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Terms in this set (39)

A. Environmental variability: A fluctuating environment over time can promote diversity. Temporal fluctuation prevents competitive exclusion by altering competitive hierarchy. Example: plankton and algae species coexist because conditions rarely favor one species long enough to exclude others.
B. Foundation species: have a large effect on community composition because of their size/abundance, example: kelp forest/trees -increases diversity - creates habitat that allows other species to live there.
C. Resource Availability: Adding nutrients typically decreases diversity. Paradox of Enrichment: Increased nutrients leads to higher productivity and biomass, but decreased diversity and increased dominance. example: Rothamsted, England. Adding different types of resources might favor diversity though.
D. Habitat Complexity: example:Niche partitioning. A more complex habitat could potentially house more animals.increases diversity by allowing resource partitioning by habitat type. habitat alteration (loss and fragmentation) decreases diversity. More diverse systems are more productive. More diverse systems are better able to withstand environmental variability. And prevent invasion.
E. Competition: generally reduces diversity by removing inferior competitors
F. Keystone predators: increases diversity by allowing inferior competitors to coexist with prey items that are competitively dominant (remove competitive dominant). Prevents competitive exclusion. Example: Otter's eat sea urchins, controlling their population. If the otters didn't eat the urchins, the urchins would eat up the habitat's kelp. Kelp, is a major source of food and shelter for the ecosystem.
G. Disturbance: Community composition can be reset with disturbance. Low diversity with low disturbance AND high disturbance. Highest diversity with mediated disturbance and coexistence. The Intermediate Disturbance Hypothesis (IDH) justifies that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At high levels of disturbance, all species are at risk of going extinct. At intermediate levels of disturbance, diversity is maximized because species that thrive at both early and late successional stages can coexist. Disturbances act to disrupt stable ecosystems and clear species' habitat. As a result, disturbances lead to species movement into the newly cleared area. Once an area is cleared there is a progressive increase in species richness and competition takes place again. Once disturbance is removed, species richness decreases as competitive exclusion increases.
-Allopatric speciation:there is a physical barrier that subdivides a species range into 2 or more populations that species can no longer freely exchange genes. It is much more common because there are clear extrinsic barriers to gene flow. Things like geographical or spatial barriers make gene flow restricted, and thus is more common to see populations diverge. Events that divide a species range are called VICARIANT events.Once vicariance has occurred, the separated populations can independently accumulate genetic differences either through (1) diversifying selection or (2) genetic drift (or both). Since there is a physical barrier, there is not going to be any selection to avoid mating because there is no opportunity to mate with other species.
-In sympatric speciation however it is more subtle, and there needs to be microhabitats within the overlapping habitat. There is no obvious geographic barrier to gene flow. For there to be divergence, there would have to be essentially no gene flow between microhabitats.This could happen if a species had very little dispersal potential, or a species exhibited very strong preferences to mate with like phenotypes(positive assortative mating).In sympatric speciation, species have overlapping ranges (so it is possible for them to interact and therefore exchange genes). Sympatry should promote gene flow since there is overlap. Have the opportunity to mate with others and selection should favor the evolution of prezygotic isolation mechanisms.

Sympatric species have much higher levels of prezygotic isolation than allopatric. This suggests that Reinforcement when populations become sympatric, shifts from POSTzygotic to PREzygotic.
Speciation among the crater lake cichilid fish happened because the 11 species diverged in sympatry (overlap). Based on the phylogeny, the 11 species are monophyletic. The mechanism used was Multiple Niche Polymorphism - genetic variants in population consume different prey that select for different traits. This subdivides the population, selecting against mates that don't specialize on what they specifically eat (positive assortative mating occurs). Results in genetic differences and reproductive isolation. In the cichlids, we see that all 11 of these lake species are sister species. When we see sister species among species that are all inhabiting the same area, it suggests sympatric speciation
The association of a certain microhabitat, with a particular feeding niche, and the preference to mate with individuals of their own kind reduces gene flow to such an extent that in fact these species of cichlids have been able to diverge within the lake despite their being the potential for gene flow. → Their behaviors reduce gene flow sort of like an extrinsic barrier.

- For snapping shrimp, there was a physical barrier to divide a species into two or more populations where they can't freely exchange genes (allopatry). Isthmus of Panama shut off gene flow between the Pacific Ocean and Caribbean Sea (vicariant event). Seven sister species in the shrimp resulted on opposite sides of barrier (they are not monophyletic). If we look at the snapping shrimp's phylogeny, we see that the majority of sister species are inhabiting opposite sides of the Isthmus of panama. When we see sister species among species that are living in distinctly different areas, it suggests allopatric speciation