Concept 41.5: Human Activities Affect Ecological Systems on a Global Scale

Some say that we are entering a new geological period, dubbed the “Anthropocene,” or Age of Humans, because our activities are altering ecological systems on a global scale. Others say that the new age should be called the “Homogenocene,” or Homogeneous Age, because the net effect of our activities is to make ecological systems less complex and more uniform. We affect ecological systems by using species for food or timber, by altering landscapes, by spreading organisms across the globe without regard to natural barriers to dispersal, and as Concept 45.4 will describe, by changing Earth’s climates.

We are altering natural ecosystems as we use them

When we use natural ecosystems—for example, through hunting, fishing, grazing, or logging—we remove individuals of particular species and thereby change their abundances. If we remove too many, we can even cause some species to disappear entirely—to go extinct. The resulting shifts in the relative abundances of components of natural ecosystems change the patterns of interaction among species, and thereby change how entire ecosystems function. The decreased abundance of grasses in relation to shrubs in the U.S.–Mexico Borderlands, for example, has reduced the ability of these lands to support grass-eating animals, whether domesticated or not.

We are replacing natural ecosystems with human-dominated ones

We have now converted almost half of Earth’s land area into human-dominated ecosystems. Cities occupy only about 1 percent of the land area (about 1.5 million km²), but an area roughly the size of South America (about 18 million km²) is devoted to agriculture, and another roughly the size of Africa (about 35 million km²) is managed as pasture or used as rangeland for cattle and other livestock.

Human-dominated ecosystems contain fewer interacting species than natural ecosystems and are therefore less complex. Pastures or heavily grazed rangelands, for example, often contain fewer species than do natural grasslands or forests. And when an area is converted to agriculture, monocultures (plantings of single crops) often replace species-rich natural plant communities (FIGURE 41.16). Furthermore, agricultural systems worldwide are dominated by a relatively few plant species. Some 19 species comprise 95 percent of total global crop production, and overall crop diversity is a tiny fraction of the approximately 350,000 named plant species on Earth.

Figure 41.16: Human Agricultural Practices Produce a Uniform Landscape The cultivation of crop plants for food and other uses diminishes the diversity of ecosystems. This monoculture is a field of soybean plants (Glycine sp.) in Mississippi.

Human activities are similarly reducing the complexity of natural landscapes. For example, when rivers are dammed to control flooding or tapped to irrigate crops, species-rich natural streamside ecosystems disappear. Similarly, conversion of land to human uses not only reduces the total area of natural vegetation that remains, but also breaks it into smaller fragments. Such changes, and others that we will discuss in the following chapters, simplify natural ecosystems and change the way they function.

We are blurring biogeographic boundaries

Humans are also moving organisms around on a global scale, sometimes deliberately—as when Spanish settlers brought domesticated cattle to the U.S.–Mexico Borderlands—and sometimes inadvertently. Only about 1 percent of inadvertent introductions result in self-sustaining populations in the new locality, but their cumulative effect on geographic distributions is huge. Of the insects found in both Europe and North America, for example, half are estimated to have been transported between the two continents by humans. Similarly, more than half of the plant species on many oceanic islands are not native, and in many continental areas the figure is 20 percent or more. The pace of introductions is astonishing: for example, in California’s San Francisco Bay, one new species on average became established every 12 weeks during the 1990s. Introducing new species to communities changes their species composition and adds novel interactions to the original system that may precipitate further changes in species abundances—even the complete loss of some species from the system.

This human-assisted biotic interchange is homogenizing the biota of the planet, blurring the differences among communities that evolved during long periods of continental isolation. One wonders how Alfred Wallace would draw the boundaries of biogeographic regions if he were using present-day data!

Science provides tools for conserving and restoring ecological systems

With the dawning of the Anthropocene, ecologists are called upon as never before to understand how humans are influencing Earth’s ecological systems and how we can preserve their ability to sustain life on our planet. Accordingly, several new subdisciplines of ecology have emerged. The goal of conservation ecology is to understand the process of extinction and devise ways to prevent the extinction of vulnerable species. Often this requires that we protect the ecosystems of which these species are a part. The goal of restoration ecology is to restore the health of damaged ecosystems. These two fields are related because natural systems are sometimes altered so strongly that extinction will occur unless the systems are restored.

To achieve their goals, conservation and restoration ecologists deploy the same tools of scientific inquiry that all other biologists (and scientists) use: observation, questioning, logic, and experimentation (see Concept 1.5). They often begin with natural history—the observation of nature outside of a formal, hypothesis-testing investigation—because knowledge of nature is fundamental to all stages of ecological inquiry. Natural history observations are the source of new questions and hypotheses and are critical for the design of good experiments—ones that do not mislead us because they use unnatural conditions or leave out an important feature of the system. A lack of natural history knowledge often limits our ability to answer important questions. For example, we cannot fully answer the question “Has the introduction of honey bees caused declines in native bee species?” without knowing what bee species were present before honey bees were introduced and how abundant they were. It turns out that such information is available for only a handful of places on Earth.

As we saw in Chapter 1, scientists test hypotheses by seeing if their predictions are true. Sometimes we can use simple rules of logic to develop predictions. But with systems as complex as ecological systems, we often need to use a mathematical model, or perhaps even a computer simulation. What is critical is that we base the model on natural history knowledge about the system. In the case of the degraded rangeland system of the Borderlands, for example, predicting the conditions that would restore desirable grasses might require a complex model of the many factors that affect the growth of grasses relative to shrubs.

Conservation and restoration ecology are only two examples of the important practical applications of ecology in the Anthropocene. The chapters that follow will highlight numerous additional examples.

Now that we have explored some of the properties of ecological systems, we are ready to take a closer look at them, beginning with the smallest scales in the ecological hierarchy: individual organisms and populations.

ANSWER The physical environment of the Borderlands is strongly influenced by Earth’s climate patterns: its position near 32° N latitude, where the warm, dry air of the Hadley cells descends, makes it an arid place (Concept 41.2). Aridity, combined with other features of the physical environment, determines the characteristics of organisms that live in the Borderlands (Concept 41.3). Ecological communities of the Borderlands are also affected by the region’s history of geographic isolation from or connection with other regions (Concept 41.4). All of these principles prepare us to realize that grasslands in different locations may look similar but may respond very differently to human activities.

As we’ve seen, communities dominated by grasses do differ from one another (Concept 41.1, Figure 41.12). Some occupy moist and cool environments, whereas others, like the Borderlands, occupy dry and hot ones. Some, including many meadows and pastures in Europe and eastern North America, were once forests that were cleared to make way for grasses; managing the size of cattle herds in these places can conserve or restore the grassland ecosystem. Others, such as the tropical savannas of Africa, have long histories of evolution with grazing mammals, and so tolerate grazing well. Some grasslands in more arid regions, such as the Borderlands, originally experienced natural wildfires and may require periodic burning for their maintenance. Still others, such as the spinifex grasslands of Australia, grow on nutrient-poor soils that support sparse growth of grasses at best.

Armed with this understanding, you may see several possible reasons why the grasslands in the Borderlands were lost and why “resting the range” by itself has not restored them. Perhaps periodic burning is needed. Perhaps the original landscape, without deep gullies that carry water away, must be re-created to return to the soil the water and nutrients that grasses require. Perhaps an interaction between fire and nutrients influences which vegetation type will be favored. And another thought may occur to you: perhaps it will be necessary to begin with drastic restoration measures—for example, removal of the existing shrubs—before other measures will have a chance of success. Ecologists are now using natural history knowledge, experiments in natural grassland plots, and modeling as scientific tools (Concept 41.5) to explore all of these possibilities.