BIODIVERSITY

• BIODIVERSITY
• ALPHA, BETA, AND GAMMA DIVERSITY
• SPECIES RICHNESS CURVES
• PATTERNS OF SPECIES RICHNESS
• COMPETITIVE EXCLUSION PRINCIPLE
• SPECIES RICHNESS HYPOTHESES
• RICHNESS AND SUCCESSION
• MAJOR CAUSES FOR DECLINES IN BIODIVERSITY

Biodiversity consists of the variety of life on earth.

It contains several components:

• Genetic diversity (eg. heterozygosity, # alleles)
• Species diversity (eg. richness, H')
• Community diversity (many measures)
• Landscape diversity (many measures)

 

Species richness is the number of species in a given area.

Equitability (=evenness): (in glossary) Uniformity of abundance in an assemblage of species. Equitability is greatest when species are equally abundant.

Diversity index: some overall measure of diversity. Usually combines aspects of richness and evenness. The most commonly used index is the Shannon index (H')

H'= - S {p_ilog(p_i)}

Where the summation is over all species and p_i is the relative abundance of species i. H' is high if there are many species, or if evenness is high.

See an example calculation  

ALPHA, BETA, AND GAMMA DIVERSITY

• alpha diversity: the diversity within a site, or quadrat. a.k.a. local diversity.
• beta diversity: the change in species composition from site to site. a.k.a. species turnover
• gamma diversity: the diversity of a landscape, or of all sites combined. a.k.a. regional diversity

 

SPECIES RICHNESS CURVES

species-individual curve: a plot of the cumulative number of species encountered vs. the cumulative number of individuals captured.

species-area curve: a plot of the (cumulative) number of species encountered as a function of area.

Species-area curves can be used to compare different regions:

PATTERNS OF SPECIES RICHNESS

Species richness is not constant through space. SR (for plants and animals) is negatively related to:

• latitude
• altitude

positively related to:

• area
• environmental variability

and have a complex relationship with:

• time since disturbance
• nutrients
• predation rate
• productivity

Islands tend to have low SR 

COMPETITIVE EXCLUSION PRINCIPLE AGAIN

Gause's principle, Gause's law, competitive exclusion principle, or CEP (modified from glossary): "No two species can coexist indefinitely on the same limiting resource"

The CEP has been experimentally verified in simple systems, as well as in mathematical models.

However, we often observe not just two, but hundreds of species coexisting in a community. One of the major goals of community ecology is to determine the nature of coexistence in the face the CEP. This is the paradox of the plankton.

competitive exclusion: the situation in which at least one species is driven to local extinction (that is, excluded from a community) by interspecific competition.

The Reformulation of the Competitive Exclusion Principle, with its conditions

The Principle: Given a suite of species, interspecific competition will result in the exclusion of all but one species.

Conditions:

1. time has been sufficient to allow exclusion
2. the environment is temporally constant
3. the environment has no spatial variation
4. growth is limited by one resource
5. rarer species are not disproportionately favored in terms of survivorship, reproduction, or growth
6. species have the opportunity to compete
7. there is no immigration

Corollary:

The greater the degree to which these conditions are broken, the greater the number of species which can coexist. 

Examples of Species Richness Hypotheses

Environmental variability hypothesis: higher SR is expected where there is more environmental variability (violates condition #3)

Niche specialization hypothesis: higher SR is expected if species are specialists (violates #4)

Mass effect: Higher SR is expected if a region is surrounded by different habitats, because of constant dispersal from these other habitats (violates #7)

INTERMEDIATE DISTURBANCE HYPOTHESIS

High species richness is expected in regions with intermediate levels of disturbance (violates #1, #2, #3, and sometimes #5)

REASONS FOR HIGH DIVERSITY AT INTERMEDIATE DISTURBANCE:

1. More heterogeneity within patch at intermediate disturbance
2. On a landscape scale, many kinds of patches at intermediate disturbance, therefore many species
3. Overlap of early successional species and late successional species
4. Disturbance prevents competitive exclusion and thus allows high niche overlap at intermediate disturbance. However, few species can cope with high disturbance.

 

 

RICHNESS AND SUCCESSION

Must be treated on a case-by-case basis. However, many forests show the following patterns:

 

MAJOR CAUSES OF DECLINE IN GLOBAL BIODIVERSITY

1. Habitat loss
2. Habitat change
3. Competition from exotic species
4. Predation by exotic species
5. Overharvesting
6. Poisoning

Future: climate change (specialists will be at most risk).

Some "Background Extinction" is natural (probably less than 1 per century). However, documented extinctions are hundreds of times this rate, and undocumented extinctions (i.e. an extinction before a species is described) is doubtless much higher.

This page was created and is maintained by Mike Palmer.

carex@okstate.edu

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