Download Lecture 17 CH 21+23+24 SPECIES ABUNDANCE + DIVERSITY

Lecture 17 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, and human introductions.
5. Biotic interactions affecting diversity include pressure from herbivores and pathogens,
competition and niche availability (+ heterogeneity in space and time).
6. Species diversity is higher habitats that have more structural complexity and greater
productivity.
7. Intermediate levels of disturbance promote higher diversity.
8. Number of species increases with area sampled.
9. Local communities contain a subset of regional species; species are sorted out by
habitat selection.
10.Ecological release provides evidence for local interactions controlling diversity.
Species relative abundance
Most species rare; few common
Lower likelihood of sampling rare species – so underrepresented
Illustrated by dominance – diversity curves 20.2
Multiple scales of species diversity
Local
Regional
Latitudinal
Continental
Global
Local Scale
How is local community 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
Shannon-Wiener index: H = -  pi log pi
Species-area relationship: # species increases with greater area sampled 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 pg.415-6
Factors affecting distributions/presence of species
Abiotic factors, biological interactions, dispersal limitation, human introduction,
chance
Abiotic factors
Ecological heterogeneity in space and time + habitat productivity
 greater number of species 20.7, 20.8
Greater solar energy input and precipitation enhances species richness 20.10, 20.11
Biotic interactions affecting species diversity
Herbivore and pathogen 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
Competition pg. 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
Add more species to community by
increase in total niche space 20.17, 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
If few species in region
populations show ecological release (larger realized niche) 20.16
each species more abundant and lives in more habitats
local diversity and beta diversity decrease
Disturbance and gap dynamics  new habitats for specialization pg. 429-30
Intermediate disturbance hypothesis:
Diversity peaks at intermediate levels of disturbance
Low disturbance competitive exclusion takes over
High disturbance few species adapted to this extreme
Recruitment limitation
All species do not have equal probability of colonizing gap
Nearest neighbors win gaps, not best competitor
Equilibrium in species number when: additions = subtractions
Differences among communities in species richness due to differences in these rates
Random ecological drift (Hubbell’s neutral model) 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
Measures of diversity at different scales
Determined by ecology and regional pool
 (alpha): local number of species in homogeneous habitat pg. 419
 (beta) : difference in number of species (turnover) between habitats 20.12
Sorensen similarity = C / (S1 + S2)/2
C = # in common divided by average number of species in each community
Turnover more rapid in N-S (than E-W) direction; steeper climate gradient 20.12
Decreases from S to N latitudes in North America 20.13
 (gamma): (regional): total number of species in all habitats in barrier-free area
mean  x  =  or  = /
Determined by evolution
 (delta): available species pool within dispersal distance (up to continental scale)
Local communities assembled from regional species pool
Species sorting = processes that determine local community composition 20.14, 20.15
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 greater when regional species pool is largest
When small pool, less competition 
Ecological release = species expansion into habitats; > population density
Provides evidence that local interactions control species diversity
Summary: 2, 4-12, 14-16