Interspecific Competition

Question 1

Intraspecific Competition

Answer 1

competition among members of the same species, a density dependent population control mechanism

Question 2

Interspecific Competition

Answer 2

Competition among members of different species

Question 3

Interference Competition

Answer 3

Competition involving direct interaction between competitors such that the action of one species inhibits other species

ex. production of toxins by competing plants (allelopathy)
ex. occupation and control of a limited space and aggressive encounters

Question 4

Resource (exploitation) Competition

Answer 4

competition involving more indirect inhibitory effects such as those arising from reduced availability of a resource, this in turn affects competing species’ growth, reproduction, or survivorship

Question 5

What do alpha and beta represent in the Lotka-Volterra equations?

Answer 5

competition coefficients, conversion/scaling factors to convert species 2 individuals into an equivalent number of species 1 individuals
-alpha is the conversion factor for expressing the inhibitory effect of an individual of species 2 on species 1
beta is the conversion factor for expressing the inhibitory effect of an individual of species 1 on species 2

Question 6

what does it mean if alpha is greater than 1 in the lotka-volterra equations?

Answer 6

the competitive effect of an individual of species 2 on the population growth rate of species 1 is greater than that of an individual of species 1

Question 7

What do the L-V zero growth curves for species 1 and 2 represent and how are they determined?

Answer 7

• The conditions under which both species are at equilibrium and their growth rates are 0
  o N1=K1-αN2 (for species 1)
  o Need X and Y intercepts to plot this curve
  o When dN1/dt=0 and dN2/dt=0 (N1=βN2 and N2=αN1)

Question 8

What happens to the population of species 1 inside of the zero growth curve? Outside of the curve?

Answer 8

• Solve for X and Y intercepts of 0 growth curve of species 1 (when species 2 =0 and when species 1 =0)
  o N1=K1- αN2
• Populations inside (to the left) of the diagonal line will increase in size and come to equilibrium at some point defined by a combination of N1 and N2 values
• Populations outside (to the right) of this line will decrease until they reach the line

Question 9

What happens to the population of species 2 below the zero growth curve? Above the curve?

Answer 9

• Can solve for 0 growth curve for species 2: N2=K2- βN1, find X and Y intercepts
• Populations inside (below) the diagonal line will increase in size and come to equilibrium at some point defined by a combination of N1 and N2 values
• Populations outside (above) this line will decrease until they reach the line

Question 10

How are the joint population trajectories of species 1 and 2 determined when both graphs are superimposed?

Answer 10

• The joint population trajectories can be determined by using vector addition

Question 11

Given a Lotka-Volterra graph showing the zero growth curves for two species, be able to determine the outcome of competition between the species.

Answer 11

• Species 1 wins, species 2 wins, outcome determined by initial numbers of species ½, both species coexist in a stable equilibrium

Question 12

Provide a biological interpretation of each of the four possible outcomes of the L-V graphs.

Answer 12

• Species 1 or 2 wins—strong interspecific competitors can exclude weak interspecific competitors under all conditions
• Vectors directed away from cross point, winner determined by initial densities of species—unstable equilibrium—when interspecific competition is more important than intraspecific competition, and the competitors are fairly equal in their competitive abilities, outcome will most likely depend on the initial densities of each species
• Vectors directed toward cross point, both populations coexist at densities below their carrying capacities—stable equilibrium—when interspecific competition is less important than intraspecific competition, two species may coexist at a stable equilibrium because each species reaches its carrying capacity before reaching a population level that can threaten the other species, but they are both below their carrying capacities so, in a competitive situation, neither population reaches densities as high as it does without competition, supporting that the sign of the interaction is always negative

Question 13

What was the outcome of Gause’s competition with protozoans?

Answer 13

• When grown together, P. caudatum was eliminated, competitively excluded by P. aurelia

Question 14

What type of competition was shown by the conell’s barnacles experiment?

Answer 14

• Interference competition—fund niche of Chthamalus extended into Balanus zone, Balanus restricted Chthamalus to realized niche higher on shoreline via competitive exclusion
• Interspecific competition—the few Chthamalus that could survive were much smaller, produced fewer offspring, reducing fecundity

Question 15

Why was Chthamalus restricted to a realized niche higher up the shoreline in the conell’s barnacles experiment?

Answer 15

• They were restricted by Balanus by competitive exclusion, Balanus smothered or damaged any Chthamalus that attempted to grow in their zone

Question 16

Which of the two species (Chthamalus or Balanus) has a broader fundamental niche in Conell’s barnacles experiment?

Answer 16

• Chthamalus

Question 17

What characteristic enabled Chthamalus to survive in the higher intertidal zone in conell’s barnacels experiment?

Answer 17

• Tolerance to desiccation (dryness)

Question 18

What type of competition was shown by tilman et al’s diatoms experiment?

Answer 18

• Exploitation (resource) competition—Synedra maintained concentration of silicate at a level too low for Asterionella to survive and reproduce

Question 19

How did the fundamental niches differ between the species examined in Tilman et al’s diatom experiment?

Answer 19

• When grown together, Synedra had a realized niche while Asterionella did not, competitive exclusion

Question 20

How did Synedra competitively exclude Asterionella from the culture medium in tilman et al’s diatom experiment?

