COMMUNITY STRUCTURE AND ECOLOGICAL SUCCESSION

Transcription

1 COMMUNITY STRUCTURE AND ECOLOGICAL SUCCESSION

2 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?

3 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

4 What influences species richness and diversity?! Area! Opportunities for immigration --Islands 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

5 In general, the tropics are the regions with the highest species richness

6 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

7 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

8 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

9 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

10 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

11 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

12 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