Lecture 17 Chap 20 SPECIES ABUNDANCE + DIVERSITY 1
MAJOR CONCEPTS
1. Most species are moderately abundant or rare; few are very abundant.
2. Species diversity is quantified by combining the number of species (species
richness) and their relative abundance.
3. Species diversity is defined at multiple spatial scales (local to global).
4. Local diversity is affected by abiotic factors, biological interactions, dispersal
limitation, human introductions, and chance.
5. Biotic interactions affecting diversity include heterogeneity in space and time,
pressure from herbivores and pathogens, and competition/niche availability
6. Intermediate levels of disturbance promote higher diversity.
7. Diversity reaches an equilibrium when factors removing species equals factors adding species.
8. Local communities contain a subset of regional species; species are sorted out by
environmental filters (abiotic and biotic).
9. Speciation + immigration vs. extinction determine regional species diversity.
10. Number of species increases with increased area.
I. Species relative abundance 413-4
Most species rare; few common
Lower likelihood of sampling rare species – so underrepresented
Illustrated by rank-abundance plot 20.2
II. How is community structure quantified and compared?
Species richness (S): number of species
Species differ in abundance and thus in role in community…so…
Species diversity
Weights species by their relative abundance (evenness)
Shannon-Wiener index: H = -  pi log pi
Measures of diversity at different geographic scales 419-20
Determined by ecology and regional pool
 (alpha): local number of species in homogeneous habitat 419
 (beta) : difference in number of species (turnover) between habitats 20.12
III. Local Scale Species Diversity
Factors affecting distributions/presence of species
Abiotic factors, biological interactions, dispersal limitation, human introduction, chance
1. Abiotic factors
Influence habitat selection/specialization
> ecological heterogeneity in space/ time greater number of species 429 20.7, 20.8, 20.9
2. Biotic interactions affecting species diversity
A.Competition 424-5
High species richness  less competitive exclusion?  more species? Why?
If greater specialization (narrower niches)
greater resource availability (greater niche space; less niche overlap)
reduced resource demand (smaller populations)
intensified predation (populations small below K)
The niche and diversity 423-425
Add more species to community by
increase in total niche space 20.18
greater number of niche axes
longer niche axes
increase in niche overlap
decrease in niche breadth
Greater niche diversitygreater morphological diversity more species 20.17
B. Herbivore and pathogen (host-specific) pressure 20.24, 20.25
A mechanism to maintain species diversity
Pests of plants affect common species more than rare ones;
gives advantage to rare specie so more species coexist
Density- and distance-dependent survival of offspring, especially of common
species, allow rare species to occupy space near parent of common species
C. Dispersal and Recruitment Limitation
All species do not have equal probability of colonizing openings in community
Nearest neighbors win gaps, not best competitor
D. Disturbance and gap dynamics  new habitats for specialization
Intermediate disturbance hypothesis: 429-430
Diversity peaks at intermediate levels of disturbance
Low disturbance competitive exclusion takes over
High disturbance few species adapted to this extreme
E. Equilibrium in species number when: additions = subtractions 426
Differences among communities in species richness due to differences in these rates
Random ecological drift (Hubbell’s neutral equilibrium model) 432-3 20.26
Random replacement of dead individuals occurs.
Individuals of all species have equal probabilities of dying or filling openings in
community
Result: Community with N individuals filled with descendants of single individual in
an average of N generations (assumes no speciation or immigration)
Local----Regional Scale 421-3
Regional diversity sets limit on local diversity
Local communities assembled from regional species pool
Saturation hypothesis: local interactions among species limit entry into community
453-4; 21.14, 21.15
Above saturation point, regional diversity has no effect on local diversity
Species sorting = processes that determine local community composition 20.14, 20.15
Environmental filers eliminate species that can’t tolerate local conditions
Abiotic factors: habitat selection via adaptations to local environmental conditions
Environmental filters eliminate some species
Biotic factors: species must persist in face of negative biotic interactions
Species sorting is greatest where regional species pool is largest 423; 20.16
With more species, each species has narrower niche breadth and beta diversity is high
Regional Scale
Diversity depends on balance of speciation/immigration vs. extinction/emigration
Comparing diversities between communities in similar habitats in different regions
reveals regional effects
Species-area relationship: # species increases with greater area sampled 414-5 20.3
S = cAZ or log S = log c + z log A (S = # species; A = area; c, z = constants)
What explains this relationship?
Larger areas give large samples
Sample more types of habitats (greater habitat heterogeneity)
Larger islands are bigger target for immigrants
Populations large enough to prevent stochastic extinction
Slope of species-area relationship is influenced by different processes on different scales 20.4
Local: relatively steep slope and influenced by sample size
Regional: slope remains constant as samples incorporate greater variety of habitat types
Global: slope increases as samples incorporate biotas of different continents
Compare range of ‘island’ sizesslope of species-area is higher on mainland than islands 415-6
Summary: 2, 4-12, 14-16