Answer

1. • Predation, Mutualism, and Competition

     Although humans receive direct benefits from thousands of species, most threatened and endangered species are probably of little or no use to people. Although birders and biologists would mourn its extinction, the loss of the rufus red knot would not likely cause direct harm to anyone.

     In reality, most species are beneficial to humans because they are part of a biological community, consisting of all the organisms living together in a particular habitat area. Within a community, each species occupies a particular ecological niche, which can be thought of as the role or “job” of the species. The complex linkage among organisms inhabiting different niches in a community is often referred to as a food web (Figure 16.11). As with a spider’s web, any disruption in one strand of the web of life is felt by other portions of the web. Some tugs on the web cause only minor changes to the community, and others can cause the entire web to collapse. Most commonly, losses of strands in the web are felt by a small number of associated species. The story of the horseshoe crab and rufus red knot already shows us that. But some disruptions caused by the loss of seemingly insignificant species have the potential to be felt even by humans.

     Visualize This

     Predict what would happen to the other species in this web if baleen whales went extinct.

     Figure 16.11 The web of life.

     Species are connected to other species in food chains—a network of food chains forms a food web. This drawing shows the feeding connections among species in the Antarctic Ocean. Black arrows represent feeding relationships; for example, penguins eat fish and in turn are eaten by leopard seals.

      

     Figure 16.11 Full Alternative Text

     Mutualism: How Bees Feed the World

     An interaction between two species that benefit each other is called mutualism. Mutualism can be contrasted with commensalism, a relationship in which one species benefits and the other is unaffected—for instance, the relationship between cattle egrets (a species of bird) and domestic cattle. The egrets follow the cattle as they graze, feeding on insects stirred up from the ground by these animals (Figure 16.12). The cattle do not appear to benefit or be harmed by the egrets’ presence.

     Figure 16.12 Commensalism.

     Cattle egrets are found in close association with grazing mammals such as this water buffalo, as well as domestic cattle. Birds follow the grazers eating insects stirred up by the activity. The cattle and water buffalo are not affected.

      

     We find examples of mutualism in many environments. Cleaner fish that remove and consume parasites from the bodies of larger fish, fungal mycorrhizae that increase the mineral absorption of plant roots while consuming the plant’s sugars, and ants that find homes in the thorns of acacia trees and defend the trees from other insects are all examples of mutualism. The mutualistic interaction between plants and bees is perhaps the most important to humans.

     Bees occupy a very important ecological niche as the primary pollinators of many species of flowering plants. The role of pollinators is to transfer sperm, in the form of pollen grains, from one flower to the female reproductive structures of another flower. The flowering plant benefits from this relationship because insect pollination increases the number of seeds that the plant produces. The bee benefits by collecting excess pollen and nectar to feed itself and its relatives in the hive (Figure 16.13).

     Figure 16.13 Mutualism.

     Honeybees transfer pollen, allowing a plant to “mate” with another plant some distance away. Both species benefit.

     Wild bees pollinate at least 80% of all the agricultural crops in the United States, providing a net benefit of $10 to $15 billion. In addition, populations of wild honeybees have a major and direct impact on many more billions in agricultural production around the globe.

     Bees in the United States and northwestern Europe have suffered dramatic declines in recent years. Although it is not unusual for beekeepers to lose approximately 20% of their hives over winter, since 2006 from 30 to 45% of captive bee colonies have been lost each year. The exact causes of these die-offs vary: from an increased level of bee parasites (infectious organisms that cause disease or drain energy from their hosts), to competition with the invading Africanized honeybees (“killer bees”), pesticide pollution, and habitat destruction. The prolonged decline of populations of either wild or domesticated bees that are mutualists of crop plants would be extremely costly to humans.

     Predation: How Songbirds May Save Forests

     A species that survives by eating another species is typically referred to as a predator. The word conjures up images of some of the most dramatic animals on Earth: cheetahs, eagles, and killer whales. You might not picture wood warblers, a family of North American bird species characterized by their small size and colorful summer plumage, as predators; however, these beautiful songsters are voracious consumers of insects (Figure 16.14a). The hundreds of millions of individual warblers in the forests of North America collectively remove thousands of kilograms of insects from forest trees and shrubs every summer.

