About Module 10

Biodiversity is a measure of variation and richness of living organisms at a particular scale. In this module, we are going to learn some of the important roles that biodiversity plays in human systems. The module begins by explaining what biodiversity is, what causes biodiversity, and why we care about it. The module then discusses biodiversity loss throughout history and around the world today. Human activity is causing extensive and alarming biodiversity loss, with many species going extinct. But humans are also active in conserving biodiversity. The module closes with a discussion of threats to the extinction of one particularly noteworthy species: humans.

What will we learn in Module 10?

By the end of Module 10, you should be able to:

• define biodiversity and explain the value of biodiversity in both ecocentric and anthropocentric terms;
• describe geographic trends in biodiversity, including factors that influence biodiversity and biodiversity hotspots;
• discuss how changes in biodiversity are influenced by processes at many spatial and temporal scales;
• describe problems associated with biodiversity loss, as well as progress in protecting biodiversity.

What is due for Module 10?

There are two reading assignments and a Written Assignment associated with Module 10. 

Questions?

If you have any questions, please post them to our Course Q & A discussion forum in Canvas. I will check that discussion forum often to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate. If you have a more specific concern, please send me a message through Inbox in Canvas.

What is Biodiversity?

“The most wonderful mystery of life may well be the means by which it created so much diversity from so little physical matter. The biosphere, all organisms combined, makes up only about one part in ten billion of the earth’s mass. It is sparsely distributed through a kilometer-thick layer of soil, water, and air stretched over a half billion square kilometers of the surface.”

The variety of life on Earth is immense and wondrous, as this quote by famed ecologist E.O. Wilson suggests. About two million species have been described by scientists. On an average day, about 300 new species are documented. Some scientists estimate that there are as many as 50 million species alive in the world today.

Biodiversity is a measure of variation and richness of living organisms at a particular scale. It can be measured on an extremely small scale, such as the number of organisms living in a spoonful of soil, or on a large scale, such as the whole earth. Biodiversity can also be thought of on several levels of biological variation, ranging from genetic diversity within a species to species richness within whole biomes. The biodiversity of a particular place, region, or landscape is influenced by climate, topography, and geologic history, as well human and non-human disturbances.

Why biodiversity matters

Humans have many reasons to value biodiversity, including anthropocentric reasons and ecocentric reasons.

Anthropocentric reasons to value biodiversity include the many potentials for different lifeforms to provide scientific information, recreational benefits, medicine, food, or other materials that are useful to us. Even if we don’t know what exactly some species or ecological community might be useful for, we may choose to protect it, just in case it turns out to be useful.

Ecosystem services are the services that ecosystems perform for humanity. They are a popular way of characterizing the variety of anthropocentric values surrounding parts of nature, including biodiversity. Animals, plants, and other components of every ecosystem do many things for humans such as purifying water and air, pollinating crops, maintaining a proper heat balance in the atmosphere, and cycling critical nutrients.

When we speak of these natural processes as ecosystem services, often we are imagining them in an economic context. For example, we might consider how much it would cost for flood control and soil erosion prevention if the vegetation in a particular region wasn’t providing the “services” of absorbing much of the rainfall, slowing down runoff, and holding the soil in place with its roots. In the case of biodiversity, one team of researchers from Minnesota recently showed that prairies with high species richness are more drought-resistant than those with fewer species. In many cases, high biodiversity enables a region to be more resilient and to continue providing important basic “ecosystem services.”

Ecocentric reasons to value biodiversity are based on the idea of biodiversity having intrinsic value irrespective of any potential human uses (refer back to Module 3 for a refresher). An ecocentric perspective on valuing biodiversity would include conserving coral reefs or redwood forests on the basis that these ecosystems have a right to exist, irrespective of how, if at all, they might benefit human society.

Given the value of biodiversity, its protection has become a primary conservation concern during the past several decades. As you work through this module, consider whether you think that anthropocentric or ecocentric arguments are more likely to be successful in conserving biodiversity. What are the strengths and weaknesses of each? What type of argument would you construct if you wanted to save a particular species or protect a natural area?

Conserving biodiversity is the main issue driving the rapid growth of protected areas around the world, places like biosphere reserves, national parks, and wildlife refuges. Sometimes protected areas are developed to protect one particular species, or to keep certain types of habitat intact. Learning about protected areas is important not only because they are important for protecting biodiversity, but also because these areas are often laboratories for studying human-environmental change. These are the sites where new ideas about environmental management are tried out, where mistakes are made, and where lessons are learned about how to balance the needs of humans with the values of biodiversity.

What factors influence biodiversity?

Biodiversity is heavily influenced by both human and non-human factors. Throughout the module, we will spend a significant amount of time studying the negative impacts of humans on biodiversity. However, it’s important to remember that humans have a very complicated relationship with biodiversity. In some cases, human activities enhance biodiversity through habitat modification or periodic disturbance. In others, certain types of biodiversity are favored over others because of human influences. For these and other reasons, it is helpful to always think of biodiversity as part of a human-environment coupled system.

Biodiversity varies significantly among different regions. Polar icecaps and tropical deserts are almost devoid of life, while tropical rainforests and coral reefs are extremely biodiverse. A forest in the mid-latitudes, in places like Pennsylvania, might have 30-40 tree species per square kilometer, whereas a square kilometer of tropical rainforest in Borneo or Ecuador might have 300-400 species.

Geographers, ecologists, and conservation biologists have learned a great deal about the patterns of biodiversity around the planet. One pattern that’s important to recognize for our purposes is that the number of species is much higher near the equator and decreases as you move toward the poles. This is known as the latitudinal gradient of species richness and is largely shaped by the availability of energy and water in each respective region. This general pattern is apparent on every continent of the world. It may also exist in the oceans, although researchers have not yet collected enough data about oceanic biodiversity.

