Organisms and Environments Lesson

Food Chains & Webs

The energy flow through an ecosystem can be shown in two main ways: food chains and food webs.

Food Chains
A food chain describes the eating relationships and energy flow between species within an ecosystem.
The ultimate source of energy for all ecosystems is the Sun. Producers receive energy from the Sun and make food. Producers are the beginning of a food chain because all of the other organisms in the food chain depend on the food energy that is made by producers. The next organisms in the food chain are primary consumers, which eat producers. Next come secondary consumers, then tertiary consumers, and so forth until the top carnivore is reached.


The food chain above shows the flow of energy from a producer, algae, to the consumers in the ecosystem. Minnows are primary consumers, salmon are secondary consumers, and bears are tertiary consumers.

The arrows in a food chain represent the direction of energy flow. The arrow points from the organism that is being consumed to the organism that is receiving energy. For example, in the food chain above, an arrow points from the algae to the minnow. This means that the minnow is eating the algae and receiving energy from it.
All organisms in the food chain are eventually broken down by decomposers. Examples of decomposers include fungi, bacteria, and worms.

Food Webs
A food web is a group of interconnected food chains. Organisms in a food web can belong to multiple feeding levels within a food web.
A food web shows interrelated food chains for an ecosystem. Organisms within a food web can belong to more than one feeding level. For example, in the food web below, krill are both primary and secondary consumers. Krill are primary consumers because they eat phytoplankton, which are producers. Krill are also secondary consumers because they eat zooplankton, which are primary consumers.


An Antarctic food web is shown in the picture above. Organisms in food webs can belong to more than one feeding level.

Predator and Prey

There is a variety of relationships among the organisms within a food web. One important example is the predator-prey relationship.

Predator—an organism that hunts and kills another organism for food
Prey—an organism that gets hunted and eaten by another organism

For example, when a frog eats a fly, the frog is the predator and the fly is the prey.

Parasite / Host Relationships

Parasite—an organism of one species that benefits from, and causes harm to, an organism of a different species through a close relationship with that organism
Host—the organism that is exploited by a parasitic organism >

In a parasitic relationship, an organism of one species harms an organism of a second species. In the parasitic relationship between tapeworms and animals or humans, the worms are parasites, and the humans or animals are hosts.
Tapeworms benefit from parasitic relationships because the worms, living in the intestines of the hosts, can easily absorb food that the hosts have predigested. However, humans or animals with tapeworms do not benefit from the parasitic relationship. They may suffer from stomach pain or malnutrition as a result of the tapeworm's presence.
Parasites cause harm to their hosts over long periods of time, but they generally do not kill their hosts. The death of a host would result in the death of the parasite also, unless it could quickly find another host organism.

Carrying Capacity

In any particular environment, the growth and survival of living things depends on the biotic and abiotic factors present.

The term biotic refers to the living portion of the ecosytem. It includes all of the plants, animals, microbes, and fungi. Abiotic factors are the nonliving parts of an ecosystem, such as climate.

For living things to grow and survive, they must have the appropriate levels of the folowing abiotic factors:

  • temperature range
  • amount of water
  • amount of minerals
  • amount of soil
  • amount of air
  • amount of space
  • amount of food or sunlight

Although populations of living things could grow to be unlimited in size, they may not have enough resources to do so. The carrying capacity, or the actual number of living things that an ecosystem can support, is limited by the available energy, water, air, space, food, and minerals. It is also limited by the ability of the ecosystems to recycle dead organisms through the activities of decomposers, such as bacteria and fungi.

Competition

Competition takes place when two or more organisms or species within an ecosystem seek the same resource. Food, water, sunlight, and space are examples of resources that organisms compete for.

All types of organisms may compete, including animals, bacteria, and plants. For example, in tropical rainforests, trees can grow as high as 200 feet tall. The taller the tree, the more sunlight it can receive. This is an example of competition for sunlight.

