Chap.8 Competition and coexistence

鄭先祐生態主張者 Ayo

Japalura@hotmail.com

chap08 Competition and coexistence 2

1. Forms of competition: Interspecific and intraspecific

2. Intraspecific competition– Common in nature

– Described by the 3/2 thinning law

3. Interspecific competition– Common in nature

– Outcome affected by• Physical environment

• Other species

Road Map

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4. Competition– Exists among 55-75% of the species

– Mechanism: over use of the same resource

5. Mathematical models, called Lotka-Volterra models, predict four outcomes of competition– One species eliminated

– The other species is eliminated

– Both species coexist

– Either species is eliminated, depending on starting conditions

6. Competing species can coexist through partitioning of resources

Road Map

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8.1 Species Interactions

• Herbivory, predation, parasitism(Table 8.1)

• Positive for one population– Negative for the other population

• Batesian mimicry– Mimicry of a non-palatable species by a palatable one

– Positive for one population

– Negative for the other population

• Amensalism– One-sided competition

– One species had a negative effect on another, but the reverse is not true.

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Species Interactions• Neutralism

– Coexistence of noninteracting species

– Probably rare

• Mutualism and commensalisms– Less common

– Symbiotic relationships

– Species are intimately associated with one another

– Both species may NOT benefit from relationship

– Not harmful, as is the case with parasitism

• Competition– Negative effect for both species

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Summary of biotic interactions

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Types of competition

• Types of competition – Interspecific

– Intraspecific

• Characterizing competition– Resource competition

• Organisms compete for a limiting resource

– Interference competition• Individuals harm one another directly by

physical force

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Aphid suckingleaf sap

Caterpillarchewing leaf

Intraspecific competitionbetween members of thesame species.

Interspecific competition between different species.

Interference competition: Each caterpillar physically intimidates the others

Resource competiton: each caterpillar chews as much leaf as it can

Fig.8.2 the different types of competition in nature

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8.2 Intraspecific competition plants vs. animals• Quantifying competition in plants vs.

animals– For plants, expressed as change in biomass

– For animals, expressed as change in numbers

– Plants can not escape competition

– Animals can move away from competition

– Yoda (1963)• Quantify competition between plants

• Yoda's Law or self-thinning rule; 3/2 power rule

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Law or self-thinning• Yoda (1963) (cont.).

– Describes the increase in biomass of individual plants as the number of plant competitors decrease.

– Log w = -3/2 (log N) + log c

– w = mean plant weight

– N = plant density

– C = constant

– w = cN3/2

– Figure 8.3

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Mean d

ry w

eig

ht

per

pla

nt,

g

10

10

1

10

10

10

10

10

10

10 1 10 10 10 10 10

1

2

3

4

5

6

-1

-2

-1 1 2 3 4 5

Number of plants per m 2

Fig. 8.3 Self-thinning in plants .

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8.3 Interspecific competition

• Field experiments– Organisms can interact with all other

organisms

– Natural variations in the abiotic environment is factored in

• Laboratory experiments– All important factors can be controlled

– Vary important factors systematically

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Thomas Park competition experiments

– Tribolium castaneum and Tribolium confusum (Figure 8.4a)

– Large colonies of beetles can be grown in small containers

– Large number of replications

– Observed changes in population sizes over two-three years

– Waited until one species became extinct

– Cultures were infested with a parasite Adelina• T. confusum won 89% of the time

– Without the parasite, no clear winner

– Microclimate effects (Figure 8.4)

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10

20

30

40

50

60

70

80

90

100

0

Hot Temperate

Wet

Cold Hot Temperate

Dry

Cold

T. confusum

T. castaneum

Perc

en

t w

ins

T. confusum generally wins in dry conditions

T. castaneum did better in moist environments

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T.castaneum

T.confusum

bI bII bIII bIV bI bII bIII bIV bI bII bIII bIV bI bII bIII bIV0

10

20

30

40

50

60

70

80

90

100Perf

ect

win

s

CI CII CIII CIV

Genetic strain of beetle

Fig. 8.5 Results of competition between different strains of flour beetles.