On a smaller scale, other factors have a significant influence as well. Factors that seem to foster an increase in biodiversity include:

Physically diverse habitats. If a region has a variety of different “microclimates” caused by variability in topography, water availability, and sunlight, it’s likely to have more biodiversity than a more uniform landscape.

Moderate disturbance. Disturbances include weather or geological events, fires, or insect outbreaks. If disturbances are too extreme, such as a volcanic eruption that kills all the vegetation in a region, then biodiversity is reduced, but a moderate amount of disturbance helps create a variety of habitats and fosters evolution. Humans practicing slash-and-burn agriculture in a tropical rainforest can create this type of moderate disturbance in some cases.

Large area. Regions that are a part of a large, connected land mass are likely to have higher biodiversity than those that are geographically isolated. Small islands that are far away from the mainland of a continent will have fewer species than large islands that are near the coast.

Longevity of system. If a particular region has been spared from extreme disturbance events like being covered by glaciers or volcanic ash, or being clear-cut or plowed by humans, it is likely to have a higher level of biodiversity. This is true even on a very long time-scale. For example, there are 85% more coral species in the Pacific Ocean than in the Atlantic Ocean, because the Pacific is a much older ocean basin.

So, if you are in a continental region of the world that receives a lot of sunlight and rainfall and is buffered from extreme disturbance events, you can expect it to be highly biodiverse. On the other hand, in isolated regions or those with low water or sunlight availability, or those that are subject to frequent extreme disturbance, you’ll likely find quite a bit fewer species.

One goal of this overview is to emphasize how important the concept of scale is to understanding and studying biodiversity. It’s important to both think about biodiversity on a very large scale, such as a biome or continent, and on a very small scale, such as a farmer’s field or a particular section of a stream. Understanding the factors that shape biodiversity on these different scales is quite challenging but also incredibly interesting and important.

Biodiversity Hotspots

A biodiversity hotspot is a region with a high amount of biodiversity that experiences habitat loss by human activity. In order to qualify as a biodiversity hotspot, according to Conservation International, “a region must contain at least 1,500 species of vascular plants (>0.5% of the world’s total) as endemics, and it has to have lost at least 70% of its original habitat.” Today, 34 hotspots have been identified around the world. While these areas once covered about 16% of the Earth’s land surface, today 86% of their habitat has been destroyed. Even though now hotspots only cover about 2% of the land, 50% of the world's vascular plants and 42% of land vertebrates are endemic to a hotspot. To get a better understanding of the distribution of biodiversity hotspots around the world, please view the following biodiversity hotspots map produced by Conservation International.

Figure 10.1 Conservation International Hotspots 

Credit: Biodiversity Hotspots Map by CEPF is licensed under CC BY-SA 4.0

The biodiversity hotspot concept highlights the coupledness of biodiversity and humanity. The concept, first suggested in 1988 by Norman Myers, arose from growing concern among ecologists and environmentalists about the rapid loss of habitat in areas of high biodiversity and endemism. Endemism means that a species only lives in a particular region of the world, which means that if it is wiped out there, it’s lost forever. For example, the now-extinct Dodo bird was endemic to Mauritius, a small island in the Indian Ocean.

One example of a hotspot is the Irano-Anatolian region, which forms the boundary between the ecosystems of the Mediterranean Sea and the plateaus of Western Asia and includes 2,500 endemic plants. Its original extent was about 900,000 square kilometers, stretching from Turkey to Turkmenistan and Iran, but today only about 135,000 square kilometers of original vegetation are left. The most significant threats to this hotspot are large-scale irrigation projects, overgrazing, and unsustainable timber harvesting. The human population has doubled in this region since the 1970s, so even traditional livestock grazing and wood-gathering practices have put increased pressure on the region’s resources. Huge areas of swamps have been drained and converted to growing sugar beets and other crops using industrial agricultural methods. The political instability of the region and active military conflicts also undermine conservation efforts.

A Historical Perspective on Biodiversity Loss

About 99.9% of species that have ever lived on earth are now extinct, but at the same time, there are likely more species alive during the current era in geological history than at any previous time. Why is this?

Since the first cellular life appeared about 3.8 billion years ago, new life forms have been constantly evolving and some species have been going extinct. Since life on Earth is so old, most of the species that have ever lived are now gone, even if they persisted for millions of years. There have been periods of biodiversity explosions, as well as periods of mass extinctions, but generally, the trend has been toward an increase in the variety of life forms on this planet. Speciation rates (the rates of new species coming into existence) are high following mass extinction events and have been increased by the evolution of body types that allow animals to inhabit all types of habitats like deserts, soils, thermal ocean vents, and the sky. Also, the breaking up of Pangaea into separate continents has fostered an explosion in the number of species on Earth.

We should realize that humans are not responsible for most of the extinctions that have happened on Earth. At the same time, humans have been influencing biodiversity for a long time, and human-caused extinctions are not a new thing at all.

Early Anthropogenic Extinctions

During the end of the last ice age (known as the Pleistocene), about 10,000 to 15,000 years ago, many of the large mammals, birds, and reptiles, collectively known as megafauna, went extinct in North and South America. Mastodons, mammoths, giant beavers, and saber-toothed tigers, along with many other species, disappeared in a fairly short period of geologic time.

While we do not have direct evidence of what caused their extinction, most researchers believe that overharvesting of wildlife by humans played a decisive role in many extinctions. The extinctions roughly coincide with the arrival of humans into the Americas, and a similar story is apparent in Australia, although human arrival there was much earlier.

It is important to note that during this period the climate was warming rapidly (due to natural, not human causes), and vegetation was changing as a result. Therefore, humans were not the only stress that may have damaged populations of these megafauna species. On the other hand, these species had persisted through significant climate fluctuations in the past, and the major new factor when they became extinct was the presence of humans.

