Lecture 18 Ch 21 + 23/24 Species Abundance and Diversity
Local communities assembled from regional species pool
Species sorting = processes that determine local community composition
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
Measures of diversity at different scales
Determined by ecology and regional pool
 (alpha): local number of species in homogeneous habitat
 (beta) : number of species that turnover between habitats
 (gamma): (regional): total number of species in all habitats in barrier-free area
 x  =  or  = /a
Determined by evolution
 (delta): available species pool within dispersal distance (up to continental scale)
Species-area relationship: # species increases with greater area sampled
What explains this relationship?
Larger areas give large samples
Sample more types of habitats
Larger islands are bigger target for immigrants
Populations large enough to prevent stochastic extinction
S = cAZ or log S = log c + z log A (S = # species; A = area; c, z = constants)
z = slope = 0.2 to 0.35
Less in continental areas than islands
Rapid dispersal within continental areas prevents local extinction in small
areas
Dispersal less often between islands; each island has local extinctions.\
Species relative abundance
Most species rare; few common
Lower likelihood of sampling rare species – so underrepresented
Illustrated by dominance – diversity curves
Spatial variation in species diversity
Hypotheses to explain:
Heterogeneity in space and time
Habitat heterogeneity can override productivityvegetation structural complexity
and food diversity  > no. species
Productivity: > primary production  all trophic levels with larger populations
Predation: higher productivity  larger predator populations
reduce competition among prey  more species coexist
Herbivore and pathogen pressure
A mechanism to maintain species diversity
Pests affect common species more than rare ones; give advantage to rare species
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: less in tropics? So less competitive exclusion?
If greater specialization (narrower niches)
greater resource availability
reduced resource demand (smaller populations)
intensified predation (smaller populations)
The niche and diversity
Add more species to community by
Increase in total niche space
Greater number of niche axes
Longer niche axes
Increase in niche overlap
Decrease in niche breadth
Greater niche diversitygreater morphological diversity
If few species in region
populations show ecological release (larger realized niche)
each species more abundant and lives in more habitats
local diversity and beta diversity decrease
Disturbance and gap dynamics  new habitats for specialization
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 : additions = subtractions
How is community structure quantified and compared?
Species richness (S): number of species
Biased by sample size so do rarefaction: subsample same # of individuals/sampl
Species differ in abundance and thus in role in community…so
Species diversity
Weights species by their relative abundance
Simpson’s index: D = 1/ pi2
Shannon-Wiener index: H = -  pi log pi
Evenness: ED = D/Dmax where Dmax = S
EH = H/Hmax where Hmax = ln S