Ecology

THE EARTH IS A GREAT WEB OF INTERACtion between various biotic organisms and nonliving, abiotic factors that make up their environment. The study of this web, and of the interactions that shape both living organisms and the environment in which they live, is called ecology.

Ecology is a critical component of biology; in some sense, it is the place where everything we have learned up until now fits together and functions in the real world. Up until this chapter we have studied biology in an increasing hierarchy:

  1. The Cell
  2. Biochemistry
  3. Tissue
  4. Organ
  5. Organism

Ecology takes individuals and puts them into larger contexts:

  1. Population
  2. Community
  3. Ecosystem
  4. Biome
  5. Biosphere

Ecology is important on the SAT II Biology for another reason: it makes up about 13 percent of the questions on the core of the test. In addition, if you choose to take the Biology E version of the test, then another 25 percent of the test will have some relation to ecology. In other words, this chapter and the material it covers are crucial.


Populations

Ecologists are interested in the interactions between organisms. Since it takes more than one organism to have an interaction, the basic unit of ecology is the population. A population is a group of individuals that interbreed and share the same gene pool. While every individual in a species has the capacity to interbreed with any other individual, a population is a group of organisms that exist in the same specific geographic locale and actually are interbreeding. All the killer whales in the ocean make up a species, but only the killer whales that actually live and migrate together—only the killer whales that actually interbreed—make up a specific population.

Populations are much more than the sum of their parts: a population displays patterns and concerns that are not applicable to an individual organism. Whereas an individual is concerned with living for as long as possible and having as many offspring as it can, a population is concerned with maintaining its number given the resources at hand.

Population Growth

A vital characteristic of a population is the rate at which it grows. The rate of population growth depends on a variety of factors, including birth rate, death rate, initial population size, and resources. With unlimited resources, a population can expand very rapidly. Two rabbits that live in Rabbit Utopia and have five male and five female offspring every four months will produce a population of 12 rabbits after four months and 72 rabbits after eight months. Sounds like nothing, right? After one year, the population will be 432 rabbits. After two years, there will be 93,312 rabbits. And after three years, the population will be more than 20 million rabbits. This rabbit population is following the trend of exponential population growth, in which there is nothing to limit the growth of a population and that population correspondingly grows by exponential factors. A graph of exponential growth looks like this:



Perhaps Rabbit Utopia can grow enough lettuce to support 20 million rabbits, but normal nature cannot. In nature, when a population is small, the resources surrounding it are relatively large and the population will grow at near exponential levels. But as populations grow larger, they need more food and take up more space, and resources become tight. Within the population, competition for food and space grows fierce, predators move in to sample some of the bounty, and disease increases. These factors slow the growth of the population well before it reaches stratospheric levels. Eventually, the rate of population growth approaches zero, and the population comes to rest at a maximum number of individuals that can be maintained within a given environment. This value is the carrying capacity of the population, the point at which birth and death rates are equal.


The carrying capacity of an environment will shift as an environment changes. When there is a drought and less vegetation, the carrying capacity of rabbits in a population will decrease since the environment will not be able to produce enough food. When there is a lot of rain and lush vegetation, the carrying capacity will increase.

Population Growth and Types of Reproduction

Population growth is affected by species’ methods of reproduction. The two most important types of reproduction are asexual and sexual reproduction. Each type of reproduction has benefits and costs.

Asexual reproduction—such as that found in plants that reproduce by shoots or organisms that reproduce through parthenogenesis—requires less energy than its sexual counterpart. Because it requires less time and effort, asexual production allows a population to grow very quickly. For example, parthenogenesis occurs when an unfertilized egg develops offspring. Parthenogenesis creates female organisms that are identical to their mothers; the eggs of these female organisms undergo parthenogenesis and produce more females. By eliminating the necessity of males from the reproductive equation, parthenogenesis doubles the rate at which a population can grow. However, by eliminating males and sexual reproduction, populations that employ asexual reproduction limit their gene pool and the resulting diversity among members. In times when an environment is changing or competitive, the lack of variation damages these populations’ ability to survive.

Sexual reproduction exhausts more energy and therefore progresses slowly. A population that reproduces through sexual reproduction will not grow as rapidly as an asexually reproducing population, but the sexual population will maintain the diversity of its gene pool. A sexually reproducing population is therefore more fit to survive in a changing or competitive environment.

