Community structure Flashcards
Predictability
When studying anything you want some control
So how predictable are communities
Clements
climax community, time = stability, predictable developmental changes, disturbance leads to the same community
Gleason
communities are unstable, any interactions that occur are due to proximity, chance if similar relationships develop twice, what juveniles and seeds happen to be present after a disturbance determines new community
Are communities predictable?
Based on history and chance so you need to account for both
Current and past species distributions
If communities are predictable then the same species should always be found together
However species go not link themselves with one another
Exp : fossil records of pollen show the movement of trees to and from communities the trees do not go in groups
Gleason is closer to being right
Keystone species
An organism that has a strong influence on the structure and abundance of the organisms surrounding it
Removing the organism can cause drastic problems in the ecosystem
When the keystone changes in abundance then
the entire community is affected
disturbance
An event that removes biomass from a community
Exp: fires, floods, fall of a large tree
when it happens, then the community changes
The impact changes according to:
- The type of disturbance
- Its frequency
- severity
can be a part of a pattern
Exp: forest fires
The recovery of it can be predicted in this case
To study this scientists give a community a disturbance regime
Disturbance regimes
Is determined by a short term analysis of any disturbances in the area and extrapolating long term data
Exp: 1% boreal forest burns every year
So every piece of boreal forest has a 1/100 chance of burning every year
So the average time between fires in one location is every 100 years
Why are regimes important
allow for some disturbance
This can increase biodiversity due to empty niches
Some plants and animals are evolved to use the disturbance to their advantage
The maintain healthy communities then the regime must be maintained
After disturbances
surccession
primary
secondary
After disturbances
primary
removes soil, and organisms in and on the soil
After disturbances
succession
the pattern of organism growth in a community after a disturbance
After disturbances
secondary
removes organism but soil is intact
The species at each stage is determined by
the traits, interactions and historical/environmental circumstances
Pioneering species
Weeds are adapted for disturbed soil
Good dispersal species
Short lived and small species
Little competitive ability
High fecundity
Can tolerate severe abiotic conditions
Species interactions
Colonization has occurred now what
The species that exist will affect what other species move in
facilitation
tolerance
inhibition
Facilitation
a species make the environment more suitable for living
Tolerance
existing species do not hinder other species movement into the territory
Inhibition
one species inhibits the other’s growth
Succession
can be predictable
Species richness
count of the number of species
Species diversity
species abundance as well as it’s presence
Island biogeography
Island have less species than areas of similar size on the main land
The theory was that speciation takes so long so much of the species richness comes from immigration and extinction
what happens with Immigration
slows as the number of species increases, because the chances are higher that anything immigrating is already there,
also competition should increase decreasing immigrates chances of establishing a population
what happens with extinction
should increase with richness because of niche overlap and competition is more intense
when it increases species and competition decrease
when species increase, competition and this increase
larger the island
higher the immigration
closer the island
land the higher the immigration
further away and smaller the island
more extinction
Measuring species diversity
Counting
But that doesn’t take into account abundance
Evenness: a similar abundance of all species present
For this equations have been made to calculate evenness, and number
Shannon index
just need to know how to define the equation
Pi : proportion of individuals of species i
H = the species diversity
H = - ∑pi ln pi
Global patterns
Latitudinal gradient
Not true for all but for most
to be able to affect diversity
The factor must affect immigration, emigration, speciation, and extinction
Abiotic(non living): earths shape,
sunlight, temperature
High productivity hypothesis
= high diversity
Increase biomass increases herbivores increases carnivores etc
However: estuaries and experiments show that high productivity does not mean high diversity
Energy hypothesis
High temperatures increase productivity and the likelihood that animals can tolerate the physical conditions of the region
new and not well studied
But gastropods diversity increases with temperature
Area and age hypothesis
No ice disturbance
However there was drying in the ice age so there is a possibility that the rain forest are younger than some polar habitats
Intermediate disturbance hypothesis
Mid successional
With some disturbance all the time pioneering species don’t die increasing species diversity
However no study shows that disturbances happen more in the tropics
Always 3 sections
Disturbance effects the biomass and everything else
High diversity - high disturbance
Tree fall, more place for plants to grown
