14: Community Organization & Structure

Two views of Communities?

1) Association

2) Individualistic Continuum

Association (community)

a distinct natural unit in which certain species are associated w/ each other

ex// mixed-evergreen forest has black oak, ponderosa pine, redwoods, etc

Individualistic continuum (community)

a gradient of species distributed independently of each other, each w/ its own response to the resource gradient

Are borders between communities usually gradual or abrupt?

gradual

The understory of trees/forests are well developed if a canopy is ______, BUT underdeveloped if a canopy is ______

relatively open(more sunlight) ; relatively closed (less sunlight)

Canopy vs. Ground vs. Root layer

Canopy→major photosynthetic layer
Ground→important for decomposition & nutrient recycling
Root→important for nutrient uptake

What factors determine where grasslands can occur?

1) soil or rainfall not enough to support tree growth

2) environment is unstable

What is the relationship between foliage height diversity and animal diversity ?

positively correlated

ex// more species of animals occur in forests than grasslands

What factors cause habitats to vary horizontally (horizontal stratification)?

soil, moisture, temperature, light, topography, & microclimate

What is an edaphic community?

example?

community in which plant growth is limited by soil rather than climate

ex// serpentine chapparal

Serpentine Chaparal soil & vegetation

Soil→high Mg and heavy metals & low levels of Calcium/nutrients
vegetation→plants are stunted w/ smaller leaves

Zonation

ex??

series of communities occurring along ecological gradient

ex// intertidal, pond, forests on mountains

Edge ?

ex?

An abrupt meeting of 2 or more communities

ex// edge between forest & clearing

Ecotone?

ex?

gradual intermingling of 2 or more communities

ex// gradual edge between forest & chaparral

What is an edge effect?

example?

increased species richness in edge or ecotone

-species richness is greatest when contrast between 2 communities is greatest

ex// forest & grassland

edge species vs. interior species

edge species→prefer habitat edges with abrupt changes

interior species→prefer interior of habitats away from abrupt changes

Are dominant species usually generalists or specialists? What about subdominant species?

generalists, displacing other species from their own wide niche

subdominant species→specialists, occupy narrow niches unexploited by other species

what is the best measure of dominance?

Biomass= mass of individuals of species A/ area sampled

best measure of relative dominance?

mass of individuals of Species A/ Mass of individuals of ALL species

Jaccard coefficient of similarity

species shared/ total # of species in both communities that are different

-most frequently used

Simpson coefficient of similarity

species shared/ # of species in smaller of the 2 communities (S/A)

-emphasizes similarity between regions

define S,A,B variables in community similarity?

S→# of species in both communites(shared)

A→# of species in community w/ less species

B→# of species in 2nd community

Define Succession. Examples?

orderly progressive replacement of 1 community by another until a relatively stable community persists

ex// sand dunes & beaches replaced by grasslands

ex// lakes being replaced by grasslands

What is a sere?

A series of successional communities, leading to a climax community

What is a seral community?

each community in succession

Define pioneer community. What type of organsisms usually comprise of this community? example?

1st seral community in succession

-usually comprised of r-strategists

ex// weeds

Define climax community? characteristics?

relatively stable, self-maintaining community at the end of a sere

1)vegetation tolerant of environmental conditions

2)K-strategists

True or false: Succession is unidirectional & predictable

TRUE

True or false: Succession cannot be reversed from later seral community to earlier seral community.

False: Succession can be reversed.

ex// trees can be eaten by elephants & become a savanna

Primary succession vs. Secondary succession

examples?

Primary→succession from lifeless substrate

ex// volcanic eruption destroying all life on island

Secondary→succession following a disturbance, that doesn’t eliminate all life

ex// forests blown down by hurricane/fire

Autogenic environmental changes? example?

results from changes caused by organisms WITHIN a community

ex// growth of shade trees reduces shade intolerant species in understory

Allogenic environmental changes? example?

results from changes in the physical environment

ex// increasing temp, salinity, water depth, etc