Module #3: Community Theories Flashcards
A community
An assemblage of interacting organisms or populations within an ecosystem
Ecological Community
A group of trophically similar, sympatric species that compete (or potentially can compete) for resources within an area
Community Assembly
An unceasing process of species arrivals, persistence, increase, decrease, and extinctions played out in an individualistic way.
Niche-Assembly
Communities are groups of interacting species whose presence/absence & relative abundance can be deduced from “assembly rules” that are based on the ecological niches or functional roles of each species.
–> big competition
–> predicts an equilibrium assemblage of particular species, usually SMALLER scales
Dispersal-Assembly
Communities are open, non-equilibrium assemblages of species largely thrown together by chance, history, and random dispersal. Species come and go, their presence/absence is dictated by random dispersal, speciation, and stochastic local extinction. Usually LARGER scales.
ex: island biography theory (IBT) and unified neutral theory of biodiversity (UNTB)
Island Biogeography Theory (IBT) by MacArthur & Wilson
An equilibrium number of species on an island is the outcome of a balance between immigration of new species from the nearest mainland & extinction of species on the island.
Equilibrium number = balance b/n immigration and extinction
As the number of species on the island increases,
- rate of immigration drops
- rate of extinction rises
Revisions to the IBT
- immigration rates decrease with increasing distance from a source area– equil # will be higher on nearer islands
- extinction rates vary inversely with island size
Species-Area Effect
As the area increases, the # of species increases.
On islands: the species doubles when the habitat increases by a factor of 10, but on mainlands the rate of increase of species is much lower
–> Island curve of area vs. species is much more steep than mainland
Habitat Diversity Hypothesis vs. Area-Alone Hypothesis
Habitat-diversity hypothesis: Larger areas have more habitats, and therefore more species
Area-alone hypothesis: Larger areas have more species, regardless of habitat diversity
–> Kohn & Walsh (1994): direct relationship b/n species + island area, but independent of habitat diversity
Simberloff + Wilson (1970)
They asked, if an island is wiped of life, will it return to an equilibrium #? They fogged keys islands with methyl bromide and watched life return. They found YES, there is an equilibrium species #
Diamond (1969)
Diamond surveyed bird populations on islands off the coast of California. He found that there was species turnover (about 30%), but species richness stayed the same.
Criticisms of IBT
- Autochthonous speciation (evolution on the island) may be significant
- Immigration rate may depend on island area as well as distance from the source (ex. migrating species are more likely to encounter a larger island)
- Extinction may depend of distance from a source, because dwindling species may be rescued by migrants + their genes
- Habitat diversity and its environmental factors will affect species richness
- The equilibrium # will change with varying environmental conditions (such as colonization and speciation)
DESPITE these, studies support that species numbers are a function of area and distance
Relaxation
Decrease in species richness
Great Basin Sky Islands example: environmental change and island diversity
Species richness was higher in the pleistocene, because the area was much wetter and not desert like it is now, so animals were able to come down from peaks and change locations
Lomolino (2000)
Proposed that different size islands encounter different circumstances for species richness.
Small islands: dominated by chance events
Medium islands: beyond an ‘ecological threshold,’ factors such as habitat diversity, carrying capacity, extinction and immigration dynamics predominate
Largest islands: Over an ‘evolutionary threshold,’ autochthonous evolution of island endemics bolsters species #s