Sexually reproducing organisms have two reproductive substrategies. Organisms such as insects have many small offspring that receive very little or no parental care, reach sexual maturity at a young age, and reproduce only one or a few times. In an environment with abundant resources, this life-history strategy allows species to quickly reproduce and exploit opportunities for population growth. The disadvantage of this strategy is that it produces high mortality and great instability when resources dwindle. The alternative strategy is to bear fewer and larger offspring that receive intensive parental attention, mature gradually, and reproduce several times. Humans employ this strategy and are better suited to thrive in a competitive environment, exhibiting lower mortality rates and longer life spans. The disadvantage here is that the concerted investment of time and energy into a few individuals makes it difficult for a population to surmount large decreases in population size due to disasters or disease.


Communities

Just as individuals live within a population, populations exist within communities. A community refers to all the populations that interact with each other in a given environment and geographical area. The specific role and way of life of each population is called a niche. When populations have overlapping niches, a variety of types of interaction may occur, including competition, symbiosis, predation, and other food relationships. Communities are shaped over time by ecological succession.

The Niche

Each population in a community plays a unique role in the community. This role, the population’s niche, ranges from where the members of a population live, what they eat, when they sleep, how they reproduce, and every other characteristic that defines a population’s lifestyle within a community. You can think of the niche as a sort of node in the network of interactions that make up a community. Wherever the niches of two populations overlap, interaction follows.

Competition

When two populations share some aspect of a niche, such as a nesting site or a food source, competition results. There are two basic outcomes of competition between populations:

  • One population will be a more effective competitor. The population that is more effective will eventually “win” and drive the second, less effective population from their niche. With the niche freed, the winning population will grow to the carrying capacity of the niche.
  • The two populations will evolve into less competitive niches. If two populations compete on even terms, it may be beneficial for both populations to modify their niches so that the populations’ niches overlap less or not at all. In these cases, natural selection will favor individuals in both populations that have non-overlapping niches, and over time the two populations will evolve into different niches.

Symbiosis

Symbiosis refers to an intimate association between organisms called symbionts. The symbiotic relationship may or may not be beneficial to the organisms involved. There are three kinds of symbiosis: parasitism, commensalism, and mutualism. Each type of symbiosis describes a different relationship of benefits between the two symbionts. A tapeworm is a parasite that lacks a digestive tract and therefore infects a host and steals predigested food; parasites benefit while their hosts suffer. In commensalism, one species benefits and the other remains unaffected. Barnacles and whales live in a commensal relationship. Finally, in mutualism, both species benefit from the presence of and interactions with each other. Lichens, which consist of a fungus and alga that provide for each other, respectively, moisture and food through photosynthesis, are a good example of a mutualist relationship.

Predation

Predation refers to one organism eating another. Predation does not only refer to carnivores. Just as an eagle eating a rodent is a form of predation, so is a rodent munching on some grass. In fact, predation doesn’t always result in the death of the prey. An antelope that gets eaten by a lion will die, but a tree that loses a few leaves to a hungry giraffe will go right on living.

Carrying capacity shifts in a periodic manner based on the cycles of predation. When the population of rabbits increases, the population of coyotes that eat the rabbits will also increase, as there’s more food for the coyotes. However, at some point, there will be so many coyotes eating so many rabbits that the rabbit population will fall in number. The coyotes’ great success in eating rabbits has eliminated their food source, and as the rabbit population declines, so will the coyote population. But as the coyote population dwindles, the lack of predators allows the rabbit population to grow again, and so the cycle continues.

Evolution Caused by Predation

The change in a population due to a shift in environment is one of the engines of evolution. Imagine the rabbits and their predators, the coyotes. As the coyotes increase in number, the rabbit population ceases to grow, and many rabbits are caught and eaten. As the coyotes increase in number, the carrying capacity of the rabbit population shrinks. But it is important to notice that not all rabbits are caught by the coyotes. The faster rabbits escape capture by the coyotes far more often than the slower rabbits. Fast rabbits survive and breed and have offspring, while slower rabbits get eaten. The next generation of rabbits will therefore be faster because they are descended from faster parents—this is directional selection in action. The population of increasingly fast rabbits means that the coyotes must be faster in order to catch the rabbits. More fast coyotes catch rabbits and live to reproduce, creating a next generation of faster coyotes. When two populations affect their mutual evolution in this manner it is called coevolution.

