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Competition

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.

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Competition

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  1. Competition Dan Bebber Department of Plant Sciences

  2. 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

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

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

  5. 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?

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

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

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

  9. b = -1 Log10 individual mass Log10 density Constant final yield • Competition reduces growth of competitors • Few small or many large individuals

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

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

  12. b>1 k b=1 b<1 b=0 Log10 density 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

  13. Modelling competition • Discrete time models for seasonal reproduction • R = fundamental net reproductive rate • Nt+1 = NtR • i.e. exponential growth

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

  15. Model behaviour

  16. 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

  17. 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)

  18. 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

  19. Slope -3/2 Self thinning • Why -3/2? • L=N=Constant • =aD2 • m=bD3 • m=b(L/a)3/2.N-3/2 • Why not -1? • Greater efficiency of resource acquisition • Disturbance resets the clock Log10 individualmass Log10 density

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

  21. Tree rings

  22. Logging reduces competition

  23. 8m 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 In Out

  24. relative growth rate total basal area 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?

  25. 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

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