COMMUNITY STRUCTURE ANDECOLOGICAL 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

- - Is lands tend to have fewer species

! Habitat heterogeneity

--Physical and biological diversity provides

opportunity

! Predator behavior

- -Predators eat competitive dominants

! Amount of disturbance

-- Intermediate levels give maximum diversity

! Climate stress

- -Remember 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

( roots 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 pre-

existing community, e.g., fires, floods, hurricanes,

landslides, tree falls, agriculture

! May be started by pre-existing germs (spores,

seeds, insect pupae)

! Some 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 Climax species

small dispersive propagules larger propagules

fast growing slow growing

herbaceous woody

short-lived annuals perennials

r-selected 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 climateand the frequency of disturbance

•The tropics have the highest species richness and diversity

•Ecological succession is a predictable pattern of changes in a communityfollowing a disturbance

•Some communities are more resistant to disturbance; some are resilientand recover more quickly than others