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Interspecific Competition

Interspecific Competition. Population interactions. # individuals. Time. Logistic Curve. Logistic model. Logistic equation dN/dt = rN((K-N)/K) Assumptions of the logistic model: Each individual has identical ecological properties Instantaneous response to environmental change

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Interspecific Competition

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  1. Interspecific Competition

  2. Population interactions

  3. # individuals Time Logistic Curve

  4. Logistic model • Logistic equation • dN/dt = rN((K-N)/K) • Assumptions of the logistic model: • Each individual has identical ecological properties • Instantaneous response to environmental change • Constant upper limit to population size and rate of gain is directly related to remaining gap • Limited space and constant food supply • Age distribution is stable

  5. Lotka/Volterra model • dN1/dt = r1N1[(K1-N1- 12N2)/K1] •  is the competition coefficient • 12 is effect of species 2 on species 1 • dN2/dt = r2N2[(K2-N2- 21N1)/K2] • if 12 >1, then individuals of sp. 2 have a greater inhibitory effect on individuals of sp. 1 than does species 1 on species 1. • if 12 <1, then sp. 2 individuals have less effect

  6. Lotka/Volterra assumptions • Environment is homogeneous and stable • Migration is unimportant • Coexistence requires a stable equilibrium point • Competition is the only important biological interaction

  7. Model with isocline Growth is 0 along the line See Fig. 12.1 and 12.2 Species 1 growth Species 2 growth

  8. Finding K/ • dN1/dt = r1N1[(K1-N1- 12N2)/K1 • if growth is 0, and r is >0, then the term (K1-N1- 12N2) must be 0 • therefore N1 = K1 - 12N2 • when N1 = 0, N2 = K1/12

  9. Possible outcomes Fig. 12.3

  10. Possible outcomes Fig. 12.3

  11. Competition - 2 species

  12. Competition - 3 species

  13. Diffuse competition • Effect of several species

  14. Diffuse competition • Effect of several species • Exclusion may occur

  15. How does coexistance occur? • Unpredictable gaps: the poorer competitor is a better colonizer • Unpredictable gaps: the pre-emption of space • Fluctuating environments • Ephemeral patches with variable life-spans • Aggregated distributions

  16. If we observe niche differences in the field, what does it mean? • Current competition • Evolutionary avoidance through competition • Evolutionary avoidance (without competition)

  17. DeWitt diagrams • Plants sown at different densities (40:0, 30:10, 20:20, 10:30, 0:40) • Seeds harvested

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