Species Interactions:
Competition (Ch. 13)
Competition (Ch. 13)
• Definition:
– Individuals attempt to gain more resource in
limiting supply
– (-,-) interaction: both participants get less
• Intraspecific: Within species.
• Interspecific: Between species.
Competition
• Interference Competition:
– Individuals interact with each other
• Resource (Exploitation) Competition:
– Individuals interact with resource
• Ex: alfalfa
Competition: plants
Competition: plants
• Ex: alfalfa
• Type competition?
Competition: plants
• Self-Thinning Rule: Biomass increases, mortality
reduces density (size per survivor increases)
intraspecific
competition
Note log: log scale
Can plants do
Interference?
Interference competition: plants
• Allelopathy: Plant makes chemical reduces
growth/survival (other plant)
• Ex, Salvia (sage)
Animal Competition
• Plant hoppers (insects:
Homoptera)
– Tap plant vascular tissue
– Experiment: density vs insect
performance measure
– Type competition?
Intraspecific competition: animals
• Wood lice (isopod): Terrestrial crustacean
Intraspecific competition: animals
• Study: high density & high mortality
– Adding more food: no increase survival
– Why??
Hint….
Intraspecific competition: animals
• Study: high density & high mortality
– Adding more food: no increase survival
– Why??
Interspecific competition
• Classic experiments: Gause
– Demonstrated resource competition with
Paramecium caudatum & Paramecium aurelia
Q: Why was Gause always in a hurry??
Paramecia Lab Experiments
• Grown alone: K
determined by
intraspecific
competition.
dN/dt = rmaxN (1-N/K)
Paramecia Lab Experiments
– Together, P. caudatum declined.
Flour Beetle Experiments
• Classic experiments: Tribolium beetles (eat
grain)
– Park studied T. confusum and T. castaneum
Flour Beetle Experiments
• Grown separately…
• Grown together: interspecific competition
Field Experiments
• Connell: barnacles
• Removal experiment
Field Experiments
• No effect Balanus
removal upper
intertidal
Competition Between Barnacles
• Big effect middle intertidal
Barnacles
• Chthamalus
zonation also
involved predation
(lower intertidal)
• Tolerance,
competition,
predation involved!
Competition of Small Rodents
• Brown studied competition:
Chihuahuan Desert
– Removal experiment
Competition of Small Rodents
• Chihuahuan Desert.
Competition of Small Rodents
• Chihuahuan Desert.
Pocket mouse
(Perognathus)
Competition of Small Rodents
• Chihuahuan Desert.
Grasshopper
mouse
Why does competition occur?
• Overlap in resource use: niche overlap
• Niche: Summarizes environmental factors that
influence growth, survival, and reproduction.
Niche (review)
• Hutchinson defined niche as:
– n-dimensional hypervolume
• n = number environmental factors important to survival
and reproduction
– Fundamental niche - hypervolume
– Realized niche - includes biotic factors (like
competition!)
Niche overlap
• Gause: Principle of Competitive Exclusion
– 2 species with identical niches cannot coexist
indefinitely (niche overlap important!)
Mathematical Models
• Scientific models:
– Abstractions/simplifications
– Provide insights into nature
Example (male) model…
Lotka Volterra
• Recall Logistic Growth Equation:
– dN/dt=Nrmax (1-N/K)
– Same as:
– dN/dt=Nrmax (K-N)/K
Lotka Volterra
• (K-N)/K contains effects
intraspecific competition
• K reflects resource availability
• N reflects resource use
Lotka Volterra Equations
• Competition between 2 species.
• Customize Logistic Growth Eqn. for sp. 1:
• dN1/dt=N1rmax1 (K1-N1)/K1
–
–
–
–
dN1/dt=rate of change population sp. 1
N1=number sp. 1 individuals
rmax1 =intrinsic rate of increase sp. 1
K1=carrying capacity for sp. 1
Lotka Volterra
• dN1/dt=N1rmax1(K1-N1)/K1
• (K1-N1)/K1 includes resource use sp. 1
• How subtract resource use sp. 2 from that
available to sp. 1?
Imagination
Lotka Volterra
• Convert resource use individual sp. 2 into sp. 1
units
– Make “conversion factor” (12)
– Multiplied by N2 expresses sp. 2 resource use as
sp. 1 individuals
• 12 N2 = resources not available to sp. 1
because used by sp. 2
• (K1-N1- 12 N2) expresses total resource
availability for sp. 1
Lotka Volterra
• Equation:
• dN1/dt=N1rmax1 (K1-N1- 12 N2)/K1
• Population growth rate species 1
• No sp. 2: Logistic Growth Equation for sp. 1
– dN1/dt=N1rmax1 (K1-N1)/K1
• Lots sp. 2, dN1/dt small
Lotka Volterra
• dN1/dt=N1rmax1 (K1-N1- 12 N2)/K1
• Alpha: competition coefficient. Expresses
effect individual sp. 2 on individual sp. 1
• If 12>1, individual sp. 2 has greater effect
than individual sp. 1
Lotka Volterra
• Equation for sp. 2:
• dN2/dt=N2rmax2 (K2-N2- 21 N1)/K2
• Note: 21 and 12 usually not equal:
competition asymmetric
Competition and Niches
• Interspecific competition should lead to
directional selection (reduce niche overlap)
• Character displacement: shift in species traits
Character Displacement
• Compare allopatric (non-overlapping) & sympatric
(overlapping) populations
Character Displacement
• Ex, Galapagos finches
• Geospiza fortis & Geospiza
fuliginosa
Character Displacement
• Shift documented
Character Displacement
• Many cases suggested: hard to prove
competition cause!
Competition and Conservation
• Alien invasive species: may be better
competitors
Competition and Conservation
• Ex, mud snails: Cerithidea californica (native) vs.
Batillaria attramentaria (introduced)
“Homeboy” snail
Asian invader
Competition and Conservation
• Lots of alien snails….
Asian invader
Competition and Conservation
High densities reduce food supply (diatoms)
Competition and Conservation
Alien growth rate high
Competition and Conservation
• Population models: competitive exclusion in 5570 years
• Who cares?
– Native snail host of 17 species trematodes (mollusk
parasites): live only in Cerithidea californica!
Exploitation:
Predation, Herbivory,
Parasitism, and Disease
(Ch. 14)
5 main types of interactions among species:
Effect on
Effect on
species A
species B
Competition
-
-
Predation
+
-
Parasitism
+
-
Commensalism
+
0
Mutualism
+
+
Type of interaction
Symbioses
There are several types of exploiters!
• Exploitation: Interaction that enhances fitness of
one while reducing fitness of another
• (+,-) interaction. Minus to “prey/host”
• Plus to:
– Predators: Kill and consume MANY other organisms
– Grazers: Consume MANY, kill FEW or
none(Herbivores generally)
There are several types of exploiters!
• Plus to:
– Parasites: Live in/on ONE or FEW host(s) and
reduce host fitness (usually do not kill host)
• Parasitoid: insect larva that consumes host (usually much
smaller than host)
• Pathogens: induce disease (debilitating condition)