Main content Chapter Introduction A clownfish gains protection by living among sea

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Chapter Introduction
A clownfish gains protection by living among sea anemones and helps protect the anemones from some of their predators.
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Core Case StudyThe Southern Sea Otter: A Species in Recovery
Learning Objective
LO 5.1Explain what could happen to the Pacific coast kelp forest ecosystem if the southern sea otters were eliminated.
Southern sea otters (Figure 5.1, left) live in giant kelp forests (Figure 5.1, right) in shallow waters along parts of the Pacific coast of North America. Most of the members of this endangered species are found off the California coast between the cities of Santa Cruz and Santa Barbara.
Figure 5.1
An endangered southern sea otter in Monterey Bay, California (USA) uses a stone to crack the shells of the clams that it feeds on (left). It lives in a bed of seaweed called giant kelp (right).
Left: Kirsten Wahlquist/; Right: Paul Whitted/
Southern sea otters are fast and agile swimmers that dive to the ocean bottom looking for shellfish and other prey. They swim on their backs on the ocean surface and use their bellies as a table to eat their prey (Figure 5.1, left). Each day, a sea otter consumes 20–35% of its weight in clams, mussels, crabs, sea urchins, abalone, and other species of bottom-dwelling organisms. Their thick, dense fur traps air bubbles and keeps them warm.
An estimated 16,000 southern sea otters once lived in California’s coastal waters. By the early 1900s, they had been hunted almost to extinction in this region by fur traders who killed them for their luxurious fur. Commercial fishers also killed the sea otters because they competed with them for valuable abalone and other shellfish.
The southern sea otter population grew from a low of 50 in 1938 to 1,850 in 1977 when the U.S. Fish and Wildlife listed the species as endangered. In 2018, there were 3,128 otters.
Why should we care about the southern sea otters of California? One reason is ethical: Many people believe it is wrong to allow human activities to cause the extinction of a species. Another reason is that people love to look at these appealing and highly intelligent animals as they play in the water. As a result, the otters help to generate millions of dollars a year in tourism revenues. A third reason—and a key reason in our study of environmental science—is that biologists classify the southern sea otter as a keystone species (see Chapter 4). Scientists hypothesize that in the absence of southern sea otters, sea urchins and other kelp-eating species would probably destroy the Pacific coast kelp forests and much of the rich biodiversity they support.
Biodiversity is an important part of the earth’s natural capital and is the focus of one of the three scientific principles of sustainability. In this chapter, we look at how species interact and help control one another’s population sizes. We also explore how communities, ecosystems, and populations of species respond to changes in environmental conditions.
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5.1aCompetition for Resources
Ecologists have identified five basic types of interactions among species as they share limited resources such as food, shelter, and space. These types of interactions are called interspecific competition, predation, parasitism, mutualism, and commensalism. They each have a role in limiting the population size and resource use of the interacting species in an ecosystem.
Competition is the most common interaction among species. It occurs when members of one or more species try to use the same limited resources such as food, water, light, and space. Competition between different species is called interspecific competition. It plays a larger role in most ecosystems than intraspecific competition—competition among members of the same species.
When two species compete with one another for the same resources, their ecological niches (Figure 4.9) overlap. The greater this overlap, the more they compete for key resources. If one species can take over the largest share of one or more key resources, each of the other competing species must move to another area (if possible), suffer a population decline, or become extinct in that area. The niches of two different species can overlap but they cannot simultaneously fully occupy the same niche, a concept called the competitive exclusion principle.
Humans compete with many other species for space, food, and other resources. As our ecological footprints grow and spread, we take over or degrade the habitats of many of those species and deprive them of resources they need to survive.
Species evolve to reduce competition for resources by reducing their niche overlap. An example is resource partitioning, which occurs when different species competing for similar scarce resources evolve specialized traits that allow them to “share” the same resources. Sharing resources can mean using parts of the resources, or using the resources at different times or in different ways. Figure 5.2 shows resource partitioning by insect-eating bird species. Adaptations allow the birds to reduce competition by feeding in different portions of certain spruce trees and by feeding on different insect species.
Figure 5.2
Sharing the wealth: Resource partitioning among five species of insect-eating warblers in the spruce forests of the U.S. state of Maine. Each species spends at least half its feeding time in its associated yellow-highlighted areas of these spruce trees.
After R. H. MacArthur, “Population Ecology of Some Warblers in Northeastern Coniferous Forests,” Ecology 36:533–536, 1958.
Another example of resource partitioning through natural selection involves birds called honeycreepers that live in the U.S. state of Hawaii (Figure 5.3). Figure 4.10 shows how the evolution of specialized feeding niches has reduced competition for resources among bird species in a coastal wetland.
Figure 5.3
Specialist species of honeycreepers: Through natural selection, different species of honeycreepers have shared resources by evolving specialized beaks to take advantage of certain types of food such as insects, seeds, fruits, and nectar from certain flowers.
Look at each bird’s beak and guess what sort of food that bird might eat.
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In predation, a member of one species that feeds on all or part of a member of another species is called a predator, while the species that is fed upon is called the prey. Together, they are engaged in a predator–prey relationship (Figure 5.4). Predation has a strong effect on the population sizes of the competing species.
