Competition Dan Bebber Department of Plant Sciences

Topics for discussion What is competition? Classifying competition Density dependence Quantifying competition Modelling competition Competitive exclusion principle Competition among trees Field work: Competition in Wytham Wood

A definition “The Perpetual Struggle for Room and Food” Thomas Malthus (1798). Essay on the Principle of Population, Chapter 3.

Engine of natural selection “Only the fittest of the fittest shall survive, stay alive” Bob Marley (1975). Could you be loved, Verse 1.

Intra- vs Interspecific Intraspecific Members of the same species, population. Require exactly the same resources Interspecific Members of different species Competitive exclusion principle How can similar species coexist?

Exploitation vs. Interference Exploitation is indirect Resources, e.g. food, water, light Interference is direct Access to resources, e.g. space, territory containing mates.

Symmetric vs. asymmetric Water and nutrients diffuse, therefore roots compete equally Tall tree shades, nothing small tree can do Pre-emption of resources

Effect on individuals Individuals’ growth and fecundity affected In maize, reproduction more affected than growth (Harper 1961) Density Seed mass Shoot mass Total mass Yield per unit area

Constant final yield Competition reduces growth of competitors Few small or many large individuals Log 10 density Log 10 individual mass b = -1

Density dependence Competition depends on density Flour beetle survival (Bellows 1981) At very high density, no adults produced Initial egg number Number surviving No effect Under compensation Over compensation Exact compensation

Carrying capacity Density where fecundity = mortality K Time Population size K Density Rate Birth Death

Quantifying competition Let k = log (initial density) - log (final density) or k = log (before / after) Plot log (before) vs. k Slope b = measure of competition b=0: No competition b<1: Under compensation b=1: Exact compensation b>1: Over compensation b=0 b<1 b=1 b>1 Log 10 density k

Modelling competition Discrete time models for seasonal reproduction R = fundamental net reproductive rate N t+1 = N t R i.e. exponential growth

Modelling competition a = (R - 1)/K = per capita susceptibility to crowding b = strength of competition

Model behaviour

Chaos in real populations? Hard to demonstrate in time series Many factors influence N Estimate R and b Most populations not in chaotic region But how realistic is the equation? May have been demonstrated in flour beetles

Competitive exclusion When species compete, one competitor always wins How can competitors coexist? Niche differentiation (Lack’s tits in Marley Wood) Disturbance (Paine’s mussels and sea palms) Temporal heterogeneity (Hutchinson’s plankton)

Competition among trees Tropical forests support <300 tree species ha -1 : How? Edaphic specialization Disturbance: Gap dynamics Pioneer-climax continuum Pre-emption of space Dispersal limitation Density-dependent attack The Neutral Theory

Self thinning Why -3/2? L= N=Constant =aD 2 m=bD 3 m=b(L/a) 3/2.N -3/2 Why not -1? Greater efficiency of resource acquisition Disturbance resets the clock Slope -3/2 Log 10 individual mass Log 10 density

A Canadian Example Ancient white pines Old trees stop growing: Why? Hydraulic limitation Reproductive output Senescence Compared growth rates in logged vs. unlogged areas

Tree rings

Logging reduces competition

Competition at Wytham Does competition affect tree growth in Wytham Wood? Use ECN plots Measure focal tree diameter Measure diameter of trees within 8m Calculate basal area Correlate growth with basal area 8m Out In

Calculations Total basal area = area occupied by tree trunks/area of plot Relative growth rate = absolute DBH change/original DBH Analyze relationship Correlation Linear regression GLM Any problems? total basal area relative growth rate

Conclusions All species produce too many offspring Must compete for resources Affects individuals and populations Leads to evolution by natural selection Diversity maintained by preventing competition exclusion