Natural Selection

Some organisms have favorable traits that are well-suited to their immediate environment. Organisms with this advantage are more likely to thrive, reproduce, and pass their traits to future generations than organisms without those favorable traits.
This process is known as natural selection.

Favorable Traits
Favorable traits are traits that promote an organism's success in a particular environment. Organisms with favorable traits are more likely to reproduce and pass on their traits than organisms without favorable traits. In this way, the external forces of nature determine, or "select" which traits (or even which types of organisms) will continue to exist in a population.
Favorable traits may include physical traits, such as the body of a sea lion storing extra fat, or they may be instinctive behavioral traits, such as a bird building a nest to protect its young.
If an organism dies before reproducing, then its unique traits will be eliminated from the population. On the other hand, if an organism has favorable traits that allow it to survive and produce many offspring, then its traits will be more numerous within the population.
Natural Selection Occurs Over Time
Genetic variation combined with natural selection allows species to survive by adapting to changes in the environment. If enough significant changes or adaptations occur in the inherited traits of a population, natural selection may result in a new species.
Over time, favorable traits are likely to increase within a population and unfavorable traits are likely to decrease. In this way, natural selection plays an important role in the way species evolve over time. When natural selection results in changes in a species, these changes generally develop gradually, over many generations, rather than suddenly.
The following are sources of genetic variation among living organisms.

  • Mutations, or changes to an organism's genetic material, are a significant source of genetic variation.
  • Sexual reproduction results in an offspring that has a combination of genetic material from its parents, which contributes to genetic variation.
  • Meiosis is the process by which sexually-reproducing organisms produce unique gametes, or sex cells. The unique gamete of one parent can fuse with the unique gamete of another parent to produce a unique offspring, resulting in increased genetic variation.
  • Habitat Change
  • Every organism is valuable because it plays a unique role within its ecosystem. There are natural forces—such as fire, drought, flood, and climate change—that threaten these organisms. However, human overpopulation and destruction of natural habitats also effect these organisms. If a species of organism cannot adapt to change brought about by natural forces or human activities, the species can become threatened, endangered, extinct, or have to migrate to a new area.
    NATURAL FORCES
    Natural forces can have extreme effects on the organisms living in an area. For example, a flood or fire can ruin or kill the resources needed by the organisms in an ecosystem. If all of the plants in an ecosystem are destroyed, the organisms that rely on those plants for survival must move to a new area or find a new food source.
    HUMAN FORCES
    The increasing number of humans on Earth is also increasing the amount of natural habitats destroyed for construction and other human uses. When the habitat of an animal is destroyed, they must move to a new area or adapt to living among humans. Construction often makes it difficult for animals that inhabit certain areas to move freely or to find mates.

Humans and the Oceans

Many aspects of modern human life depend on the rich resources of the oceans.

  • Food: one fifth of the animal protein that humans eat comes from the ocean.
  • Air: half the oxygen in the entire world is produced in the ocean.
  • Transportation: for most of human history, the oceans have been the main routes used for exploration, trade and shipping, and even warfare.
  • Minerals: materials that can be extracted from ocean water or mined from the bottom of the ocean include salt, potassium, magnesium, gold, tin, titanium, and diamonds. Salt from the ocean or from old ocean deposits is used in many industries, including textiles and dyeing, metal processing, rubber manufacturing, oil and gas drilling, paper making, animal hide processing and leather tanning, and soap making.
  • Weather and Climate: the ocean controls weather patterns across the entire world.

As exploration of the ocean continues, even more valuable substances may be discovered. The varied life of the ocean may hold the secret to cleaning up toxic waste or curing AIDS, if the oceans can just be preserved until these secrets can be discovered.
Life in the oceans is threatened by many things, including

  • global warming
  • pollutants
  • pesticides
  • fertilizers and runoff
  • overfishing
  • mining of the ocean floor
  • oil and chemical spills
  • noise pollution, especially from sonar