1. Competition

2. What is competition?
Competition: use or defense of a resource by one individual that reduces the availability of that resource to other individuals.
Competition among individuals may be:
intraspecific (within species)
interspecific (between species)

3. Types of competition
Competition occurs when use of a needed resource (food, water, space, light, nutrients, etc.) by one individual reduces the availability of that resource to another - whether of the same or another species – Ricklefs
Intraspecific competition most intense for common species
Interspecific competition most important for sparse species

4. What is a resource?
Resources include foods that are eaten, but also include:
open space for sessile organisms
hiding places and other safe sites
Conditions are not resources:
for example, temperature is not a resource:
it may affect growth and reproduction, but is not consumed by organisms

5. Competition?

6. Types of competition - 1 Exploitation (scramble, diffuse) Competition
Consumptive - use of renewable resource
Preemptive - based on occupation of open space
Interference (contest, direct) Competition
Overgrowth - when one grown on or over another, depriving it of resources
Chemical - toxin that acts at a distance
Territorial - defense of space
Encounter - transient interactions over a resource

7. Types of competition - 2
Symmetric competition: ability to capture resources is proportional to size.
Asymmetric competition: the large organisms capture a disproportionate amount of the resource

8. Intraspecific competition

9. Relationship between density and yield in herbaceous plants.
The “Law of constant final yield”
Note that after a critical density is reached, yield is independent of density.

10. Plant weight as a function of plant density, and its change with plant growth

11. y=w.d
1/w = d(1/y)
1/w = Ad + B

12. Survivorship in 7 populations of Loblolly Pine established in 1925 in the Duke Forest.

14. G= { ΣiΣj |yi-yj| }/(2n**2)ybar
all plants equal --> 0.0 ; all plant but one infinitely small --> 1.0

15. The -3/2 thinning rule w= c.p-3/2 area proportional to L2
weight proportional to volume proportional to L3
Therefore, area proportional to w2/3
and area proportional to 1/density
Thus, weight = c.p-3/2

16. Natural thinning in loblolly pine populations of differing initial density.

18. Limiting Resources
The potential of a resource to limit a population depends on availability relative to demand:
This concept is embodied in Liebig’s law of the minimum:
each population increases until the supply of some limiting resource becomes depleted
this law applies strictly to resources that do not interact to determine population growth rate

20. The Competitive Exclusion Principle
Results from many studies were summarized by Gause and others in the competitive exclusion principle:
two species cannot coexist indefinitely on the same limiting resource
although similar species exist, careful study usually reveals that they differ in their habitat or diet requirements

21. Lotka - Volterra model
Incorporates interspecific competition into the logistic equation as follows:

22. Two models are needed to describe the joint effects of species upon one another.
For two species to coexist, each population must reach a stable size greater than zero:
dN1/N1dt = 0 when:
N1 = K1 - αN2
dN2/N2dt = 0 when:
N2 = K2 - βN1

23. Species 2 K1
Species 1

24. K1/a
Species 2 K1
Species 1

25. K2
Species 2
K2/β
Species 1

34. G.F. Gause conducted early experimental studies of competition: he grew Paramecium aurelia and P. caudatum alone and in mixture in nutritive media:
each species grew well alone, but in mixture only P. aurelia persisted
similar experiments conducted on a wide variety of species have tended to show the same thing - one species persists and the other dies out, usually within generations

37. 24%
29%
34%
70 F
confusum
castanum
30 F
100 – 0
87 – 13

38. 1. Relative intrinsic growth rate
L minor > S. natans > L. gibba > L. polyrhiza
2. Arithmetic growth rate when crowded
S. natans > L. polyrhiza > L. gibba > L. minor
3. Asymptotic yield / culture
L. polyrhiza > L. minor > > S. natans > L. gibba
4. Succession, mixed
S natans & L gibba > L. polyrhiza >= L. minor

39. Resource Ratio Model
Dave Tilman constructed a model of competition based on differential depletion of two resources.
When there is an abundant supply of resources, the population of each species increases and the supply of resources per unit competitor declines until growth becomes zero (ZNG = Zero Net Growth)

42. EP10140.jpg

50. Tansley’s Classic Study
British ecologist A.G. Tansley was the first to experimentally demonstrate competition between closely related species:
He selected two species of the plant species Galium:
G. saxatile is normally found on acid, peaty soils
G. sylvestre is normally found on limestone hills and pastures
These two species were grown alone and in mixture with the other species on both soil types in a common garden

52. Tansley’s Results
When grown alone in common garden experiments, each species performed better on its preferred soil, although each could grow on the other soil type.
When grown in mixture, each species overgrew and shaded the other on its preferred soil type.
Tansley concluded that each species was at a disadvantage in competition when grown on the other soil type; this helped explain the observed distributions of the two species in nature.

53. Tansley’s Conclusions
Tansley’s conclusions have far-ranging implications for all competitive situations:
the presence or absence of species can be determined by competition with other species
conditions of the environment affect the outcome of competition
competition may be felt very broadly throughout the community
the present segregation of species may have resulted from past competition

54. Competition among barnacles
Experimental studies of competition among intertidal barnacles by Joe Connell enhanced our understanding of how competition can affect spatial distributions of species:
Barnacles are not food-limited, but compete intensely for limited space on rocks in the intertidal zone:
The barnacle Chthamalus is more tolerant of desiccation and thrives in the upper intertidal zone
The barnacle Balanus is less tolerant of desiccation but can displace Chthamalus in the lower intertidal zone

59. Competition may occur through exploitation or interference.
Exploitation competition occurs when individuals compete indirectly through their mutual effects on shared resources:
a common mechanism, typical of examples we’ve considered thus far
Interference competition occurs when individuals defend resources through antagonistic behaviors:
less common, requires that resources can be profitably defended