Humans influence ecological systems
For more than a century, ecologists have labored passionately to understand how nature works, from the level of the organism to the level of the biosphere. The wonders of the natural world summon our curiosity about life and our environment. For many ecologists, a curiosity about how nature works is reason enough to study ecology. Increasingly, however, ecologists find themselves struggling to understand how the rapidly growing human population, now more than 7 billion people, is affecting the planet. Our need to understand nature is becoming more and more urgent as the growing human population stresses the functioning of ecological systems. Environments dominated by humans or created by them—including urban and suburban living places, agricultural fields, tree farms, and recreational areas—are also ecological systems. The welfare of humanity depends on maintaining the proper functioning of these systems.
The human population currently consumes massive amounts of energy and resources, and produces large amounts of waste. As a result, virtually the entire planet is strongly influenced by human activities (Figure 1.19). These influences include the degradation of the natural environment and the disruption of many important functions that natural environments provide to humans. Growing human consumption of natural resources has caused a number of ecological problems. For example, removing plants from their natural environment to use as house plants and exploiting animals for human consumption and the pet trade have caused the decline of many species in their native habitats. The species affected are diverse, including the cacti of the American Southwest that are collected for sale as house plants, several species of reptiles and amphibians that are sold in the pet trade, and many species of fish and whales that are overexploited by commercial fishing. As commerce has become more global, species have been introduced unintentionally to new locations at an increasing rate. Some of these species such as rats, snakes, and pathogens can have devastating effects on local species. To feed 7 billion people, we have converted a large amount of land for agricultural use. This conversion has brought with it a number of challenges including loss of natural habitats, pollution from fertilizers and pesticides, and questions about growing genetically modified crops. Some crops, such as corn, are now increasingly being used as sources of fuel, also known as biofuels, causing even more land to be converted to agricultural use. Humans also need land for housing, business, and industry. This has further reduced the amount of natural habitat available for other species, and has been a major contributor to the decline and extinction of many species. We will deal with these issues in greater detail throughout the book.
Greenhouse gases Compounds in the atmosphere that absorb the infrared heat energy emitted by Earth and then emit some of the energy back toward Earth.
Another suite of ecological challenges has been caused by wastes produced by human activity. For example, untreated sewage and industrial processes can damage the air, water, and soil. In addition, the use of nuclear power plants to generate electricity produces substantial amounts of nuclear waste. Of all human wastes, perhaps none has a higher public awareness than the greenhouse gases responsible for global warming. Greenhouse gases are compounds in the atmosphere that absorb the infrared heat energy emitted by Earth and then emit some of the energy back toward Earth. In doing so, the gases prevent much of this energy radiated from the surface of Earth from escaping into space. Greenhouse gases include many different compounds, but an important player is CO2, which is produced by burning fossil fuels in the cars we drive and in the coal-powered electricity-generating plants that provide electricity to so many of our homes and businesses. As the human population continues to grow and demands for energy increase, we burn more fossil fuels and produce more greenhouse gases. The more greenhouse gases we put into the atmosphere, the warmer our Earth becomes.
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Because ecological systems are inherently complex, it is difficult to predict and manage the effects of a growing human population on ecological systems at every level. At the level of the organism, we might want to know how a pesticide sprayed in the environment could affect each of the many tissues and organ systems of an animal’s body, leading to changes in behavior, growth, and reproduction. At the community level, we might ask how a decrease in abundance of one species caused by commercial harvesting could affect the populations of other species in that community. At the biosphere level, we would like to quantify the large number of sources that emit CO2 into the atmosphere and understand the processes that take CO2 out of the atmosphere. Each of these cases presents a set of complex questions that are not easy to answer. Yet we need a solid understanding of how the ecological system operates before we can predict the outcome of anthropogenic impacts on the system and recommend ways to minimize damages to the system.
