Lecture 18: Community change

1. Lecture 18: Community change EEES 3050

2. Succession • Succession: • Definition: • The process of directional change in vegetation during ecological time. • Primary Succession: • Succession on new areas • Secondary Succession: • The recovery of disturbed new sites.

4. Example: Dune succession • Henry Cowles (1899) • Studied sand dune vegetation on the shores of Lake Michigan. • Brief history of Lake Michigan Lake levels.

5. Historic Lake Levels

8. Example: Dune succession • “ideal” system for studying succession. • Why? • Same initial substrate. • Same relief • Same available flora and fauna. • Thus, only differences between dunes should be time, biological processes of succession, and random events.

9. Example: Dune succession • Primary Assumption • Exchanging space for time.

10. Stages of dune succession • Bare soil • Bare sand produced by drop of lake and blowouts • Wind continually shifts sands.

12. Stages of dune succession • 1) Bare soil • Bare sand produced by drop of lake and blowouts • Wind continually shifts sands. • 2) Colonization • Establishment of grasses (marram grass). • Usually by rhizomes • “Likes” moving sands • Not found in areas after about 20 years.

14. Stages of dune succession • 3) Colonization of additional grasses. • Help stabilize dunes.

16. Stages of dune succession • Colonization of additional grasses. • Help stabilize dunes. • 4) Colonization by woody species • 1st usually cottonwoods • Also willow and sand cherry. • Dune becomes stabilized

18. Stages of dune succession • 5) After stabilization • Jack pine & white pine • 100 to 150 years later • Black oak trees.

21. What other systems can be used by exchanging time with space? • Retreating glaciers. • Examples of succession • Old farm field • Post-burn • Tree gaps • Volcanic lava flows • Disease • Human disturbance

22. Succession Happens – How?

23. Succession: an historical perspective. • 4 major hypotheses: • Relay floristics/monoclimax hypothesis • Initial floristic composition • Tolerance Model • Random colonization

24. Succession: an historical perspective. • Clements view: Monoclimax Hypothesis • Called relay floristics by Egler

25. Clements view: Monoclimax Hypothesis • Key assumption: • Species replace each other through each stage because they change the environment such that it is more suitable for the next species. • Climax community in any region is determined by climate.

26. Nature and Structure of the Climax by Frederic Clements (1936) • Has a category for everything! • Climax state • Proclimax • Disclimax • Subclimax • Climax community definition: • Final or stable community in a successional series. It is self-perpetuating and in equilibrium with the physical and biotic environments.

28. Succession • Initial floristic composition • Proposed by Elger (1954) • Succession is heterogeneous. • Development depends on who gets there first.

30. Succession • Initial floristic composition • Still species have no competitive advantage. • Dominant community merely a matter of who gets there first and lives the longest. • Succession proceeds from short-lived to long-lived species.

31. Succession • Tolerance model. • Could lead to Tilman’s resource ratio hypothesis. • Species are replaced by other species that are more tolerant of limiting resources.

32. Random Colonization

33. Random Colonization • Succession involves only the chance survival of different species and the random colonization by new species. • Thoughts? • Not a realistic scenario, but established a null model to test against. • If not random, than some process must be at work.

34. Back to the dunes

36. What model fits the dune scenario? • Monoclimax? • Initial floristic composition? • Tolerance? • Random Colonization? • We only examined the dominate species! • Perhaps a mix of climax and random colonization.

37. Example 2: Glacier Bay

39. Glacier Bay Movie

40. Changes in pH

41. Changes in Nitrogen

42. Types of species • pioneers (r-species) • short-lived • high reproduction • poor competitors • emphasize dispersal • climax species (K-species) • long-lived • low reproduction • strong competitors • emphasize survival

43. Type of Succession • primary: • new site, never before home to a community • Examples: ash flow, new sediments, retreat of glacier • secondary: • disturbed site • most common • Examples: abandoned farm field, flood, forest fire, disease/insect outbreak. • regenerative: • replacement with same species • Examples: Periodic disturbance

44. Community Change • Patch Dynamics • Small-scale changes in a community • Examples • Degeneration of plants • Tree fall gap • Dispersal of organisms • Wave action • Burrowing animals

45. Treefall gap