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 In consequence, the loser is excluded, at least locally, unless…
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.
Categorizing niches: dietary segregation amongst local granivores SpeciesHabitatOther foods? juncofloor berries, insects (esp. ants and beetles) chickadeecanopyinsects Douglas squirrelcanopy insects, mushrooms, flowers, birds’ eggs deer mousefloorinsect larvae (esp. moths)
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’.
Determining niche compression A. Field experiments: reciprocal transplants Scirpus Carex low tide high tide competitive refuge? Two-year transplant experiment was inconclusive. Both species grew well in other species zone. (Mike Pidwirny)
Dominance hierarchy A B C dominant sp. subdominant sp. resource gradient in the absence of competition with competition refuge exclusion refuge zone resource gradient
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
Flexible dominance hierarchies resource gradient inundation/ waterlogging salinity B A C
Redwood forest niches
Competitive ‘release’ or are niches and habitat occupance more-or-less fixed? NB: hypothetical !
Can niches be vacant? mainland vs. island absent new invader? evolution of new species? competitive release OR
Does evolution fill a finite number of jobs? (e.g. community wants burrower?) Is there a restricted “guild”? Burrower Placental mammals Australian marsupials
Parallel (or convergent) evolution of animals inhabiting African (right) and S. American (left) tropical forest
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
Stickleback pairs in coastal lakes of SW British Columbia Source: BC Min. Environment Land and Parks, “Wildlife in BC: At Risk” brochure benthics feed on lake bed, limnetics in water column
Stickleback pairs A single episode of colonization of coastal lakes by a marine stickleback about to 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?
Tapirus bairdii (Belize) Situations vacant? Large generalist herbivore wanted??
Niches and diversity 6 species ‘community’ 10 species ‘community’ original state more resources 10 species ‘community’ more specialization
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.
“The Panama Canal Experiment: fishes” Fish censused in ; canal completed in 1914; fish re-censused in 2002 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,
Island invasions and community saturation: plants Sax D.F. and Gaines S.D PNAS 105 : NY Times, Sept. 09, 2008
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 Austral Ecology 29,
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 Austral Ecology 29,
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 The Unified Neutral Theory of Biodiversity and Biogeography. Princeton U.P. Graphic: New Scientist, 9 February 2002.
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).
Community structure Closed vs. open communities Ecotones (community boundaries) The continuum concept Biogeoclimatic zones
Are communities closed, or open? E = ecotone = a continuum? ** * *community named after dominant(s): e.g. Douglas fir, hemlock- cedar. fidelity? species abundance
Characteristics of open and closed communities OPENCLOSED Early proponentH.A. GleasonF.E. Clements OrganizationIndividualisticHolistic BoundariesDiffuseDistinct Species rangesIndependentCoincident CoevolutionUncommonProminent
Testing the community concept in montane forests of the American West
Plant associations Environmental gradient trees shrubs mosses * Ass: *An association is a local grouping (a sub-community)
Local plant association mapping UBC Research Forest, Haney (after Klinka, 1975)
Map of plant associations in part of UBC Research Forest, Haney, BC 100 m
Plant associations as environmental indicators
Biogeoclimatic zones and subzones purple = mountain hemlock; green = coastal western hemlock; yellow = coastal Douglas- fir