Species Interactions

Dandelion GentianFinch

CactusShark RemoraOak Gypsy moth

Lion

Zebra

Tapeworm

Types of Interactions Between Organisms

- 0 +

- - - (Competition)

- 0 (Amensalism)

- + (Predation Parasitism Herbivory)

0 0 – (Amensalism)

0 0 0 + (Commensalsim)

+ + - (Predation

Parasitism Herbivory)

+ 0 (Commensalsim)

+ + (Mutualism)

I. The Niche

• Each niche is occupied by only one species.

• Joseph Grinnell (1917)

• Charles Elton (1927)

• G. Evelyn Hutchinson (1957)

G.E. Hutchinson (1957)Uses range of tolerance for each resource

Hutchinsonian Niche

• We can continue to include resources until we have all possible resources

• The niche is described as an – nth dimensional

hypervolume

Hutchison’s n-dimensional hypervolume

Niche

• Fundamental Niche • Realized Niche

Niche Breadth

The concept of niche breadth can then be employed to exam niche overlap

• Fundamental vs Realized Niche

• Which one is greater for each species?

• Is interspecific competition occurring?

• Who wins?

NICHE SPACE – No overlap No competition

SPECIES A

SPECIES B

LIGHT

HU

MID

ITY

NICHE SPACE – Overlap; Species B wins

SPECIES A

SPECIES B

LIGHT

HU

MID

ITY

Region of Overlap

SPECIES A

SPECIES B

LIGHT

HU

MID

ITY

Region of Overlap

NICHE SPACE – Overlap; Species A wins

NICHE SPACE – Complete overlapSpecies A wins

SPECIES B

LIGHT

HU

MID

ITY

SPECIES A

• Exploitation Competition

Types of Competition

• Interference Competition (contest)

• Diffuse Competition

Competition

• Intraspecific– Between individuals of

the same species

• Interspecific– Between individuals of

different species

Competitive Exclusion

Gause’s Competitive Exclusion Principle

Experiments with Paramecium

III. How does one obtain evidence of competition?

• Experimental studies– J.H. Connell 1961 - barnacles

Connell Results: Middle IntertidalFundamental vs. Realized Niche

Interspecific Competition

IV. Effects of CompetitionNiche Shifting

One species shifts its niche.

Niche variable

Niche variable

Observational studies

Manipulation is not always possible

J.M. Diamond 1975

Inferred competition resulted in the distributional patterns he observed for dove species

Lack – “Ghost of competition past”

Robert MacArthur - warbler study

Niche partitioning

IV. Effects of CompetitionCharacter Displacement

a morphological (or physiological) change in areas of sympatry

We are assuming that competition for a resource is the only thing which effects this character

Character Displacement

Beak size in Darwin’s finches from the Galapagos Islands. Beak sizes given for Geospiza fortis and G. fuliginosa on islands where these two species occur together (upper three sets of islands) and alone (lower two islands). Geospiza magnirostris is a large finch that occurs on some islands.

Lotka-Volterra Model of Competition

2 speciesfor

1 speciesfor

2

2222

2

1

1111

1

K

NKNr

dt

dN

K

NKNr

dt

dN

Population size in the presence of intraspecific competiton

How do we incorporate interspecific competiton?

Lotka-Volterra Model of Competition

2 speciesfor

1 speciesfor

2

2222

2

1

1111

1

K

NKNr

dt

dN

K

NKNr

dt

dN

dN1dt

r1N1K1N1 12N2

K1

for species 1

dN2dt

r2N2K2N2 21N1

K2

for species 2

Population size in the presence of intraspecific competiton

How do in incorporate interspecific competiton?

We need to convert one species into the equivalent of another – add competition coefficients, α

What would be the outcome of competition based on the Model?

• Does one species have to win?

Lotka-Volterra Model of Competition

dN1dt

r1N1K1 N1

K1

for species 1

dN2dt

r2N2K2 N2

K2

for species 2

dN1dt

r1N1K1 N1 12N2

K1

for species 1

dN2dt

r2N2K2 N2 21N1

K2

for species 2

Population size in the presence of intraspecific competiton

How do in incorporate interspecific competiton?

We need to convert one species into the equivalent of another

Intraspecific competition

Interspecific competition

Competition

dN1

dtr1N1

K1 N1 12N2

K1

dN2

dtr2N2

K2 N2 21N1

K2

• Lotka-Voltera Interspecific competiton– Convert individuals of species 1

into species 2 equivalents.

-α12 Amount of spp.1’s nicheoverlapped by spp 2’s niche, > or < 1

- α 21 Amount of spp.2’s niche overlapped by spp 1’s niche, > or < 1

Competition – Isocline Analysis

0

0

2

1212222

2

1

2121111

1

K

NNKNr

dt

dN

K

NNKNr

dt

dN

• Rearrange equations when = 0• Predict population growth for

the two species will stop

– Graph of these = straight lines = isoclines = dN/dt = 0

– Zero Growth Isoclines– Above: Population

decreasing– Below: Population

increasing

12122

21211

NKN

NKN

Competition

K1/α12

K2

N2

K2/α21K1N1

• Isoclines don’t cross?– One species

excludes the other

K1/α12

K2

N2

K2/α21K1N1

• Isoclines cross?– Coexistence

possible

Pp 331-332

Competition

• * = all sp 1, no sp 2• ** = all sp 2, no sp 1• What happens to

species 1 in the presence of species 2?

K1N1

N2

K1/α12

dN1/dt =0

**

*

Competition

• What happens to species 2 in the presence of species 1?

K2

N1

N2

K2/α21

dN2/dt =0

Competition

K2

N2

K1/α12

K2/α21 K1N1

Species 1 wins

K2

N2

K2/α21K1N1

K1/α12

Species 2 wins

Isocline Analysis

N2

N1K1

K2

N2

N1K1

K2

Species 1 wins Species 2 wins

• Sp. 1 isocline above

• Sp. 2 most vulnerable to interspecific competition

• Sp. 2 isocline above

• Sp. 1 most vulnerable to interspecific competition

K2/α21K2/α21

K1/α12

K1/α12

Isocline Analysis

N2

N1K1

K2

Unstable Coexistence

• K1 and K2 outside

• Inter > Intra for both species

K2/α21

K1/α12

N2

K2

K2/α21 K1N1

K1/α12

Isocline Analysis

K2/α21

N2

N1K1

K2

Stable Coexistence

•K1 and K2 inside

•Intra > Inter for both species

K1/α12K1/α12

K2

N2

K2/α21K1N1

Intraspecific competition > interspecific competition

What would be the outcome of competition based on the Model?

• Species 1 wins

• Species 2 wins

• Both species win

• We don’t know who is going to win, but one species goes extinct