Answer 20

• Synedra maintained concentration of silicate at a level too low for Asterionella to survive and reproduce

Question 21

What is “diffuse competition” and its possible consequences for a species?

Answer 21

• When a species experiences minimal competitive interactions along several resource gradients with a number of species, resulted by competitive exclusion
• Can competitively exclude a species or greatly reduce its numbers

Question 22

According to Pielou, what are four conditions that must be met for competitive exclusion to occur?

Answer 22

• The competitors must remain genetically unchanged for a sufficiently long period for one species to exclude the other
  o Can’t find a way to coexist by avoiding competition (favored over time)
• Immigrants from areas with different conditions can’t move into the population of the losing species, there can be no replacements for those “killed in battle”
• The environment must remain constant, rules can’t be changed during the game
• Competition must continue long enough for an equilibrium to be reached, game has to be played long enough for a winner to be determined

Question 23

How does niche differentiation promote coexistence between two species?

Answer 23

• It can prevent or reduce interspecific competition by concentrating competitive effects more within species than between them (case 4 of the L-V model)
• Can be accomplished by resource partitioning where organisms living in the same habitat may utilize different resources
• Or by showing different microhabitat preferences that lead to differential resource utilization
  o Feed at different parts of the same tree

Question 24

Provide an example of coexistence due to resource partitioning in nature.

Answer 24

• Different bill sizes and shapes allow Galapagos finches to feed on different food resources

Question 25

How might temporal separation promote coexistence in nature?

Answer 25

• Potential competitors feed at different times

o Ex. gray squirrels feed during the day, flying squirrels feed at night

Question 26

What is Hutchinson’s “Paradox of the Plankton” and how did he suggest a solution to the paradox?

Answer 26

• Numerous species of planktonic algae coexist in simple environments with little apparent opportunity for niche differentiation
• Although simple, the environment is continually changing, particularly on a seasonal basis, shifting the balance of the competitive interactions among the species

Question 27

How might the formation of unpredictable gaps promote coexistence between two species?

Answer 27

• Creation of unoccupied space created by fires, landslides, wave action, etc.
• As long as gaps are created with the appropriate frequency, possible for fugitive species and highly competitive, K-selected species to coexist at a given site as a whole
• Fugitive species colonizes new gaps, establish self, reproduce, then get displaced (competitively excluded) by slower growing, more efficient competitors

Question 28

What are “fugitive species”?

Answer 28

• An r-selected, colonizing species

Question 29

How might a predator promote coexistence between two potentially competing species?

Answer 29

• Predators force competing species into spatially segregated portions of the available habitat by their activities leading to coexistence via the resource partitioning that accompanies such segregation

Question 30

How did a predatory fish promote coexistence between the two species in Brooks and Dodson’s work with Daphnia?

Answer 30

• Fish are visual predators, they have a minimum size they can detect and small prey can’t be retained by the gill rakers
• Larger zooplankton exist in refuges near shore (fish can’t get to) where they exclude smaller zooplankton by virtue of greater feeding efficiency
• Smaller species exists in open waters where they are free from predation and competition

Question 31

In Brooks and Dodson’s work with Daphnia, what did they find in lakes that did not have the predatory fish? Why?

Answer 31

• Only the larger species of zooplankton exist

Question 32

Give three ways in which observed niche differences in the field might have arisen.

Answer 32

• Current competition
• Ghost of competition past
• Independent evolutionary changes

Question 33

If two species are currently competing in the same habitat, what would you expect to happen to the realized niche breadth of one of the species if its competitor were removed from the habitat?

Answer 33

• They retain their ability to occupy the whole of their fund niche in absence of competitors

Question 34

What did Connell mean by the “Ghost of Competition Past”?

Answer 34

• Through natural selection, originally competing species might have evolved niche differences that are not, therefore, the result of current competition
• Niche differences involve permanent (evolutionary) change in fund niche dimensions of potentially competing species
• Wouldn’t expect to see any niche expansion if either of the competing species were removed

Question 35

What is competitive release?

Answer 35

• Niche expansion in the competitor’s absence

Question 36

How can competitive release be used to infer competition between two species?

Answer 36

• If the competitor is removed and the species expands its niche, then the removed species was a competitor

Question 37

What evidence for competition did Diamond find with his work with ground doves in New Guinea?

Answer 37

• Three species of ground doves each occupied different habitats, on smaller islands in the archipelago that have only one species of dove, that particular species occupies all three habitats

Question 38

What is character displacement?

Answer 38

• A term to describe the tendency for differences to become exaggerated among closely relate species living in sympatry

Question 39

Give a possible example of character displacement in nature.

Answer 39

• Several species of Galapagos finches appear to show displacement in bill size on the islands on which they are in sympatry but similar bill sizes on islands occupied by one species

Question 40

What is the probable role of character displacement in reducing competition between two species?

Answer 40

• The exaggerated differences can serve to reduce competition among species especially if they relate to feeding morphology

Question 41

What role can character displacement play in the evolution of reproductive isolating mechanisms?

Answer 41

• Character displacement can occur as a way of reinforcing reproductive isolating mechanisms
  o Ex. character displacement observed in breeding calls of some anuran amphibians