Species Interactions
Lion
Tapeworm
Zebra
Oak
Gypsy moth
Dandelion
Gentian
Finch
Cactus
Shark
Remora
Types of Interactions Between Organisms
-
-
0
+
--
-0
-+
(Amensalism)
(Predation
Parasitism
Herbivory)
(Competition)
0
0–
00
(Commensalsim)
(Amensalism)
+
0+
+-
+0
++
(Predation
Parasitism
Herbivory)
(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
NICHE SPACE – Overlap; Species B wins
Region of Overlap
SPECIES
A
SPECIES B
LIGHT
NICHE SPACE – Overlap; Species A wins
Region of Overlap
SPECIES
A
SPECIES B
LIGHT
NICHE SPACE – Complete overlap
Species A wins
SPECIES
A
SPECIES B
LIGHT
• 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 Intertidal
Fundamental vs. Realized Niche
Interspecific Competition
IV. Effects of Competition
Niche 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”
Niche partitioning
Robert MacArthur
- warbler study
IV. Effects of Competition
Character 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
Population size in the presence of intraspecific competiton
 K1  N1 
dN1

 r1 N1 
dt
 K1 
for species1
 K2  N2 
dN2
 for species 2
 r2 N 2 
dt
 K2 
How do we incorporate interspecific competiton?
Lotka-Volterra Model of
Competition
Population size in the presence of intraspecific competiton
 K  N1 
dN1

 r1 N1  1
dt
 K1 
for species1
 K  N2 
dN2
 for species 2
 r2 N 2  2
dt
 K2 
How do in incorporate interspecific competiton?
We need to convert one
species into the
equivalent of another –
add competition
coefficients, α
K1N1  12 N 2 
dN1
 r1N1

dt
K1


for species 1
K 2 N 2  21N1 
dN2
 r2 N 2 
 for species 2
dt
K2


What would be the outcome of
competition based on the Model?
• Does one species have to win?
Lotka-Volterra Model of
Competition
Population size in the presence of intraspecific competiton
K1  N1 
dN1
 r1N1

dt
 K1 
for species 1
Intraspecific competition
K 2  N 2 
dN2
 r2 N 2 
 for species 2
dt
 K 2 
How do in incorporate interspecific competiton?

We need to convert one
species into the
equivalent of another
K1  N1  12 N 2 
dN1
 r1N1

dt
K1


for species 1
Interspecific
competition
K 2  N 2  21N1 
dN2
 r2 N 2 
 for species 2
dt
K2


Competition
K  N   N 
dN1
1
1
12 2
 r1N1



dt
K1


K2  N 2   21N1 
dN2
 r2 N 2 



dt
K2


• Lotka-Voltera Interspecific
competiton
– Convert individuals of species 1
into species 2 equivalents.
-α12 Amount of spp.1’s niche
overlapped 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
 K  N1  12 N 2 
dN1
  0
 r1 N1  1
dt
K1


 K  N 2   21 N1 
dN2
  0
 r2 N 2  2
dt
K2


N1  K1  12 N 2
N 2  K 2   21 N1
• 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
Competition
K2
• Isoclines don’t
cross?
N2
K1/α12
N1
K1 K2/α21
K1/α12
K2
N2
– One species
excludes the
other
• Isoclines
cross?
– Coexistence
possible
N1
K1
K2/α21
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?
K1/α12**
dN1/dt =0
N2
N1
*
K1
Competition
• What happens to
species 2 in the
presence of species 1?
K2
N2
dN2/dt =0
N1
K2/α21
Competition
K1/α12
K2
K2
K1/α12
N2
N2
N1
K2/α21
Species 1 wins
K1
K1
N1
Species 2 wins
K2/α21
Isocline Analysis
Species 1 wins
K1/α12
Species 2 wins
K2
K1/α12
K2
N2
N2
N1
K2/α21
K1
N1 K1
K2/α21
• Sp. 1 isocline above
• Sp. 2 isocline above
• Sp. 2 most vulnerable to
interspecific competition
• Sp. 1 most vulnerable to
interspecific competition
Isocline Analysis
Unstable Coexistence
K2
K2
N2
K1/α12
K1/α12
N2
K2/α21
N1
K1
• K1 and K2 outside
• Inter > Intra for both species
N1
K2/α21
K1
Isocline Analysis
Stable Coexistence
K1/α12
K2
K1/α12
K2
N2
N2
N1
K1
K2/α21
•K1 and K2 inside
•Intra > Inter for both species
N1
K1
K2/α21
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