1. Gause’s competitive exclusion principle and “the paradox of the plankton”
713/813 Lecture 10

2. Gause’s law
Two species competing for the same resource cannot coexist if all other ecological factors are constant (

3. How do we define “niche”
Class: “A set of conditions (resource, environmental, biotic) that an organism exploits/inhabits best to avoid competition”
Hutchinson: “an N-dimensional hypervolume”

4. But why are some ecosystems so diverse? The paradox of the plankton
The problem that is presented by the phytoplankton is essentially how it is possible for a number of species to coexist in a relatively isotropic or unstructured environment all competing for the same sorts of materials

5. The phytoplankton of which Hutchinson speaks
Prochlorococcus, the most abundant photosynthetic organism on the planet

6. How is diversity maintained?
How can a liter of seawater have at least 1,000,000 bacteria and 20,000 bacterial species?

7. Prochlorococcus ecotype specificity

8. Possible explanations
Predation and parasitism -related tradeoffs
Frequency-dependent selection (advantage when rare)
Resource partitioning and cross-feeding
Ecological subdivision (spatial structure, niche subdivision)
Dispersal creates environmental variability (could speed up evolution)
Symbioses and co-evolution

9. Diversity resulting from predation/parasitism

10. Diversity: parasitism
With phage Without phage

11. Diversity: frequency-dependence

12. Diversity: resource partitioning (illustrating periodic selection)

13. Diversity: spatial structure
Why doesn’t only 1 type prevail?

14. A tangled bank: laboratory biofilm evolution mimics the ecology of chronic infections