2 Principles of Ecology
The Science of Ecology
Ecology is the study of how living things interact with each other and with their environment. It is a major branch of biology, but has areas of overlap with geography, geology, climatology, environmental science, and other sciences. This chapter introduces fundamental concepts in ecology related to organisms and the environment.
The Importance of Energy
Energy is defined as the ability move things, do work, or transfer heat, and comes in various forms, including light, heat, and electricity. There is Low-quality energy that comes in dispersed forms and high quality energy comes in condensed forms. Thermodynamics is the study of energy and the laws of thermodynamics, as you already learned about them, can be applied to energy flow in ecosystems. Remember: The first law of thermodynamics, or, the conservation of energy principle, states that energy may change from one form to another, but the total amount of energy will remain constant. That is to say that energy is not destroyed or created; it just changes form. For example, when wood is burned, the energy that was stored in the wood is not lost. It is given off as heat, smoke, and ash. The final amount of energy is the same just in new forms. The second law of thermodynamics is also important to environmental science and states that disorganization, or entropy, increases in natural systems through any spontaneous process. This means that as energy is used it is degraded to lower forms of energy.
These two laws are important to environmental science in the following ways:
1. First, and very important: we live in a closed system, the Earth’s ecosphere. Nearly all of the organisms on Earth obtain their energy from the sun, and the sun composes the primary level of most ecosystem food chains, save a few deep-water thermal vents and some geyser bacteria. Since energy is neither created nor destroyed, as stated by the first law of thermodynamics, we can conclude that other than the sun’s energy, the energy present is what we have to work with, including the food you live on
2. Second, when humans use non-renewable resources (such as oil) they are converting them into less-useful energy, as stated by the second law of thermodynamics. When those energy sources are depleted, they are gone. Use of these energy sources often also releases different elements back into the environment. For example, the combustion of oil releases carbon back into the air, and this offsets the carbon cycle ( which you learn about ). This helps contribute to climate change.
What these examples attempt to illustrate is that there are inputs and outputs to all energy types, and also benefits and costs to each kind. and each is controlled and limited within the laws of both ecology and thermodynamics.
Organisms and the Environment
Organisms are individual living things. Despite their tremendous diversity, all organisms have the same basic needs: energy and matter. These must be obtained from the environment. Therefore, organisms are not closed systems. They depend on and are influenced by their environment. The environment includes two types of factors: abiotic
1. Abiotic factors are the nonliving aspects of the environment. They include factors such as sunlight, soil, temperature, and water.
2. Biotic factors are the living aspects of the environment. They consist of other organisms, including members of the same and different species.
Levels of Organization
Ecologists study organisms and their environments at different levels. The most inclusive level is the biosphere. The biosphere consists of all the organisms on planet Earth and the areas where they live. It occurs in a very thin layer of the planet, extending from about 11,000 meters below sea level to 15,000 meters above sea level. An image of the biosphere is shown in Figure 2.1. Different colors on the map indicate the numbers of food-producing organisms in different parts of the biosphere. Ecological issues that might be investigated at the biosphere level include ocean pollution, air pollution, and global climate change.
FIGURE 2.1 – Image of the biosphere
Ecologists also study organisms and their environments at the population level. A population consists of organisms of the same species that live in the same area and interact with one another. You will read more about populations in the Populations chapter. Important ecological issues at the population level include:
• rapid growth of the human population, which has led to overpopulation and environmental damage;
• rapid decline in populations of many nonhuman species, which has led to the extinction of numerous species.
Another level at which ecologists study organisms and their environments is the community level. A community consists of populations of different species that live in the same area and interact with one another. For example, populations of coyotes and rabbits might interact in a grassland community. Coyotes hunt down and eat rabbits
for food, so the two species have a predator-prey relationship. Ecological issues at the community level include how changes in the size of one population affect other populations. The Populations chapter discusses population interactions in communities in detail.
An ecosystem is a unit of nature and the focus of study in ecology. It consists of all the biotic and abiotic factors in an area and their interactions. Ecosystems can vary in size. A lake could be considered an ecosystem. So could a dead log on a forest floor. Both the lake and log contain a variety of species that interact with each other and with
abiotic factors. Another example of an ecosystem is pictured in Figure 2.2.
FIGURE 2.2 – Desert Ecosystem. What are some of the biotic and abiotic factors in this desert ecosystem?