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Interspecific competition: Natural systems• Assessing the importance of competition

– Remove species A and measure the response of species B

– Difficult to do outside of laboratory• Migration problems

• Krebs or Cage effect (p.115)

– Examples in nature• Parasitic wasps

• Figure 8.6

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40

80

60

100

20

0

40

80

60

100

20

0

40

80

60

100

20

0

A. chrysomphali

A. melinusA. lagnanensis

A. chrysomphali displaced byA. lagnanensis on oranges

No competitive displacement

Competitive displacement ofA. lagnanensis

(a) Orange County

(b) Santa Barbara

(mild)

(c) San Fernando Valley (hot)

1 2 3Year

Perc

ent

of

ind

ivid

uals

Fig. 8.6 interspecific competition: replacement of one parasite by another in the orange gwoves of California

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The Frequency of Competition

• Joe Connell (1983)

– Competition was found in 55% of 215 species surveyed (Figure 8.7)

– Effects of number of competing species• Single pairs: competition was almost always

reported (90%)

• Multiple species, competition was reported in 50% of the studies

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ABBCCD

ABACADBCBDCD

A B C D

A C

Ant Beetle Mouse Bird

Ant Mouse

Resource spectrum,(for example grain size)

Reso

urc

e u

tiliz

ati

on

Reso

urc

e s

upp

ly

a)

b)

Fig. 8.7

•倘若有 ABCD 四種，其可能的互動有 c(4,2)=6個，但實質會有重疊到的互動只有 3 個，所以可能發生 competition 的機率是 50% (3/6) 。

倘若只有兩種，互動的可能就只有一個。

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Differing opinions - Schoener (1983)• Common flaws of studies

– Positive results tend to be more readily

– Scientists do not study systems at random - may work in systems where competition is more likely to occur

• Failure to reveal the true importance of competition in evolution and ecological time– Most organisms have evolved to escape competition

and lack of fitness it may confer

– Competition may only occur infrequently and in years where resources are scarce

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Freshwater

Marine Habitat

Terrestrial

Vertebrates

Invertebrates

Taxa

Carnivores

Herbivores

Plants

70 60 50 40 30 20 10 0

Percent competition Fig. 8.8

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Mechanisms of interspecific competition

1. Consumptive (exploitative)

2. Preemptive

3. Overgrowth

4. Chemical

5. Territorial

6. Encounter •P.120-121

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Table 6.2 Mechanisms of interspecific competition

Consumptive competition is the most common form of competition, occurring in 37.8% of cases.

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Amensalism

• Asymmetric competition is often called amensalism and may be particularly important in plants, wherein one species might secrete chemicals from its roots which inhibit the growth of other plants that do not secrete such chemicals.

• Allelopathy

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Differing views of competition

• Gurevitch et al. 1992– Examined on 93 species

– Primary producers and carnivores did not show strong effects of competition as did filter feeders and herbivores. (fig. 8.9)

– No differences in the effects of competition in terrestrial, freshwater, or marine systems, for plants or carnivores, or in high-productivity versus low-productivity systems.

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Fig. 8.9 mean size of the effect of competition on biomass for carnivores, filter feeders, herbivores, and primary producers.