Figure 5.4
Predator–prey relationship: This brown bear (the predator) in the U.S. state of Alaska has captured and will feed on this salmon (the prey).
Steve Hilebrand/U.S. Fish and Wildlife Service
Grizzly Bears and Moths
During the summer months, the grizzly bears of the Greater Yellowstone ecosystem in the western United States eat huge amounts of army cutworm moths, which huddle in masses high on remote mountain slopes. In this predator–prey interaction, one grizzly bear can dig out and lap up as many as 40,000 cutworm moths in a day. Consisting of 50–70% fat, the moths offer a nutrient that the bear can store in its fatty tissues and draw on during its winter hibernation.
In a giant kelp forest ecosystem, sea urchins prey on kelp, a type of seaweed (Science Focus 5.1). As a keystone species, southern sea otters (Core Case Study) prey on the sea urchins and prevent them from destroying the kelp forests. An adult southern sea otter can eat as many as 1,500 sea urchins a day.
Science Focus 5.1
Threats to Kelp Forests
A kelp forest contains large concentrations of seaweed called giant kelp. Anchored to the ocean floor, its long blades grow toward the sunlit surface waters (Figure 5.1, right). Under good conditions, the blades can grow 0.6 meter (2 feet) in a day and the plant can grow as tall as a 10-story building. The kelp blades are flexible and can survive all but the most violent storms and waves.
Kelp forests support many marine plants and animals and are one of the most biologically diverse marine ecosystems. These forests also reduce shore erosion by blunting the force of incoming waves and trapping some of the outgoing sand.
Sea urchins, such as the purple urchin (Figure 5.A), prey on kelp plants. Large populations of these predators can rapidly devastate a kelp forest because they eat the bases of young kelp plants. Scientific studies by biologists, including James Estes of the University of California at Santa Cruz, indicate that the southern sea otter (Core Case Study) is a keystone species that helps sustain kelp forests by controlling populations of purple and other sea urchin species.
Figure 5.A
The purple sea urchin inhabits the coastal waters of the U.S. state of California and feeds on kelp.
Another threat to kelp forests is polluted water running off the land. The pollutants in runoff can include pesticides and herbicides that can kill kelp plants and other species and upset the food webs in these aquatic forests. Another runoff pollutant is fertilizer. Its plant nutrients (mostly nitrates) can cause excessive growth of algae and other aquatic plants. This growth blocks some of the sunlight needed to support the growth of giant kelp.
Some scientists warn that the warming of the world’s oceans is a growing threat to kelp forests, which require cool water. If coastal waters get warmer during this century, as projected by climate models, many or most of California’s coastal kelp forests could disappear.
Critical Thinking
List three ways in which we could reduce the degradation of giant kelp forest ecosystems.
Predators use a variety of ways to capture prey. Herbivores can walk, swim, or fly to the plants they feed on. Many carnivores, such as cheetahs, use their speed to chase down and kill prey, such as zebras. Eagles and hawks have keen enough eyesight to spot their prey from the air as they fly. Some predators such as female African lions work in groups to capture large or fast-running prey.
Other predators use camouflage to hide in plain sight and ambush their prey. For example, praying mantises (see Figure 4.4, right sit on flowers or plants of a color similar to their own and ambush visiting insects. White ermines (a type of weasel), snowy owls, and arctic foxes (Figure 5.5) hunt their prey in snow-covered areas. Some predators use chemical warfare to attack their prey. For example, some spiders and poisonous snakes use venom to paralyze their prey and to defend against their predators.
Figure 5.5
A white arctic fox hunts its prey by blending into its snowy background to avoid being detected.
Paul Nicklen/National Geographic Image Collection
Prey species have evolved many ways to avoid predators. Some can run, swim, or fly fast and some have highly developed senses of sight, sound, or smell that alert them to the presence of predators. Other adaptations include protective shells (abalone and turtles), thick bark (giant sequoia trees), spines (porcupines and sea urchins), and thorns (cacti and rose bushes).
Other prey species use camouflage to blend into their surroundings. Some insect species resemble twigs (Figure 5.6a), or bird droppings on leaves. A leaf insect can be almost invisible against its background (Figure 5.6b), as can an arctic hare in its white winter fur.
Figure 5.6
These prey species have developed specialized ways to avoid their predators: (a, b) camouflage, (c, d, e) chemical warfare, (d, e, f) warning coloration, (f) mimicry, (g) deceptive looks, and (h) deceptive behavior.
Prey species also use chemical warfare. Some discourage predators by containing or emitting chemicals that are poisonous (oleander plants), irritating (stinging nettles and bombardier beetles, Figure 5.6c), foul smelling (skunks and stinkbugs), or bad tasting (buttercups and monarch butterflies, Figure 5.6d). When attacked, some species of squid and octopus emit clouds of black ink, allowing them to escape by confusing their predators.
Learning from Nature
Researchers have studied the bombardier beetle’s high-pressure combustion chamber in its abdomen, used to expel a poison that forces predators to vomit the beetle after eating it. Engineers hope to apply this research to industrial or medical spray technology.
