Download community structure and ecological succession

COMMUNITY STRUCTURE AND
ECOLOGICAL SUCCESSION
What is “Community structure”?
! In a given region, how many species occur
together?
! How many inter-relationships can be supported?
How do these affect the number of species?
! How do changes in species composition occur?
How quickly?
How many species occur together?
Example: raptors in Yolo County
(1) Count species: "Species richness" S
S = 4 for both region A and B
(This calculation may underestimate effects of dominance;
also the risk potential extinctions)
(2) Count species and weight the count by proportions:
“Species diversity” Simpson's Diversity Index D
D = 4 for region A, 1.53 for region B: D is maximized by
large number of species and even distribution
What influences species richness and diversity?
! Area
! Opportunities for immigration
- - I s l a nds tend to have fewer species
! Habitat heterogeneity
--Physical and biological diversity provides
opportunity
! Predator behavior
- - P r e dators eat competitive dominants
! Amount of disturbance
--Intermediate levels give maximum diversity
! Climate stress
- - R e m e m ber the trade-offs: adaptations for fast
reproduction, competition, or tolerating stress
In general, the tropics are the regions with the highest species richness
Why are the tropics regions of high richness?
Intermediate levels of disturbance:
! Tree-fall gaps frequent, because roots are shallow
( r o o t s are shallow because of high rainfall)
! Low abiotic stress--not glaciated like temperate
areas (low extinction, high immigration?)
! High density-dependent predation of seeds
Agriculture increases disturbance, artificially lowers
richness
Ecological succession—a predictable change in a
community over time (<1000 years), following a
disturbance
Primary succession: starts on "new" environment, e.g.,
lava flows, upthrust ocean bottom, exposed land
from melting glacier
! R equires immigration and soil formation
Ecological succession
Secondary succession: starts from a disturbance of preexisting community, e.g., fires, floods, hurricanes,
landslides, tree falls, agriculture
! May be started by pre-existing germs (spores,
seeds, insect pupae)
! S ome organisms (r-selected) evolve to take
advantage of "predictable" disturbances
Ecological succession continues through a
predictable series of stages, depending on the
disturbance and the environment
But the climax community depends on abiotic factors
(temperature, rainfall), not the initiating disturbance
Patterns of succession
! Increase in biomass
! Change in types of species
! For plants, see a change in these characteristics:
Pioneers
small dispersive propagules
fast growing
herbaceous
short-lived annuals
r-selected
Climax species
larger propagules
slow growing
woody
perennials
K-selected, competitive
Stability/fragility of ecosystems (susceptibility
to disturbance)
Resistance to change—withstanding disturbance
! Wet tropical forests are pretty resistant to fire,
flood; but they take a long time to recover
from forestry
Resilience—recovery from disturbance
! dry tropical forests do not resist fire, but they
recover quickly
Different communities show different degrees of
resistance and resilience
Next hour: compare different types of
communities (“biomes”) in terms of resistance
and resilience
Summary
•Species richness is a count of the number of species in a community
•Species diversity is richness, weighted by the proportion of each species
•Species richness and diversity depend on many factors, including climate
and the frequency of disturbance
•The tropics have the highest species richness and diversity
•Ecological succession is a predictable pattern of changes in a community
following a disturbance
•Some communities are more resistant to disturbance; some are resilient
and recover more quickly than others