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Interactions

Populations do not exist in isolation.

All populations are tightly linked to other populations
that share the same habitat.

Categories of interaction:
• parasitism +/• predation +/• herbivory +/• competition -/-
• mutualism +/+
Parasitism

Parasites are extremely diverse. (Fig. 49.1)

All parasites acquire resources from their host.
This is always detrimental to the host. (Fig. 49.2)

A “coevolutionary arms race” exists between parasites
and their hosts.
• Parasites develop better ways to attack and use the host,
while hosts develop better defenses.
• Example: Plasmodium and the human immune system.
(Fig. 49.3a,b)
• Parasites can manipulate the behavior of their host.
(Fig. 49.4)
Figure 49.1 left
159 nm
Figure 49.1 middle
1437 nm
Figure 49.1 right
833 µm
Figure 49.2
Mosquito host
1. Sporozoites are injected from
salivary gland of mosquito into human.
Human host
2. Sporozoites
reproduce
asexually
to form
merozoites in
human liver.
8. After meiosis, resulting
cells develop into sporozoites
and migrate to salivary glands.
7. Male and female
gametocytes fuse
in mosquito’s gut.
Gametocytes
Sporozoites
in salivary
glands
Gut
6. When a mosquito bites
the human host, gametocytes
enter the mosquito as part
of a blood meal.
3. Merozoites
are released
into bloodstream where
they infect red
blood cells.
4. Merozoites reproduce
asexually until they
cause red blood cells to
rupture (causing anemia
5. Merozoites surviving human in human host).
immune system become male
or female gametocytes.
Figure 49.3a
HUMAN IMMUNE DEFENSE AGAINST PLASMODIUM
Plasmodium
sporozoites
Cytotoxic
T cell
Cytotoxic
T cell
cp26 protein
from Plasmodium
HLA-B53
HLA-B53
Healthy
liver cell
Infected liver cell
1. Healthy liver
cell has an
HLA-B53 protein
on its membrane.
2. HLA-B53 protein
displays cp26
protein from
Plasmodium,
indicating that
the liver cell is
infected.
Infected liver cell
3. Cytotoxic T cell
recognizes HLA-B53
and cp26 complex.
Dead liver cell
4. Cytotoxic T cell
kills liver cell
before merozoites
are produced.
Figure 49.3b
In The Gambia, West Africa, different strains of Plasmodium have different versions
of the cp protein. How successful are these different strains at infecting people?
Plasmodium
strain
cp26
Infection
rate
Interpretation
Low
HLA-B53 binds to these proteins.
Immune response is effective.
cp29
Low
cp26 and cp29
strains together
High
cp27
High
cp28
Average
Immune response fails when these
strains infect the same person.
HLA-B53 does not bind to these
proteins. Immune response is not
as effective.
Figure 49.4
Birds that prey on snails are
the next host for the parasite
Infected snails move to open
sunny areas; tentacles wiggle.
Uninfected snails stay in shaded
areas; tentacles do not wiggle.
Predation

When predation occurs, a predator kills and consumes
a prey individual.

Predators can regulate prey populations and/or reduce
them to below carrying capacity. (Fig. 49.5a,b)

Prey have a wide array of mechanisms that they use to
defend themselves from predation. (Fig. 49.7)

Keystone predators are those that have an exceptionally
great impact on all the other surrounding species.
(Fig. 49.9, 49.10)
Figure 49.5a
Regulated prey population
Prey population size
Carrying capacity (the max # that can be supported by available resources)
Population fluctuates within a narrow range
Time
Figure 49.5b
Predator behavior
Time
Predation rate (number of moose
killed/moose density)
Figure 49.6
20
15
10
5
0
High
Medium
Moose density
Low
Figure 49.7 upper
Camouflage
Figure 49.7 center
Mimicry
Figure 49.7 lower
Weapons
Figure 49.8a
Prey and predator
Figure 49.8b
Shell mass (g)
Attachment strength (N)
Correlation between predation rate and prey defense
Low predation
High predation
Site type
Low predation
High predation
Site type
Figure 49.8c
Is prey defense induced by presence of predator?
Are mussel defenses induced
by the presence of crabs?
Are mussel defenses induced by the
presence of broken mussel shells?
Seawater
Crab
(fed fish, not mussels)
Mussels
Shell thickness HIGH
Yes
Seawater
No crab
Broken mussel
shells
Intact mussel
shells
Mussels
Mussels
Mussels
Shell thickness LOW
Shell thickness HIGH
Shell thickness LOW
Yes
Figure 49.9 left
Keystone predator present
Figure 49.9 right
Keystone predator absent
Figure 49.10 left
Keystone predator present
Figure 49.10 right
Keystone predator absent
Herbivory


