Interactive note taking-Interactions Among Populations in a Community

Interactions Among Populations in a Community

We have learned that the Earth is filled with ecosystems.  An ecosystem is defined as the living organisms within a specific area and the physical environment or abiotic factors that support them.  Scientists divide ecosystems further into communities.  A community consists of the biotic or living factors in an ecosystem.  This would include the different plants and animals that interact with each other.  Communities are broken down further into populations.  A population is a group of organisms of the same species that live in a particular area.  Organisms that are the same species are so similar, they can mate and produce offspring that can also mate and produce offspring.  

The populations that make up a community interact with each other.  These interactions help the different populations survive and contribute to the flow of energy and matter through the ecosystem.  Energy resources and important compounds and elements such as water, carbon, and nitrogen are shared through the interactions of producers, consumers, and decomposers in a community.  We have also learned that all food chains and food webs begin with producers or green plants.  Producers are then consumed by first-order consumers.  The food web continues as second-order consumers eat first-order consumers and then are eaten by third-order consumers.  

Regardless of its position in the food web, every organism in a community fills a specific role or niche.  What an organism eats and what eats it is called its niche.  In addition to its position in the food web, an organism’s niche includes other interactions it has with the populations within its community. 

For example, the honey produced in a bee hive high in a pine tree will serve as food for other populations such as bears. Bear droppings will put nutrients back in the soil.  These nutrients will be used by the pine tree community to produce branches and leaves. The branches will support the bee population’s hive and the leaves will be used by the squirrel population to line their nests. Each organism and population fills a specific role that is unique and necessary for the survival of the community.  Organisms or populations that depend on each other for basic needs such as food and shelter form interdependent communities.

Often an organism’s physical characteristics are clues to the niche it fills in a community.  For example, thorny plants often serve as protective habitats for small animals to discourage first-order consumers from feeding on them.  The keen eyes and powerful wings of a hawk identify it as a second- or third-order consumer that must hunt its prey.  Can you infer the niche of any other organisms based on their physical characteristics?

In an ecosystem, basic resources such as food, water, and shelter are in limited supply.  Therefore, populations within a community interact in three main ways:  competition, predation, and symbiosis


Because of limited resources, populations of one species may compete with populations of other species for these resources.  For example, the pronghorn antelope found in the western part of the United States, is in competition with grazing animals such as cattle and sheep for food. Plant populations mainly compete for space.  A plant’s space provides access to water, nutrients, and sunlight.  A good example of plant competition occurs in tropical rainforests where climbing vines attach themselves to trees in order to reach the sunlight in the upper canopy.  These vines can cover a tree, blocking sunlight while competing for water and nutrients at ground level. Competition can lead to one population being driven out of a community in search of resources or in the dying out or extinction of a population.


Another interaction that occurs between different populations in a community is predation.    Predation is the interaction between consumers that hunt other consumers for food.  The animal that hunts and eats another animal is called a predator.  The animal that is eaten is called the prey. This is called a predator-prey relationship.  In a food web, an animal can be both a predator and prey.  For example, the spider that eats a cricket caught in its web can be breakfast for a robin.


Symbiosis is another interaction that occurs when two or more organisms of different populations live and work together to benefit the community. Three types of symbiosis include mutualism, commensalism, and parasitism.  

  1. Mutualism is a relationship in which both organisms benefit from the interaction.A good example of mutualism is the relationship between a hummingbird and a flower.The hummingbird takes food in the form of nectar from the flower.At the same time the hummingbird transfers pollen that has attached to its body from flower to flower.This transfer of pollen results in the production of seeds by the plant
  2. Commensalism is a relationship in which one organism benefits while the other organism is not affected.An example of commensalism occurs with moss and lichen.These plants often live on the trunks or branches of trees.Although the tree is not harmed, these plants take in nutrients and water that are found on its bark.Animal populations also benefit from commensalism. For example, clown fish find shelter from predators among the stinging tentacles of sea anemones
  3. Parasitism is a relationship in which one organism benefits and the other organism is harmed.Ticks, fleas, and lice are examples of parasites that feed on the body of another animal called a host.Parasites can also live on the inside of their hosts.For example, the tapeworm is an internal parasite that lives in the digestive system of its host.