Another striking example of human-caused biodiversity loss from before the modern era comes from Polynesia in the southern Pacific Ocean. Humans caused the extinction of over 2000 species of birds as they colonized these tropical islands between 1000 and 3000 years ago. Among the factors causing extinction were direct harvesting, habitat alteration, and the introduction of predators like pigs and rats. Flightless birds were particularly vulnerable to human and non-human hunters, and many of them went extinct.

One important lesson to draw from these two examples is that even people whom we identify as “native” or “indigenous” to a place can cause extinctions. It can be tempting to imagine that Western civilization, capitalism, or other “modern” ideas or technologies are the root cause of biodiversity loss, but that belief is not supported by this history. It is vital that we view indigenous peoples not as somehow “one with nature” or in perfect harmony with their ecosystems, but as dynamic and diverse human cultures that have long played important roles in shaping the landscapes that they inhabit. That said, there are valuable lessons that we can learn from indigenous cultures about how to maintain functioning ecosystems and biodiversity while providing for basic human needs.

European Colonialism

The above example of Polynesian colonialism was a precursor to the massive colonial efforts by European nations from the 1400s through the 1800s. European colonialism had massive impacts on biodiversity through the exchange of species between Europe and colonized regions, the conversion of habitat, and over-harvesting of species that led to extinction.

The transfer of plants, animals, and microbes between continents during this era is known as the “Columbian Exchange.” One of the most dramatic impacts of this exchange was the introduction of European diseases into Native American populations that had no immunities to them. These diseases caused declines in indigenous populations of up to 90% in some cases, crippling social systems and subsistence harvesting, altering long-established practices like burning and agriculture, and leading to large cities simply disappearing in many parts of the Americas. Because of these diseases, much of the interior regions of North and South America became much less populated than they had been for thousands of years.

The “hollowing out” of the interiors of these continents had a serious impact on the processes of colonial settlement, both in the past and today. In North America during the 1700s and 1800s, many European settlers interpreted the regions they were moving into as an “untouched wilderness,” when, in many cases, those areas had a long history of habitation and alteration by Native American groups.

In South America, the impacts of European diseases are perhaps even more evident today. We don’t have precise data on population levels in the Amazon River Basin prior to European settlement, but the best estimates are that about 10 million people lived in the region. There were cities, villages, and intensive agricultural areas, as was true of many other biologically rich places in the Western Hemisphere at that time. During the 1600s and 1700s, diseases brought by European explorers wiped out 90% of Amazon residents, leaving less than one million. During the subsequent centuries, descendants of the Spanish and Portuguese colonists have built large cities like Rio de Janeiro, Lima, and São Paulo along the coasts, while the population of the interior Amazon region remains low. The world map of population distribution below shows that South America remains a “hollow continent” today.

Figure 10.2 World Population Density

Credit: World Population Density 1994 by the USDA is licensed under CC0. The colors of the original image were altered for clarity by User: XyKyWyKy on Wikimedia.

In this light, we should not think of the Amazon rainforest as a “virgin wilderness,” but rather as a long-humanized landscape that has only recently grown back into a wild state. Without the influence of European diseases, South America’s demographics and environments would look much different. This is a reminder that we can never ignore history when trying to understand complex human-environment systems.

European colonialism also led to habitat modifications on an unprecedented scale, which had serious negative impacts on biodiversity. One key example is deforestation in North America. Native Americans made noticeable alterations to the temperate forests of North America through burning the understory and clearing patches of forest to grow maize and other crops, but their modifications are eclipsed by the systematic destruction of forests by European colonists.

Consider This: Deforestation in the United States

Deforestation was driven largely by a desire for cleared agricultural land, but also by the needs of manufacturing industries. In Pennsylvania (literally “Penn’s Woods”), much of the forest was cleared and turned into charcoal to fuel iron furnaces. While today much of Pennsylvania has reforested, during the past 200 years almost every forest in the state has been cleared, some multiple times. While biodiversity has benefited from forest regrowth in many places in North America, often new forests do not have the same level of biodiversity as their predecessors, and some areas remain agricultural lands or urban developments with low levels of biodiversity.

Consider the map of the history of deforestation in the United States below. The map focuses on areas of “virgin forests,” otherwise known as old-growth or primary forest, and so it doesn’t show us where forests have grown back. Nevertheless, it’s useful because it mirrors a process that is going on today: the deforestation of the Amazon rainforest in South America. While many people in the U.S. bemoan the destruction of the Amazon today, that deforestation follows in the footsteps of the U.S., Europe, and other “developed” nations. We should be careful not to point fingers of blame at developing countries in the tropics as the main causes of deforestation because that would ignore our own history.

Figure 10.3 Deforestation in the US from 1620 to 1992.

Credit: Area of Virgin Forest by William B. Greeley and the US Forest Service, Wikipedia (Public Domain)

One of the most important lessons that we should learn from biodiversity hotspots is that biodiversity cannot be fully understood without considering factors like human population, agricultural techniques, military activities, and political systems. Biodiversity is entangled with human influences. At the same time, human economic, social, and political systems cannot be understood outside the context of the diverse life forms that support our existence.

“Extinction is the most irreversible and tragic of all environmental calamities. With each plant and animal species that disappears, a precious part of creation is callously erased.” -Michael Soulé, noted American conservation biologist

It is estimated that the current rate of species extinction is between 1,000 and 100,000 times more rapid than the average rate during the last several billion years. The growth of human populations, consumption levels, and mobility is the root of most of the serious threats to biodiversity today.

While learning about the negative impacts of humans on biodiversity, please keep a few things in mind. First, it is rare that humans intend to make a species go extinct or to threaten biodiversity in some other way. Usually, those impacts are the unfortunate by-products of people trying to provide a decent living for themselves or to serve some other purpose. Second, in the last 30 years or so, efforts to protect and preserve biodiversity have expanded exponentially. We will explore those efforts later in the module. As you learn about the current threats to biodiversity, resist the temptation to conclude that humans are simply foolish or short-sighted or greedy, and instead consider the larger pressures and systems that lead toward biodiversity loss.

