Day 7 Community Structure & Dynamics

Frederick Clements theory

communities are stable with a highly predictable composition - highly integrated and interdependent

Henry Gleasons theory

communities are a loose, ephemeral association of species, determined by chance

Communities affected by

1. keystone species

2. succession

3. biogeography

Keystone species

has a major effect on the composition of a community

Example of keystone species

Pisaster ochraceous starfish in intertidal communities

Effect if you remove Pisaster ochraceous starfish

1. barnacle and mussel populations increase

2. less open space for algae

3. chitons and limpets leave

4. mussels outcompete barnacles

5. end result is a mussel dominated community

What Pisaster ochraceous starfish mainly eat

barnacles and mussels

Disturbance

event that removes biomass (organisms) from communitythe 

• predictable frequency

Disturbance regime

has predictable frequency - disturbance

Succession

predictable progression of species replacements in a region

Types of succession

1. Primary succession

2. Secondary succession

Primary succession

succession pattern after a disturbance removes both organisms and soil

Secondary succession

succession pattern after disturbance removes some or all of organisms, but not soil

Pioneer species

first species to invade habitat

Pioneer species in secondary succession

R-selected

• rapid growth

• reach reproductive maturity quickly

• produce many seeds

• short life span

• tolerate challenging abiotic conditions

Early successional community in secondary succession

weedy species are replaced by longer lived herbaceous species

Mid-successional community in secondary succession

shrubs and short-lived trees begin to invade

climax community in secondary succession

final stage in succession - stable community

• long lived tree species at mature age

Pioneer species in primary succession

pioneer species may be slower growing but very tolerant of nutrient-poor substrate

Fire effect on communities

fire can cause communities to not reach a climax stage unless fires stop

What fires favors

fire favors grasses over non-woody plants and woody plants

Fires majorly impact these communities

invertebrate communities

Fewer fires today due to

human interference

Conservation/restoration

managed burns to maintain natural state

Species richness

number of species present in community

Species evenness

measure of similarity in abundance of each species

Species diversity

measure that incorporates richness and evenness

Shannon index

H = Shannon index of diversity

n = number of species present

i = identities of each species 

pi = fraction of all species that is species i