Competition between individuals Lecture 4 Nick Brown PB 1.2

Competition between individuals

Lecture 4

Nick Brown

PB 1.2

Objective: To describe the effects of

competition on the quality and quantity of individuals.

Competition: a definition“An interaction brought about by a shared requirement for a resource in limited supply.

Competition leads to a reduction in: growth reproduction and/or survivorship

of at least some of the individuals.”

Begon, Harper & Townsend (1996)

1

2

3

The effects of intraspecific competition are density dependent

• Number

• Proximity

• Size

The opportunities for a plant to capture light, water and nutrients depend on:

of neighbours already capturing them.

Density

Density dependent mortality

Plant density

Num

ber

of d

eath

s A

B

Is thisdensity dependent

mortality?

Density dependence in natural plant populations

Most plants do not experience “average” population density

Density dependence in natural animal populations

In island Roe deer (Capreolus capreolus) populations, body size strongly affects the number of fawns produced but density explained <10% of the variation in body size Andersen & Linnell (2001)

The law of constant final yield

Plant population density

Tot

al y

ield

(kg

m-2)

Plants at high densities are unable to grow to a large size. High density is offset by poor individual growth.

The reciprocal yield law

Log10 population density

Log

10 m

ean

plan

t we i

ght

100 days after sowing

50 days after sowing

10 days after sowing

Self-thinning

Log10 number of survivors (ha-1)

Log

10 m

ean

dry

wei

ght p

er p

lant Path followed by a growing

population of plants

Shade?Infertile soil?

Self-thinning rule3

2

Log10 number of survivors (ha-1)

Log

10 m

ean

dry

wei

ght p

er p

lant

W CN

3

2

log log logW C N 3

2

Log

10 v

olum

e (c

m3 )

Log10 number in box

Forestry

• Spacing determines quality and quantity

• High density = high total production

• Low density = high quality

• Thinning: is it worth it?

Negative competition: the Allee effect

Death rate

Birth rate

Density

Per

cap

ita

birt

h/de

ath

rate

Intraspecific competition drives evolutionary diversification

• Competition in a population can drive niche expansion onto new resources for which competition is less severe

Bolnick, D. (2001) Intraspecific competition favours niche width expansion in Drosophila melanogaster. Nature 410 (6827): 463-466