Community Ecology

Question 1

Species interactions

Answer 1

Interactions between species are often classified
by their outcome (+ or -)
* An over-simplification, but a useful starting point
* Competition hurts
both species
* Predation benefits
predators, but hurts
prey (host-parasite
and plant-herbivore
interactions also +/-)
* Mutualism helps
both species

Question 2

Intra-specific competition:

Answer 2

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

Question 3

Inter-specific competition:

Answer 3

competition among
members of different species (i.e., among
heterospecifics) for resources

Question 4

Scramble/exploitative competition:

Answer 4

depletion of a
shared resource

Question 5

Contest/interference competition:

Answer 5

direct
interactions, such as battles over territory

Question 6

Exploitative competition

Answer 6

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 7

Lotka-Volterra equations

Answer 7

Simple outgrowth of logistic equation:
* Logistic already a has braking term
for intra-specific competition
* Just add a second braking term for
inter-specific competition

Question 8

add the a looking coeffiecent

Answer 8

Notice that αij = per-capita effect on i by j =
“competition coefficient”
Fixed for a particular
pair of species (in
theory, anyways)
* α12N2 converts
individuals of species 2
into an equivalent
number of individuals
of species 1
* Ex. A squirrel can eat a
lot more seeds than a
sparrow; α measures
how many sparrows-
worth of seeds a single
squirrel eats

Question 9

outcomes of Lotka-Volterra
competition

Answer 9

Equilibrium = N’s no longer changing
- Four possible equilibria:
* The two species may stably coexist
* Species 1 may always win (N 1 = K 1 , N 2 = 0)
* Species 2 may always win (N 2 = K 2 , N 1 = 0)
* Identity of winner may depend on starting N’s
* Outcomes depend on values of K’s and α’s
* Coexistence requires both species to inhibit their
own growth more than they inhibit each other’s
* Can expand to consider n species

Question 10

equilibrium

Answer 10

For a population: population size not changing over
time (dN/dt = 0)
* For a community: a community not changing over time
* In the strict sense: all populations in a community at
equilibrium
* More generally: constant species composition

Question 11

Stability

Answer 11

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

Question 12

Coexistence

Answer 12

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

Question 13

Principle of competitive exclusion

Answer 13

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 out-compete the
other

Question 14

Character displacement

Answer 14

Coexisting
similar species
evolve
differences

Question 15

Paradox of the
plankton

Answer 15

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 16

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

Answer 16

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
* Remove Chthamalus, Balanus does not extend upwards