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Gause’s competitive exclusion principle
and “the paradox of the plankton”
713/813 Lecture 10
Gause’s law
Two species competing for the same
resource cannot coexist if all other
ecological factors are constant
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”
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
The phytoplankton of which Hutchinson speaks
Prochlorococcus, the most
abundant photosynthetic organism
on the planet
How is diversity maintained?
• How can a liter of seawater have at least
1,000,000 bacteria and 20,000 bacterial
Possible explanations
• Predation and parasitism -related tradeoffs
• Frequency-dependent selection (advantage
when rare)
• Resource partitioning and cross-feeding
• Ecological subdivision (spatial structure, niche
• Dispersal creates environmental variability
(could speed up evolution)
• Symbioses and co-evolution
Diversity resulting from
Diversity: parasitism
With phage
Without phage
Diversity: frequency-dependence
Diversity: resource partitioning
(illustrating periodic selection)
Diversity: spatial structure
Why doesn’t only 1 type prevail?
A tangled bank: laboratory biofilm evolution
mimics the ecology of chronic infections
t = 1000