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Grime-Tilman debate

• Grime 1979– Competition unimportant for plants in

unproductive environments

• Tilman 1988– Competition occurs across all productivity

gradients

• Gurevitch’s result supports Tilman

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8.5 Modeling Competition

• Based on logistic equations for population growth

• Growth equations for two populations coexisting independently

– For species 1; dN1 /dt = r1N1 [(K1- N1) / K1]

– For species 2; dN2 /dt = r2N2 [(K2 - N2) / K2]

• r = per capita rate of population growth

• N = population size

• K = carrying capacity

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Modeling Competition

– For species 1; dN1 /dt = r1N1 [(K1 - N1 - N2) / K1]

– For species 2; dN2/dt = r2N2 [(K2 - N2 - N1)/ K2] = per capita competitive effect of species 2 on

species 1

• = per capita competitive effect of species 1 on species 2

• dN1 /dt = 0: zero-growth isocline

• Four possible outcomes (Figure 8.12)

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Fig. 8.10 conceptualization of conversion factors and

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Fig. 8.11 changes in the population size of species 1 when competing with species 2.

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N

NN

2 N 2

N

NN

N2 2

Region of increase of N only1

Region of increase of N only2

2Region of increase of N and N1

1.

2.

3.

Species 2 eliminated Species 1 eliminated

Either species 1 or species 2 eliminated

Both species coexist

2

2

2

2

1

1

1

1

1

00

0 0

1

1

1

2

2 2

21 1

11

1

dN

dt2

1dN

dt= 0

= 0

1dNdt

= 0

1dNdt

= 0

1dNdt

= 01dN

dt= 0

2

>

<

2

1

Fig. 8.12

chap08 Competition and coexistence 33

2

4

6

8

10

12

14

0.2

1.0

1.8

Alc

ohol

conce

ntr

ati

on (

%)

10 20 30 40 50 60 70

Pure populations

K = 13.0

Mixed populations

Pure populations

(a)

Volume of yeast, purepopulations

Volume of yeast, mixed populations

Alcohol concentration, pure populations

1

2

3

4

5

6

20 40 60 80 100 120 140

Pure populations K = 5.82

Mixed populations

Schizosaccharomyces

Time (hr)

Saccharomyces

(b)

Volu

me o

f yeast

1

Test of equations

chap08 Competition and coexistence 34

Lotka-Volterra 公式的缺陷

• The maximal rate of increase, the competition coefficients, and the carrying capacity are all assumed to be constant

• There are no time lags

• Field tests of these equations have rarely been performed

• Laboratory tests have shown divergence– Figure 8.14

• Mechanisms that drive competition are not specified

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N increase

N increase

N and N increase

1

2

1 2

Equilibrium

K 1

1N

N2

K 2

K1/

2/K

•Fig. 8.14 Nonlinear Lotka-Volterra isoclines.

chap08 Competition and coexistence 36

R star concept

• R* - Tilman (1982, 1987) alternative– Need to know the dependence of an

organism's growth on the availability of resources

– Figure 8.15

chap08 Competition and coexistence 37

Gro

wth

or

loss

rate

Gro

wth

or

loss

rate

Pop

ula

tion s

ize

(a)

(b)

(c)100

Species A

Species B

Species A

Species B

Loss

100 R*A

0 R*B

Loss

Growth

Resource level (R)

Time0 0

Growth

10

R*BR*

Reso

urc

e level (R

)

•Fig. 8.15 Tilman’s R star concept of competition between two species A and B, based on their resource utilization curves.

•Initially grows faster than species B

•Outcompetes species A, as resources become more scarce.

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8.6 Coexistence of species

• Niche– Grinnell (1918): a subdivision of a habitat that contains

an organism's' dietary needs, its temperature, moisture, pH, and other requirements

– Elton (1927) and Hutchinson (1958): an organism's role within the community

• Gause: two species with similar requirements could not live together in the same place– Hardin (1960): Gause's principle, known as competitive

exclusion principle, where direct competitors cannot coexist

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Coexistence of species

• David Lack: Competition and coexistence in about 40 pairs of birds, mediated by habitat segregation.– Figure 8.16

• Examples of coexistence– Darwin's finches on the Galapagos

– Terns on Christmas Island (Ashmole 1968)

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By h

ab

itat

0

10

20

Num

ber

of

speci

es

pair

sSegre

gati

ng a

cross

diff

ere

nt

axes

By g

eogra

phy

By f

eedin

g h

abit

at

By s

ize

By w

i nt e

r ra

nge

No s

epara

tion

•Fig. 8.16 Type of separation among 40 species.

chap08 Competition and coexistence 41

Resource partitioning

• Ranks for resource partitioning

– (Schoener 1974)

– Macrohabitat (55%)

– Food type (40%)

– Time of day or year (5%)

• Habitat was of most importance in separation species.