Unlike predators, herbivores are plant-eaters that remove
tissue from their prey, but rarely kill them.
Why don’t herbivores eat more of the available plants
than they do?
• Top-down hypothesis (Fig. 49.11)
• Poor nutrition hypothesis
• Plant defense hypothesis (Fig. 49.12)
Figure 49.11
Predator
Herbivore
Primary producer
(plants)
Competition

Competition is detrimental to both of the individuals or
species involved because it reduces available resources.

Every species has a unique niche, or set of habitat
requirements.
• Competition occurs when niches overlap.
• Competitive exclusion results when niches completely overlap.
(Fig. 49.13c)
• Coexistence is possible if niches do not overlap completely
and the species involved partition the available resources.
(Fig. 49.13b)
• Coexistence is also possible if other factors serve to limit the
better competitor in some way.
Figure 49.13a
Number consumed
One species eats seeds of one size range
Seed size
Figure 49.13b
Partial niche overlap: competition for seeds of
intermediate size
Species 2
Number consumed
Species 1
Seed size
Figure 49.13c
Complete niche overlap
Number consumed
Species 1: Strong competitor
Species 2: Weak competitor,
driven to extinction
Seed size
Figure 49.14 upper left
Consumptive competition occurs when organisms compete for the same resources.
These trees are competing for nitrogen and other nutrients.
Figure 49.14 upper right
Preemptive competition occurs when individuals occupy space and prevent access
to resources by other individuals. The space preempted by these barnacles is
unavailable to competitors.
Figure 49.14 middle left
Overgrowth competition occurs when an organism grows over another, blocking
access to resources. This large fern has overgrown other individuals and is
shading them.
Figure 49.14 middle right
Chemical competition occurs when one species produces toxins that negatively
affect another. Note how few plants are growing under these Salvia shrubs.
Figure 49.14 lower left
Territorial competition occurs when mobile organisms protect a feeding or
breeding territory. These red-winged blackbirds are displaying to each other
at a territorial boundary.
Figure 49.14 lower left
Encounter competition occurs when organisms interfere directly with each other’s
access to specific resources. Here, spotted hyenas and vultures fight over a kill.
Figure 49.15a
Intertidal competitors
Chthamalus
in upper
intertidal zone
Mean tidal level
Balanus
in lower
intertidal zone
Figure 49.15b
COMPETITION EXPERIMENT
Upper
intertidal
1. Transplant rocks
containing young
Chthamalus to
lower intertidal.
Lower intertidal
2. Let Balanus
colonize the rocks.
Chthamalus
Balanus
3. Remove Balanus
from one-half of
each rock.
Monitor survival
of Chthamalus
on both sides.
On which side of the rocks do
Chthamalus survive better?
Figure 49.15c
Chthamalus survives better without competition.
100
Competitor present
80
Competitor present
60
Competitor
absent
40
Competitor
absent
20
0
Young Chthamalus
Older Chthamalus
Mutualism

Mutualism is a type of interaction that is beneficial to both
species involved.

It does not involve altruism. The benefits are a by-product of
each species’ own self-interest.

The costs and benefits of mutualism vary widely between
partners, over time, and from one area to the next.
(Fig. 49.16a–c)
Figure 49.16b
Mutualism between fish
Figure 49.17a
Treehopper excreting honeydew, which is harvested by ants
Figure 49.17b
Are ants beneficial to treehoppers?
1000 m2
study plot
Plants with
ants
Plants with
ants removed
Figure 49.17c
Which treatment contained more treehoppers?
Average number of young
treehoppers per plant
100
80
Plants
with
ants
60
Plants
without
ants
40
20
0
20
25
July
30
5
10
August
20