1. Ecological succession
How communities and ecosystems change

2. Constant change or stability?
Do ecosystems stay the same?
Or, they constantly changing?
Human time scales are short, may not notice change

3. Community Change or Stability
Communities are constantly changing, they are in nonequilibrium – many are in some state of recovery from disturbance
Drought
Flood
Clearcut
Fire

4. What Is Disturbance? A disturbance
Is an event that changes a community
Removes organisms from a community
Alters resource availability

5. Fire A major disturbance in most terrestrial ecosystems
Is often a necessity in some communities
(a) Before a controlled burn. A prairie that has not burned for several years has a high propor- tion of detritus (dead grass).
(b) During the burn. The detritus serves as fuel for fires.
(c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.
Figure 53.21a–c

6. Yellowstone fires, 1988
Communities can often respond very rapidly to a massive disturbance
(a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.
(b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.
Figure 53.22a, b

7. Ecological succession
Changes in community composition over time
In a disturbed community = secondary succession
In a new community = primary succession

8. Succession Secondary More common Where an ecosystem previously existed
Examples?
Flood
Storm
Earthquake
After volcano destroys existing ecosystem

9. Succession Primary Occurs where no ecosystem existed before
On a sand dune
On a cliff
After a glacier retreats
On a new volcanic island

11. Pioneer species
Species, typically plants or lichens, that are able to colonize bare ground
Must disperse easily
Must live on poor soils
Hardy organisms, with adaptations such as long roots, or symbiotic with nitrogen-fixing bacteria

12. Pioneer species Lichens are important Composed of 2 different species:
Fungus – absorbs nutrients and holds water
Alga – photosynthesis
On rock, lichen can grab hold and begin to break the rock

13. Succession Moraines in Glacier Bay, Alaska
Follows a predictable pattern of change in vegetation and soil characteristics
- Some species improve environment for followers
(b) Dryas stage
(c) Spruce stage
(d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content.
Soil nitrogen (g/m2)
Successional stage
Pioneer
Dryas
Alder
Spruce 10 20 30 40 50 60
(a) Pioneer stage, with fireweed dominant

14. Succession

15. Ecological Succession
Succession after the Yellowstone fires.
Ecological Succession
Ecological succession is the set of changes in community composition that occur over time in a new or disturbed community.
Succession at Mt. St. Helens.

16. Retreating Glaciers at Glacier Bay Alaska Make It a Natural Laboratory for Studying Primary Succession
Primary succession occurs when organisms colonize a barren environment.

17. Primary Succession at Glacier Bay, Alaska
A climax community is the stable community at the final stage of succession.

18. Succession Succession shows some general trends that include:
1) Biomass increase over time.
2) An increase in the number and proportion of longer-lived species.
3) Increased species diversity.
Succession on Mt. St. Helens – another site of intense study.

19. Mt. St. Helens

20. Mt. St. Helens – 1980 eruption: May 18, 1980

21. Meadow vegetation immediately after eruption

22. 1980

23. 1981

24. 1982

25. 1985

26. 1989

27. 1992

28. 1994

30. 1981

31. 1985

32. 1989

33. 1992

34. 1994

35. 1998

36. Ecological Succession
Eastern US

37. Yellowstone wolves

38. Dying aspen grove

39. Streamside elk grazing

40. Hey, what’s that noise?

42. Yellowstone Ecosystem

43. Lunch grew back! Beaver dams
Can transform landscapes on a very large scale
Figure 53.18

45. Pioneer species
Species, typically plants, that are able to colonize barren ground
Must disperse easily
Must live on poor soils
So: pioneer species are hardy, with adaptations such as long roots, symbiotic with nitrogen-fixing bacteria