The Role of Ecologists
The plight of individual species headed toward extinction arouses us emotionally. However, ecologists increasingly realize that the only effective means of preserving the species of the world is through the conservation of ecosystems and the management of large-scale ecological processes. Individual species, including those that humans rely on for food and other products, are themselves dependent on the maintenance of environmental support systems. Local effects of human activities on ecological systems can often be managed once we understand the underlying mechanisms responsible for change. Increasingly, however, our activities have led to multiple, widespread effects that are more difficult for scientists to characterize and for legislative and regulatory bodies to control. For this reason, a sound scientific comprehension of environmental problems is a necessary prerequisite to action.
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The media is filled with reports of environmental problems: disappearing tropical forests, depleted fish stocks, emerging diseases, global warming, and wars that cause environmental tragedies and human suffering. But it is important to know that there are success stories as well. Many countries have made great strides in cleaning up their rivers, lakes, and air. Fish are once again migrating up major rivers in North America and Europe to spawn. Acid rain has decreased, thanks to changes in the combustion of fossil fuels. The release of chlorofluorocarbons, which damage the ozone layer that shields the surface of Earth from ultraviolet radiation, has decreased dramatically. The inevitability of global warming caused by increasing atmospheric CO2 concentrations has provoked global concern and set off an international research effort. Conservation programs, including breeding endangered species in captivity, have saved some animals and plants from certain extinction. They have also heightened public awareness of environmental issues, and sometimes sparked public controversy.
These successes would not have been possible without a general consensus founded on evidence produced by scientific study of the natural world. Understanding ecology will not by itself solve our environmental problems, because these problems also have political, economic, and social dimensions. However, as we contemplate the need for global management of natural systems, our effectiveness in this enterprise critically depends on our understanding of their structure and functioning—an understanding that depends on knowing the principles of ecology.
This book introduces you to the study of ecology by building an understanding of all aspects of the discipline. We begin by looking at the individual level, including how species have adapted to the challenges of the aquatic and terrestrial environments. We will then explore the topic of evolution, including how species have evolved various strategies for mating, reproducing, and living in social groups. Next we move to the population level with a discussion of population distributions, population growth, and population dynamics over space and time. With a firm understanding of populations, we move on to examine species interactions, communities, and ecosystems. Finally, we consider ecology at the global level and investigate global patterns of biodiversity and global conservation.
ECOLOGY TODAY CONNECTING THE CONCEPTS
THE CALIFORNIA SEA OTTER
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At the end of each chapter, we want to reflect on the topics covered and explore how these topics are interconnected. In this first chapter, we have examined a wide range of topics including the hierarchy of perspectives in ecology, the biological and physical principles that govern natural systems, the variety of roles that different species play, the multiple approaches to studying ecology, and the influence of humans on ecological systems. To help you see how these topics interconnect, let’s examine a case study of the sea otter (Enhydra lutris) off the Pacific coast. Humans have affected sea otter populations for hundreds of years. Several scientific approaches have been taken to understand these impacts and to help reverse them.
The sea otter was once abundant, with a geographic range that extended around the northern Pacific Rim, from Japan up to Alaska and down to Baja California. However, in the 1700s and 1800s, intense hunting for otter pelts reduced the population to near extinction and the fur industry subsequently collapsed. When a small population was discovered off the coast of central California in the 1930s, the otters were placed under protection. As a result, the population increased to several thousand individuals by the 1990s, though in more recent years, the otter has again experienced population declines. These changes in the size of otter populations presented an opportunity for scientists to examine a natural experiment in action.
Ecologists quickly realized that to understand the causes and consequences of the sea otter’s fluctuations in abundance, they needed to use a range of ecological approaches, from the individual to the ecosystem. Taking an individual approach, ecologists established that the sea otter was a predator on a wide range of prey species including abalone, spiny lobsters, small fish, crabs, sea urchins, and small snails. Among these prey items, observations of otter feeding behavior revealed that otters prefer certain prey such as abalone, a large species of sea snail. They will only eat other small species of snails when their preferred prey becomes rare.