When it comes to energy, ecosystems are not closed. They need constant inputs of energy. Most ecosystems get energy from sunlight. A small minority get energy from chemical compounds. Unlike energy, matter is not constantly added to ecosystems. Instead, it is recycled. Water and elements such as carbon and nitrogen are used
over and over again.
One of the most important concepts associated with the ecosystem is the niche. A niche refers to the role of a species in its ecosystem. It includes all the ways that the species interacts with the biotic and abiotic factors of the environment. Two important aspects of a species’ niche are the food it eats and how the food is obtained. Look at
Figure 2.3. It shows pictures of birds that occupy different niches. Each species eats a different type of food and obtains the food in a different way.
FIGURE 2.3 – Bird Niches. Each of these species of birds has a beak that suits it for its niche. For example, the long slender beak of the nectarivore allows it to sip liquid nectar
from flowers. The short sturdy beak of the granivore allows it to crush hard, tough grains.
Another aspect of a species’ niche is its habitat. The habitat is the physical environment in which a species lives and to which it is adapted. A habitat’s features are determined mainly by abiotic factors such as temperature and rainfall. These factors also influence the traits of the organisms that live there. Consider a habitat with very low temperatures. Mammals that live in the habitat must have insulation to help them stay warm. Otherwise, their body temperature will drop to a level that is too low for survival. Species that live in these habitats have evolved fur, blubber, and other traits that provide insulation in order for them to survive in the cold. Human destruction of habitats is the major factor causing other species to decrease and become endangered or go extinct. Small habitats can support only small populations of organisms. Small populations are more susceptible to being wiped out by catastrophic events from which a large population could bounce back. More than 1,200 species face extinction during the next century due mostly to habitat loss and climate change.
Flow of Energy: Producers and Consumers
Energy enters ecosystems in the form of sunlight or chemical compounds. Some organisms use this energy to make food. Other organisms get energy by eating the food.
Producers are organisms that produce food for themselves and other organisms. They use energy and simple inorganic molecules to make organic compounds. The stability of producers is vital to ecosystems because all organisms need organic molecules. Producers are also called autotrophs. There are two basic types of autotrophs:
photoautotrophs and chemoautotrophs.
1. Photoautotrophs use energy from sunlight to make food by photosynthesis. They include plants, algae, and certain bacteria (see Figure 2.6).
2. Chemoautotrophs use energy from chemical compounds to make food by chemosynthesis. They include some bacteria and also archaea. Archaea are microorganisms that resemble bacteria.
In some places where life is found on Earth, there is not enough light to provide energy for photosynthesis. In these places, producers called chemoautotrophs use the energy stored in chemical compounds to make organic molecules by chemosynthesis. Chemosynthesis is the process by which carbon dioxide and water are converted to
carbohydrates. Instead of using energy from sunlight, chemoautotrophs use energy from the oxidation of inorganic compounds, such as hydrogen sulfide (H2S). Oxidation is an energy-releasing chemical reaction in which a molecule, atom, or ion loses electrons. Chemoautotrophs include bacteria called nitrifying bacteria, which live underground in soil. They oxidize nitrogen-containing compounds and change them to a form that plants can use. Chemoautotrophs also include archaea. Archaea are a domain of microorganisms that resemble bacteria. Most archaea live in extreme environments, such as around hydrothermal vents in the deep ocean. Hot water containing
hydrogen sulfide and other toxic substances escapes from the ocean floor at these vents, creating a hostile environment for most organisms. Near the vents, archaea cover the sea floor or live in or on the bodies of other organisms, such as tube worms. In these ecosystems, archaea use the toxic chemicals released from the vents to produce organic compounds. The organic compounds can then be used by other organisms, including tube worms. Archaea are able to sustain thriving communities, like the one shown in Figure 2.4, even in these hostile environments. Some chemosynthetic bacteria live around deep-ocean vents known as “black smokers.” Compounds such as
hydrogen sulfide, which flow out of the vents from Earth’s interior, are used by the bacteria for energy to make food. Consumers that depend on these bacteria to produce food for them include giant tubeworms, like these pictured in Figure 2.5. Why do bacteria that live deep below the ocean’s surface rely on chemical compounds instead of
sunlight for energy to make food?
