding and loss of genetic diversity.

As we’ll see below, understanding the effect of fragmentation makes it possible for wildlife refuge designers to prevent population bottlenecks from happening. How? Make each population’s territory connect with an adjacent population’s territory. In the case of lions, that would allows for genes to flow between prides. Then, instead of there being several isolated small gene pools, there would be one larger gene pool: large enough to keep the population’s genetic diversity intact.

5d. 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 at left, fragmented areas have much more edge habitat. This habitat experiences much more disturbance, which creates stress for the organisms living at the edge.

What kind of disturbances? In a forest, these include wind, weather, and dryness. In one long term study of the fragmented areas in a Brazilian rainforest, researchers found that forest areas up to 2.7 kilometers away from a clearing had much less moisture than interior areas further away from the edge. In addition, the force of wind on the trees at the forest’s edge led to higher tree mortality (source: Brazil’s Fragmented Forests).

5e. 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. Factors like the edge effect and fragmentation lead populations to become small and isolated.
2. Low population size causes a population bottleneck, which decreases genetic diversity. In the diagram, this is shown in the boxes that are labeled as “genetic drift, inbreeding” and “loss of genetic diversity.”
3. Loss of genetic diversity makes the population less fit and less able to adapt to change.
4. With lowered fitness, the population can’t reproduce, shown in the box labeled “less reproduction, higher mortality.”
5. Less reproduction and higher mortality leads to higher population decline.

The extinction vortex is a vicious cycle: a positive feedback loop with increasingly negative consequences As a result species that enter the vortex often quickly decline toward extinction.

5f. Habitat Fragmentation Flashcards

6. Threats to Biodiversity (part 1): Checking Understanding