2. Two populations interacting: Species 1 Species 2 Effect of species 1 density on species 2 per cap. growth rate Effect of species 2 density on species 1 per cap. growth rate Effect of species 1 density on its own per cap. growth rate Effect of species 2 on its own per cap. growth rate (intraspecific interactions) interspecific interactions

3. N Intraspecific interactions are characterized by the form of density- dependence: dN Ndt Density-indendence N Negative density-indendence or competition N Positive density-indendence or cooperation

4. Similarly, one species can have a positive, a negative or no effect on another species. Species 1 Species 2 +,- or 0

5. N2N2 Intraspecific interactions are characterized by the form of density- dependence: dN 1 N 1 dt Density-indendence N2N2 Negative density-indendence or competition N2N2 Positive density-indendence or cooperation

6. Effect of spc 1 on 2 Effect of spc 2 on 1 Mutualism Competition Exploitation +--+-- +-++-+ Amensalism Commensalism Neutralism -+0-+0 000000 Two-species interactions table:

7. Species 1 Species 2 + + Each population has a positive effect on the other. Example: hummingbird and a hummingbird pollinated plant: MUTUALISM (+,+)

8. Species 1 Species 2 - - Each population has a negative effect on the other. Example: two or more annual plants compete for soil resources in spring: COMPETITION (-,-)

9. Species 1 Species 2 + - One population has a negative effect on the other, but the other benefits from the first. Example: one species eats the other (predation): EXPLOITATION (+,-)

10. Species 1 Species 2 + - One population has a negative effect on the other, but the other benefits from the first. Example: grazing: EXPLOITATION (+,-)

11. Species 1 Species 2 + - One population has a negative effect on the other, but the other benefits from the first. Example: parasitism: EXPLOITATION (+,-)

12. COMMENSALISM (+,0) Example: where one species creates another’s habitat Species 1 Species 2 + 0 One population has a positive effect on the other, but the other has no effect on the first.

13. AMENSALISM (-,0) Species 1 Species 2 - 0 One population has a negative effect on the other, but the other has no effect on the first. Example: competition between very unequal competitors.

14. NEUTRALISM (0,0) Species 1 Species 2 0 0 Neither population affects the other’s growth rate. Example: two species without any direct or indirect ecological relationship.

15. What’s the interaction?

16. Task for the next few weeks: 1)How do two populations grow when they affect each other’s growth rates? 2)What factors determine the population sizes of species engaged in an interaction? 3)When do interacting populations coexist? When does one population wipe out the other? When do populations wipe out each other?

17. Species 1 Species 2 - - Each population has a negative effect on the other. Example: two or more annual plants compete for soil resources in spring: COMPETITION (-,-)

18. Testing the consequences of species interactions: Georgii Frantsevich Gause (b. 1910) Paramecium caudatum Paramecium aurelia

20. Gause’s competitive exclusion principle: Two species competing for the same resources cannot stably coexist if other ecological factors are constant. One of the two competitors will always overcome the other, leading to the extinction of this competitor: Complete competitors cannot coexist. If two species utilize sufficiently separate niches, the competitive effects of one species on another decline enough to allow stable coexistence. Overcoming Gause’s exclusion principle: