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Section 14.4: Interactions within Communities
Community: all populations in a given ecosystem at a time

Organisms work together for survival and cannot live alone
Ecological niche: an organism’s biological characteristics, including use of and interaction with abiotic
and biotic resources in its environment

What an animal eats, what eats it, the way it reproduces and other factors that describes its
pattern of living
Fundamental niche: biological characteristics of the organism and the set of resources individuals in the
population are theoretically capable of using under ideal conditions
Interspecific competition: interactions between individuals of different species for an essential common
resource that is in limited supply
Realized niche: biological characteristics of the organism and the resources individuals in a population
actually use under the prevailing environmental conditions
- Intraspecific interactions occur between the same species
- Interactions between two species and their effects on population density can be classified into 5
categories
Symbiosis: various interactions in which two species maintain a close association; includes
commensalism, parasitism, mutualism
- Interspecific competition occurs between individuals of different populations

Can occur in two ways:
o Interference competition: involves aggression between individuals of different species
who fight over the same resources
o Exploitative competition: involves consumption of shared resources by individuals of
different species, where consumption by one species may limit resource availability to
other species
Resource partitioning: avoidance of, or reduction in, competition for similar resources by individuals of
different species occupying different nonoverlapping ecological niches
- Predation is an example of an interspecific interaction in which the population density of one species
increases while the other decreases
- When prey population increases, there is more food for predators and can increase predator
populations
- When predator population increase, the prey population decreases
- Predator-prey relationships have resulted in defence mechanisms that plants and animals have evolved
- Plants use morphological defences (thorns, hooks, spines and needles) and chemical defences against
herbivores
Symbiosis:
Mutualism: a symbiotic relationship in which both organisms benefit (neither are harmed)

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Bacteria that live in the stomachs of cows
Obligatory mutualism: a symbiotic relationship in which neither species involved could survive
without the other
Commensalism: a symbiotic relationship in which one organism benefits and the other organism is
unaffected