H.I.P.P.O.

There are many threats to biodiversity today. The biggest ones can be remembered by using the acronym H.I.P.P.O.: Habitat Loss, Invasive Species, Pollution, Human Population, and Overharvesting.

Habitat Loss

This occurs when a particular area is converted from usable to unusable habitat. Industrial activities, agriculture, aquaculture, mining, deforestation, and water extraction are all central causes of habitat loss. This includes deforestation for wood for cooking food. Habitat fragmentation, the loss of large units of habitat, is also a serious threat to biodiversity. The picture below shows an example of habitat fragmentation in the Amazon rainforest.

Figure 10.4 Habitat Fragmentation in the Amazon Rainforest

Deforestation in the Amazon River Basin often occurs in a “fish-bone” pattern, meaning that larger areas of habitat are fragmented and degraded than are actually cleared for agricultural use.

Credit: Amazonie Deforestation by NASA found at Wikimedia Commons (Public Domain)

Invasive Species

When an animal, plant, or microbe moves into a new area, it can affect the resident species in several different ways. New species can parasitize or predate upon residents, hybridize with them, compete with them for food, bring unfamiliar diseases, modify habitats, or disrupt important interactions. One famous and striking example of an invasive species is the brown tree snake in Guam. Native to Australia, the snake was accidentally transported to Guam in ship cargo following World War II. Because Guam had basically no predators to keep the snake population in check, it rapidly multiplied and caused the extirpation of most of the resident bird species. Extirpation means extinction within a region: the species survives elsewhere, but not in that region.

Figure 10.5 Brown Tree Snake

Credit: Brown Tree Snake Photo by G. Rodda/U.S. Fish and Wildlife Service (Public Domain)

Pollution

The discharge of toxic synthetic chemicals and heavy metals into the environment has a huge impact on species abundance and can lead to extinctions. It’s important to remember that substances that are “natural” can become pollution when they are too abundant in a certain area. For example, nitrogen and phosphorous are important nutrients for plant growth, but when they concentrate in water systems after being applied as agricultural fertilizers, they can cause “dead zones” that are uninhabitable for fish and other wildlife. Also, carbon dioxide is a “natural” component of the atmosphere but is considered a pollutant when emitted by human industrial activities.

Bioaccumulation is an important concept connected with pollution. This is the process of chemicals becoming increasingly concentrated in animal tissues as they move up the food chain. Killer whales provide an example of how bioaccumulation can be a serious problem for biodiversity, and especially for marine mammals. Many agricultural and industrial chemicals are persistent organic pollutants (POPs), which do not seem to cause biological damage at very low concentrations. However, these POPs are easily incorporated into organisms like bacteria, phytoplankton, and other invertebrates at the bottom of marine food chains. As those organisms are eaten by fish, and fish are eaten by marine mammals, the POPs move up the food chain. If a killer whale eats 100 king salmon, she incorporates all the POPs that were in those salmon into her body tissues, meaning that over time, the concentrations of POPs in her body can become quite high. At these higher concentrations, many POPs have been shown to cause disruptions to hormone levels and immune systems, and increase birth defects. Anything that eats high on the food chain (such as humans!) is at risk of impacts from bioaccumulation of toxins.

Human Population

In the year 1800, there were fewer than 1 billion people on Earth, and today there are about 8.1 billion. Even without the vast increases in per capita resource use that have occurred during this period, the pressures on biodiversity would have increased during this time period simply based on population growth. While the impacts that each human has on biodiversity varies widely depending on the types and amounts of resources that they use (remember the I=PAT equation!), overall, increasing populations have lead to increasing threats to biodiversity.

Overharvesting

This includes targeted hunting, gathering, or fishing for a particular species as well as incidental harvesting such as bycatch in ocean fisheries. The megafauna extinction example earlier was an example of overharvesting causing biodiversity loss.

Ocean fisheries have been particularly vulnerable to overharvesting during the post-WWII period because of technological developments like refrigeration, sonar, larger nets, and onboard processing. The cod fishery in the Northwestern Atlantic Ocean was an important commercial fishery for hundreds of years, but only a few decades of intense harvesting using these new technologies in the late twentieth century led to a population collapse. The population declined by over 90%, and fishing for the species was closed in both Canada and the United States. The loss of a top predator like cod, along with reductions of other top predator fish populations like haddock and flounder, has led to an explosion in prey fish populations like herring, capelin and shrimp. Cod populations have not recovered, despite fishing pressures ceasing, and this observation has made researchers speculate that the ecosystem may now be in an alternative stable state that will prevent the recovery of cod populations any time in the near future.

Interactions among drivers of biodiversity loss

As explained above, in most places, more than one of these factors is having an impact on biodiversity. It often requires a closer look at a particular place to understand the interplay between habitat loss, invasive species, human population, pollution, overharvesting, and other factors that affect biodiversity. For example, an increasing human population with high meat-consumption patterns and loose environmental regulations may increase deforestation rates for agriculture and cattle grazing, resulting in habitat loss and nitrogen pollution from synthetic fertilizers. Arguably, human population is not a driver of biodiversity loss in and of itself, but it tends to intensify and interact with other drivers.

Climate Change & Biodiversity Loss

In Module 9, we saw that climate change is impacting ecosystems in several ways, including via temperature shifts. These shifts are making it difficult or even impossible for many species to survive. As the climate changes more and more, biodiversity will face ever greater threats. Likewise, efforts to conserve biodiversity will face ever greater challenges. Indeed, some are starting to speak about conservation triage as a situation in which not all species can be saved, forcing conservationists to decide which species to protect. This use of the term triage is adapted from its use in medical crises, such as in emergency response to natural disasters.

