5R – Terms used in ecology

Supporting article R: Biotic relationships and terms that are important the field of ecology


Biotic Relationships

All organisms fit into a:
• Habitat – the physical area in which an organism lives.
• Niche – the way of life of an organism.

Energy enters an ecosystem from the sun and flows between organisms as one eats another.
Trophic levels: the flow of energy through an ecosystem. Energy is always lost from one trophic level to the next.
• Producers – autotrophic organisms using solar or chemical energy to produce all the organic nutrients for an ecosystem.
• Consumers – heterotrophic organisms that cannot make their own food. They get energy from the chemical bonds in the nutrients they eat.
• Primary consumers (herbivores) – eat primary producers (plants).
• Conversion efficiency: only 10 to 20 percent of the available energy passes from producers to primary consumers.
• Secondary consumers (carnivores) – eat primary consumers (herbivores).
• Conversion efficiency: only 5 to 10 percent of the available energy passes from primary consumers to secondary consumers.
• Tertiary consumers (carnivores) – eat secondary consumers (carnivores).
• The conversion efficiency for tertiary consumers may be as low as 1%.
• Omnivores – eat both plants and animals.
• Decomposers – break down dead tissues and wastes.

Primary productivity in an ecosystem – the rate at which solar energy is converted into organic compounds. The units of productivity are kilocalories per square meter per year.
• Gross primary productivity – the total amount of energy produced, including the energy used by the plants for their own respiration.
• Net primary productivity – the rate at which plants store energy that is not used in plant respiration.

Competition – within any ecosystem, some organisms utilize resources and reduce the availability of those resources to other organisms.
• Intraspecies competition – between organisms of the same species.
• Interspecies competition – between organisms of different species.

Predation – refers to the relationship between a predator and its prey.


Predator and prey are often tied together in many ways. Each has traits that attempt to take advantage of the traits of the other. Moths are a good meal for many birds. Most moths are active at night, a time when most birds are not hunting. But the moth has to hide during the day to escape the birds. The protective coloration of some moths allow them to hide in plain view. Do you see the moth on the tree bark here? Would the moth be as well hidden on just any tree?

Have you ever seen a rabbit run onto the road in front of a car and suddenly stop? While this is not a good reaction at the time, it is the behavior that will most often save the rabbit from being caught by a predator. How?
Even predator and prey populations are related. If the predator population is low, the numbers of the prey species will increase. Most predator species will reproduce in larger numbers if food is abundant. As the numbers of the predator species increase, the prey population begins to decline.

Symbiosis the close association between two dissimilar organisms.
• Parasitism – one organism obtains its nutrition from another organism to the harm of the host.
• Commensalism – one organism benefits from another organism while that organism neither benefits nor is harmed.
• Mutualism – the relationship benefits both organisms equally.

Ecological pyramid – a graph representing trophic level numbers within an ecosystem. The primary producer level is at the base of the pyramid with the consumer levels above.
• Numbers pyramid – compares the number of individuals in each trophic level.
• Biomass pyramid – compares the the total dry weight of the organisms in each trophic level.
• Energy pyramid – compares the total amount of energy available in each trophic level. This energy is usually measured in kilocalories.


Population a group of individuals of the same species living in the same area.                      World population information

• Population density – the number of individuals in a population in a given area at a given time.
• Carrying capacity – the maximum number of individuals that an ecosystem is capable of supporting.
As population numbers increase, certain factors in the environment become limits to growth. All living things need a certain amount of space in which to live, food, water, and oxygen.
• Biotic potential – the rate at which a population will grow if all individuals survive and reproduce at their maximum capacity. This is a theoretical number that is never realized in nature. While the term is not usually applied to human populations, health and census data are available for such calculations.
• Realized intrinsic rate of growth – a measure of the difference between natality (birth rate) and mortality (death rate).
= r = n – m (Since environmental conditions are rarely ideal, the maximum growth rate is almost never achieved in nature. For this reason, the realized intrinsic rate of growth more closely represents the actual growth of organisms in nature than does the biotic potential.)