1. Ecological succession How communities and ecosystems change

2. Balance of nature Is there a “balance of nature”? What does that even mean? –Suggests “stability” –Suggests “lack of change” Do ecosystems stay the same? –Or are they constantly changing? –May be hard to tell because human time span is short

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

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

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

6. 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

7. Yellowstone fires, 1988 Communities can often respond very rapidly to a massive disturbance Figure 53.22a, b (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.

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

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

10. Succession Primary –Occurs where no ecosystem existed before or where no trace remains On a sand dune On a cliff After a glacier retreats On a new volcanic island Where rocks are laid bare by erosion

12. 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

13. 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

14. Succession Moraines in Glacier Bay, Alaska –Follows a predictable pattern of change in vegetation and soil characteristics (b) Dryas stage (c) Spruce stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. Soil nitrogen (g/m 2 ) Successional stage Pioneer Dryas Alder Spruce 0 10 20 30 40 50 60 (a) Pioneer stage, with fireweed dominant - Some species improve environment for followers

15. 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. Ecological Succession Succession after the Yellowstone fires.

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.

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

20. 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.

21. Mt. St. Helens - 1976

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

23. Meadow vegetation immediately after eruption

24. 1980

25. 1981

26. 1982

27. 1985

28. 1989

29. 1992

30. 1994

31. 1981

32. 1985

33. 1989

34. 1992

35. 1994

36. 1998

37. Eastern US Ecological Succession

38. Yellowstone wolves

39. Dying aspen grove

40. Streamside elk grazing

41. Hey, what’s that noise?

43. Yellowstone Ecosystem

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