It is arguable that predation is actually helpful to the prey population. Since predators want to capture prey with the least possible effort, the weakest members of the prey population are usually targeted. In this way, the predators often remove from the gene pool of a population those prey animals that have the weakest and least fit alleles.


Food Relationships

Every organism needs food in order to live and has to get that food from somewhere. Every organism can be classified by where it fits into the food chain. Most broadly, all organisms fit into one of three camps: producers, consumers, and decomposers.

Producers

Producers are able to produce carbohydrates from the energy of the sun through photosynthesis or, in some instances, from inorganic molecules through chemosynthesis. Because they can produce their own food, producers are also called autotrophs. Producers form the foundation of every food chain because only they can transform inorganic energy into energy that all other organisms can use. On land, plants and photosynthetic bacteria are the main producers. In marine environments, green plants and algae are the main producers. In deep water environments near geothermal vents, chemosynthetic organisms are the main producers.

Consumers

Consumers cannot produce the energy and organic molecules necessary for life; instead, consumers must ingest other organisms in order to get these materials. Consumers are also called heterotrophs because they must consume other organisms in order to get the energy necessary for life. There are three types of consumers; the categories of consumers are based on which organisms a particular consumer preys on. Primary consumers, such as sheep, grasshoppers, and rabbits, feed on producers. Since all producers are plants or plantlike, all primary consumers are herbivores, which is the name for a plant-eating animal. Secondary consumers eat primary consumers, making them carnivores—animals that eat other animals. Foxes and insect-eating birds are examples of secondary consumers. Tertiary consumers eat secondary consumers and are therefore carnivores. Polar bears that eat sea lions are tertiary consumers. Consumers that eat both producers and other consumers are called omnivores.

Decomposers

Also called saprophytes, decomposers feed on waste or dead material. Since they must ingest organic molecules in order to survive, decomposers are heterotrophs. In the process of getting the energy they need, decomposers break down complex organic molecules into their inorganic parts—carbon dioxide, nitrogen, phosphorus, etc.

Food Chains and Food Webs

All predatory interactions between producers and consumers in a community can be organized in food chains or more complex and realistic food webs. A food chain imagines a strictly linear interaction between the levels of producers and consumers we described above. An abstract food chain appears below on the left, with examples of animals that fit each category appearing on the right:


Each step in the food chain is referred to as a trophic level.

Food chains are simple and help us to understand the predation interactions between organisms, but because they are so simple, they aren’t really accurate. For instance, while sparrows do eat insects, they also eat grass. In addition, the food chain makes it seem as if there are only four populations in a community, when most communities contain far more. Most organisms in a community hunt more than one kind of prey and are hunted by more than one predator. These numerous predation interactions are best shown by a food web:


In fact, the more diverse and complicated the food relationships are in a community, the more stable that community will be. Imagine a community that was correctly described by the food chain grassinsectssparrowshawks. If some blight struck the grass population, the insect population would be decimated, which would destroy the sparrow population, and so on, until the very top of the food chain. A more complex food web is able to absorb and withstand such disasters. If something were to happen to the grass in the food web, the primary consumers would all have some other food source to tide them over until the grass recovered.

Food Webs and Energy Flow

Each trophic level in a food web consumes the lower trophic level in order to obtain energy. But not all of the energy from one trophic level is transferred to the next. At each trophic level, most of the energy is used up in running body processes such as respiration. Typically, just 10 percent of the energy present in one trophic level is passed along to the next. If the energy present in the producer trophic level of a food web is  kcal, you could draw an energy pyramid to show the transfer of energy from one trophic level to the next:


The energy lost between each trophic level affects the number of organisms that can occupy each trophic level. If the secondary consumer trophic level contains 10 percent of the energy present in the primary consumer level, it follows that there can only be about 10 percent as many secondary consumers as there are primary consumers. The energy pyramid is therefore also a biomass pyramid that shows the number of individuals in each trophic level.

Biological Magnification

Because biomass drops so dramatically from one trophic level to the next, any chemical present in a lower trophic level becomes heavily concentrated in higher trophic levels. Beginning in the 1940s, a pesticide called DDT was sprayed on crops to stop invading insects. The concentration of DDT in any local area was enough to kill insects, but not enough to hurt any of the larger organisms. But as each predator ate its prey, the DDT became concentrated in successive trophic levels. The small levels of DDT found in the insects became much more concentrated as it was swallowed and digested by predators. Eagles, sitting at the top of the food web, took in massive amounts of DDT in the course of eating their prey. The DDT caused the eagles to lay soft eggs that could not protect the developing embryos inside, which led to a severe population decline.