Case Study: The Amazon Rainforest

The Amazon in context

Tropical rainforests are often considered to be the “cradles of biodiversity.” Though they cover only about 6% of the Earth’s land surface, they are home to over 50% of global biodiversity. Rainforests also take in massive amounts of carbon dioxide and release oxygen through photosynthesis, which has also given them the nickname “lungs of the planet.” They also store very large amounts of carbon, and so cutting and burning their biomass contributes to global climate change. Many modern medicines are derived from rainforest plants, and several very important food crops originated in the rainforest, including bananas, mangos, chocolate, coffee, and sugar cane.

Figure 10.6 Amazon Tributary

Credit: K. Zimmer © Penn State University is licensed under CC BY-NC-SA 4.0

In order to qualify as a tropical rainforest, an area must receive over 250 centimeters of rainfall each year and have an average temperature above 24 degrees centigrade, as well as never experience frosts. The Amazon rainforest in South America is the largest in the world. The second largest is the Congo in central Africa, and other important rainforests can be found in Central America, the Caribbean, and Southeast Asia. Brazil contains about 40% of the world’s remaining tropical rainforest. Its rainforest covers an area of land about 2/3 the size of the continental United States.

Figure 10.7 Areas of tropical wet forests

Credit: Public Domain

There are countless reasons, both anthropocentric and ecocentric, to value rainforests. But they are one of the most threatened types of ecosystems in the world today. It’s somewhat difficult to estimate how quickly rainforests are being cut down, but estimates range from between 50,000 and 170,000 square kilometers per year. Even the most conservative estimates project that if we keep cutting down rainforests as we are today, within about 100 years there will be none left.

How does a rainforest work?

Rainforests are incredibly complex ecosystems, but understanding a few basics about their ecology will help us understand why clear-cutting and fragmentation are such destructive activities for rainforest biodiversity.

Figure 10.8 Lateral plane roots. Trees have developed lateral plane roots in the rainforest to ensure stability because the lack of soil fertility discourages deep tap root growth for this purpose.

Credit: K. Zimmer © Penn State University is licensed under CC BY-NC-SA 4.0

High biodiversity in tropical rainforests means that the interrelationships between organisms are very complex. A single tree may house more than 40 different ant species, each of which has a different ecological function and may alter the habitat in distinct and important ways. Ecologists debate about whether systems that have high biodiversity are stable and resilient, like a spider web composed of many strong individual strands, or fragile, like a house of cards. Both metaphors are likely appropriate in some cases. One thing we can be certain of is that it is very difficult in a rainforest system, as in most other ecosystems, to affect just one type of organism. Also, clear cutting one small area may damage hundreds or thousands of established species interactions that reach beyond the cleared area.

Pollination is a challenge for rainforest trees because there are so many different species, unlike forests in the temperate regions that are often dominated by less than a dozen tree species. One solution is for individual trees to grow close together, making pollination simpler, but this can make that species vulnerable to extinction if the one area where it lives is clear cut. Another strategy is to develop a mutualistic relationship with a long-distance pollinator, like a specific bee or hummingbird species. These pollinators develop mental maps of where each tree of a particular species is located and then travel between them on a sort of “trap-line” that allows trees to pollinate each other. One problem is that if a forest is fragmented then these trap-line connections can be disrupted, and so trees can fail to be pollinated and reproduce even if they haven’t been cut.

The quality of rainforest soils is perhaps the most surprising aspect of their ecology. We might expect a lush rainforest to grow from incredibly rich, fertile soils, but actually, the opposite is true. While some rainforest soils that are derived from volcanic ash or from river deposits can be quite fertile, generally rainforest soils are very poor in nutrients and organic matter. Rainforests hold most of their nutrients in their live vegetation, not in the soil. Their soils do not maintain nutrients very well either, which means that existing nutrients quickly “leech” out, being carried away by water as it percolates through the soil. Also, soils in rainforests tend to be acidic, which means that it’s difficult for plants to access even the few existing nutrients. The section on slash and burn agriculture in the previous module describes some of the challenges that farmers face when they attempt to grow crops on tropical rainforest soils, but perhaps the most important lesson is that once a rainforest is cut down and cleared away, very little fertility is left to help a forest regrow.

What is driving deforestation in the Amazon?

Many factors contribute to tropical deforestation, but consider this typical set of circumstances and processes that result in rapid and unsustainable rates of deforestation. This story fits well with the historical experience of Brazil and other countries with territory in the Amazon Basin.

Population growth and poverty encourage poor farmers to clear new areas of rainforest, and their efforts are further exacerbated by government policies that permit landless peasants to establish legal title to land that they have cleared.

At the same time, international lending institutions like the World Bank provide money to the national government for large-scale projects like mining, construction of dams, new roads, and other infrastructure that directly reduces the forest or makes it easier for farmers to access new areas to clear.

The activities most often encouraging new road development are timber harvesting and mining. Loggers cut out the best timber for domestic use or export, and in the process knock over many other less valuable trees. Those trees are eventually cleared and used for wood pulp, or burned, and the area is converted into cattle pastures. After a few years, the vegetation is sufficiently degraded to make it not profitable to raise cattle, and the land is sold to poor farmers seeking out a subsistence living.

Regardless of how poor farmers get their land, they often are only able to gain a few years of decent crop yields before the poor quality of the soil overwhelms their efforts, and then they are forced to move on to another plot of land. Small-scale farmers also hunt for meat in the remaining fragmented forest areas, which reduces the biodiversity in those areas as well.

Another important factor not mentioned in the scenario above is the clearing of rainforest for industrial agriculture plantations of bananas, pineapples, and sugar cane. These crops are primarily grown for export, and so an additional driver to consider is consumer demand for these crops in countries like the United States.

