Review topics

1
Review topics
Whats the difference between territorial
competition and encounter competition??
Territorial Direct, aggressive exclusion of
competitor from unit of space.
Encounter No territories. Time and energy that
could be used in reproduction instead used in
aggressive encounters (example of parasitoids,
(also suggested Tribolium beetles - but that is
more akin to intraguild predation).
2
What is the definition of the R value?
ZNPG
R represents the resource level at which
population growth is balanced by population loss
(death or biomass loss in plants). At resource
levels below the R population growth is
negative. Maximum growth rate doesnt influence
species coexistence
3
2 species - one limiting resource What is the
outcome of competition?
Resource level where sp A pop stops growing
Sp A pop stops growing --represents R for that
species
4
Why should we expect species to differ in their
R values?

• Grime argument Strong competitor for one
  resource strong for all.
• Tilman argument Resource allocation trade-offs
  mean that strong competitive ability for one
  resource poor ability for another.

5
Resource allocation trade-off More resources
allocated to light capture means less resources
available for soil-borne resource capture
6
How is the direction of the consumption vector
determined?
resource supply point R1, R2 in habitat
Resource level will depend on resource supply and
resource consumption rates. Direction of the
consumption vector - results from trade-offs in
resource uptake (doesnt necessarily have to do
with R value)
Resource supply vector
Resource 2 availability
Resource consumption vector
Resource 1 availability
7
What happens in zone 1 and zone 2?
Which spp survive if resource supply point is in
zones 1,2 and 3?
Resource 2 availability
Zone 3
ZNPG for sp 2
Zone 2
Zero Net population growth for sp 1
Zone 1
Resource 1 availability
8
Two species coexistence
Species A consumption vector
Species B consumption vector
Resource 2 availability
Species A zero gwth
Species B zero gwth
Resource 1 availability
9
Which species wins in zone 4?
Species A consumption vector
4
2
6
Species B consumption vector
Resource 2 availability
5
3
1
Resource 1 availability
10
Which sp wins in zone 4?
Species A consumption vector
4
2
6
Species B consumption vector
Resource 2 availability
Answer A Because the consumption vector will
pass through the sp B zero growth isocline before
reaching the species A isocline. Resource 1 is
most limiting to BOTH species
5
3
1
Resource 1 availability
11
Which sp wins in zone 4?
Species A consumption vector
4
2
6
Species B consumption vector
Resource 2 availability
Answer A Because the consumption vector will
pass through the sp B zero growth isocline before
reaching the species A isocline. Resource 1 is
most limiting to BOTH species
5
3
1
Resource 1 availability
12
Two species coexistence
Species A consumption vector
4
2
6
Species B consumption vector
Resource 2 availability
5
Species A zero gwth
3
Species B zero gwth
1
Resource 1 availability
13
Why do species coexist in zone 6?
Species A consumption vector
4
2
6
Species B consumption vector
Resource 2 availability
5
3
1
Resource 1 availability
14
Why do species coexist in zone 6?
Species A consumption vector
4
2
6
Species B consumption vector
Resource 2 availability
Answer In zone 6 the resource most limiting to
species A is resource 2. The resource most
limiting to species B is resource 1. Each species
is limted by the resource for which it has the
highest R value
5
3
1
Resource 1 availability
15
Could the vectors be switched? YES!!
Species B consumption vector
4
2
6
Species A consumption vector
Resource 2 availability
Red arrows show direction of species A
consumption from zone 6. Moves resource supply
point towards zone 4
5
3
1
Resource 1 availability
16
Could the vectors be switched? YES!!
Species B consumption vector
4
2
6
Species A consumption vector
Resource 2 availability
5
3
1
Resource 1 availability
17
Could the vectors be switched? YES!!
Species B consumption vector
4
2
6
Species A consumption vector
Resource 2 availability
Outcome of competition is conditional on starting
conditions
3
1
Resource 1 availability
18
How can R promote multi-species coexistence?
Resource allocation trade-offs mean that a low R
for one resource implies a high R for
another. If resource supply rates vary then
species with differ R ratios will be favored in
different sites
Species differences in R across gradients of
resource availability
19
Can R promote multi-species coexistence?
Tilman (1988) Fig. 2.10a
20
What does the big blue ball represent?
21
Even if initial resource supply rates are high
22
Might also account for successional change?
23
What is the functional response of a predator?
?V represents the type I functional response of
the predator (this describes how the rate of prey
capture is affected by prey abundance)
24
Type II or III without carrying capacity are not
stable
Why is this unstable?
High P pop V pop cannot grow until pop much lower
25
Most appropriate model yields humped prey
isocline Why humped?
26
Inefficient predator populations oscillate to
equilibrium point
27
Efficient predator predator drives prey to
extinction (paradox of enrichment)
Greater opportunity for predator to drive down
prey density despite increased prey carrying
capacity
28
Paradox of enrichment (Rosenzweig 1971 Science
171385-387)
depends on the assumption of a vertical predator
isocline
29
Cyclic dynamics of the arctic collared
lemming Gilg et al. (2003, 2006)
Lemmings live in high arctic tundra (NW Greenland)
4 predators stoats, snowy owl, skua and fox
30
Measured lemming densities, over several years,
and predation rates (observations, skat samples,
pellet samples, lemming fur in borrows)
No evidence that food or space limits lemming
pop. growth
Skua Fox
Functional response Lemmings eaten/day
Lemming density
What kind of functional response is this?
31
Skua Fox
Numeric response Young produced/yr
Stoat
Stoats occupy lemming nests in winter. Only
predator to show a delayed response - highest
number year after lemming pop peak
Numeric response
32
Observed data
Squares lemmings Circles nests
Model prediction
33
Are food webs interval? Cohen suggested that
often yes, overlaps among predators in the prey
that they consume can be represented by a series
of line segments arranged in a single dimension.
Argued that this means that a single niche
dimension is sufficient to describe feeding
relations among predators.
An interval food web what does this represent?
Some dimension
Prey consumed by different predators
34
Paine found that most interactions are weak -
i.e. algae did not respond strongly to removal of
most grazers, however there are a few very strong
interactions (per capita effects of removal of an
urchin and a chiton species). Interactions
strengths were not normally distributed by
heavily skewed toward weak interactions (ie
highly uneven). Only 2 of 7 herbivores species
had strong, significantly negative effects on
algae. Nice example of how theory does,
occasionally, inspire field experiments in
ecology!
35
What are the take home messages of food web
lecture?
Food web theory too much of an abstraction from
reality. How can we assess the importance of food
web architecture if we dont know the structure
of real webs? Measuring links (qualitative webs)
is not enough. Need to measure interaction
strength and the evenness of interactions across
webs. Detailed measurements of interaction
strengths restricts food web analysis to small
webs or components of them Looking at
interaction strength distributions may provide
additional dimension on community organization
that species richness does not reveal.
36
Zero growth isoclines - combination of abundances
of N1 and N2 at which growth of one species is
zero
1000/0.6
-ve
ve
37
For any starting value of N1 and N2 what is the
predicted equilibrium population sizes??
38
Predicted popn sizes
39
(No Transcript)