- Distinguish between abiotic and biotic factors.
- Define ecosystem and other ecological concepts.
- Describe how energy flows through ecosystems.
- Explain how food chains and webs model feeding relationships.
- Identify trophic levels in a food chain or web.
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, and other sciences. This lesson introduces fundamental concepts in ecology, beginning with organisms and the environment.
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 and biotic.
- Abiotic factors are the nonliving aspects of the environment. They include factors such as sunlight, soil, temperature, and water.
- Biotic factors are the living aspects of the environment. They consist of other organisms, including members of the same and different species.
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 below.
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 below. 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.
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.
Competitive Exclusion Principle
A given habitat may contain many different species, but each species must have a different niche. Two different species cannot occupy the same niche in the same place for very long. This is known as the competitive exclusion principle. If two species were to occupy the same niche, what do you think would happen? They would compete with one another for the same food and other resources in the environment. Eventually, one species would be likely to outcompete and replace the other.
Flow of Energy
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.
- Photoautotrophs use energy from sunlight to make food by photosynthesis. They include plants, algae, and certain bacteria (see Figure below).
- Chemoautotrophs use energy from chemical compounds to make food by chemosynthesis. They include some bacteria and also archaea. Archaea are microorganisms that resemble bacteria.
Different types of photoautotrophs are important in different ecosystems.
Consumers are organisms that depend on other 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.
Pitcher Plant. Virtually all plants are producers. This pitcher plant is an exception. It consumes insects. It traps them in a sticky substance in its “pitcher.” Then it secretes enzymes that break down the insects and release nutrients. Which type of consumer is a pitcher plant?
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 below). 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.
Dung Beetle. This dung beetle is rolling a ball of feces to its nest to feed its young.
KQED: Banana Slugs: The Ultimate Recyclers
One of the most beloved and iconic native species within the old growth redwood forests of California is the Pacific Banana Slug. These slimy friends of the forest are the ultimate recyclers. Feeding on fallen leaves, mushrooms or even dead animals, they play a pivotal role in replenishing the soil. QUEST goes to Henry Cowell Redwoods State Park near Santa Cruz, California on a hunt to find Ariolomax dolichophallus, a bright yellow slug with a very big personality. See http://www.kqed.org/quest/television/science-on-the-spot-banana-slugs-unpeeled for more information.
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 below. Food chains are generally simpler than what really happens in nature. Most organisms consume—and are consumed by—more than one species.
This food chain includes producers and consumers. How could you add decomposers to the food chain?
A musical summary of food chains can be heard at http://www.youtube.com/watch?v=TE6wqG4nb3M (2:46).
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 below.
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 below. 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
Energy is passed up a food chain or web from lower to higher trophic levels. However, only about 10 percent of the energy at one level is available to the next level. This is represented by the pyramid in Figure below. What happens to the other 90 percent of energy? It is used for metabolic processes or given off to the environment as heat. This loss of energy explains why there are rarely more than four trophic levels in a food chain or web. Sometimes there may be a fifth trophic level, but usually there’s not enough energy left to support any additional levels.
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?
Energy pyramids are discussed at http://www.youtube.com/watch?v=8T2nEMzk6_E&feature=related (1:44).
Trophic Levels and Biomass
With less energy at higher trophic levels, there are usually fewer organisms as well. Organisms tend to be larger in size at higher trophic levels, but their smaller numbers result in less biomass. Biomass is the total mass of organisms at a trophic level. The decrease in biomass from lower to higher levels is also represented by Figure above.
- Ecology is the study of how living things interact with each other and with their environment. The environment includes abiotic (nonliving) and biotic (living) factors.
- An ecosystem consists of all the biotic and abiotic factors in an area and their interactions. A niche refers to the role of a species in its ecosystem. A habitat is the physical environment in which a species lives and to which it is adapted. Two different species cannot occupy the same niche in the same place for very long.
- Ecosystems require constant inputs of energy from sunlight or chemicals. Producers use energy and inorganic molecules to make food. Consumers take in food by eating producers or other living things. Decomposers break down dead organisms and other organic wastes and release inorganic molecules back to the environment.
- Food chains and food webs are diagrams that represent feeding relationships. They model how energy and matter move through ecosystems.
- The different feeding positions in a food chain or web are called trophic levels. Generally, there are no more than four trophic levels because energy and biomass decrease from lower to higher levels.
Lesson Review Questions
1. Define biotic and abiotic factors of the environment. Give an example of each.
2. How do ecologists define the term ecosystem?
3. State the competitive exclusion principle.
4. Identify three different types of consumers. Name an example of each type.
5. Describe the role of decomposers in food webs.
6. Draw a terrestrial food chain that includes four trophic levels. Identify the trophic level of each organism in the food chain.
7. Compare and contrast the ecosystem concepts of niche and habitat.
8. What can you infer about an ecosystem that depends on chemoautotrophs for food?
9. Explain how energy limits the number of trophic levels in a food chain or web.
Points to Consider
In this lesson, you learned how matter is transferred through food chains and webs. Producers make food from inorganic molecules. Other organisms consume the producers. When organisms die, decomposers break down their remains and release inorganic molecules that can be used again by producers. In this way, matter is recycled by the biotic factors in ecosystems.
- Do you think that abiotic factors in ecosystems might also play a role in recycling matter? In what way?
- What abiotic factors might be involved in recycling matter? For example, what abiotic factors might be involved in recycling water?