FIGURE 2.4 – Red tube worms, each containing millions of archaea microorganisms, grow in a cluster around a hydrothermal vent in the deep ocean floor.
Archaea produce food for themselves (and for the tube worms) by chemosynthesis.
FIGURE 2.5 – Tubeworms deep in the Gulf of Mexico get their energy from chemosynthetic bacteria. The bacteria actually live inside the worms.
Phototautotrophs are organisms that use energy from sunlight to make food by photosynthesis. Photosynthesis is the process by which carbon dioxide and water are converted to glucose and oxygen, using sunlight for energy. Glucose, a carbohydrate, is an organic compound that can be used by autotrophs and other organisms for energy. As shown in Figure below, photoautotrophs include plants, algae, and certain bacteria. Plants are the most important photoautotrophs in land-based, or terrestrial, ecosystems. There is great variation in the plant kingdom. Plants include organisms as different as trees, grasses, mosses, and ferns. Nonetheless, all plants
are eukaryotes that contain chloroplasts, the cellular “machinery” needed for photosynthesis. Algae are photoautotrophs found in most ecosystems, but they generally are more important in water-based, or aquatic, ecosystems. Like plants, algae are eukaryotes that contain chloroplasts for photosynthesis. Algae include single-celled eukaryotes, such as diatoms, as well as multicellular eukaryotes, such as seaweed. Photoautotrophic bacteria, called cyanobacteria, are also important producers in aquatic ecosystems. Cyanobacteria were formerly called blue-green algae, but they are now classified as bacteria. Other photosynthetic bacteria, including purple photosynthetic bacteria, are producers in terrestrial as well as aquatic ecosystems. Both cyanobacteria and algae make up phytoplankton. Phytoplankton refers to all the tiny photoautotrophs found on or near the surface of a body of water. Phytoplankton usually is the primary producer in aquatic ecosystems.
Figure 2.6 – Different types of photoautotrophs are important in different ecosystems.
Consumers are organisms that depend on the producers (phototrophs or chemotrophs) organisms for food. They take in organic molecules by essentially “eating” other living things. They include all animals and fungi. (Fungi don’t really “eat”; they absorb nutrients from other organisms.) They also include many bacteria and even a few plants, such as the pitcher plant in Figure below. Consumers are also called heterotrophs. Heterotrophs are classified by what they eat:
- Herbivores consume producers such as plants or algae. They are a necessary link between producers and other consumers. Examples include deer, rabbits, and mice.
- Carnivores consume animals. Examples include lions, polar bears, hawks, frogs, salmon, and spiders. Carnivores that are unable to digest plants and must eat only animals are called obligate carnivores. Other carnivores can digest plants but do not commonly eat them.
- Omnivores consume both plants and animals. They include humans, pigs, brown bears, gulls, crows, and some species of fish.
FIGURE 2.7 – Pitcher Plant. Virtually all plants are producers. This pitcher plant is an exception. It consumes insects. It traps them in a substance that digests
them and absorbs the nutrients.
When organisms die, they leave behind energy and matter in their remains. Decomposers break down the remains and other wastes and release simple inorganic molecules back to the environment. Producers can then use the molecules to make new organic compounds. The stability of decomposers is essential to every ecosystem. Decomposers are classified by the type of organic matter they break down:
- Scavengers consume the soft tissues of dead animals. Examples of scavengers include vultures, raccoons, and blowflies.
- Detritivores consume detritus—the dead leaves, animal feces, and other organic debris that collects on the soil or at the bottom of a body of water. On land, detritivores include earthworms, millipedes, and dung beetles (see Figure 2.8). In water, detritivores include “bottom feeders” such as sea cucumbers and catfish.
- Saprotrophs are the final step in decomposition. They feed on any remaining organic matter that is left after other decomposers do their work. Saprotrophs include fungi and single-celled protozoa. Fungi are the only organisms that can decompose wood.
FIGURE 2.8 – Dung Beetle. This dung beetle is rolling a ball of feces to its nest to feed its young.
Food Chains and Food Webs
Food chains and food webs are diagrams that represent feeding relationships. They show who eats whom. In this way, they model how energy and matter move through ecosystems.
A food chain represents a single pathway through which energy and matter flow through an ecosystem. An example is shown in Figure 2.9. Food chains are generally simpler than what really happens in nature. Most organisms consume—and are consumed by—more than one species.
