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SNC 1D/2D
Name: ____________________
Unit 1: Sustainable Ecosystems
Date: ____________________
Population Changes and Interactions Among Species
Some Definitions
Term
Ecological niche
Population
Density
Predation
Competition
Symbiosis
Mutualism
Parasitism
Today’s
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Definition
An ecological niche
 is the way that an organism occupies a position in an ecosystem, including all of the
necessary biotic and abiotic factors it requires (e.g. food, water, shelter, mating needs,
predators, competitors… and temperature, pH, sunlight, etc. needs)
 includes all of the services an organism provides to its ecosystem – e.g. regulation of the
population size of other organisms, matter cycling, energy flow…
 is unique to a species – no two species can occupy the exact same ecological niche or
provide the exact same services to an ecosystem because no two species can live in exactly
the same way.
Population Density refers to the number of organisms of one species found in a defined area.
[For example: The population density of mice in Metcalfe is 3 mice per 100 m2.]
When considering changes to populations it is important to look at the following ideas relating
to population density and its effect on population growth:
a) density-dependent factors affecting population growth
The effect of these factors on population growth may change depending on the number of
organisms in the original population (population density). For example: If there is too large
a population of deer in one area, they will compete for food much more than in an area with
just a few deer.]
Density-dependent factors that may change population density include:
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levels of limiting factors like resource needs (e.g. food, water, shelter, mates) and
other abiotic factors (e.g. temperature, pH, air quality) [See the handout “Populations
and Resources” .]
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biotic relationships/interactions (i.e. levels of predation, competition, symbiosis)
between species can also restrict populations to particular places, roles and sites in
ecosystems
b) density-independent factors affecting population growth [The effect of
these factors on population growth is not dependent on how large the original population is.]
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Natural disasters such as drought, tornadoes and forest fire alter population
sizes/population densities and the carrying capacity of an ecosystem without any
consideration for the size of the original population.
 Predators (organisms that hunt, kill and consume prey) and prey (organisms hunted, killed
and eaten by predators) can interact in three basic ways to regulate their population sizes.
These interactions are not always completely clear cut.
1) Bottom-Up Population Regulation
 Occurs when a shortage of a plant resource at the base of a food chain causes a decline in
the size of animal populations in higher trophic levels.
2) Top-Down Population Regulation
 Occurs when an increase in the number of predators in an area leads to a decrease in the
number of prey.
3) Too Complicated to Call
 Often, the interactions between predator and prey are more complicated than the above
descriptions because the population fluctuations rely on all kinds of other factors besides
the one between one predator and one prey organism.
Competition occurs when two organisms are vying for the same resource in the ecosystem.
Competition can help to control population growth because as it becomes more common and/or
more fierce in a given community, the “loss” to a competitor sometimes means death and a
decrease in population growth.
 Intraspecific competition occurs between organisms of the same species.
[e.g. Two rabbits are competing to use the same grassy patch as a food source.]
 Interspecific competition occurs between organisms of different species.
[e.g. A fox and a coyote are both hunting the same rabbit.]
 When species rely on one another in a symbiotic relationship (either mutualism or parasitism),
their populations need one another to be able to increase.
 If a symbiotic relationship is affected by changing biotic and/or abiotic factors then the
ecological niche of a species may be affected. [For example: If the habitat of the organisms
involved in the relationship is altered, then one or both of the organisms involved in the
relationship may face changes to it ecological niche.
A symbiotic relationship in which both members involved derive some kind of benefit. [e.g.
Nitrifying bacteria live in the root nodules of legume plants like alfalfa. The bacteria secure a safe
place to live and unlimited access to the glucose (food) made by the legume plant. The plant thrives
on the nitrogen fixed by the bacteria. Both populations derive a benefit in this mutalistic
relationship.]
A symbiotic relationship in which one member definitely benefits (the parasite) but the other may
not benefit and may even be harmed (the host). [For example: Some types of tubeworms
(parasites) live in the guts of vertebrates (hosts). The tubeworms feed off the materials in the
intestines of the hosts and sometimes consume so much material that the host suffers from
malnutrition.
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Big Ideas
Each species occupies an ecological niche, which has biotic and abiotic components.
Many species occupy narrow niches for which they are well adapted.
Predation, competition, mutualism and parasitism are four major kinds of biotic relationships between
species. [Note that mutualism and parasitism are two kinds of symbiotic relationship.]
Predation, competition, mutualism and parasitism relationships help to define a species’ niche and
influence the distribution and abundance of the species
1.
2.
