Ricklefs ed. 6: Chapter 20: Biodiversity Summary
Obvious Trend=number of species increases toward the equator
Why? Two views…
1) diversity if a matter of history (stable versus unstable environments)
2) diversity reaches an equilibrium (more recent evidence supports this)
-how do diversity-dependent processes balance each other?
-how does this differ from place to place?
Note: Climate and topography (local factors) obscures latitudinal trend of species richness
Variation in the Relative Abundance of Species Influences Concepts of Biodiversity:
Biodiversity= variation among organisms and ecological systems at all levels, including…
-genetic variation within populations
-morphological and functional differences between species
-variation in biome structure and ecosystem processes (terrestrial and aquatic)
-*can be simplified to “Species Richness”=# of species within an area
Measures of species “presence”=Relative Abundance
Note: Universal trend that there are few dominant species and many rare species (fundamental logic)
Diversity Indexes-must normalize species richness in relation to size of sample
1) Margalef’s-assumes species richness increases as the logarithm of sample size
2) Menhinick’s (not covered)
Heterogeneity Measure-combines species richness with variation in abundance of species (species evenness)
-ex: Simpson’s Index
The Number of Species Increases with the Area Sampled:
Species-area relationship: S=cA^z, where c & z are constants (can also be written logS=logc+zlogA)
Trend: “when areas are large enough so that sample pick up most of the local species, the species-area relationship depends primarily on new habitat types begin included in progressively larger areas”
Endemic species=species found nowhere else (ex island)
Factors likely to increase species richness:
1) Habitat diversity
2) Physical Size (larger target for organism to find)
3) Size of population/diversity (more stable, longer time to diversify)
Results can be seen in nature with meerkat's that share many social bonds. Measure their willingness to risk themselves to warn other members of their group can be related to how similar the DNA could be.
"I'll jump in the river to save two brothers or 4 cousins".
Large Scale Patterns of Diversity Reflect Latitude, Environmental Heterogeneity and Productivity:
(…with trees, mammals, birds, and amphibians; but not so with reptiles which are mainly latitudinal)
“habitats with simple vegetation structure tend to have fewer species than more complex habitat with similar productivity levels”
Energy input from the sun and water input from precipitation predict species richness reasonably well in most groups of organisms…”energy diversity hypothesis”
Diversity had Both Regional and Local Components:
1) Local/Alpha= # of species in a small area of homogeneous habitat
2) Regional/Gamma= total # of species observed in all habitats within a geographical area that includes no significant barriers to dispersal of organisms
3) Beta Diversity= difference/turnover in species from one habitat to another (or rate at which similarity decreases with the distance between two samples
4) *Whitaker: Regional diversity = local diversity * Beta diversity
Species Pool= species that occur within a region
Species Sorting= species present in pool are sorted into different communities based of their adaptations and interactions (“environmental filters”)
Ecological Release= island species that attain greater densities and expand into more habitats then on mainland
Trend: “as size of regional species pool decreases, the realized niche of each species becomes broader”
Diversity Can Be Understood in Terms of Niche Relationships:
-higher habitat diversity = lower competition (specialization)
-higher niche diversity = higher niche diversity (ex: headwaters to mouth of river)
Equilibrium Theories of Diversity Balance Factors the Add and Remove Species:
Steady-state models resemble density-dependent models
Equilibrium Theory of Island Biogeography = the # of species on an island balances regional processed governing immigration against local processes governing extinction
-if smaller populations have a higher probability of extinction, then extinction curves should be higher for small island than for large island and small island should support fewer species at the steady state than large islands.
- if the rate of immigration to islands decreases with increasing distance from mainland source of colonists, then far islands should receive fewer immigrants than near islands and should therefore support fewer species
Explanations for High Tree Species Richness in the Tropics Focus on Forest Dynamics:
Why are there so many different kinds of trees in the tropics?
1) Environmental Heterogeneity
3) Herbivores and Pathogens afflict common species more than rare species
4) Random ecological drift
*NONE fully explain reality!