FIGURE 2.9 – This food chain includes producers and consumers. How could you add decomposers to the food chain?
A food web represents multiple pathways through which energy and matter flow through an ecosystem. It includes
many intersecting food chains. It demonstrates that most organisms eat, and are eaten, by more than one species. An
example is shown in Figure 2.10.
FIGURE 2.10 – Food Web. This food web consists of several different food chains. Which organisms are producers in all of the food chains included in the food web?
The feeding positions in a food chain or web are called trophic levels. The different trophic levels are defined in Table 1.1. Examples are also given in the table. All food chains and webs have at least two or three trophic levels. Generally, there are a maximum of four trophic levels.
|Trophic Level||Where It Gets Food||Example|
|1st Trophic Level: Producer||Makes its own food||Plants make food|
|2nd Trophic Level: Primary Consumer||Consumes producers||Mice eat plant seeds|
|3rd Trophic Level: Secondary Consumer||Consumes primary consumers||Snakes eat mice|
|4th Trophic Level: Tertiary Consumer||Consumes secondary consumers||Hawks eat snakes|
Many consumers feed at more than one trophic level. Humans, for example, are primary consumers when they eat plants such as vegetables. They are secondary consumers when they eat cows. They are tertiary consumers when they eat salmon.
Trophic Levels and Energy Transfer
The different feeding positions in a food chain or web are called trophic levels. The first trophic level consists of producers, the second of primary consumers, the third of secondary consumers, and so on. There usually are no more than four or five trophic levels in a food chain or web. Humans may fall into second, third, and fourth trophic levels of food chains or webs. They eat producers such as grain, primary consumers such as cows, and tertiary consumers such as salmon. Energy is passed up the food chain from one trophic level to the next. However, only about 10 percent of the total energy stored in organisms at one trophic level is actually transferred to organisms at the next trophic level. The rest of the energy is used for metabolic processes or lost to the environment as heat. As a result, less energy is available to organisms at each successive trophic level. This explains why there are rarely more than four or five trophic levels. The amount of energy at different trophic levels can be represented by an energy pyramid like the one in Figure 2.11.
FIGURE 1.11 – This pyramid shows the total energy stored in organisms at each trophic level in an ecosystem. Starting with
primary consumers, each trophic level in the food chain has only 10 percent of the energy of the level below it.
The pyramid makes it clear why there can be only a limited number of trophic levels in a food chain or web.
FIGURE 2.12 – Ecological Pyramid. This pyramid shows how energy and biomass decrease from lower to higher trophic levels. Assume that producers in this pyramid have 1,000,000 kilocalories of energy. How much energy is available to primary consumers?
Biomes as different as grasslands and estuaries share something extremely important. They have populations of interacting species. Moreover, species interact in the same basic ways in all biomes. For example, all biomes have some species that prey on other species for food. Species interactions are important biotic factors in ecological
systems. The focus of study of species interactions is the community.
What Is a Community?
In ecology, a community is the biotic component of an ecosystem. It consists of populations of different species that live in the same area and interact with one another. Like abiotic factors, such as climate or water depth, species interactions in communities are important biotic factors in natural selection. The interactions help shape the evolution of the interacting species. Three major types of community interactions are predation, competition, and symbiosis.
Predation is a relationship in which members of one species (the predator) consume members of other species (the prey). The lions and cape buffalo in Figure 2.13 are classic examples of predators and prey. In addition to the lions, there is another predator in this figure. Can you find it? The other predator is the cape buffalo. Like the lion, it
consumes prey species, in this case species of grasses. Predator-prey relationships account for most energy transfers in food chains and webs.
FIGURE 2.13 – An adult male lion and a lion cub feed on the carcass of a South African cape buffalo.
Types of Predators
The lions in Figure 2.13 are true predators. In true predation, the predator kills its prey. Some true predators, like lions, catch large prey and then dismember and chew the prey before eating it. Other true predators catch small prey and swallow it whole. For example, snakes swallow mice whole.
Some predators are not true predators because they do not kill their prey. Instead, they graze on their prey. In grazing, a predator eats part of its prey but rarely kills it. For example, deer graze on plants but do not usually kill them. Animals may also be “grazed” upon. For example, female mosquitoes suck tiny amounts of blood from
animals but do not harm them, although they can transmit disease.