These cycles of land use, which are driven by poverty and population growth as well as government policies, have led to the rapid loss of tropical rainforests. What is lost in many cases is not simply biodiversity, but also valuable renewable resources that could sustain many generations of humans to come. Efforts to protect rainforests and other areas of high biodiversity is the topic of the next section.

Globalization of Biodiversity Concerns

The realization that global biodiversity is seriously threatened by human activities emerged as a primary international concern in the 1970s, although the history of human efforts to protect rare species is much older.

National Parks and Biodiversity Conservation

In the United States, efforts were made to prevent the extinction of the American bison at the end of the 1800s. Yellowstone National Park, the first national park in the world, was established in 1872, and it provided habitat to the only wild bison herd during that era. In 1900, the U.S. federal government passed the Lacey Act, which forbade interstate commerce in illegally harvested animals or their body parts, and likely helped prevent the extermination of snowy egrets and other birds that were being harvested for their feathers.

The national park system in the United States grew rapidly during the late 1800s and early 1900s. Its model for protecting nature was to draw a boundary around a particular area and restrict human uses within it. Most early parks were focused on places with geological, not biological, wonders, so they weren’t especially good at protecting biodiversity, but they established an important model for nature protection.

With adequate enforcement, the national park model can be very effective for conserving biodiversity, but it also raises questions of social justice. Even during the 1800s when the first parks were established, local residents complained about lost access to resources because of the restrictions that parks imposed. Among those most disenfranchised were Native American groups, such as the Blackfeet of Montana who lived within today’s Glacier National Park; they were told they were no longer allowed to do traditional hunting, fishing, and gathering within the park boundaries. Despite the social injustices that were a part of the U.S. national park movement, this model of nature conservation was adopted by many European nations that established national parks in their African colonies. Below, you will learn about efforts to balance biodiversity protection in key areas with the needs of humans who live nearby, and those efforts stem from social justice concerns about the original “fortress” model for nature conservation exemplified by national parks.

These early efforts were quite minimal compared to the global boom in protected areas since the 1960s. Today, there are over 100,000 individual protected areas that cover about 12% of the Earth’s total land surface. Over half of this area was protected just in the last decade.

Within the field of geography, particularly in a subfield called political ecology, there has been a lot of research in protected areas on the issue of balancing biodiversity protection with human needs. Political ecologists have looked at the political and economic interests of humans in protected areas, and how those interests relate to biodiversity and other ecological processes. The establishment of a new protected area invokes social justice concerns about the way that the fortress model of conservation displaces local people from their land and resources. At the same time, some parks have been operational for over 100 years and have their own unique set of political and ecological issues. An example can be seen in Yosemite National Park, where park visitation levels have become so high that efforts are underway by park managers to establish a park “capacity” for visitation to certain parts of the park with the goal of limiting human impacts on ecological processes (recall the “carrying capacity” concept from Module 2). As visitation to protected areas increases, the interface between environmental protection and levels of visitation becomes increasingly complex, and innovative management strategies are required to meet the given objective of a protected area.

IUCN Protected Area Categories

It’s important to remember, however, that protected areas receive very different levels of protection, and may have many more purposes than simply protecting biodiversity. The International Union for the Conservation of Nature (IUCN) has identified six different levels of protected areas:

Category 1: Strict Nature Reserves. These areas restrict motorized vehicles and extractive uses. They may be open to indigenous people for traditional gathering and hunting, but, in most cases, the only human activities are scientific research and monitoring and low-impact recreation. The federal wilderness system in the United States, established in 1964, is an example of this kind of reserve.

Category 2: National Parks. These are areas intended to balance ecosystem protection with human recreation, which is often a very difficult mandate for the managing agency to achieve. Extractive uses in these areas are prohibited. Many national parks, such as Tubbataha National Park in the Philippines, are sources of ecotourism income as well as breeding grounds for commercially important species. One problem with national parks in many developing countries is that there is little or no enforcement of regulations. One study showed that only 1% of parks in Africa and Latin America have adequate enforcement. We might think of these as “paper parks” that exist on a map but, in reality, are not protected.

Category 3: Natural Monuments. Protecting interesting natural or cultural features is the goal in these areas, but they are smaller than the areas in the two previous categories.

Category 4: Habitat/Species Management Areas. These are areas that are relatively heavily utilized by humans for agriculture or forestry but have been designated as important habitats for a particular species or natural community. Management plans and continual monitoring are important components to ensure that conservation goals are achieved.

Category 5: Protected Landscape/Seascape. These areas are intended to protect historically important interactions between people and nature. Examples include traditional farming areas, homelands of indigenous peoples, and significant religious landscapes. Endemic and rare species in these regions are often best protected by maintaining the traditional human land uses that have existed alongside them for many generations.

Category 6: Managed Resource Protected Area. Similar to Category 5, these areas are managed for the long-term sustainable use by humans. In the Ngorongoro Crater Conservation Area in northern Tanzania, Masai pastoralists graze cattle on most of the land while living alongside Africa’s largest concentrations of megafauna.

One system of protected areas that has become particularly important for conserving biodiversity is “Biosphere Reserves.” In 1971, The United Nations’ Educational, Social and Cultural Organization (UNESCO) started the Man and the Biosphere Programme. Its major focus has been building a network of biosphere reserves. There are over 400 reserves in almost 100 countries today. Each reserve has to be large enough to contain three different “zones”: (1) a core area where the national government restricts essentially all human activities except scientific monitoring and research, (2) a buffer zone where tourist recreation and local resident usage for agriculture, sustainable logging, grazing, hunting, and fishing are allowed, as long as they don’t threaten the core area, and (3) a transition zone where more intensive uses of land are permitted. This model seeks to balance the needs of humans and the biosphere, as its name implies.