Ecological Succession

Just as the people living in your neighborhood can come and go, ecological communities change over time. One way a community can change is if external conditions shift. If the weather in a certain geographical area suddenly gets colder, certain populations will be better off and will thrive, while others will shrink and disappear.

However, change in communities is not always caused by external factors: populations can change environments simply by living in them. The success of a particular population in a particular area will change the environment to the advantage of other populations. In fact, the originally successful population often changes the environment to its own detriment. In this way, the populations within a community change over time, often in predictable ways. The change in a community caused by the effects of the populations within it is called ecological succession.

The first population to move into a geographical area is referred to as a pioneer organism. If this pioneer population is successful in its new location, it will change the environment in such a way that new populations can move in. As populations are replaced, changing plant forms bring with them different types of animals. Typically, as a community moves through the stages of succession, it is characterized by an increase in total biomass, a greater capacity to retain nutrients within the system, increasing species diversity, and increasing size and life spans of organisms. Eventually, the community will reach a point where the mixture of populations creates no new changes in the environment. At this point, the specific populations in the stable community are said to make up a climax community. While individuals within a climax community will come and go, the essential makeup of the populations within the climax community will stay constant.

Which species are dominant in a particular climax community is determined by unique factors of that geographical area, such as temperature, rainfall, and soil acidity. Since a climax community does not change the environment, it also does not affect its own dominance; a climax community will remain dominant unless destroyed by a significant change in climate or some catastrophic event such as a fire or volcanic eruption.

Succession in Action

Imagine a catastrophic event: a forest fire rages through the Green Mountains of Vermont. The fires burn everything and leave behind a barren, rocky expanse.

The population of trees that once lived in this area can’t grow back because the fire has changed the ground composition. Without tree roots to act as anchors, rain washes away the soil and the ground becomes rocky and barren. This rocky ground, however, proves ideal to lichens, the pioneer population. The lichens colonize the rocks and thrive. As part of their life process, lichens produce acids that break down rock into soil. Lichens need solid places to survive: they are victims of their own success. Mosses and herbs are well suited to living in the shallow soil environment created by the lichen, and they replace the lichen as the dominant population.

The mosses and herbs continue to build up the soil. As the soil deepens, the conditions favor plants with longer roots, such as grasses. Eventually the land becomes suitable for shrubs and then for trees. The early dominant trees in the community will be species like poplar, which thrive in bright, sunlit conditions. As more trees grow in the area, however, there is less sunlight, and maples, which grow in shade, supplant the sun-starved poplars. The maples eventually dominate the community, because they don’t change the soil composition and thrive in their own shade. The community has reached its climax community, with maple as the dominant species. Don’t forget that during all this, the changing vegetation has brought with it various changes in animal populations.

The SAT II Biology is most likely to test your knowledge of ecological succession in an originally rocky area, as we just covered, or in a pond. Succession in a pond follows a similar pattern. Originally, the pond will contain protozoa, some small fish, and algae. As individual organisms die and water runs into the pond, sediment builds up at the bottom and the pond grows shallower. The shallower pond becomes marshlike and fills with reeds and cattails. The standing water eventually disappears, and the land is merely moist: grasses and shrubs dominate. As the land grows even less moist, it becomes woodland. And as trees come to dominate, the climax community will arise from a species that can grow in the shade of its neighbors.

Ecological Succession vs. Evolution

For the SAT II Biology, do not get confused between ecological succession and evolution. In ecological succession, the populations that make up a community change, but the characteristics of the individuals within the population will not change over time. Ecological succession is something that happens to communities, while evolution happens to populations. Although succession has different rates, it is much faster overall than evolution.


Ecosystems

The dominant species in a climax community interact with and depend on nonliving (abiotic) factors in that environment. The most important abiotic factors in an environment, and on the SAT II Biology, are the chemical cycles, the availability of sunlight and oxygen, the character of the soil, and the regulation of these various phenomena. These abiotic elements, along with living matter, make up an ecosystem.