Predation and Populations
True predators help control the size of prey populations. This is especially true when a predator preys on just one species. Generally, the predator-prey relationship keeps the population size of both species in balance. This is shown in Figure 2.14. Every change in population size of one species is followed by a corresponding change in the population size of the other species. Generally, predator-prey populations keep fluctuating in this way as long as there is no outside interference.
As the prey population increases, the predator population starts to rise. With more predators, the prey population starts to decrease, which, in turn, causes the
predator population to decline. This pattern keeps repeating. There is always a slight lag between changes in one population and changes in the other population.
Some predator species are known as keystone species, because they play such an important role in their community. Introduction or removal of a keystone species has a drastic effect on its prey population. This, in turn, affects populations of many other species in the community. For example, some sea star species are keystone species in
coral reef communities. The sea stars prey on mussels and sea urchins, which have no other natural predators. If sea stars are removed from a coral reef community, mussel and sea urchin populations would have explosive growth, which in turn would drive out most other species and destroy the reef community. Sometimes humans deliberately introduce predators into an area to control pests. This is called biological pest control. One of the earliest pests controlled in this way was a type of insect, called a scale insect. The scale insect was accidentally introduced into California from Australia in the late 1800s. It had no natural predators in California and was destroying the state’s citrus trees. Then, its natural predator in Australia, a type of beetle, was introduced into California in an effort to control the scale insect. Within a few years, the insect was completely controlled by the predator. Unfortunately, biological pest control does not always work this well. Pest populations often rebound after a period of decline.
Adaptations to Predation
Both predators and prey have adaptations to predation. Predator adaptations help them capture prey. Prey adaptations help them avoid predators. A common adaptation in both predator and prey species is camouflage, or disguise. One way of using camouflage is to blend in with the background. Several examples are shown in Figure 2.15.
Another way of using camouflage is to look like a different, more dangerous animal. Using appearance to “mimic” another animal is called mimicry. Figure 2.16 shows an example of mimicry. The moth in the figure has markings on its wings that look like the eyes of an owl. When a predator comes near, the moth suddenly displays the markings. This startles the predator and gives the moth time to fly away.
FIGURE 2.15 – Can you see the crab in the photo on the left? It is camouflaged with algae. The preying mantis in the middle photo looks just like the dead leaves in the
background. The stripes on the zebras in the right photo blend the animals together, making it hard to see where one zebra ends and another begins.
Some prey species have adaptations that are the opposite of camouflage. They have bright colors or other highly noticeable traits that serve as a warning for their predators to stay away. For example, some of the most colorful butterflies are poisonous to birds, so birds have learned to avoid eating them. By being so colorful, the butterflies are
more likely to be noticed—and avoided—by their predators.
FIGURE 2.16 – The moth on the left mimics the owl on the right.
Predation, Natural Selection, and Co-evolution
Adaptations to predation come about through natural selection (see the Evolution in Populations chapter). When a prey organism avoids a predator, it has higher fitness than members of the same species that were killed by the predator. The organism survives longer and may produce more offspring. As a result, traits that helped the prey
organism avoid the predator gradually become more common in the prey population. Evolution of traits in the prey species leads to evolution of corresponding traits in the predator species. This is called co-evolution. In co-evolution, each species is an important factor in the natural selection of the other species. Predator-prey co-evolution is illustrated by rough-skinned newts and common garter snakes, both shown in Figure 2.17. Through natural selection, newts evolved the ability to produce a strong toxin. In response, garter snakes evolved the ability to resist the toxin, so they could still safely prey upon newts. Then, newts evolved the ability to produce higher levels of toxin. This was followed by garter snakes evolving resistance to the higher levels. In short, the predator-prey relationship led to an evolutionary “arms race,” resulting in extremely high levels of toxin in newts.
FIGURE 2.17 – The rough-skinned newt on the left is highly toxic to other organisms. Common garter snakes, like the one on the right, have evolved resistance to the toxin.
Competition is a relationship between organisms that strive for the same limited resources. The resources might be
food, nesting sites, or territory. Two different types of competition are intraspecific and interspecific competition.
- Intraspecific competition occurs between members of the same species. For example, two male birds of the same species might compete for mates in the same territory. Intraspecific competition is a necessary factor in natural selection. It leads to adaptive changes in a species through time (see the Evolution in Populations chapter).