Figure 10.9 Distribution of global "biosphere reserves" (number of biosphere reserves per country)

Credit: Biosphere Reserves by Mehmet Karatay from Wikimedia Commons (Public Domain)

If you were designing a set of protected areas with the goal of preserving biodiversity, here are a few concepts that you would want to keep in mind:

Comprehensiveness: Include samples of different types of habitats and ecological processes.

Representativeness: It’s unlikely that you will be able to preserve much of each habitat type, so protect an area that is representative of the ecological processes contained within it.

Risk Spreading: Natural disasters, wars, or other disturbances can harm even the most well-protected areas, so it may be wise to not have all of your reserves connected and nearby each other.

Connectivity: On the other hand, maintaining connections between protected areas is very important for several reasons, including the dispersal of genetic material, the ability for migrating and wide-ranging species to persist, and the possibility for species to adapt to climate changes or adjust their ranges after disturbance events.

Examples of Biodiversity Conservation Practices

Of course, creating a theoretical set of protected areas is much easier than doing it in the real world, but here are several examples of how these ideas are being implemented or advocated for in different parts of the world.

Costa Rica is perhaps the best example of a biodiversity-rich country making a commitment to protecting its natural endowments. While it is a small country, about the size of West Virginia, it is home to about 500,000 plant and animal species. Though Costa Rica experienced very serious deforestation driven by cattle ranching during the 1960s and 1970s, it has worked for the last 30 years to protect about 25% of its land in national parks and other forms of reserves. The protected areas are designed to ensure the survival of at least 80% of Costa Rica’s remaining biodiversity. Efforts have been made to facilitate connectivity between reserves and to ensure that they are as representative as possible. Beyond the reserves, the Costa Rican government has also halted subsidies that encourage forest clearing and has encouraged investment in ecotourism. Today, tourism is the largest industry in Costa Rica, and is very substantially focused on activities within and surrounding these reserves. Tourism has become so popular that the Costa Rican government and conservation biologists are now concerned about the impacts that so many visitors are having on the country’s biodiversity. Nevertheless, Costa Rica remains an example of the benefits that protected areas can have for biodiversity and local economies.

Protected Areas in Costa Rica (map)

But connectivity between reserves is often necessary on a larger than national scale, and that was the goal of advocates for the “Paseo Pantera” (Panther Path) in Central America. Now known as the “Mesoamerican Biological Corridor,” this system of protected areas and corridors stretches from Mexico to Panama.

The Rewilding Institute advocates for the creation of large-scale connectivity between important ecosystems in North and Central America, focusing on the necessity for large carnivores like wolves, mountain lions, and grizzly bears to travel the long distances they require.

The Rewilding Institute’s “megalinkages” (map)

The primary goal of all of these corridor-based projects is to ensure landscape permeability, which means that even if a particular place is not designated as a protected area, wildlife is able to use the habitat and to travel freely through it. Elements that ensure landscape permeability include laws that regulate or restrict wildlife hunting or trapping, designing roads and railroads so that animals can cross safely, and establishing relationships between government wildlife agencies and local communities so that everyone feels that they benefit from protecting the biological integrity of the region.

Biodiversity and Ecosystem Resilience

Ecosystems involve many complex interactions between members of different species. These interactions often create negative feedback loops, keeping the ecosystem in approximately the same state. For example, if the population of a certain type of plant starts to grow, then the population of an animal that eats this plant may also start to grow, thereby lowering the population of the plant. Ecosystems contain many interactions like this. These interactions are crucial to understanding the importance of individual species in biodiversity.

Suppose the animal species described above goes extinct, perhaps because of human hunting. This destroys the negative feedback loop. When the plant population grows, there is nothing to stop it from continuing to grow. The plant may then deplete resources that are crucial for a different species, which then starts to die out. As that species dies out, it can affect still other species. Indeed, removing just one species can have huge consequences for all other species in the ecosystem, sending the entire ecosystem into a completely different system state. In other words, removing just one species can be a disturbance so great that it exceeds the ecosystem's resilience. But this doesn't always happen. Sometimes, when one species is removed, the ecosystem does not respond in such dramatic fashion.

For this reason, ecologists often explain the role of biodiversity in ecosystem resilience using the metaphor of the house of cards. When we remove cards from the house, one of two things can happen. If the card was not essential for the house's structure, then the house remains standing. Or, if the card was essential to the house's structure, then the entire house falls down. There is often no middle ground in which part of the house remains standing. To be sure, ecosystems are more complex than card houses, and removing a species can result in a partial collapse of the ecosystem. But the metaphor still works well because it emphasizes the importance of interactions between species and the role that one species can play in the overall function of the entire ecosystem.

Figure 10.10 A House of Cards: metaphor for biodiversity.

Credit: House of cards by Meghana Kulkami from Flickr is licensed under (CC BY-NC 2.0)

One clever Flickr user noted that biodiversity is actually more similar to the game of Jenga than it is to a house of cards because in Jenga, we start with the structure intact and actively remove pieces instead of starting with no structure and building it up. This is a fair point, though in Jenga we intentionally remove pieces, whereas people rarely intentionally remove species from ecosystems.

Figure 10.11 Biodiversity Jenga

Credit: Biodiversity Jenga by Martin Sharman from Flickr is licensed under (CC BY-NC-SA 2.0)

Human Extinction

Throughout this module, we've been focusing on biodiversity loss and species extinctions, in which the species going extinct are species other than humans. But what about us?

It turns out that there are threats to the existence of the human species. Some of them have already been discussed in this course. Human extinction would also have major impacts on natural systems. The possibility of human extinction raises profound ethical issues, including issues of sustainability.