Many bad-tasting, bad-smelling, toxic, or stinging prey species flash a warning coloration that eating them is risky. Examples are the brilliantly colored, foul–tasting monarch butterflies (Figure 5.6d) and poisonous frogs (Figure 5.6e). When a bird eats a monarch butterfly, it usually vomits and learns to avoid monarchs.
Some butterfly species gain protection by looking and acting like other, more dangerous species, a protective device known as mimicry. For example, the nonpoisonous viceroy butterfly (Figure 5.6f) mimics the monarch butterfly. Other prey species use behavioral strategies to avoid predation. Some attempt to scare off predators by puffing up (blowfish), spreading their wings (peacocks), or mimicking a predator (Figure 5.6h). Some moths have wings that look like the eyes of much larger animals (Figure 5.6g). Other prey species gain some protection by living in large groups such as schools of fish and herds of antelope.
Biologist Edward O. Wilson (Individuals Matter 4.1) proposed two criteria for evaluating the dangers posed by various brightly colored animal species. First, if they are small and strikingly beautiful, they are probably poisonous. Second, if they are strikingly beautiful and easy to catch, they are probably deadly.
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Over time, a prey species develops traits that make it more difficult to catch. Its predators then face selection pressures that favor traits that increase their ability to catch their prey. Then the prey species must get better at eluding the more effective predators.
This back-and-forth adaptation is called coevolution, a natural selection process in which changes in the gene pool of one species lead to changes in the gene pool of another species. It can play an important role in controlling population growth of predator and prey species. When populations of two species interact as predator and prey over a long time, coevolution can help the predator succeed and it can help the prey avoid being eaten.
For example, bats prey on certain species of moths (Figure 5.7) that they hunt at night using echolocation. They emit pulses of high-frequency sound that bounce off their prey and capture the returning echoes that tell them where their prey is located. Over time, certain moth species have evolved ears that are sensitive to the sound frequencies that bats use to find them. When they hear these frequencies, they drop to the ground or fly evasively. Some bat species evolved ways to counter this defense by changing the frequency of their sound pulses. In turn, some moths evolved their own high-frequency clicks to jam the bats’ echolocation systems. Some bat species then adapted by turning off their echolocation systems and using the moths’ clicks to locate their prey. This is a classic example of coevolution.
Figure 5.7
Coevolution: This bat is using ultrasound to hunt a moth. As the bats evolve traits to increase their chances of getting a meal, the moths evolve traits to help them avoid being eaten.
Michael Duham/Minden Pictures/Superstock
Learning from Nature
Bats and dolphins use echolocation to navigate and locate prey in the darkness of night and in the ocean’s murky water. Scientists are studying how they do this to improve our sonar systems, sonic imaging tools for detecting underground mineral deposits, and medical ultrasound imaging systems.
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5.1dParasitism, Mutualism, and Commensalism
Parasitism occurs when one species (the parasite) lives in or on another organism (the host). The parasite benefits by extracting nutrients from its host. The parasite weakens its host but rarely kills it, since doing so eliminates the source of its benefits. Parasites can be plants, animals, or microorganisms.
Tapeworms are parasites that live part of their life cycle inside their hosts. Others such as mistletoe plants and blood-sucking sea lampreys (Figure 5.8) attach themselves to the outsides of their hosts and suck nutrients from them. Some parasites move from one host to another (fleas and ticks) while others (such as certain protozoa) spend their adult lives within a single host. Parasites help keep their host populations in check.
Figure 5.8
Parasitism: This blood-sucking, parasitic sea lamprey has attached itself to an adult lake trout from one of the Great Lakes of the United States and Canada.
Great Lakes Fishery Commission
In mutualism, two species interact in ways that benefit both species by providing each with food, shelter, or some other resource. An example is pollination of flowering plants by species such as honeybees, hummingbirds, and butterflies that feed on the nectar of flowers. These pollinators get food in the form of nectar and spread the pollen from flower to flower, which helps the flower species produce seeds and reproduce.
Figure 5.9 shows an example of a mutualistic relationship that combines nutrition and protection. It involves birds that ride on the backs or heads of large animals such as elephants, rhinoceroses, and impalas. The birds remove and eat parasites and pests (such as ticks and flies) from the animals’ bodies and often make noises warning the animals when predators are approaching.
Figure 5.9
Mutualism: This red-billed oxpecker feeds on parasitic ticks that infest animals such as this impala and warns of approaching predators.
Uwe Bergwitz/
Another example of mutualism involves clownfish, which usually live within sea anemones (see chapter-opening photo), whose tentacles sting and paralyze most fish that touch them. The clownfish, which are not harmed by the tentacles, gain protection from predators and feed on the waste matter left from the anemones’ meals. The sea anemones benefit because the clownfish protect them from some of their predators and parasites.
Mutualism might appear to be a form of cooperation between species. However, each species is acting for its own survival.
Commensalism is an interaction between two species in which one species benefits and the other species is unaffected. For example, plants called epiphytes (air plants), attach themselves to the trunks or branches of trees (Figure 5.10) in tropical and subtropical forests. The epiphytes gain better access to sunlight, water from the humid air and rain, and nutrients falling from the tree’s upper leaves and limbs. Their presence apparently does not harm the tree. Similarly, birds benefit by nesting in trees, generally without harming them.