- Interspecific competition occurs between members of different species. For example, two predator species might compete for the same prey. Interspecific competition takes place in communities of interacting species. It is the type of competition referred to in the rest of this section.
Interspecific Competition and Extinction
When populations of different species in a community depend on the same resources, there may not be enough resources to go around. If one species has a disadvantage, such as more predators, it may get fewer of the necessary resources. As a result, members of that species are less likely to survive, and the species will have a higher death
rate than the other species. Fewer offspring will be produced and the species may eventually die out in the area. In nature, interspecific competition has often led to the extinction of species. Many other extinctions have occurred when humans introduced new species into areas where they had no predators. For example, rabbits were introduced into Australia in the mid-1800s for sport hunting. Rabbits had no predators in Australia and quickly spread throughout the continent. Many species of Australian mammals could not successfully compete with rabbits and went extinct.
Interspecific Competition and Specialization
Another possible outcome of interspecific competition is the evolution of traits that create distinct differences among the competing species. Through natural selection, competing species can become more specialized. This allows them to live together without competing for the same resources. An example is the anolis lizard. Many species of anolis live and prey on insects in tropical rainforests. Competition among the different species led to the evolution of specializations. Some anolis evolved specializations to prey on insects in leaf litter on the forest floor. Others evolved specializations to prey on insects on the branches of trees. This allowed the different species of anolis to
co-exist without competing.
Symbiosis is a close association between two species in which at least one species benefits. For the other species, the outcome of the association may be positive, negative, or neutral. There are three basic types of symbiotic relationships: mutualism, commensalism, and parasitism.
Mutualism is a symbiotic relationship in which both species benefit. Lichen is a good example. A lichen is not a single organism but a fungus and an alga. The fungus absorbs water from air and minerals from rock or soil. The alga uses the water and minerals to make food for itself and the fungus. Another example involves goby fish and shrimp (see Figure 2.17). The nearly blind shrimp and the fish spend most of their time together. The shrimp maintains a burrow in the sand in which both the goby and the shrimp live. When a predator comes near, the fish touches the shrimp with its tail as a warning. Then, both fish and shrimp retreat to the burrow until the predator is gone. Each gains from this mutualistic relationship: the shrimp gets a warning of approaching danger, and the fish gets a safe home and a place to lay its eggs. Co-evolution often occurs in species involved in mutualistic relationships. Many examples are provided by flowering plants and the species that pollinate them. Plants have evolved flowers with traits that promote pollination by particular species. Pollinator species, in turn, have evolved traits that help them obtain pollen or nectar from certain species of flowers.
Figure 2.17: The multicolored shrimp in the front and the green goby fish behind it have a mutualistic relationship. The shrimp shares its burrow with the fish, and the fish warns the shrimp when predators are near. Both species benefit from the relationship.
Comensalism is a symbiotic relationship in which one species benefits while the other species is not affected. In commensalism, one animal typically uses another for a purpose other than food. For example, mites attach themselves to larger flying insects to get a “free ride,” and hermit crabs use the shells of dead snails for shelter. Co-evolution explains some commensal relationships. An example is the human species and some of the species of bacteria that live inside humans. Through natural selection, many species of bacteria have evolved the ability to live inside the human body without harming it.
Parasitism is a symbiotic relationship in which one species (the parasite) benefits while the other species (the host) is harmed. Some parasites live on the surface of their host. Others live inside their host, entering through a break in the skin or in food or water. For example, roundworms are parasites of the human intestine. The worms produce huge numbers of eggs, which are passed in the host’s feces to the environment. Other humans may be infected by swallowing the eggs in contaminated food or water. This usually happens only in places with poor sanitation. Some parasites eventually kill their host. However, most parasites do not. Parasitism in which the host is not killed is a successful way of life and very common in nature. About half of all animal species are parasitic in at least one stage of their lifecycle. Many plants and fungi are parasitic during some stages, as well. Not surprisingly, most animals are hosts to one or more parasites. Species in parastic relationships are likely to undergo co-evolution. Host species evolve defenses against parasites, and parasites evolve ways to evade host defenses. For example, many plants have evolved toxins that poison plant parasites such as fungi and bacteria. The microscopic parasite that causes malaria in humans has evolved a way to evade the human immune system. It hides out in the host’s blood cells or liver where the immune system cannot find it.