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.
One clever Flickr user noted that biodiversity is actually more similar to the game Jenga than it is to a card house 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.
Reading Assignment: Honey Bees and Colony Collapse Disorder
One of the clearest and most important examples of the importance of one species to others is that of the honey bee and its role in agricultural ecosystems. Honey bees help many important crops by performing pollination, including many fruits, vegetables, nuts, and other species. Just some examples include almonds, apples, broccoli, cotton, grapes, lemons, onions, soybeans, tomatoes, and walnuts. However, recently many honey bees have been dying, a phenomenon known as colony collapse disorder, thereby threatening these crops.
Please read the article What's killing the honey bees? This article discusses the collapse of honey bee populations and the role of Penn State researchers in understanding and addressing the phenomenon. As you read it, consider the following. What is colony collapse disorder? What are the causes of it? What are the consequences of it? What actions are being taken to address it?
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 lead to some of the most severe disasters in human history, such as the bubonic plague and the "Spanish" flu. Another pandemic could occur. Indeed, in recent years there have been several near-pandemics, including severe acute respiratory syndrome (SARS) and new flu strains. Due to genetic diversity within the human population, it is unlikely for one pathogen to kill everyone. Probably some people will happen to have immunity. But this is not guaranteed, and, meanwhile, the devastation from a major pandemic could be so severe that civilization never recovers.
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 warfare 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 (which is 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 major interconnections between human systems and environmental systems, we should expect human extinction to have major environmental consequences. Here are the main reasons why this is indeed the case.
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 equally, except for 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 about them. 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 about your world and about yourself. No matter what you end up doing with your life, the topics covered in this course will be in some way relevant. This is because of how closely interconnected different parts of the world are.