Figure 5.10
Commensalism: This pitcher plant is attached to a branch of a tree without penetrating or harming the tree. This carnivorous plant feeds on insects that become trapped inside it.
Watch this animation to see the difference between primary and secondary succession.
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5.2aEcological Succession
The types and numbers of species in biological communities and ecosystems change in response to changing environmental conditions. The gradual change in species composition in a given community or ecosystem is called ecological succession.
There are two major types of ecological succession, depending on the conditions present at the beginning of the process. Primary ecological succession involves the gradual establishment of communities of different species in lifeless areas—in terrestrial systems with no soil or in aquatic systems with no bottom sediments. Examples include bare rock exposed by a retreating glacier (Figure 5.11), an abandoned highway or parking lot, and a newly created shallow pond or lake (Figure 5.12). Primary succession can take hundreds to thousands of years because of the need to build up fertile soil or aquatic sediments to provide the nutrients needed to establish a community of producers.
Figure 5.11
Primary ecological succession: Over almost a thousand years, these plant communities developed, starting on bare rock exposed by a retreating glacier on Isle Royal, Michigan (USA), in western Lake Superior. The details of this process vary from one site to another.
Figure 5.12
Primary ecological succession in a lake basin in which sediments and plants have been gouged out by a glacier. When the glacier melts, the lake basin begins accumulating sediments and plant and animal life. Over hundreds to thousands of years, the lake can fill with sediments and become a terrestrial habitat.
Pioneer species are the first species to occupy the barren environment and are often carried there by wind or water. Common pioneer species are mosses and lichens (Figure 5.12) because they can grow on rock. They spread and break the rock into pieces that start the long soil formation process. When they die and decompose, they provide nutrients for the thin soil layer.
The other, more common type of ecological succession is called secondary ecological succession, in which a community or ecosystem develops on the site of an existing community or ecosystem and replaces or adds to the existing set of resident species. This type of succession begins in an area where an ecosystem has been disturbed, removed, or destroyed, but where some soil or bottom sediment remains. Candidates for secondary succession include abandoned farmland (Figure 5.13), burned or cut forests, heavily polluted streams, and flooded land. Because some soil or sediment is present, new vegetation can begin to grow, usually within a few weeks. On land, growth begins with the germination of seeds already in the soil and seeds imported by wind or in the droppings of birds and other animals.
Figure 5.13
Secondary ecological succession: Natural restoration of disturbed land on an abandoned farm field in the U.S. state of North Carolina. It took 150 to 200 years after the farmland was abandoned for the area to become covered with a mature oak and hickory forest. Primary and secondary ecological succession are examples of natural ecological restoration.
Watch this animation to see the difference between primary and secondary succession.
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Ecological succession is an important ecosystem service that tends to enrich the biodiversity of communities and ecosystems by increasing species diversity and interactions among species. Such interactions enhance an ecosystem’s sustainability by promoting population control and by increasing the complexity of food webs, which enhances energy flow and nutrient cycling.
Ecologists have identified three factors that affect how and at what rate ecological succession occurs. One is facilitation, in which one set of species makes an area suitable for species with different niche requirements, and often less suitable for itself. For example, as lichens and mosses gradually build up soil on a rock in primary succession, herbs and grasses can move in and crowd out the lichens and mosses (Figure 5.11).
A second factor is inhibition, in which some species hinder the establishment and growth of other species. For example, needles dropping off some pine trees make the soil beneath the trees too acidic for most other plants to grow there. A third factor is tolerance, in which plants in the late stages of succession succeed because they are not in direct competition with other plants for key resources. Shade-tolerant plants, for example, can live in shady forests because they do not need as much sunlight as the trees above them do (Figures 5.11 and 5.13).
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5.2bIs There a Balance of Nature?
According to the traditional view, ecological succession proceeds in an orderly sequence along an expected path until a certain stable type of climax community (Figures 5.11 and 5.13), which is assumed to be in balance with its environment, occupies an area. This equilibrium model of succession is what ecologists once meant when they talked about the balance of nature.
Over the last several decades, many ecologists have changed their views about balance and equilibrium in nature based on ecological research. There is a general tendency for succession to lead to more complex, diverse, and presumably more resilient ecosystems that can withstand changes in environmental conditions if the changes are not too large or too sudden. However, the current scientific view is that we cannot predict a given course of succession or view it as inevitable progress toward an ideally adapted climax plant community or ecosystem. Rather, ecological succession reflects the ongoing struggle by different species for enough light, water, nutrients, food, space, and other key resources. In other words, research shows that there is no balance of nature consisting of a permanent and stable state.
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5.2cLiving Systems Are Sustained through Constant Change
All living systems, from a cell to the biosphere, are constantly changing in response to changing environmental conditions. Living systems have processes that interact to provide some degree of stability, or sustainability. However, this stability, or the capacity to withstand external stress and disturbance, is maintained by constant change in response to changing environmental conditions. Nature is dynamic, not static, and is not fragile as revealed by how the earth’s life has changed and evolved for 3.8 billion years in response to drastic changes in environmental conditions.
