Download Topic 3: Relations Between Organisms

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
Scientists have attempted to divide natural
population regulating factors into two main
categories:
 Density-Dependent Factors
 Density-Independent Factors
englishexercises.org
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.
ibc.lynxeds.com
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 densityindependent 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.
en.wikipedia.org
Competition
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.
imzaroncikgusains.blogspot.com
Competition
chrisalba-enchantedoak.blogspot.com
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 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.
northsydney.nsw.gov.au
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.
oikoumene.org
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’
ozanimals.com
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’
jennytalia.com
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.
elec-intro.com
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.
web.mac.com
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.
mtkembla-e.schools.nsw.edu.au
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.
flickr.com
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.
flickr.com
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
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 viceversa.
prairiestateoutdoors.com
Predation
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.
adelaidenow.com.au
Parasitism
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.
doctortipster.com
Parasitism
A good example of a hostparasite 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.
http://en.wikipedia.org/wiki/Rabbits_in_Australia
Parasitism
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.
fark.com
Symbiosis
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.
scott-macleod.blogspot.com
Symbiosis
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.
en.wikipedia.org
Commensalism
Commensalism is a common form of symbiosis. This
is where one member of the association benefits while
the other is neither helped nor harmed.
science.howstuffworks.com
Mutualism
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.
karimahscuisina.wordpress.com
Homework
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.
1. Define predator and prey
2. Identify a predator-prey relationship in a local ecosystem
and discuss factors that influence the population sizes.
3. Complete handout questions 2.2.3 and 2.3