Foxes follow caribou herds when looking for food because they expose lichens for fox while they
walk in the snow
Parasitism: a symbiotic relationship in which one organism (parasite) benefits at the expense of another
organism (host), which is often harmed but not usually killed
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Microparasites: parasites that are too small to see with the naked eye
Macroparasites: larger parasites that are readily visible
Endoparasites: parasites that live and feed within the host’s body
Ectoparasites: parasites that live and feed on the outside surface of the host
Social parasites: parasites that complete their life cycle by manipulating the social behaviour of
their hosts
Try This Activity pg. 686
Parasite: Tapeworm
1.
2.
3.
4.
5.
Eggs are found in feces and passed into the environment
Large animals get infected by eating contaminated vegetation
Eggs hatch and penetrate the intestinal wall
Humans get infected by ingesting raw/undercooked infected meat
The process is repeated
Explore an Issue pg.687
Foreign species: Asian carp
(a) Asian carp have an effect on interspecific interactions because they can limit the food supply in
the water for other species that live there. They can also take away food from other predators
that prey on the same animals that they do. The introduction of these fish has impacted the
food supply of the other species that share a habitat with the carp.
(b) Asian carp affect the economy and health because they cause damage to property. Asian carp
jump out of water when they are startled by the sound of motorboats. This causes them to fly
out of the water and cause damage to property such as boats. This results in people needing to
pay to fix the damage that the fish have caused to their boats. The fish are also dangerous and
can seriously injure people when they jump into the boats. The Asian carp also leave excrement
on everything that it comes in contact with. This is not sanitary and could be an issue to human
health
(c) A way to deal with the invasion of this species is to block off an entrance to where they live by
blocking the canals in which they enter from.
(d) An invasive species that resides in the Great Lakes is the Asian carp. These fish enter the Great
Lakes through two different canals located in Chicago. These large fish get startled when they
hear the noise of motorboats. Since the Great Lakes are where many people spend time doing
recreational activities, there are many boats found on the lakes. When the fish are startled, they
can jump into the air resulting in them jumping into boats and causing damage to the boats as
well as damaging property and injuring people. These fish can destroy the ecosystem, economy,
property and can pose a risk to boaters and their passengers.
Section 14.4 Questions pg.688
1a) exploitative competition (one species limits resources for the other)
1b) interference competition (one species kills the other)
1c) interference competition (aggression between individuals)
1d) exploitative competition (consumption of shared resources)
2) A fundamental niche is the biological characteristics of the organism and the set of resources
individuals in the population are theoretically capable of using under ideal conditions. A realized niche is
the biological characteristics of the organism and the resources individuals in a population actually use
under the prevailing environmental conditions. For example, with barnacle species Balanus balanoides
and Chthamalus stellatus, these characteristics are being shown. Balanus concentrates on rocks in the
lower portion of the shore whereas Chthamalus concentrates on the upper rocks of the shores. When
Balanus is removed from the rocks, Chthamalus could survive on rocks lower down. This shows that
under ideal conditions, Balanus can outcompete Chthamalus.
3) This example shows that more marine invertebrates are found in an area where starfish and mussels
are present together rather than where mussels are found alone. This could be because there are more
marine invertebrae that prey on starfish than mussel. When mussels are by themselves, no species prey
on them so they are not found in the area where mussels are by themselves. However, when starfish are
in a location with mussel, marine species are found because starfish are one of the species that they
consume.
4) These observations regarding the average bill lengths of birds when they inhibit islands separately
and together could be the result of evolution and competition. The bird with the longer beak on the
island where both species of birds lived had an advantage by getting more resources because of the
longer beak that they possessed. However when the bird with the longer beak lived on their own
separate island, it had a shorter beak. This could be because of competition between the two birds
because they shared the same resources. One bird would have to find a way to get more resources than
the other in order for them to survive.
5a) Camouflage
5b) Chemical toxins
6) Predation is an interaction where the predator catches and kills prey to consume. Parasitism is an
interaction in which one organism benefits off another organism which is usually harmed but not killed.
7) The interaction between termites and zoomastigotes is like a commensalism interaction. The
zoomastigotes benefit from the food that the termites ingest but does not harm the termite in any way.
The two organisms help each other survive without causing any harm to each other.
Lab Exercise 14.4.1 pg.691
(a) There will be a large effect on the white-tailed deer population as a result of the introduction of
wolves into their habitat.
(b) Wolf predation is not a limiting factor in the forest reserve because the wolves have plenty of
deer that they are able to feed on and they will have an abundant food supply for many years.
(c) Other factors that might limit the deer population are a small food supply and the spread of
diseases.
(d) The number of wolves in the reserve is influenced by the size of the deer population because
when the number of deer begins to increase, the number of wolves increases as well. However,
when the number of deer decreases, the number of wolves decreases too.
(e) A way that marine biologists might study changes to the Atlantic cod population to determine
whether the reduction of the harp seal population would be an effective solution is to track the
cod with a tracking system and to see where the cod end up. If overfishing is the issue, they
could see how many cod are actually being caught by identifying the mark on the fish from the
tracking system. If equipment on the ocean floor is disturbing the fish and making them flee to
other areas, a tracking piece would be able to detect where the fish have relocated. If
overfishing and disturbing equipment is not the problem and fish are not being tracked, then
they could consider the harp seals as a real problem and could decide whether or not reducing
the harp seal population would be an effective solution. If they do decide to reduce the harp
seal population, they could consider only reducing a small amount to begin with instead of
reducing a large amount to ensure that they are the main problem of the cod population
reduction.
Changes in White-tailed Deer and Wolf
Populations
Wolves
Year
1998
1996
1994
1992
1990
Whitetailed deer
19
19
18
21
24
28
22
16
12
10
1972
1952
1916
1968
2094
2244
2360
2500
2300
2000
Number of Wolves and White-tailed Deer
Case Study: The Struggle for Survival pg.678
1) In each of the paired experiments, the species grown in combination do not reach the same
density as the species grown independently.
2) P. caudatum does not grow past the population density of 125 when grown individually and can
only reach 50 when grown in combination. P. aurelia does not grow past the density of 160 when
grown individually and can 170 when grown in combination.
3) This is an example of exploitative competition because when the two organisms are sharing the
same resources, one of them, P.aurelia, is consuming the shared resources more than P.caudatum
which results in a limited supply for P.caudatum.
4) Both species of paramecium could have survived when grown in combination because one could
have benefitted off of the other without harming the other. An interaction like commensalism could
have been achieved in this situation which would have resulted in the survival of both species of
paramecium.
5) P.caudatum and P.aurelia feed on similar organisms. The location of where the species live could
have an effect which allows one species to acquire food more effectively than the other.