Ecologists distinguish between two aspects of sustainability in ecosystems. Ecological inertia, or persistence is the ability of an ecosystem to survive moderate disturbances. A second factor, resilience, is the ability of an ecosystem to be restored through secondary ecological succession after a severe disturbance.
Evidence suggests that some ecosystems have one of these properties but not the other. However, once a large tract of tropical rain forest is cleared or severely damaged, the resilience of the degraded forest ecosystem may be so low that the degradation reaches an ecological tipping point. Once it exceeds that point, the forest might not be restored by secondary ecological succession. One reason is that most of the nutrients in a tropical rain forest are stored in its vegetation, not in the topsoil. Once the nutrient-rich vegetation is gone, frequent rains can remove most of the remaining soil nutrients and thus prevent the return of a tropical rain forest to a large cleared area.
By contrast, grasslands are much less diverse than most forests. Thus, they have low inertia and can burn easily. Because most of their plant matter is stored in underground roots, these ecosystems have high resilience and can recover quickly after a fire because their root systems produce new grasses. Grassland can be destroyed only if its roots are plowed up and something else is planted in its place, or if it is severely overgrazed by livestock or other herbivores.
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5.3aPopulations Can Grow, Shrink, or Remain Stable
A population is a group of interbreeding individuals of the same species (Figure 5.14). Most populations live together in clumps or groups such as packs of wolves, schools of fish (Figure 5.14), and flocks of birds. Living in groups allows them to cluster where resources are available, provides some protection from predators, and helps some predator species to find and capture prey.
Figure 5.14
A population, or school, of Big Eye Trevally Jack in Baja California Sur, Mexico.
Leonardo Gonzalez/
Population size is the number of individual organisms in a population at a given time. Four variables—births, deaths, immigration, and emigration—govern changes in population size. A population increases through birth and immigration (the arrival of individuals from outside the population). Populations decrease through death and emigration (the departure of individuals from the population):
Scientists use sampling techniques to estimate the sizes of large populations of species such as oak trees that are spread over a large area and squirrels that move around and are hard to count. Typically, they count the number of individuals in one or more small sample areas and use this information to estimate the number of individuals in a larger area.
A population’s age structure—its distribution of individuals among various age groups—can have a strong effect on how rapidly its numbers grow or decline. Age groups are usually described in terms of organisms not mature enough to reproduce (the pre-reproductive stage), those capable of reproduction (the reproductive stage), and those too old to reproduce (the post-reproductive stage).
The size of a population will likely increase if it is made up mostly of individuals in their reproductive stage, or soon to enter this stage. In contrast, the size of a population dominated by individuals in their post-reproductive stage will tend to decrease over time.
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5.3bSeveral Factors Can Limit Population Size
Each population in an ecosystem has a range of tolerance—a range of variations in its physical and chemical environment within which it is most likely to survive. For example, a trout population (Figure 5.15) will thrive within a narrow band of temperatures (optimum level or range), although a few individuals can survive above and below that band (Figure 5.15). If the water becomes too hot or too cold, none of the trout can survive.
Figure 5.15
Range of tolerance for a population of trout to changes in water temperature.
Various physical or chemical factors can determine the number of organisms in a population and how fast a population grows or declines. Sometimes one or more factors, known as limiting factors, are more important than other factors in regulating population growth.
Learning from Nature
Biomimicry researchers are hoping to learn how plants that have a high tolerance for salty seawater can teach us how to design better ways of providing fresh drinking water in drought-prone areas.
On land, precipitation often is the limiting factor. Low precipitation levels in desert ecosystems limit desert plant growth. Lack of key soil nutrients limits the growth of plants, which in turn limits populations of animals that eat plants, and animals that feed on such plant-eating animals.
Limiting physical factors for populations in aquatic systems include water temperature (Figure 5.15), depth, and clarity (allowing for more or less sunlight). Other important factors are nutrient availability, acidity, salinity, and the level of oxygen gas in the water (dissolved oxygen content).
Too much of a physical or chemical factor can also be limiting. For example, too much water or fertilizer can kill land plants. If acidity levels are too high in an aquatic environment, some of its organisms can be harmed.
An additional factor that can limit the sizes of some populations is population density, the number of individuals in a population found within a defined area or volume. Density-dependent factors are variables that become more important as a population’s density increases. In a dense population, parasites and diseases can spread more easily, resulting in higher death rates, and competition for resources such as food and water can intensify. On the other hand, a higher population density can help sexually reproducing individuals to find mates more easily to produce offspring. Other factors such as drought, and climate change are considered density-independent factors, because they can affect population sizes regardless of density.
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5.3cNo Population Can Grow Indefinitely: J-Curves and S-Curves
The populations of some species, such as bacteria and many insect species, have an ability to increase their numbers exponentially. For example, with no controls on its population growth, a species of bacteria that can reproduce every 20 minutes would generate enough offspring to form a 0.3-meter-deep (1-foot-deep) layer over the surface of the entire earth in only 36 hours. Plotting such numbers against time yields a J-shaped curve of exponential growth (Figure 5.16, left). Members of such populations typically reproduce at an early age, have many offspring each time they reproduce, and reproduce many times with short intervals between generations.