Once scientists understood the sea otter’s niche, they were better able to protect it. However, not everyone was happy about the resurgence of sea otters through the 1990s. California anglers became upset; they argued that the growing otter population would cause a dramatic change in the marine community, including a drastic reduction in the populations of commercially valuable fish, abalone, and spiny lobsters—all harvested for human consumption. However, scientists who took a community approach to ecology found that an increasing otter population was also having other dramatic effects on the marine community. For example, the otter’s consumption of sea urchins—marine invertebrates that eat kelps—was causing an increase in kelps (Figure 1.8). Kelps can be harvested for making fertilizer, food, and pharmaceuticals. Thus, the growing otter population caused sea urchins to decrease, kelps to increase, and the commercial harvesting of kelps to increase. It turns out that the increase in kelps also provided young fish with a refuge from predators and a place to feed. Thus, the sea otter plays a key role in determining the community composition of coastal marine ecosystems.
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In the 1990s, the sea otter population mysteriously began to decline. To understand these declines, scientists have used individual, community, and ecosystem approaches. In 1998, researchers showed that populations of otters in the vicinity of the Aleutian Islands, Alaska, had declined precipitously during the 1990s. The reason was that killer whales, or orcas (Orcinus orca), which previously had not preyed on otters, had begun to come close to shore where they consumed large numbers of otters. Why did killer whales adopt this new behavior? The researchers pointed out that populations of the principal prey of killer whales—seals and sea lions—collapsed during the same period, perhaps inducing the whales to hunt the otters as an alternative food source. Why did the seals and sea lions decline? One can only speculate at this point, but intense human fisheries have reduced fish stocks exploited by the seals to levels low enough to seriously threaten seal populations.
There also were declines in otter populations along the California coast. Initially, declines in sea otters were attributed to the use of gill nets along the coast to exploit a new fishery that inadvertently killed otters in substantial numbers. Subsequent legislation moved the fishery farther offshore to help protect the otters. In this same region, the otters also were dying from infections by two protist parasites, Toxoplasma gondii and Sarcocystis neurona. These parasites cause a lethal inflammation of the brain. In 2010, for example, 40 dead and dying sea otters were found near Morro Bay, California, and 94 percent were infected with S. neurona. This was a surprising observation because the only known hosts of these parasites are opossums (Didelphis virginiana) and several species of cats. Given that these mammals live on land, how did sea otters become infected?
Scientists hypothesized links between the terrestrial and marine ecosystems that are allowing the parasites to infect sea otters. To date, two potential links have been suggested. First, cats that spend time outside defecate on land and their feces contain the parasites; when it rains, the parasites get washed into local streams and rivers, and eventually make their way to the ocean. Second, when humans flush cat feces and kitty litter down the toilet and into the sewer system, the waste water eventually enters the ocean. Although manipulative experiments found that the protists do not infect marine invertebrates and cause illness, the invertebrates can take the parasites into their bodies inadvertently while feeding. When invertebrates infested with parasites are consumed by otters, the otters get infected. New research indicates that abalone do not carry the parasites whereas small marine snails do. Thus, when otters have an abundance of their preferred food, such as abalone, they have a low risk of being infected by the deadly parasite. When abalone is scarce, however, the otters are forced to feed on small snails that carry the parasite, which dramatically increases the risk of infection and death.
The story of the sea otter highlights the importance of understanding ecology from multiple approaches using both manipulative and natural experiments. It also underscores the multiple roles that species can play in communities and ecosystems and how humans can dramatically influence the outcome. This understanding can then be used to take action to reverse harmful impacts on the environment. In the case of the sea otter, education campaigns now encourage the public to keep their cats inside more and to put used cat litter into the trash rather than flushing it down the toilet.
SOURCES: Johnson, C. K., et al. 2009. Prey choice and habitat use drive sea otter pathogen exposure in a resource-limited coastal system. Proceedings of the National Academy of Sciences 106: 2242–2247.
Miller, M. A. 2010. A protozoal-associated epizootic impacting marine wildlife: Mass mortality of southern sea otters (Enhydra lutris nereis) due to Sarcocystis neurona infection.Veterinary Parasitology 172: 183–194.
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