Human Extinction Hazards

Recall from Module 8 that a hazard is an extreme event that causes harm to humans. A human extinction hazard is thus an event that causes human extinction. For better or worse, there exist quite a few human extinction hazards. Here are some important ones:

Climate change. We already know that the climate is changing and that these changes are harming humanity. What we don't know is exactly how harmful climate change will be. We can hope that climate change will be relatively mild and easy to adapt to. However, it might not be. Worst-case scenarios for climate change are frightening, including the possibility that large portions of Earth's land mass will become too warm for mammals to survive. Many species would go extinct under these worst-case scenarios. Humans could be one of them. But it is important to understand that such scenarios would unfold over time scales of decades or centuries. Exactly what the impacts end up being could depend heavily on what else is going on in society during this time. This means that we should view climate change as being part of the human society system. That said, the worst-case scenarios for climate change really are so severe that they could cause human extinction.

Biodiversity loss. The following video summarizes the relationship between biodiversity and human wellbeing and why biodiversity loss is a concern. As more species go extinct, it becomes more likely for ecosystems to collapse. Given how many species are endangered, it is difficult to put an upper limit on how severe the ecosystem collapses could be. The collapses could be so severe that human extinction is threatened. The current honey bee colony collapse situation illustrates this. Without honey bees, humans would struggle - and perhaps fail - to grow many important crops. As more biodiversity is lost, we may find ourselves learning the hard way how important it is to our civilization and indeed our very survival.

Pandemics. In Module 8, we saw that biological hazards have led to some of the most severe disasters in human history, such as the bubonic plague and the "Spanish" flu. The COVID-19 pandemic has altered almost every aspect of our society. Politics have changed to reflect concerns about virus transfer. Economic systems have been upended by falling consumption and unemployment. People are transforming the way that they interact socially in order to minimize infection risks. In the coming years, we might begin to see that some of the changes made in response to the pandemic continue even after the risk for infection disappears as a way to increase society's resilience.

Nuclear warfare. In Module 2, we learned that arms races are examples of positive feedback loops. During the Cold War, the arms race between the United States and the Soviet Union was so extensive that it produced enough nuclear weapons to cause destruction worldwide. The Cold War is now over, but many of these weapons still exist. Meanwhile, other countries are pursuing nuclear weapons. The destruction from even a regional nuclear war would be global, because smoke from the weapon detonations would blacken the sky, reducing the amount of sunlight available for crops (a phenomenon sometimes known as nuclear winter). The threat of nuclear warfare is lower now than it was during the Cold War (when there were a few near-misses). But as long as nuclear weapons still exist, the threat will not be zero. The question is, will the world's nuclear powers continue to act collectively to avoid global destruction?

Asteroids and comets. In Module 8, we noted that asteroids and comets are examples of global-scale natural hazards, and that NASA (among other space agencies around the world) is working on monitoring the skies for them. If a large enough asteroid or comet hits Earth, then it could cause human extinction, as well as the extinction of many, many other species. The destruction would come from the impact itself (which could cause massive tsunamis) and from the large amount of dust that kicks up into the atmosphere (similar to the effects of nuclear weapon detonation). Fortunately, large asteroid and comet impacts are not likely to happen anytime soon. In general, the most likely human extinction scenarios are those related to human activity, including all of the other scenarios discussed on this page.

Environmental Consequences of Human Extinction

Given the many interconnections between human systems and environmental systems, we should expect human extinction to have major environmental consequences.

Impacts of the extinction event. Depending on how humans go extinct, environmental systems could also be significantly affected. If there is a pandemic that only infects humans, then the extinction event itself would not have much effect on the environment. However, for other extinction scenarios, the impacts would be quite large. As we've seen in this course, climate change and biodiversity loss harm natural systems at least as much as they harm human systems, with many non-human species going extinct. The explosions and atmospheric dust accumulation from nuclear weapon detonation or asteroid or comet impact would affect all species, except perhaps those in deep-sea ecosystems that get their energy from hydrothermal vents instead of sunlight. While it is unlikely that any one event would end all life on Earth, the event would probably eliminate a significant portion of the species now alive.

Consequences of Earth without humans. Human impact on Earth systems is so large that this era of Earth's history is known as the Anthropocene. Without further human influence, ecosystems would evolve in very different directions. Ecosystems would not return to exactly how they were before humans. If nothing else, the many lingering artifacts of human civilization would prevent this from happening. But some return would occur, as would other novel changes. Some of these consequences are explored in the book The World Without Us by Alan Weisman. To get an overview of the ideas presented there, please view this timeline from The World Without Us.

Ethical Issues With Human Extinction and Some Concluding Remarks

Please read the article Long-Term Sustainability. What are the ethical issues raised here? What are the arguments being made? Do you agree or disagree with them? What other issues does human extinction raise, both for us as individuals and for society at large? How much of a priority should avoiding human extinction be relative to other issues we face? And above all, what do you think should be done?

All of these are difficult but important questions. How you answer them depends on what your ethical views are, as well as your understanding of the nature of human-environment systems and of what can be done to ensure their sustainability. It is not the intent of this course to tell you what your views should be. Instead, it is hoped that the course has given you the opportunity to learn about and reflect on some important topics that will be relevant to you and the world around you, no matter what you end up doing with your life. This is because of how closely interconnected the environment and society are in this ever-changing world.

Summary

Biodiversity refers to the variation and richness of living organisms. Biodiversity provides many important services to humanity and is also often considered to be valuable for its own sake. Biodiversity tends to be concentrated in certain regions, with rainforests having more biodiversity than any other type of ecosystem on land. While there have been species extinctions throughout history, both because of human activities and for other reasons, today, human activities are threatening extinctions at unprecedented rates. To be sure, humans are also active in protecting biodiversity, such as through protected areas and other projects focused on conservation. But the overall threat to biodiversity loss is so great that conservationists face conservation triage, in which they must decide which species to protect. If too much biodiversity is lost, ecosystems could collapse, threatening the extinction of one more species: humans. The threat of human extinction, whether due to biodiversity loss or other events, raises some very profound ethical issues, issues which connect deeply to the topics in this course.