Interactions Populations do not exist in isolation.

All populations are tightly linked to other populationsthat 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 parasitesand 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)

159 nm

Figure 49.1 left

1437 nm

Figure 49.1 middle

833 µm

Figure 49.1 right

Human host1. Sporozoites are injected from salivary gland of mosquito into human.

2. Sporozoites reproduce asexually to form merozoites in human liver.

3. Merozoites are released into blood- stream where they infect red blood cells.

5. Merozoites surviving human immune system become male or female gametocytes.

6. When a mosquito bites the human host, gametocytes enter the mosquito as part of a blood meal.

8. After meiosis, resulting cells develop into sporozoites and migrate to salivary glands.

Mosquito host

7. Male and female gametocytes fuse in mosquito’s gut.

GametocytesSporozoitesin salivaryglands

Gut

4. Merozoites reproduce asexually until they cause red blood cells to rupture (causing anemia in human host).

Figure 49.2

CytotoxicT cell

Infected liver cell

HUMAN IMMUNE DEFENSE AGAINST PLASMODIUM

Plasmodiumsporozoites

HLA-B53

Healthyliver cell

HLA-B53

Infected liver cell

cp26 proteinfrom Plasmodium

1. Healthy liver cell has an HLA-B53 protein on its membrane.

2. HLA-B53 proteindisplays cp26 protein from Plasmodium, indicating that the liver cell is infected.

3. Cytotoxic T cellrecognizes HLA-B53and cp26 complex.

4. Cytotoxic T cellkills liver cell before merozoitesare produced.

Dead liver cell

CytotoxicT cell

Figure 49.3a

In The Gambia, West Africa, different strains of Plasmodium have different versionsof the cp protein. How successful are these different strains at infecting people?

Infectionrate

Plasmodiumstrain

Interpretation

cp26 Low

HLA-B53 binds to these proteins.Immune response is effective.

cp29 Low

cp26 and cp29strains together

HighImmune response fails when thesestrains infect the same person.

HLA-B53 does not bind to theseproteins. Immune response is notas effective.

cp27

cp28

High

Average

Figure 49.3b

Birds that prey on snails arethe next host for the parasite

Infected snails move to opensunny areas; tentacles wiggle.

Uninfected snails stay in shadedareas; tentacles do not wiggle.

Figure 49.4

Predation

When predation occurs, a predator kills and consumesa prey individual.

Predators can regulate prey populations and/or reducethem to below carrying capacity. (Fig. 49.5a,b)

Prey have a wide array of mechanisms that they use todefend themselves from predation. (Fig. 49.7)

Keystone predators are those that have an exceptionallygreat impact on all the other surrounding species. (Fig. 49.9, 49.10)

Regulated prey population

Time

Population fluctuates within a narrow range

Figure 49.5a

Carrying capacity (the max # that can be supported by available resources)

Pre

y p

op

ula

tio

n s

ize

Time

Predator behavior

Pre

y p

op

ula

t io

n s

ize

Figure 49.5b

15

20

10

Pre

dat

ion

rat

e (n

um

ber

of

mo

ose

kille

d/m

oo

se d

ensi

ty)

5

0

High Medium Low

Moose density

Figure 49.6

Camouflage

Figure 49.7 upper

Mimicry

Figure 49.7 center

Weapons

Figure 49.7 lower

Figure 49.8a

Prey and predator

Figure 49.8b

Correlation between predation rate and prey defense

Att

ach

men

t st

ren

gth

(N

)

Sh

ell m

ass

(g)

Low predation High predation High predationLow predation

Site type Site type

Figure 49.8cIs prey defense induced by presence of predator?

Are mussel defenses inducedby the presence of crabs?

Are mussel defenses induced by the presence of broken mussel shells?

Seawater Seawater

Crab(fed fish, not mussels)

Mussels Mussels Mussels Mussels

No crabBroken musselshells

Intact musselshells

Shell thickness HIGH Shell thickness LOW Shell thickness HIGH Shell thickness LOW

Yes Yes

Keystone predator present

Figure 49.9 left

Keystone predator absent

Figure 49.9 right

Keystone predator present

Figure 49.10 left

Keystone predator absent

Figure 49.10 right

Herbivory

Unlike predators, herbivores are plant-eaters that removetissue from their prey, but rarely kill them.

Why don’t herbivores eat more of the available plantsthan they do?

• Top-down hypothesis (Fig. 49.11)

• Poor nutrition hypothesis

• Plant defense hypothesis (Fig. 49.12)

Predator

Herbivore

Primary producer(plants)

Figure 49.11

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 completelyand 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.

One species eats seeds of one size range

Nu

mb

er c

on

sum

ed

Seed size

Figure 49.13a

Partial niche overlap: competition for seeds ofintermediate size

Species 1

Species 2

Nu

mb

er c

on

sum

ed

Seed size

Figure 49.13b

Complete niche overlap

Species 1: Strong competitor

Species 2: Weak competitor,driven to extinction

Nu

mb

er c

on

sum

ed

Seed size

Figure 49.13c

Consumptive competition occurs when organisms compete for the same resources. These trees are competing for nitrogen and other nutrients.

Figure 49.14 upper left

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 upper right

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 left

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 middle right

Territorial competition occurs when mobile organisms protect a feeding or breeding territory. These red-winged blackbirds are displaying to each otherat 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 49.14 lower left

Intertidal competitors

Chthamalusin upperintertidal zone

Mean tidal level

Balanusin lowerintertidal zone

Figure 49.15a

COMPETITION EXPERIMENT

Upperintertidal

Lower intertidal

1. Transplant rockscontaining youngChthamalus tolower intertidal.

2. Let Balanuscolonize the rocks.

3. Remove Balanusfrom one-half ofeach rock.Monitor survivalof Chthamaluson both sides.

On which side of the rocks doChthamalus survive better?

Chthamalus

Balanus

Figure 49.15b

100

80

60

40

20

0

Young Chthamalus Older Chthamalus

Competitorabsent

Competitor present

Competitorabsent

Competitor present

Figure 49.15c

Chthamalus survives better without competition.

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)

Mutualism between fish

Figure 49.16b

Treehopper excreting honeydew, which is harvested by ants

Figure 49.17a

Are ants beneficial to treehoppers?1000 m2

study plot

Plants withants

Plants withants removed

Figure 49.17b

100

80

60

40

20

0

Plantswithants

Plantswithoutants

20 25 30 5 10 20

AugustJuly

Ave

rag

e n

um

ber

of

you

ng

tree

ho

pp

ers

per

pla

nt

Figure 49.17c

Which treatment contained more treehoppers?