     Figure 16.14 Predation.

     (a) The black-throated blue warbler is one of many warbler species native to North American forests. These birds are active predators of plant-eating insects. (b) Insects can kill trees, as seen in this photo of a spruce budworm infestation. Warblers and other insect-eating birds likely reduce the number and severity of such insect outbreaks.

     Most of the insects that warblers eat prey on plants. By reducing the number of insects in forests, warblers reduce the damage that insects inflict on forest plants. Reducing the amount of damage likely increases the growth rate of the trees. Harvesting trees for paper and lumber production fuels an industry worth more than $230 billion in the United States alone. At least some of this wood was produced because warblers were controlling insects in forests (Figure 16.14b).

     Many species of forest warblers are experiencing declines in abundance. The loss of warbler species has several causes, including habitat destruction in their summer habitats in North America and their winter habitats in Central and South America. Warblers also face increased predation by animals whose populations benefit from human settlements, such as raccoons and housecats. Although other, less vulnerable birds may increase in number when warblers decline, these “replacement” birds are typically less insect-dependent. If smaller warbler populations correspond to lower forest growth rates and higher levels of forest disease, then these tiny, beautiful birds definitely have an important effect on the human economy.

     Competition: How a Deliberately Infected Chicken Could Save a Life

     When two species of organisms both require the same resources for life, they will be in competition for the resources within a habitat. In general, competition limits the size of competing populations. From a scientific perspective, to determine whether two species that seem to be using the same resource are competing, we remove one from an environment. If the population of the other species increases, then the two species are competitors.

     We may imagine lions and hyenas fighting over a freshly killed antelope or weeds growing in our vegetable gardens as typical examples of competition, but most competitive interactions are invisible. The least visible competition occurs among microorganisms. However, microbial competition is often essential to the health of both people and ecological communities.

     Salmonella enteritidis is a leading cause of food-borne illness in the United States. Between 2 million and 4 million people in this country are infected by S. enteritidis every year, experiencing fever, intestinal cramps, and diarrhea as a result. In about 10% of cases, the infection results in severe illness requiring hospitalization. Four to six hundred Americans die as a result of S. enteritidis infection every year.

     Most S. enteritidis infections result from consuming undercooked poultry products, especially eggs. The U.S. Centers for Disease Control and Prevention estimate that as many as 1 in 50 consumers is exposed to eggs contaminated with S. enteritidis every year. Surprisingly, most of these eggs look perfectly normal and intact. These pathogens contaminate the egg when it forms inside the hen. Thus, the only way to prevent S. enteritidis from contaminating eggs is to keep it out of hens.

     A common way to control S. enteritidis is to feed hens antibiotics—chemicals that kill bacteria. However, like most microbes, S. enteritidis strains can evolve drug resistance that makes them more difficult to kill off. But there is another way to reduce S. enteritidis infection in poultry: make sure another species is occupying its niche.

     Most S. enteritidis infections originate in an animal’s gut. If another bacterial species is already monopolizing the food and available space in a hen’s digestive system, then S. enteritidis will have trouble colonizing there. Following this principle, some poultry producers now intentionally infect hens’ digestive systems with harmless bacteria, a practice called competitive exclusion, to reduce S. enteritidis levels in their flocks. This technique involves feeding cultures of benign bacteria to 1-day-old birds. When the harmless bacteria become established in the niche of the chicks’ intestines, the chicks will be less likely to host large S. enteritidis populations (Figure 16.15). There is evidence that this practice is working; S. enteritidis infections in chickens have dropped by nearly 50% in the United Kingdom, where competitive exclusion is common practice.

     Visualize This

     Why does the total number of bacteria level off over time in both graphs?

     Figure 16.15 Competition.

     If poultry producers feed very young chicks non-disease-causing (beneficial) bacteria, the beneficial bacteria take up the space and nutrients in the intestine that would be used by S. enteritidis.