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• Hutchinson (1959)

– Seminal paper, "Homage to Santa Rosalia, or why are there so many kinds of animals?"

– Examined size differences for• Sympatric species (species occurring together)

• Allopatric species (occurring alone)

• Table 8.3

• Hutchinson's ratio, 1.3

chap08 Competition and coexistence 43

chap08 Competition and coexistence 44

Criticism of Hutchinson

• Studies that supported Hutchinson - inappropriate statistics

• Further tests showed no differences between species than would occur by chance alone.

• Size-ratio differences could have evolved for other reasons

• Biological significance cannot always be attached to ratios, particularly to structures not used to gather food. Figure 8.17

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Fig. 8.17 A ratio of 1:1.3 has been found to occur between members of sets of kitchen knives, skillets, musical recorders, and children’s bicycles.

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Support of Hutchinson

• Figure 8.18 d/w analysis for separation on continuous resource sets– Figure 8.19

– Figure 8.20

– Discontinuous resource distribution• Figure 8.21

• Figure 8.22

chap08 Competition and coexistence 47

Reso

urc

e u

tiliz

ati

on

Resource availability, K

Resource spectrum (x)

d

A B C

w

Fig. 8.19 Resource partitioning. Three species with similar, normal resource utilization curves utillization curves utilize a resource supply K.

•d= distance apart of means.

•w=standard deviation of resource utilization,

•d/w= resource separation ratio

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a

b

a

c

b

c

1

1

2

1

2

2

a

b

c

a

b

c

A B C

B C B C1 1 1 2 2 2A

First niche dimension

Seco

nd

nic

he d

imensi

on

A

•Fig. 8.20

chap08 Competition and coexistence 49

1

2

3

4

5

6

7

8

20

20

20

20

20

20

50

50

50

Leaf pairs N2

Distribution of insect A

Distribution of insect B

P.S. = 0 + 0.166 + 0.166 + 0 + 0 + 0 + 0 + 0 = 0.333

N1

•Fig. 8.21

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Species A

Species B

0.4

0.3

0.2

0.1

0 4 5 6 72 31 8

Resource set

Pro

port

ion

•Fig. 8.22

chap08 Competition and coexistence 51

Coexistence of Species

• Niche overlap between two insect species that feed on a shrub– Measured quantity

• PS = pi

• PS = proportional similarity = sum of all units, 1 to n, in resource set

• pi = proportion of least abundant member of pair

• PS < 0.70 indicates coexistence for single resource

• PS > 0.70 indicates competitive exclusion for single resource

chap08 Competition and coexistence 52

Coexistence of Species

• Proportional similarity indices for two or more resources can be combined– Multiply separate PS values to determine

overall PS value– Coexistence for two resources– 0.7 x 0.7 = 0.49 or less

–

chap08 Competition and coexistence 53

Applied Ecology

• Is the release of multiple species of biological control agents beneficial?– Control of pests in agriculture is of paramount

importance

– Biological control is seen as a preferable alternative to chemical control

• Release a variety of enemies against a pest

• Observe which enemy does the best job

chap08 Competition and coexistence 54

Biological control• Is this the best strategy?

– Intensive competition for the prey leads to lower effectiveness of the biological agents

– Greater population establishment rate with fewer enemy species (Figure 1)

– Establishment rate of single-species releases were significantly greater than the simultaneous release of two or more species (76% vs. 50%)

chap08 Competition and coexistence 55

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