Lecture 19

Question 1

Outcome of competition

Answer 1

hurts both species

Question 2

Outcome of predation

Answer 2

benefits predators, but hurts prey

Question 3

Outcome of host-parasite and plant herbivore interactions

Answer 3

same as predation - positive and negative

Question 4

Outcome of mutualism

Answer 4

helps both species

Question 5

Interactions between species are often classified by

Answer 5

their outcome (+ or -)

Question 6

Two main foci of study in ecology and evolution of species interactions

Answer 6

• population dynamics and effects on community structure (how species interactions affect these two things)
• evolutionary dynamics (adaptation and co-evolution)

Question 7

Intra-specific competition

Answer 7

competition among the members of the same species (i.e. among conspecifics) for resources

Question 8

Inter-specific competition

Answer 8

competition among members of different species (ie among heterospecifics) for resources

Question 9

Scramble/exploitative competition

Answer 9

depletion of a shared resource

Question 10

Contest/interference competition

Answer 10

direct interactions, such as battles over territory

Question 11

Give an example of interference competition

Answer 11

• Invasive Argentine ants fight a harvester ant in California
• Invasive ants (superior competitors) often drive down populations of native ants

Question 12

Exploitative competition

Answer 12

• two species do not need to directly interact or even to be active at the same time to compete
• if one consumes a resource, leaving less resource for the other, then they compete

Question 13

Example of exploitative competition

Answer 13

squirrels and birds, and bird feeders
- squirrel eats food from the feeder and leave no seeds left from the birds
- squirrel is competing successfully with birds by consuming a lot of shared resources

Question 14

model for inter-specific competition for resources

Answer 14

Lotka-Volterra equations for two species competing for resources
- is a simple outgrowth of logistic equation
- logistic already has a breaking term for intra-specific competition
- Just add a second braking term for
inter-specific competition

Question 15

give the 4 steps for arriving at the Lotka-Volterra model from a logistic model

Answer 15

1. Start with the logistic model for population growth
2. Rewrite the logistic model with subscripts to indicate species 1
3. Add a term to show effect of species 2 on species 1
4. Write matching equation for species 2

Question 16

give the equation for Lotka-Volterra model

Question 17

α(ij) =

α(ji)

Answer 17

per-capita effect on i by j

per capita effect on j by i

=> competition coefficient

Question 18

describe the competition coefficients (α’s)

Answer 18

• fixed for a particular pair of species
• α(12)N(2) converts individuals of species 2 into an equivalent number of individuals of species 1
• eg a squirrel can eat a lot more seeds than a sparrow; a measures how many sparrows-worth of seeds a single squirrel eats

Question 19

four possible equilibria outcomes of Lotka-Volterra competition

Answer 19

• the two species may stably coexist
• species 1 may always win (N1 = K1, N2 = 0)
• species 2 may always win (N2 = K2, N1 = 0)
• identity of winner may depend on starting N’s

Question 20

meaning of Equilibrium for Lotka-Volterra competition

Answer 20

N’s are no longer changing

Question 21

what do the outcomes of the Lotka-Volterra competition depend on?

Answer 21

values of K’s and α’s

Question 22

coexistence requires

Answer 22

both species to inhibit their own growth more than they inhibit each other’s

Question 23

define equilibrium

Answer 23

• for a population: size not changing over time (dN/dt = 0)
• for a community: a community not changing over time (in a strict sense: all populations in a community at equilibrium. more generally: constant species composition)

Question 24

define stability

Answer 24

the ability of a system to return to equilibrium following a perturbation or disturbance

Question 25

define coexistence

Answer 25

occurs when two or more species have non-zero population sizes at equilibrium

Question 26

Principle of competitive exclusion

Answer 26

• Lotka-Volterra predicts that for two species to coexist, competition between species must be weaker than competition within a species
• in other words, two species can’t compete too intensely (i.e. overlap too much in resource use/niche space), or one will outcompete the other
• This idea is very old: “As a result of
  competition two similar species scarcely ever
  occupy similar niches” (Gause 1934)
• Or: “Complete competitors cannot coexist”
  (Hardin 1960)

Question 27

Character displacement

Answer 27

coexisting similar species evolve differences to minimise effects of competition on their fitness
- eg Darwin’s finches and beak size
- when finches live on same island, beak size becomes different so that they can eat different sized seeds

Question 28

Paradox of the plankton

Answer 28

The principle of competitive exclusion
seems false for phytoplankton
- Hutchinson wrote “The problem that
is presented by the phytoplankton is how is it
possible for a number of species to coexist in a relatively … unstructured environment all
competing for the same sorts of materials”
- Phytoplankton need only light, CO2 , N, P, and micro-nutrients to grow

Question 29

Paradox of the tropical forest

Answer 29

• hundreds of species of trees living in very small areas, despite having the same niche
• how is this possible?
• either every species has a distinct niche or something prevents competitive
  exclusion from driving species extinct
• this is subject of intense study/debate

Question 30

how do Lotka-Volterra models relate to the real world?

Answer 30

Experiments by Gause (1930’s)
studied competition among protozoa
in artificial culture vessels; saw both stable
coexistence and competitive exclusion

Question 31

Gause’s famous competition experiments with Paramecium species in lab culture - draw graphs

Question 32

How are competitive effects manifested in nature compared to the lab?

Answer 32

• in nature, competitive exclusion is less likely to go to completion
• nonetheless, competition can drastically affect abundances and alter distributions in space
• Biological effects interact with physical
  effects: different outcomes in different
  environments

Question 33

Connell, 1961: Field experiments with two barnacle species in the marine intertidal zone

Answer 33

• Zone upper limits set by desiccation
• Lower limits set by competition for space on the rock
• Competition is asymmetrical
• Remove Balanus, Chthamalus extends its distribution down (distribution limited by competition)
• Remove Chthamalus, Balanus does not extend upwards; not competition, but simply can’t tolerate the conditions at the top of the rocks

Question 34

Resolving the paradox of the
plankton

Answer 34

Lotka-Volterra models too simple, ignore too
much reality, including:
- Most real communities are not at a
competitive equilibrium
- Real populations are kept below carrying
capacity by weather, disease, predators
- Real conditions fluctuate, favouring different species at different times (or in different places)

Question 35

Scaling up from two populations
to ecological communities

Answer 35

• Competition can affect which and how many
  species occur in an ecological community,
  which ecologists call community composition
  and species richness, respectively
• Competition is generally expected to
  decrease species diversity (e.g., if a superior
  competitor excludes other species)
• It is a real challenge to scale up from simple,
  species-poor systems (e.g., two Paramecium
  in lab cultures) to complex, species-rich
  systems (e.g., a whole tropical rainforests)