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Populations and communities Topic 3: Relations Between Organisms. Part of the Local Ecosystems Module Spotlight Biology Preliminary Text Chapter 3 Authors: D. Heffernan, J. Bastina , B. Grieve, K. Humphreys, A. Sartor Science Press 2002. Relations Between Organisms.

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Populations and communities topic 3 relations between organisms

Populations and communitiesTopic 3: Relations Between Organisms

Part of the Local Ecosystems Module

Spotlight Biology Preliminary Text Chapter 3

Authors: D. Heffernan, J. Bastina, B. Grieve, K. Humphreys, A. Sartor

Science Press 2002

Relations between organisms
Relations Between Organisms

Scientists have attempted to divide natural population regulating factors into two main categories:

  • Density-Dependent Factors

  • Density-Independent Factors


Relations between organisms1
Relations Between Organisms

Density-Dependent Factors:

  • A good example of this is competition. This is when organisms compete with one another for food and resources. The higher the population density, the more organisms compete and the more the population is affected.


Relations between organisms2
Relations Between Organisms

Density-Independent Factors:

  • These influence population growth to the same extent whatever the population density. Unpredictable physical changes to the environment often reduce populations in a density-independent manner. A good example of this is tropical storms and cyclones which kill a proportion of trees regardless of how many are growing in a forest.



Competition occurs when organisms require common resources that are limited. This can happen with organisms of the same or different species and includes resources such as: sunlight, space, shelter, food or water.



There are two major kinds of competition:

  • Intraspecific Competition (Members of the same species competing)

  • Interspecific Competition (Members of two or more species compete)


Competitive exclusion
Competitive Exclusion

Competitive Exclusion is a principle that states: two species cannot live together in the same space at the same time if they are both limited by one or more of the same limiting resources. One species always drives the other to extinction locally.


Competitive exclusion1
Competitive Exclusion

A significant scientist to study this was Australian biologist Charles Birch. In the 1950’s he observed population sizes of two species or grain beetles living in wheat.


Competitive exclusion2
Competitive Exclusion

When living alone, the beetles could survive indefinitely. However when the beetles had to share the same environment one was always driven to very low numbers or extinction. The two species cannot live together, one always drives the other to extinction ‘Competitive Exclusion’


Competitive exclusion3
Competitive Exclusion

The spinifex plant releases a chemical which inhibits the growth of other plants around it (allelopathy). This plant grows in the desert where water is scarce. Having no other plants growing near it reduces the competition for water when it rains. This is another example of ‘Competative Exclusion’


Competition in nature
Competition in Nature

Scientists are able to specifically control and observe competitive situations in the lab. It’s very challenging to do this in nature because there are so many factors we must consider.


Competition in nature1
Competition in Nature

Plants and animals on rocky seashores are arranged in distinct zones or horizontal layers called ‘zonation’. These layers create a very predictable pattern from a ‘always wet’ zone below the lowest tide line to a ‘splash zone’ above the highest tide. Certain organisms will only survive in a specific zone.


Competition in nature2
Competition in Nature

Scientists studies two species of barnacles that are found in adjacent but separate layers along a rocky shore. One species lives on the highest rocks just above normal high tide, the other species lives below this height and experiences regular tidal shifts.


Competition in nature3
Competition in Nature

Because these organisms live on rocks, they are quite easy to shift. So that’s what scientists did. They took a rock with barnacles in the high zone and swapped it with a rock with barnacles from the lower zone and observed what happened over time.


Competition in nature4
Competition in Nature

So what was the result?

-Scientists discovered that the barnacles living in the higher zone did not survive in the lower zone. They grew to slow and could not compete for food. Scientists speculate that these barnacles have adapted to survive higher, in a slightly different condition, because there was/is less competition for food.


Competition in nature5
Competition in Nature

Competition for resources can lead to short and long term changes in an ecosystem.

  • The introduction of a species may forever (long term) increase competition for resources.

  • A native mouse plague may only last a few weeks when the abundance of food is extremely high. While the population of mice is up, it may have drastic but short term affects to an ecosystem.


Predation is the killing and eating of one species by an organism from another species. In nature the relationship between predator/prey is usually balanced. As prey increases/decreases the number or predators also increase/decrease and vice-versa.



A good example of this is mice populations. When farmers have a good crop they often see an influx of mice. When the food source is exhausted, there is often a crash in the mouse population. There are many variables we must consider in studies like these as the rise and fall of populations may be caused by a number of factors.



Parasites can also act as density-dependent population regulators. A parasite is an organism that feeds on a host, harming them in some way. In nature this relationship too is often balanced.



A good example of a host-parasite interaction most Australians are well aware of is with rabbits. Rabbits were intentionally introduced in the late 1700’s and flourished in the absence of all their natural population limiting factors such as predators and disease. Their populations grew and devastated the countryside.



In an attempt to control their numbers scientists introduced a viral disease called myoxomatosis. This lethal disease spread and killed off most rabbits. However now, most of the lethal strain of the virus is gone and the rabbit population has developed some resistance. Myxomatosis and rabbits have reached an ‘equilibrium’ in which the host (rabbit) and parasite (virus) coexist.



The opposite of parasitism is symbiosis. Symbiosis is when two or more species form close relationships and they help rather than hinder one another. Sym means together, bios means life.



Many organisms involved in symbiosis totally depend on their partner and cannot live without them. Because of this, influencing the distribution of one species directly affects their partner.



Commensalism is a common form of symbiosis. This is where one member of the association benefits while the other is neither helped nor harmed.



Mutualism is where both partners benefit from the association. Our intestines contain a bacteria that assists with the digestion of food. We provide the bacteria with food and a warm moist environment and in turn they synthesise certain vitamins our bodies cant produce or obtain from food without the bacteria's help.



Answer the following questions your notebooks. Come to class prepared to discuss next lesson. Remember you will also be marked as attempting/not attempting these.

  • Define predator and prey

  • Identify a predator-prey relationship in a local ecosystem and discuss factors that influence the population sizes.

  • Complete handout questions 2.2.3 and 2.3