ch 17: community structure

community

assemblage of different populations that live together in a defined area

a group of species that occupy a given area, interacting either directly or indirectly

- more restrictive definition: a subset of species that are similar (ex. plant community, small mammal community, fish community)

what is a community

number of species occurring within the community

what is species richness

species richness

the number of different species in a community

the percentage of each species contributes to the total number of individuals of all species

what is relative abundance

relative abundance

the proportion each species represents of all individuals in the community

species evenness

the relative proportion of individuals within the different species in a given area

- indicates the distribution of species abundance

- more even = more diversity

- perfectly even would mean there are the same number of every species

what is species eveness

provide a way to quantify the relationship between species number and relative abundance

what are diversity indices

D=summation(ni/N)2

- ni= number of individuals of species i

- N= total number of individuals of all species

what is Simpson's index (D)

- D is between 0 and 1

- 0 is higher biodiversity

- 1 is lower biodiversity

what is higher and lower biodiversity in Simpson's index

- this is Simpson's index

- D= 0.3 would be higher

would D=0.7 and D=0.3 be higher biodiversity

- 1-D

- numbers are between 0 and 1

- higher numbers= higher biodiversity

what is Simpson's index of diversity

- 1/D

- the lowest possible value is 1 meaning one species in the community

- the maximum value is the species richness for the community

what is Simpson's reciprocal index or Simpson's diversity index

H=summation(pi)(lnpi)

- higher H is higher biodiversity

what is the Shannon index

a single or few species that predominates within a community

- usually identified separately for subsets of the community

- can be quantified by numbers, biomass, surface area

what are dominants

dominant species

species that are the most abundant or have the most biomass

keystone species

a species on which other species in an ecosystem largely depend, such that if it were removed the ecosystem would change drastically

- has a disproportionate impact on the community relative to its abundance

- the removal of the species initiates changes in the community structure and often results in significant loss of diversity

- often a top predator, not necessarily high in numbers

describe keystone species

- builder species (beavers, coral reefs)

- destructive species (elephants, rhinos)

- sea otters in kelp forests eat sea urchins which eat kelp

- when the otter is removed the sea urchins increase in numbers and cause the kelp numbers to decrease

examples of keystone species

groups of species that exploit a common resource in a similar fashion

- ex. nectar feeding birds, seed eating birds, forest canopy folivores

what are guilds

group of species based on their common response to the environment

- ex. plants grouped into C3, C4, and CAM, shade tolerant vs shade intolerant, phytoplankton calcifiers, phytoplankton silicifiers

what is a functional type

an associated vertical structure and physical characteristics

- community properties include the structure, abiotic factors (soil, temp, PH, climate), and biotic factors (characteristic species)

what does every community have

zonation

prominent horizontal banding of organisms that live in a particular habitat

as one moves across the landscape, the physical and biological structure of the community changes; usually includes a transition zone between communities

- organisms and habitat characteristics may be different

what is zonation

- based on similarity or difference, but can be subjective

- large scale descriptions are used for larger landscape level communities

how can you define boundaries between communities

- the community is an organism and the species are organelles

- organisms are codependent on one another

- the community is an integrated whole

describe the organismic concept of communities

- organisms are independent of one another

- organisms only live in communities because it has the characteristics they need

- it is by chance that other species also live there

- the community is merely the group of species om any particular environment

describe the individualistic (continuum) concept of communities

Frederick Clements (1916 and 1920)

viewed communities as cohesive entities, with integrated parts

Henry Gleason (1917)

proposed that most communities consist of species with independent distributions

1.) species interactions over time produced an associated of species

2.) species depend on each other

3.) community acts as an organism

4.) each species is a necessary part that must be present

5.) boundaries between communities are narrow and easy to identify

rules for the organismic concept of communities

1.) any association of species is mainly due to similar environmental tolerances, not due to dependent/evolutionary interactions of the past

2.) each species has its own individual distribution among an environmental gradient

3.) associations are due to chance

4.) boundaries between communities are wide and difficult to identify

rules for the individualistic (continuum) concept of communities