Succession and Stability

1. Succession and Stability Chapter 20

2. Outline • Introduction • Primary Succession • Secondary Succession • Disturbance • Ecosystem Recovery • Mechanisms of Succession • Community and Ecosystem Stability

3. Introduction • Succession: Gradual change in plant and animal communities in an area following disturbance. • Primary succession on newly exposed geological substrates. • Secondary succession following disturbance that does not destroy soil.

4. Introduction • Climax Community: Late successional community that remains stable until disrupted by disturbance.

5. Glacier Bay • In 1794 George Vancouver visited Glacier Bay, Alaska and found his way blocked by mountains of ice. • In 1879, John Muir explored the same area, relying on Vancouver’s accounts. • Muir found open water in this area. • He found that the glaciers had retreated 30-40 km up the valley.

6. Glacier Bay • Muir found no forests at the upper portions of the bay, although areas closest to where Vancouver saw the mountains of ice were already covered by forests.

7. Ecological sequence of succession in Glacier Bay.

8. Community Changes During Succession • Community changes during succession include increases in species diversity and changes in species composition.

9. Primary Succession at Glacier Bay • Reiners et.al. studied changes in plant diversity during succession. • Total number of plant species increased with plot age. • Species richness increased rapidly in early years of succession and more slowly during later stages.

10. Primary Succession at Glacier Bay • Not all groups increased in density throughout succession. • Most groups reached maximum diversity after 100 years. • Diversity of low shrubs continued through 1500 years.

11. Secondary Succession in Temperate Forests • Oosting found number of woody plant species increased during secondary succession at Piedmont Plateau.

12. Secondary Succession in Temperate Forests • Johnston and Odum found increase in bird diversity across successional sequence closely paralleled increase in woody plant diversity observed by Oosting.

13. Succession in Rocky Intertidal Communities • When an area of the intertidal is cleared, organisms colonize in a successional pattern over about 1.5 years.

14. Succession in Stream Communities • Fisher studied rapid succession following a flood in Sycamore Creek, AZ. • Normally, evaporation nearly equals precipitation - flows generally low and intermittent. • Subject to flash floods.

15. Succession in Stream Communities • They observed rapid changes in diversity and composition of algae and invertebrates.

16. Succession in Stream Communities • Invertebrates found refuge because many adults in aerial stage. • Re-colonized after flooding.

17. Ecosystem Changes During Succession • Ecosystem changes during succession include increases in biomass, primary production, respiration, and nutrient retention.

18. Ecosystem Changes During Succession • Chapin documented substantial changes in ecosystem structure during succession at Glacier Bay.

19. Ecosystem Changes During Succession • Total soil depth and depth of all major soil horizons show significant increase from pioneer community. • In addition, organic content, moisture, and N concentrations all increased. • Physical and biological systems are inseparable.

20. Ecosystem Changes During Succession

21. Four Million Years of Ecosystem Change • Chronosequences such as that found at Glacier Bay are limited. • Hawaiian Islands have formed over hot spots on the Pacific tectonic plate, forming an island chain varying greatly in age.

22. Four Million Years of Ecosystem Change • Hedin et.al. found differing patterns of nutrient distribution across the chronosequence. • Organic matter – absent from fresh lava flows increases over the first 150,000 years.

23. Four Million Years of Ecosystem Change • The total phosphorus content didn’t change over time, but the forms did change. • Older soils contain an unusable form of phosphorus. • Phosphorus was limiting primary production here.

24. Recovery of Nutrient Retention Following Disturbance • Bormann and Likens found felling trees in Hubbard Brook substantially increased nutrient losses. • Herbicide used to suppress regrowth.

25. Recovery of Nutrient Retention Following Disturbance • When application stopped, succession proceeded, nutrient losses decreased, and primary production increased. • Vegetation can’t account for all nutrient loss. • Reduced nutrient amounts in ecosystem.

26. Model of Ecosystem Recovery • Biomass Accumulation Model • Reorganization (10-20 yrs) • Forest loses biomass and nutrients. • Aggradation (100+ yrs) • Ecosystem reaches peak biomass. • Transition • Biomass declines from peak. • Steady-State • Biomass fluctuates around mean.

27. Succession and Stream Ecosystem Properties • Fisher’s study of succession following a flood showed a pattern similar to the biomass accumulation model. • Algal biomass increased rapidly before leveling off.

28. Succession and Stream Ecosystem Properties • Both primary production and respiration began to level off less than one month after flooding.

29. Succession and Stream Ecosystem Properties • Succession, which produces changes in species composition and diversity, also changes the structure and function of ecosystems. • Mechanisms?

30. Mechanisms of Succession • Clements (1916) • Facilitation • Connell and Slatyer (1977) • Facilitation • Tolerance • Inhibition

31. Facilitation • The facilitation model proposes many species may attempt to colonize newly available space, but only certain species will establish. • Colonizers “Pioneer Species” modify environment so it becomes less suitable for themselves and more suitable for species of later successional stages.

32. Tolerance • In the tolerance model, initial stages of colonization are not limited to pioneer species. • Early successional species do not facilitate later successional species. • Later successional species are tolerant of environment conditions present early in succession.

33. Inhibition • With the inhibition model, early occupants of an area modify the environment in a way that makes it less suitable for both early and late successional species. • Early arrivals inhibit colonization by later arrivals. • Assures late successional species dominate an area because they live a long time and resist damage by physical and biological factors.

34. Successional Mechanisms in Rocky Intertidal Zone • Sousa investigated mechanisms behind succession of algae and barnacles in intertidal boulder fields. • If the inhibition model is in effect, early successional species should be more vulnerable to mortality.

35. Successional Mechanisms in Rocky Intertidal Zone • Results showed early successional species (Ulva) had lowest survivorship and were more vulnerable to herbivores.

36. Successional Mechanisms in Rocky Intertidal Zone • Ulva had lower survivorship when exposed to higher air temperatures at low tide.

37. Successional Mechanisms in Rocky Intertidal Zone • Turner found evidence for facilitation in the rocky intertidal. • Ulva is the pioneer. • Replaced by mid-successional species (red algae). • Surfgrass is the dominant late successional species. • Surfgrass seeds require the red algae for attachment.

38. Successional Mechanisms in Forests • Secondary succession in old fields shows a mixture of mechanisms. • Horseweed inhibits the growth of asters. • Asters facilitate growth of broomsedge.

39. Successional Mechanisms in Forests • Primary succession following deglaciation also shows a mixture of mechanisms.

40. Community and Ecosystem Stability • Community stability may be due to lack of disturbance or community resistance or resilience in the face of disturbance.

41. Community and Ecosystem Stability • Stability: Absence of change. • Resistance: Ability to maintain structure and function in face of potential disturbance. • Resilience: Ability to recover from disturbance.

42. Park Grass Experiment • Hertfordshire, England • Studied effects of fertilizer treatments. • Continued for 150 years.

43. Park Grass Experiment • Silvertown investigated ecosystem stability. • Used community composition variability as measure of stability. • Represented composition as proportion of community consisting of each plant form.

44. Park Grass Experiment • Dodd showed that although community stability is present, populations of individual species can change substantially. • Stability depends on resolution an area is investigated at.

45. Desert Stream Stability • Valett studied interaction between surface and subsurface waters in Sycamore Creek. • Flash floods devastated biotic community. • Spatial relationships of zones stable. • Geomorphology of landscape.

46. Desert Stream Stability • The concentration of nitrate in surface water varied directly with vertical hydraulic gradient.

47. Desert Stream Stability • Higher nitrate in the upwelling areas was associated with higher algal production. • Indicates greater ecosystem resilience following flooding.