Chemical Cycles

Inorganic elements such as carbon, nitrogen, and water pass through the environment in various forms. These elements are vital to life: they are consumed, excreted, respired, and otherwise utilized by living things. The passages of these elements between organisms and the abiotic environment are called the chemical cycles.

The Carbon Cycle

The carbon cycle begins when plants use CO2 from the air to produce glucose, which both animals and plants use in respiration and other life processes. Animals consume some of these plants as a source of food. Animals use what they can of the carbon matter and excrete the rest as waste that decays into CO2. Plant and animal respiration releases gaseous CO2. The carbon that plants and animals do use remains in their bodies until death. After death, decay sends the organic compounds back into the Earth and CO2 back into the atmosphere.


The Nitrogen Cycle

Nitrogen is a vital component of amino acids and nucleic acids, which are the fundamental units of proteins and DNA. The nitrogen cycle begins with inert atmospheric nitrogen (N2), which is generally unusable by living organisms. Nitrogen-fixing bacteria in the soil or on the roots of legumes transform the inert nitrogen into nitrates (NO3–) and ammonium (NH4–). Plants take up these compounds, synthesize the 20 amino acids found in nature, and transform them into plant proteins; animals, typically only able to synthesize eight of the 20 amino acids, eat the plants and produce protein using the plant’s materials. Plants and animals give off nitrogen waste and death products in the form of ammonia (NH3). One of two things can happen to the ammonia: (1) nitrifying bacteria transform the ammonia into nitrites (NO2) and then to nitrates (NO3–), which reenter the cycle when they are taken up by plants; (2) denitrifying bacteria break down the ammonia to produce inert nitrogen (N2).


The cycling of water and phosphorus are also important, as these substances are limited and vital to the life processes of most organisms.

The Water Cycle

The majority of the Earth’s water resides in the oceans and lakes, which act as water storage depots. This water escapes into the atmosphere through evaporation and condenses into clouds. Precipitation in the form of rain, snow, hail, etc., returns water to the ocean and lakes and also brings water to dry land. Water on land may either return to the oceans and lakes as runoff or penetrate into the soil and seep out as groundwater.


Oxygen, Sunlight, and Competition

Oxygen and sunlight are both vital to most forms of life. The relative abundance or lack of oxygen in a particular geographic or physical locale will create competition among organisms and drive evolution. Oxygen is abundant in the atmosphere and is therefore readily available to terrestrial species. But in order to penetrate aquatic environments, oxygen must be dissolved in water, where it exists in smaller concentration.

Like oxygen, sunlight is necessary to life for most organisms. In terrestrial species, competition for sunlight has pushed evolution of plants, with some plants growing broader leaves and branching to capture more rays. Sunlight cannot travel through water as easily as it can travel through air, so at great ocean depths, light is scarce. At these sorts of depths, autotrophic organisms have to find some way to produce energy that does not use light, such as chemosynthesis.

The Soil

The nature of soil determines which populations can be sustained in a given ecosystem. High acidity inhibits most plant growth but may be ideal for some plants that are better adapted to acidic soil. The texture of the soil and amount of clay it contains affect its ability to retain water, while the presence of minerals and decaying organic matter influences the types of plant life that can be supported.


Biomes

Different climatic conditions are produced by the geography and uneven heating of the Earth. Plant and animal forms that are characteristic of a particular geographic area with a common climate constitute biomes. Each biome is characterized by specific climax communities. All the biomes together form the biosphere.

Terrestrial Biomes

The various biotic and abiotic factors at play on Earth result in six major terrestrial biomes. Terrestrial biomes are categorized according to the types of plants they support. The fundamental characteristics of each type are described in the list below.

TROPICAL RAIN FOREST

Rain forests have the highest rainfall of all biomes (100–180 inches per year), which results in the greatest animal and plant diversity. Trees form canopies that block sunlight from reaching the ground. Most animal species live in the canopy, while the forest floor is inhabited predominantly by insects and saprophytes and consists of soil low in nutrients. Decomposed products on the forest floor are washed away or quickly reabsorbed by plants. Tropical rain forests can be found in Central America, the Amazon basin in South America, Central Africa, and Southeast Asia.

SAVANNA

This biome is characterized by grassland with sparse trees, with extended dry periods or droughts. Tropical savannas generally border rain forests and receive a yearly total of 40 to 60 inches of rainfall. They support large herbivores, such as antelope, zebra, elephants, and giraffe. Most tropical savannas exist in Africa. Temperate savannas, such as the Pampas in Argentina and the prairies east of the Rocky Mountains in the United States, receive only about 10 to 30 inches of rain a year. Grasses and shrubs dominate the landscape and support insects, birds, smaller burrowing animals, and larger, hoofed animals such as bison.

