Transcript Niches
Ecological niches Niches: fundamental and realized Principle of competitive exclusion Realized niche as competitive refuge Niche crossovers Character displacement Adaptive radiation Ecological niche concept Habitat occupance = Ecological niche = “Where are you from?” “What’s your address?” “What do you do?” “Do you eat meat?” Specialized habitat occupance Niche breadth: generalist vs. specialist Categorizing niches Niche overlap? Junco Chickadee Douglas squirrel Deer mouse Deer Coyote Cougar Main food source generalist/ specialist seeds seeds & insects seeds ? specialist ? generalist ? specialist seeds ? specialist 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… 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 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) Reducing niche overlap through habitat segregation upper canopy lower canopy shrub floor resource overlap? Fundamental vs. realized niche 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 Natural experiments compression Niche compression: barnacles on Scotland’s rocky shores QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Connell, J. 1961. Ecology 42, 710-72 Sedge niches: Fraser delta high tide low tide Daily Inundation Rare H1: realized = fundamental Daily Inundation Rare H2: Scirpus occupies refuge Determining niche compression A. Field experiments: reciprocal transplants high tide low tide Two-year transplant experiment was inconclusive. Both species grew well in other species zone. (Mike Pidwirny) Dominance hierarchy dominant sp. subdominant sp. in the absence of competition resource gradient A B C with competition refuge exclusion refuge zone resource gradient Dominance hierarchies are environmentally contingent dominant sp. B in the absence of competition resource gradient subdominant sp. A C with competition exclusion refuges zone resource gradient Flexible dominance hierarchies salinity B resource gradient A C inundation/ waterlogging resource gradient Niche crossovers Character displacement Redwood forest niches Competitive ‘release’ or are niches and habitat occupance more-or-less fixed? NB: hypothetical ! Can niches be vacant? absent mainland competitive release vs. OR island new invader? evolution of new species? Placental mammals Does evolution fill a finite number of jobs? (e.g. community wants burrower?) Is there a restricted “guild”? Burrower Australian marsupi Hawaiian honeycreepers: “woodpecker” “nectar-feeder” seed-eating finch evolves into vacant niches? http://biology.swau.edu/faculty/petr/ftphotos/hawaii/postcards/birds/ Galapagos finches: opportunistic evolution Source: Lack, D. 1966. Darwin’s Finches. Harper, N.Y. Parallel (or convergent) evolution of animals inhabiting African (right) and S. American (left) tropical forest Stickleback niches in coastal lakes of SW British Columbia Pairs of stickleback species occur in these lakes Texada Is. (4 lakes) Van. Is. (1 lake) Lasqueti Is. (extinct, 1996) Stickleback pairs in coastal lakes of SW British Columbia benthics feed on lake bed, limnetics in water column Source: BC Min. Environment Land and Parks, 1999. “Wildlife in BC: At Risk” brochure 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? Situations vacant? Large generalist herbivore wanted?? QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Tapirus bairdii (Belize) Niches and diversity original state more resources more specialization 6 species ‘community’ 10 species ‘community’ 10 species ‘community’ 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 1922-2; canal completed in 1914; fish re-censused in 2002 Rio Chagres 7 additions 0 extinctions QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Invaders contradict the “saturation” model Rio Grande 5 additions 0 extinctions Smith et al. ( 2004) Proc. Roy Soc., 271, 1889-189 NY Times, Sept. 09, 2008 Island invasions and community saturation: plants Sax D.F. and Gaines S.D. 2008. PNAS 105 :11490-11497 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. 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. 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. What is a “community”?* 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). *or is it a “commutiny”? Community structure Closed vs. open communities Ecotones (community boundaries) The continuum concept Biogeoclimatic zones Are communities closed, or open? *community named after dominant(s): e.g. Douglas fir, hemlockcedar. * * * E = ecotone fidelity? = a continuum? Characteristics of open and closed communities Early proponent OPEN H.A. Gleason CLOSED F.E. Clements Organization Individualistic Holistic Boundaries Diffuse Distinct Species ranges Independent Coincident Coevolution Uncommon Prominent Testing the community concept in montane forests of the American West Plant associations * Environmental gradient 1 2 3 4 trees shrubs mosses Ass: 1 2 3 1 2 111 122 232 4 5 3 343 4 345 454 *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 Douglasfir Terrestrial biomes (plants and animals) Bioclimates (highly schematic)