Figure 5.16
According to this idealized mathematical model, populations of species can undergo exponential growth represented by a J-shaped curve (left) when resource supplies are plentiful. As resource supplies become limited, a population undergoes logistic growth, represented by an S-shaped curve (right), when the size of the population approaches the carrying capacity of its habitat.
However, there are always limits to population growth in nature. Research reveals that a rapidly growing population of any species eventually reaches some size limit imposed by limiting factors. These factors include sunlight, water, temperature, space, nutrients, or exposure to predators or infectious diseases. Environmental resistance is the sum of all such factors in a habitat.
Limiting factors largely determine an area’s carrying capacity, the maximum population of a given species that a particular habitat can sustain indefinitely. As a population approaches the carrying capacity of its habitat, the J-shaped curve of its exponential growth (Figure 5.16, left) is converted to an S-shaped curve of logistic growth, or growth that often fluctuates around the carrying capacity of its habitat (Figure 5.16, right).
However, the rate of population growth and the carrying capacity for a population are not fixed and can rise or fall as environmental conditions change the factors that promote and limit the population’s growth. Nature is constantly changing and is never in balance. In other words, the curve in Figure 5.16 is a simplified and idealized mathematical model of the growth rate and carrying capacity of populations in nature.
Some populations do not make a smooth transition from exponential growth to logistic growth. Instead, they use up their resource supplies and temporarily overshoot, or exceed, the carrying capacity of their environment. In such cases, the population suffers a sharp decline, called a dieback, or population crash, unless part of the population can switch to new resources or move to an area that has more resources. Such a crash occurred when reindeer were introduced onto a small island in the Bering Sea in the early 1900s (Figure 5.17).
Figure 5.17
Exponential growth, overshoot, and population crash of a population of reindeer introduced onto the small Bering Sea island of St. Paul in 1910.
Data Analysis:
By what percentage did the population of reindeer grow between 1923 and 1940?
Patterns of Population growth
Watch this animation to see what factors affect population size.
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5.3dReproductive Patterns
Species vary in their reproductive patterns. Species with a capacity for a high rate of population growth (r) (Figure 5.16, left) are called r-selected species. These species tend to have short life spans and produce many, usually small offspring and give them little or no parental care. As a result, many of the offspring die at an early age. To overcome such losses, r-selected species produce large numbers of offspring so a few will likely survive and have many offspring to sustain the species. Examples of r-selected species include algae, bacteria, frogs, and most insects.
Such species tend to be opportunists. They reproduce and disperse rapidly when conditions are favorable or when a disturbance such as a fire or clear-cutting of a forest opens up a new habitat or niches for invasion. Once established, their populations may crash because of unfavorable changes in environmental conditions or invasion by more competitive species. This explains why most opportunist species go through irregular and unstable boom-and-bust cycles in their population sizes.
At the other extreme are K-selected species. They tend to reproduce later in life, have few offspring, and have long life spans. Typically, the offspring of K-selected mammal species develop inside their mothers (where they are safe). After birth, they mature slowly and one or both parents care for and protect them. In some cases, they live in herds or groups until they reach reproductive age.
The population size of K-selected species tends to be near the carrying capacity (K) of its environment (Figure 5.16, right). Examples of K-selected species include most large mammals such as elephants, whales, and humans, birds of prey, large and long-lived plants such as the saguaro cactus, and most tropical rain forest trees. Many of these species—especially those with low reproductive rates, such as elephants, sharks, giant redwood trees, and California’s southern sea otters (Core Case Study and Science Focus 5.2)—are vulnerable to extinction. Most organisms have reproductive patterns between the extremes of r-selected and K-selected species. Table 5.1 compares typical traits of r-selected and K-selected species.
Table 5.1
Typical Traits of r-Selected and K-Selected Species
r-Selected Species
K-Selected Species
Reproductive potential
Population growth rate
Time to reproductive maturity
Number of reproductive cycles
Number of offspring
Size of offspring
Degree of parental care
Life span
Population size
Variable with crashes
Stable, near carrying capacity
Role in environment
Usually prey
Usually predators
Science Focus 5.2
The Future of California’s Southern Sea Otters
The population of southern sea otters (Core Case Study) has fluctuated in response to changes in environmental conditions (Figure 5.B). One change was a rise in populations of the orcas (killer whales) that feed on them. Scientists hypothesize that orcas started feeding more on southern sea otters when populations of their normal prey, sea lions and seals, began declining. In addition, between 2010 and 2015, the number of sea otters killed or injured by sharks increased, possibly because warmer ocean water brought some sharks closer to the shore.
Figure 5.B
Changes in the population size of southern sea otters off the coast of the U.S. state of California, 1983–2018.
(Compiled by the authors using data from U.S. Geological Survey.)
Another factor affecting sea otters may be parasites that breed in the intestines of cats. Scientists hypothesize that some southern sea otters are dying because coastal area cat owners flush feces-laden cat litter down their toilets or dump it in storm drains that empty into coastal waters. The feces contain parasites that can infect otters.
