6. The principle of competitive exclusion
“Two species requiring approximately the same
resources are not likely to remain long evenly
balanced in numbers in the same habitat.”
J. Grinnell (1915)
Also known as “Gause’s principle” after mathematical
formulation by Gause in 1930.
In consequence, the loser is excluded, at least locally,
unless…
7. 1. There are refuges from competition;
the potential loser hangs on in marginal
habitats; or
2. The loser can re-immigrate from
elsewhere; or
3. Disturbances in the environment
prevent the winner from gaining a
complete monopoly.
8. Categorizing niches:
dietary segregation amongst local
granivores
Species Habitat Other foods?
junco floor
berries, insects (esp. ants
and beetles)
chickadee canopy insects
Douglas squirrel canopy
insects, mushrooms,
flowers, birds’ eggs
deer mouse floor insect larvae (esp. moths)
11. Niche compression
Realized niches are narrower than
fundamental niches, therefore the
species occupies a narrower range of
habitats than it would in the absence
of competition.
The realized niche can be regarded as
a ‘competitive refuge’.
16. Determining niche compression
A. Field experiments: reciprocal transplants
low
tide
high
tide
Two-year transplant experiment
was inconclusive. Both species
grew well in other species zone.
(Mike Pidwirny)
17. Dominance hierarchy
A
B C
dominant sp.
subdominant sp.
resource gradient
in the absence of competition with competition
refuge exclusion refuge
zone
resource gradient
18. Dominance hierarchies are
environmentally contingent
dominant sp. subdominant sp.
B A C
resource gradient
in the absence of competition with competition
exclusion refuges
zone
resource gradient
23. Competitive ‘release’ or are niches and
habitat occupance more-or-less fixed?
NB: hypothetical !
24. Can niches be vacant?
mainland vs. island
absent
new invader?
evolution of new species?
competitive release OR
25. Does evolution
fill a finite
number of
jobs?
(e.g. community
wants burrower?)
Is there a
restricted
“guild”?
Burrower
Placental mammals Australian marsupi
29. Stickleback niches in coastal lakes of
SW British Columbia
Texada Is. (4 lakes) Van. Is. (1 lake) Lasqueti Is. (extinct, 1996)
Pairs of
stickleback
species
occur in
these lakes
30. Stickleback pairs in coastal
lakes of SW British Columbia
Source: BC Min. Environment Land and Parks, 1999.
“Wildlife in BC: At Risk” brochure
benthics feed on lake bed, limnetics in water column
31. Stickleback pairs
• A single episode of colonization of coastal
lakes by a marine stickleback about 11 000
to 13 000 years ago (when sea level was
higher than at present.
• Lakes colonized independently
• Divergence into benthic and limnetic niches
in each lake
• Indicates “vacant niches” in each lake?
32. QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Tapirus bairdii
(Belize)
Situations vacant?
Large generalist
herbivore wanted??
33. Niches and diversity
6 species
‘community’
10 species
‘community’
original
state
more
resources
10 species
‘community’
more
specialization
34. So, are communities ‘designed’
by natural selection for maximum
efficiency and orderly function?*
Does this only happen in stable
‘saturated’ communities?
And how do we determine that a
community is ‘saturated’?
*Source: Eric Pianka.
35. “The Panama Canal Experiment: fishes”
Fish censused in 1922-2; canal completed in 1914;
fish re-censused in 2002
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Rio Chagres
7 additions
0 extinctions
Rio Grande
5 additions
0 extinctions
Invaders contradict the
“saturation” model
Smith et al. ( 2004) Proc. Roy Soc., 271, 1889-189
37. Island invasions and community
saturation: plants
New Zealand
~2000 native plants
~2000 naturalized aliens
3 natives extinct
California
~5000 native plants
~1000 naturalized aliens
30 natives extinct
Brown, JH and Sax, DF 2004. Austral Ecology 29, 530-536.
38. Island invasions and community
saturation: fish
Hawaii
5 native freshwater fish species
40 naturalized aliens
no extinctions
124 watersheds in temperate
North America
Fish diversity increased in 100; declined in 20
Brown, JH and Sax, DF 2004. Austral Ecology 29, 530-536.
39. The challenge to classical niche theory:
Hubbell’s unified neutral theory
Hubbell champions the idea
that tree species in the
tropical forests of Panama,
are competitively equivalent
(i.e. “neutral”= red line).
Coexistence is not a function
of niche segregation across
a spatially heterogeneous
landscape (blue line).
See: Hubbell, S.P. 2001. The Unified Neutral Theory of Biodiversity and Biogeography. Prince
Graphic: New Scientist, 9 February 2002.
40. What is a “community”?*
*or is it a “commutiny”?
An ecological (or biological) community refers
to a group of interacting organisms living
together in a specific geographical area or
habitat.
An equivalent (and now somewhat
anachronistic) term is biocenosis (proposed
by Karl Möbius in 1877 to describe the
interacting organisms of the oyster- and
mussel-bearing tidal flats of the North Sea).
41. Community structure
Closed vs. open communities
Ecotones (community
boundaries)
The continuum concept
Biogeoclimatic zones
42. Are communities closed, or open?
E = ecotone
= a continuum?
* * *
*community
named after
dominant(s):
e.g. Douglas
fir, hemlock-
cedar.
fidelity?
43. Characteristics of open and
closed communities
OPEN CLOSED
Early proponent H.A. Gleason F.E. Clements
Organization Individualistic Holistic
Boundaries Diffuse Distinct
Species ranges Independent Coincident
Coevolution Uncommon Prominent