      

     Figure 16.15 Full Alternative Text

     The competitive exclusion of S. enteritidis in hens mirrors the role of some human-associated bacteria, such as those that normally live within our intestines and genital tracts. For instance, many women who take antibiotics for a bacterial infection will then develop vaginal yeast infections because the antibiotic kills noninfectious bacteria as well, including species that normally compete with yeast. Maintaining competitive interactions between larger species can be important for humans as well. For instance, in temporary ponds, the main competitors for the algae food source are mosquitoes, tadpoles, and snails. In the absence of tadpoles and snails, mosquito populations can become quite large—potentially with severe consequences because these insects may carry deadly diseases such as malaria, West Nile and Zika viruses, and yellow fever. With frogs, toads, and their tadpoles increasingly endangered, this risk is a real one.

     Keystone Species: How Wolves Feed Beavers

     Table 16.2 summarizes the major types of ecological interactions among organisms. However, this table emphasizes the effects of each interaction on the species directly involved; it does not illustrate that many of these interactions may have multiple indirect effects.

     Table 16.2 Types of species interactions and their direct effects.

      

     Table 16.2 Full Alternative Text

     Look again at the food web pictured in Figure 16.11. None of the species in the Antarctic Ocean’s biological community is connected to only one other species—they all eat something, and most of them are eaten by something else. You can imagine that penguins, by preying on squid, have a negative effect on elephant seals, which they compete with for these squid, and a more indirect positive effect on other seabirds, which compete with squid for krill. The existence of these indirect effects of varying importance has led ecologists to hypothesize that, in at least some communities, the activities of a single species can play a dramatic role in determining the composition of the system’s food web. These organisms are called keystone species because their role in a community is analogous to the role of a keystone in an archway (Figure 16.16a). Remove the keystone, and an archway collapses; remove the keystone species, and the web of life collapses. It is very difficult to predict which species in an intact ecosystem may be a keystone species, but biologists can point to several examples that became apparent after a species disappeared. One example is the population of gray wolves in Yellowstone National Park (Figure 16.16b).

     Figure 16.16 Keystone species.

     (a) The keystone in an archway helps to stabilize and maintain the arch. (b) A keystone species, such as wolves in Yellowstone National Park, helps to stabilize and maintain other species in an ecosystem.

     Gray wolves were exterminated within Yellowstone National Park by the mid-1920s because of a systematic campaign to rid the American West of this occasional predator of livestock. However, by the 1980s, thanks to insights gained from the science of ecology and a new interest in environmental health, attitudes about the wolf had changed. A new appreciation of the role of wolves in natural systems led to renewed interest in returning wolves to their historical homeland. From 1995 to 1997, 41 wolves originally trapped in Canada were released into Yellowstone National Park and surrounding areas. Thanks to protection from hunting and the wolves’ own adaptability, by the end of 2015 the number had grown to at least 500 wolves in the greater Yellowstone area, and the animal was no longer considered endangered there.

     During the time that wolves were extinct in Yellowstone Park, biologists noticed dramatic declines in populations of aspen, cottonwood, and willow trees. They attributed this decline to an increase in predation by elk, especially during winter when grasses become unavailable. However, just a few years after wolf reintroduction, aspen, cottonwood, and willow tree growth has rebounded in some areas of the park, even though the wolf population was still too low to make a major dent in elk populations. Besides the regions near active wolf dens, the areas of the park that saw the greatest recovery include places on the landscape where elk have limited ability to see approaching wolves or to escape. Thus, the elk will stay away from these areas to avoid wolf predation. Wolves, primarily by changing elk behavior, appear to be important to maintaining large populations of hardwood trees in Yellowstone Park.

     The rebound of aspen, cottonwood, and willow populations in Yellowstone has effects on other species as well. Beaver rely on these trees for food, and their populations appear to be growing in the park after decades of decline. Warblers, insects, and even fish that depend on shelter, food, and shade from these trees are increasing in abundance as well. Wolves in Yellowstone appear to fit the profile of a classic keystone species, one whose removal had numerous and surprising effects on biodiversity.