Toxic algae blooms also threaten otters. The algae thrive on urea, a nitrogen-containing ingredient in fertilizer that washes into coastal waters. Other pollutants released by human activities include PCBs and other fat-soluble toxic chemicals. These chemicals can kill otters by accumulating to high levels in the tissues of the shellfish that otters eat. Because southern sea otters feed at high trophic levels and live close to the shore, they are vulnerable to these and other pollutants in coastal waters.
Other threats to otters include oil spills from ships. The entire California southern sea otter population could be wiped out by a large oil spill from a single tanker off the central west coast or by the rupture of an offshore oil well, should drilling for oil be allowed off this coast. Some sea otters die when they are trapped in underwater nets and traps for shellfish. Others are killed by boat strikes and gunshots.
Figure 5.B shows the change in the population size of the southern sea otter since 1967, ten years before it was protected as an endangered species. In 2016, the sea otter population was 3,272 the highest it has been since 1985. In 2017, the population was 3,186 and in 2018, it was 3,128. Thus, for three years the sea otter population has averaged above 3,090, which means it can be considered for removal from the federal endangered species list. Such a delisting would be a success story for the U.S. Endangered Species Act and the otters would still be protected under a California state law.
Critical Thinking
How would you design a controlled experiment to test the hypothesis that cat litter flushed down toilets might be killing southern sea otters?
The reproductive pattern of a species may give it a temporary advantage. However, the key factor in determining the ultimate population size of a species is the availability of suitable habitat with adequate resources. Changes in habitat or other environmental conditions can reduce the populations of some species while increasing the populations of other species, such as white-tailed deer in the United States (see Case Study that follows).
Case Study
Exploding White-Tailed Deer Populations in the United States
By 1900, habitat destruction and uncontrolled hunting had reduced the white-tailed deer (Figure 5.18) population in the United States to about 500,000 animals. In the 1920s and 1930s, laws were passed to protect the remaining deer. Hunting was restricted and predators, including wolves and mountain lions that preyed on the deer, were nearly eliminated.
Figure 5.18
White-tailed deer populations in the United States have been growing.
Roy Toft/National Geographic Image Collection
These protections worked, perhaps too well for some suburbanites and farmers. Today there are over 30 million white-tailed deer in the United States. During the last 50 years, suburbs have expanded and many Americans have moved into the wooded habitat of deer. The gardens and landscaping around their homes provide deer with flowers, shrubs, garden crops, and other plants they like to eat.
Deer prefer to live in the edge areas of forests and woodlots for security and go to nearby fields, orchards, lawns, and gardens for food, so white-tailed deer populations have soared in suburban areas.
In woodlands, larger populations of the deer are consuming native ground-cover vegetation, which has allowed nonnative weed species to take over and upset ecosystem food webs. The deer also help to spread Lyme disease (carried by deer ticks) to humans. In addition, each year about 1.5 million deer–vehicle collisions injure thousands and kill more than 160 people per year, on average—the highest human death toll from encounters with any wild animal in the United States.
There are no easy solutions to the deer population problem in the suburbs. Changes in hunting regulations that allow for the killing of more female deer have cut down the overall deer population. However, this has had a limited effect on deer populations in suburban areas because it is too dangerous to allow widespread hunting with guns in such populated communities. Some areas have hired experienced and licensed archers who use bows and arrows to help reduce deer numbers, being careful not to endanger nearby residents.
Some communities spray the scent of deer predators or of rotting deer meat in edge areas to scare off deer. Others scare off deer by using electronic equipment that emits high-frequency sounds that humans cannot hear. Some homeowners surround their gardens and yards with high, black plastic mesh fencing.
Deer can be trapped and moved from one area to another, but this is expensive and must be repeated whenever they move back into an area. In addition, there are questions concerning where to move the deer and how to pay for such programs.
Darts loaded with contraceptives can be shot into female deer to hold down their birth rates, but this is expensive and must be repeated every year. Another approach is to trap dominant males and use chemical injections to sterilize them. However, this is costly and will require years of testing. In addition, ethical questions about this approach would have to be considered.
Meanwhile, suburbanites can expect deer to chow down on their shrubs, flowers, and garden plants unless they can protect their properties with fences, repellents, or other methods. Suburban dwellers could also stop planting trees, shrubs, and flowers that attract deer around their homes.
Critical Thinking
If the earth experiences significant warming during this century as projected, is this likely to favor r-selected or K-selected species? Explain.
Critical Thinking
Some people blame the white-tailed deer for invading farms and suburban yards and gardens to eat food that humans have made easily available to them. Others say humans are mostly to blame because they have invaded deer territory, eliminated most of the predators that kept deer populations under control, and provided the deer with plenty to eat in their lawns, gardens, and crop fields. Which view do you hold? Why? Do you see a solution to this problem?
