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9 July 2019

Species Interactions •  Interactions can be + or -

–  Mutualism –  Competition

•  Resource Partitioning

–  Predators and Prey •  food webs

•  Species diversity •  Richness and Evenness

•  Global patterns •  Equilibrium of colonization and

extinction

•  Succession •  Primary and secondary

Species Interactions

Competition -/- both spp. suffer

Mutualism +/+ both spp.benefit

Predation, Parasitism +/- one sp. benefits at the expense of the other

Commensalism +/0 one sp. benefits with no effect on the other

Leafcutter ants are fungus farmers

https://www.youtube.com/watch?v=RH3KYBMpxOU

Many kinds of interactions can happen at the same time

Growth with limited resources

What type of population growth is shown? Why does the population size stabilize?

What is happening to the two populations of Paramecium (P. aurelia and P. caudatum) when housed together at

•  Day 4

•  Day 8

•  Day 16

What do you predict will happen at Day 20? Why?

Questions to ponder or discuss with your neighbor:

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Competitive Exclusion Principle

When two species compete for the same resources, the species that is more efficient at using the resources will outcompete the other.

Paramecium Competition

Why can these coexist?

Removal experiments can test the role of species interactions

Chthamalus

Balanus

High tide

Chthamalus

Balanus

Ocean Low tide

High tide

Chthamalus

Ocean Low tide

When Balanus was removed, Chthamalus expanded its range

Ocean

Chthamalus

Balanus

EXPERIMENT

RESULTS

High tide

Low tide

Chthamalus realized niche

Balanus realized niche

High tide

Chthamalus fundamental niche

Low tide Ocean

Realized niche: The actual species� use of resources (as constrained by interactions with other species)

Fundamental niche: Its potential resource space.

A. ricordii

A. insolitus usually perches on shady branches.

A. distichus perches on fence posts and other sunny surfaces.

A. aliniger A. distichus

A. insolitus

A. christophei

A. cybotes A. etheridgei

Niche partitioning

Competition among lizards causes species to forage in different microhabitats.

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Resource Partitioning Can Cause Intraspecific Competition to Be Greater Than Interspecific Competition Niche evolution in lizards

Green Anole Brown Anole

Green anoles perch higher in trees and evolved larger toepads on islands where Brown anoles were introduced.

Los Hermanos

G. fuliginosa G. fortis

Beak depth

Daphne

G. fuliginosa, allopatric

G. fortis, allopatric

Sympatric populations

Santa María, San Cristóbal

Beak depth (mm)

Perc

enta

ges

of in

divi

dual

s in

eac

h si

ze c

lass

60 40 20 0

60 40 20 0

60 40 20 0

8 10 12 14 16

Sizes of Galapagos finch beaks are more different on islands where two species compete.

Fig. 53-20

Snowshoe hare

Lynx

Num

ber o

f lyn

x (th

ousa

nds)

Num

ber o

f har

es

(thou

sand

s)

160

120

80

40

0 1850 1875 1900 1925

Year

9

6

3

0

Predators and Prey

Fig. 53-19

Wolves Moose 2,500

2,000

1,500

1,000

500

Num

ber o

f moo

se

0

Num

ber o

f wol

ves

50

40

30

20

10

0 1955 1965 1975 1985 1995 2005

Year

Predator/Prey interactions Fig. 54-11

Carnivore

Carnivore

Carnivore

Herbivore

Plant

A terrestrial food chain

Quaternary consumers

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

A marine food chain Phytoplankton

Zooplankton

Carnivore

Carnivore

Carnivore

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Fig. 54-12

Humans

Smaller toothed whales

Baleen whales

Sperm whales

Elephant seals Leopard

seals Crab-eater

seals

Birds Fishes Squids

Carnivorous plankton

Copepods Euphausids (krill)

Phyto- plankton

Fig. 54-13

Sea nettle

Fish larvae

Juvenile striped bass

Fish eggs Zooplankton

A Food Web in the Grasslands of Yellowstone National Park

Removing Wolves Initiated a Trophic Cascade

With sea otters Without sea otters Another Trophic Cascade Another Trophic Cascade

Largemouth Bass removed

Source: Estes et al. 2011

Bass Planktivorous fish Zooplankton Phytoplankton

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Defense Mechanisms and Evolutionary Arms Races

Some places have high diversity

Boreal forest

Desert

Other habitats have low species diversity

“Diversity” depends on species richness and evenness

A B C D

Community 1 A: 25% B: 25% C: 25% D: 25%

Community 2 A: 80% B: 5% C: 5% D: 10%

Which of these communities is more diverse?

Tropical habitats support much larger numbers of species than do temperate and polar regions.

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Mammals and Plants show the same pattern

Plant Species Richness

Mammals

Imagine a newly formed island some distance from the mainland…

What will determine the rate of arrival of new species?

Species richness will be a balance between immigration and extinction

Number of species on island

Rat

e of

imm

igra

tion

or e

xtin

ctio

n

Extinc

tion

Imm

igration How would these curves change for a larger island? How will that affect the equilibrium number of species?

Area and Isolation Influence Species Richness on Islands

Number of plant species on the Galápagos Islands in relation to island area.

Endemic Galapagos prickly pear cactus tree

Species increase with area even at continental scales.

Area (hectares)

Num

ber o

f spe

cies

1,000

100

10

1 0.1 1 10 100 103 104 105 106 107 108 109 1010

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The Theory of Island Biogeography Can Be Tested

Experimentally remove insects from small mangrove islands. Will the species number come back to equilibrium?

The Theory of Island Biogeography Can Be Tested (Part 2)

How well does that experiment fit predictions of island biogeography theory?

Habitat Fragmentation in Tropical Forests

1950

1985

A Large-Scale Study of Habitat Fragmentation in Brazil

Which species are at most risk from fragmentation?

Other Habitat Fragmentation Experiments

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Results from Habitat fragmentation experiments Sometimes the declines in species don’t show up for several years

Species composition changes over time

Disturbance can alter the species composition of communities

(a) Soon after fire

(b) One year after fire

Primary succession starts with empty sites

1941 1907

1860 Glacier

Bay

Alaska

1760

0

Pioneer stage Dryas stage

Kilometers Spruce stage Alder stage

5 10 15

1 2

4 3

Dung Beetle Species Composition Changes over Time

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Intermediate levels of Disturbance can increase species richness

35

Num

ber o

f tax

a 30

25

20

15

10 0.9

Index of disturbance intensity (log scale) 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0