Place a checkmark beside the phrases that describe something about the ecological niche of a
big brown bat.
a) ___ the space the brown bat takes up while sleeping in a cave
b) ___ the insects the brown bat eats
c) ___ the brown and white colours of the bat
d) ___ regulation of insect populations
e) ___ the time of night the bat hunts for food
f) ___ the places the bat uses for roosting and hibernation
g) ___ the bat’s competitors (like the common nighthawk)
h) ___ the temperature range the bat can tolerate
i) ___ the size of brown bat litters (average number of babies per pregnancy)
READ: In each case, decide if the predator-prey example is most likely one of: bottom-up
population regulation; top-down population regulation, or, if population regulation is due to a
more complicated group of factors.
a)
Lynx eat snowshoe hare. Complete each sentence about the graph above.
In the far left of the graph (around 1850), the population of snowshoe hare begin to
increase dramatically. While the population of snowshoe hare is high, the population of lynx
begins to ________________ (around 1855). Lynx eat snowshoe hare, so the increase in
population of the lynx cause the snowshoe hare population to _____________________
around 1860 or so.
This is an example of ________________________________________ predation
because ___________________________________________.
b)
A plant-eating grasshopper is prey to a shrew. If the grasshoppers consume too many plants
for food, the plant population will decrease and then the grasshopper population declines due
to lack of food. This means that the shrew population will also decline. This is an example of
____________________________ predation because _________________________
____________________________________________________________________
c)
The largest herbivore in Canada’s boreal forest is the moose. Wolf eat moose. The number of
moose in an area seems to be regulated more by other factors (like very cold winters and tick
infestations) than just the population of the wolves. The population of wolves, though,
depends mostly on the population fluctuations of the population of moose. This is an example
of ______________________ predation because _______________________________
______________________________________________________________________
3. This graph shows the relationship between the number of plantain seeds planted and the number
of seeds produced on each mature plant in the area.
Background: Plantain plants develop from plantain seeds. The
more seeds one plants, the more mature trees one is likely to
get. Mature plantain trees produce seeds of their own. The
y-axis on the graph measures the number of seeds produced
per mature plant. The x-axis indicates how many seeds are
planted (the assumption being that the more seeds planted,
the more mature trees you’ll have – so if you plant 10 seeds
per m2 you’ll have fewer trees around than if you plant 100
seeds.)
b. As more plantain seeds germinate and grow into
a. The graph shows that if more seeds are planted per square
mature trees, the area becomes more crowded with
metre (meaning more mature plants are produced), each mature
plantain. This will lead to more _________________
plant develops ____________ (fewer, more) seeds than when
(symbiosis, predation, intra-specific competition,
there are fewer plants around.
inter-specific competition) between the trees.
How does this relationship between the trees lead to
the pattern observed in part a) above? __________
4.
Examine the diagrams below. Explain how and why scientists say that the niche of brook
stickleback is “broader” when it is alone in a lake and “narrowed” when nine-spine sticklebacks are
also present. _______________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Scenario A
In a lake with just brook sticklebacks, these fish
can be found feeding on insects and other
invertebrates along the bottom, among the
vegetation growing from the bottom, and, in the
shoreline waters above the vegetation.
Scenario B
In a lake with both brook sticklebacks and ninespine sticklebacks, the brook sticklebacks are
found feeding only along the bottom. The ninespines are found feeding everywhere else in the
water above the vegetation. Both species consume
insects and other invertebrates.
5.
Special photosynthetic algae live inside the tissues of most tropical reef-building corals. The
algae provide the coral host with up to 90% of its energy requirements. In return, the coral
provides the algae with protection, nutrients and a constant supply of carbon dioxide. In 1998
about 16% of the world’s tropical reefs were destroyed when the corals within them turned white.
This is known as coral bleaching. It was caused by algae leaving the area. Scientists think that
higher than normal sea temperatures caused the loss of algae. Elevated sea temperatures that
last as little as 6 weeks can lead to coral bleaching and coral death.
a) What kind of relationship exists between coral and algae?
b) Describe one abiotic and one biotic factor that affect the niche of coral.
6.
White-tailed deer often contain brainworms as eggs and young larvae. They excrete the early
larval stage of the worms which are then taken in by snails and slugs. The largest stages of the
larvae are again picked up by deer as they graze. Brainworms do not have much of an effect on
deer but they can cause moose disease in moose. Moose disease can make a moose lose motor
control and eventually die.
a) Name the kind of relationship described above between moose and brainworms.
b) How might this relationship affect the ecological niche and the population growth of moose?
7.
In your own words, explain the difference between density-dependent and density-independent
factors on population growth.