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 Number of deaths A B Is this density 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 Total yield (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 Log 10 population density Log10 mean plant weight 100 days after sowing 50 days after sowing 10 days after sowing

Self-thinning Log 10 number of survivors (ha -1 ) Log 10 mean dry weight per plant Path followed by a growing population of plants Shade? Infertile soil?

Self-thinning rule Log 10 number of survivors (ha -1 ) Log 10 mean dry weight per plant

Log 10 volume (cm 3 ) Log 10 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 capita birth/death 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):