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5.3eSurvivorship Curves
Individuals of species with different reproductive strategies tend to have different life expectancies. This can be illustrated by a survivorship curve, which shows the percentages of the members of a population surviving at different ages. There are three generalized types of survivorship curves: late loss, early loss, and constant loss (Figure 5.19). A late loss population (K-selected species such as elephants and rhinoceroses) typically has high survivorship to a certain age, and then high mortality. A constant loss population (such as many songbirds) typically has a constant death rate at all ages. For an early loss population (many r-selected species and annual plants), survivorship is low early in life. These generalized survivorship curves only approximate the realities of nature.
Figure 5.19
Survivorship curves for populations of different species, obtained by showing the percentages of the members of a population surviving at different ages.
Critical Thinking:
Which type of survivorship curve applies to the human species?
Top: Gualtiero boffi/ Center: IrinaK/ Bottom: ultimathule/
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5.3fHumans Are Not Exempt from Nature’s Population Controls
Humans are not exempt from population crashes. In 1845, Ireland experienced such a crash after a fungus destroyed its potato crop. About 1 million people died from hunger or diseases related to malnutrition and millions more migrated to other countries, sharply reducing the Irish population.
During the 14th century, bubonic plague spread through densely populated European cities and killed at least 25 million people—one-third of the European population. The bacterium that causes this disease normally lives in rodents. It was transferred to humans by fleas that fed on infected rodents and then bit humans. The disease spread like wildfire through crowded cities, where sanitary conditions were poor and rats were abundant. Today several antibiotics can be used to treat bubonic plague.
So far, technological, social, and other cultural changes have expanded the earth’s carrying capacity for the human species. We have used large amounts of energy and matter resources to occupy formerly uninhabitable areas. We have expanded agriculture and controlled the populations of other species that compete with us for resources. Some say we can keep expanding our ecological footprint in this way indefinitely because of our technological ingenuity. Others say that at some point, we will reach the limits that nature eventually imposes on any population that exceeds or degrades its resource base. We discuss these issues in Chapter 6.
Big Ideas
Certain interactions among species affect their use of resources and their population sizes.
The species composition and population sizes of a community or ecosystem can change in response to changing environmental conditions through a process called ecological succession.
No population can escape natural limiting factors and grow indefinitely.
Patterns of Population growth
Watch this animation to see what factors affect population size.
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Tying It All TogetherSouthern Sea Otters and Sustainability
fred goldstein/
The southern sea otters of California are part of a complex ecosystem made up of large underwater kelp forests, bottom-dwelling creatures, and other species that depend on one another for survival. The sea otters act as a keystone species, mostly by feeding on sea urchins and keeping them from destroying the kelp.
In this chapter, we focused on how biodiversity promotes sustainability, provides a variety of species to restore damaged ecosystems through ecological succession, and limits the sizes of populations. Populations of most plants and animals depend, directly or indirectly, on solar energy, and all populations play roles in the cycling of nutrients in the ecosystems where they live. In addition, the biodiversity in different terrestrial and aquatic ecosystems provides alternative paths for energy flow and nutrient cycling, better opportunities for natural selection as environmental conditions change, and natural population control mechanisms. When we disrupt these paths, we violate the three scientific principles of sustainability.
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Chapter Review
Critical Thinking
What difference would it make if the southern sea otter (Core Case Study) became extinct primarily because of human activities? What are three things we could do to help prevent the extinction of this species?
Use the second law of thermodynamics (Chapter 2) and the concept of food chains and food webs to explain why predators are generally less abundant than their prey.
How would you reply to someone who argues that we should not worry about the effects that human activities have on natural systems because ecological succession will repair whatever damage we do?
How would you reply to someone who contends that efforts to preserve species and ecosystems are not worthwhile because nature is largely unpredictable?
What is the reproductive strategy of most species of insect pests and harmful bacteria? Why does this make it difficult for us to control their populations?
List three examples of how your life might be affected if changing environmental conditions favor r-selected species during the latter half of this century.
List two factors that may limit human population growth in the future. Do you think that we are close to reaching those limits? Explain.
If the human species were to suffer a population crash, name three species that might move in to occupy part of our ecological niche. What are three species that would likely decline as a result? Explain why these other species would decline.
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Chapter Review
Doing Environmental Science
Visit a nearby land area, such as a partially cleared or burned forest, grassland, or an abandoned crop field, and record signs of secondary ecological succession. Take notes on your observations and formulate a hypothesis about what sort of disturbance led to this succession. Include your thoughts about whether this disturbance was natural or caused by humans. Study the area carefully to see whether you can find patches that are at different stages of succession and record your thoughts about what sorts of disturbances have caused these differences. You might want to research the topic of ecological succession in such an area.
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Chapter Review
Data Analysis
The graph below shows changes in the size of an Emperor penguin population in terms of numbers of breeding pairs on the island of Terre Adelie in the Antarctic. Scientists used this data along with data on the penguins’ shrinking ice habitat to project a general decline in the island’s Emperor penguin population, to the point where they will be endangered in 2100. Use the graph to answer the following questions.
If the penguin population fluctuates around the carrying capacity, what was the approximate carrying capacity of the island for the penguin population from 1960 to 1975? What was the approximate carrying capacity of the island for the penguin population from 1980 to 2010?
What was the overall percentage decline in the penguin population from 1975 to 2010?
What is the projected overall percentage decline in the penguin population between 2010 and 2100?
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