DESERT

Deserts are the driest biome, receiving less than 10 inches of rain per year. They exhibit radical temperature changes between day and night. Animals of the desert such as lizards, snakes, birds, and insects are typically small and have adapted to the dry, hot climate by being nocturnally active. Plants, such as cactus, have evolved waxy cuticles, fewer stomata, spiky leaves, and seeds capable of remaining dormant until sufficient resources are available. Deserts exist in Asia, Africa, and North America.

TEMPERATE DECIDUOUS FOREST

Rainfall in temperate deciduous forests is evenly distributed throughout the year. The biome has distinct summer and winter seasons. It has long growing seasons during the summer. In winter, the deciduous plants drop their leaves and enter a period of dormancy. Beech and maple dominate in colder variations of this biome, while oak and hickory are more prevalent where temperatures are warmer. Animals in deciduous forests are both herbivorous and carnivorous, such as deer, fox, owl, and squirrel. The forest floor is fertile and contains fungi and worms. Temperate deciduous forests exist mainly on the east coast of North America and in central Europe.

TAIGA

The taiga is a forest biome but is colder and receives less rainfall than deciduous forests. Coniferous (cone-bearing) trees, especially spruce, dominate the taiga. The trees also have needle-shaped leaves that help conserve water. Taiga forests sustain birds, small mammals such as squirrels, large herbivorous mammals such as moose and elk, and large carnivorous mammals such as wolves and grizzly bears. Taiga exist mainly in Russian and northern Canada.

TUNDRA

This biome is located in the far north and is covered by ice sheets for the majority of the year. The soil, down to a few feet, remains permanently frozen, though in the summer, the topsoil can melt and support a short growing season. Very few plants grow in the northernmost parts of the tundra, but lichens, mosses, and grasses occupy some more southern areas. Animals must be well suited for extreme cold or must migrate. The tundra supports large herbivores such as reindeer and caribou, large predators such as bear, and some birds.

Aquatic Biomes

Aquatic biomes account for 70 percent of the Earth’s surface and contain the majority of plant and animal life. Aquatic biomes also account for a vast portion of the photosynthesis, and therefore oxygen production, that occurs on Earth. There are two types of aquatic biomes, based on the type of water found in each: marine and freshwater.

MARINE

Marine biomes refer to the oceans that all connect to form a single, great body of water. Since water has an immense capacity to absorb heat with little temperature increase, conditions remain uniform over these large aquatic bodies. Marine biomes are divided into three zones: intertidal/littoral, neritic, and pelagic.



The intertidal zone, also called the littoral zone, is the region where land and water meet. It experiences periodic dryness with changing tides and is inhabited by algae, sponges, various mollusks, starfish, and crabs.

The neritic zone extends to 600 feet beneath the water’s surface and sits on the continental shelf, hundreds of miles from shores. Algae, crustaceans, and numerous fish inhabit this region.

The pelagic zone consists of a photic zone (reaching 600 feet below sea level) and below that an aphotic zone. Light penetrates the photic zone, which is why it contains photosynthetic plankton. The photic zone also is home to heterotrophs such as bony fish, sharks, and whales that prey on these producers as well as on each other. No light penetrates the aphotic zone, which is a kind of watery circus of the bizarre, where extreme cold water, darkness, and high pressure have spurred strange evolutionary paths. The region is home to some chemosynthetic autotrophs. Other denizens of the deep are scavengers that feed on dead organic matter falling from the higher realms and predators who feed on each other.

FRESHWATER

Freshwater biomes include rivers, lakes, and marshes. Life here is affected by temperature, salt concentration, light penetration, depth, and availability of dissolved CO2 and O2. Freshwater biomes are much smaller than marine biomes, so conditions are less stable. Organisms that live in these regions must be able to handle the greater extremes. The very nature of freshwater also demands special characteristics of the organisms that live within it. In freshwater environments, the salt concentration within the cell of an organism is higher than the salt concentration in the water. A concentration exists between the interior of cells and the exterior environment: water from the environment is constantly diffusing into the organism. Organisms in freshwater need homeostatic systems to maintain proper water balance.

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