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

Competition: plants Ex: alfalfa

Competition: plants Ex: alfalfa Type competition?

Competition: plants Self-Thinning Rule: Biomass increases, mortality reduces density (size per survivor increases) Note log: log scale intraspecific competition 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 = r max N (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=Nr max (1-N/K) –Same as: –dN/dt=Nr max (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: dN 1 /dt=N 1 r max1 (K 1 -N 1 )/K 1 –dN 1 /dt=rate of change population sp. 1 –N 1 =number sp. 1 individuals –r max1 =intrinsic rate of increase sp. 1 –K 1 =carrying capacity for sp. 1

Lotka Volterra dN 1 /dt=N 1 r max1 (K 1 -N 1 )/K 1 (K 1 -N 1 )/K 1 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 N 2 expresses sp. 2 resource use as sp. 1 individuals  12 N 2 = resources not available to sp. 1 because used by sp. 2 (K 1 -N 1 -  12 N 2 ) expresses total resource availability for sp. 1

Lotka Volterra Equation: dN 1 /dt=N 1 r max1 (K 1 -N 1 -  12 N 2 )/K 1 Population growth rate species 1 No sp. 2: Logistic Growth Equation for sp. 1 –dN 1 /dt=N 1 r max1 (K 1 -N 1 )/K 1 Lots sp. 2, dN 1 /dt small

Lotka Volterra dN 1 /dt=N 1 r max1 (K 1 -N 1 -  12 N 2 )/K 1 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: dN 2 /dt=N 2 r max2 (K 2 -N 2 -  21 N 1 )/K 2 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 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 Type of interaction species A species B Competition - - Predation + - Parasitism + - Commensalism + 0 Mutualism + + 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)