Succession and Stability

1. Succession and Stability Seals

2. Chapter 20

3. Reduced snowfall in ice field sources • Warmer temp. during winter • Reduction in cloud cover and rain in summer

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

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

6. Primary Succession at Glacier Bay • Ecologist 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. • Not all groups increased in density throughout succession.

7. Margerie Glacier

9. Primary Succession at Glacier Bay

10. Secondary Succession in Temperate Forests • Oosting found number of woody plant species increased during secondary succession at Piedmont Plateau. • Johnston and Odum found increase in bird diversity across successional sequence closely paralleled increase in woody plant diversity observed by Oosting.

11. Secondary succession occurs where a plant community has been disrupted. A common example is when abandoned farmland is allowed to revert to the wild. In deserted farm fields, the pioneer plants are often grasses, shrubs, weeds, and tree saplings. Eventually, taller trees move in and alter the environment by creating moist and shaded conditions. Ultimately, a relatively stable climax community will be attained.

13. Buell- Small Succession Study

14. Succession in Stream Communities • Fisher studied rapid succession in Sycamore Creek, AZ. • Evaporation nearly equals precipitation - flows generally low and intermittent. • Subject to flash floods. • Observed rapid changes in diversity and composition of algae and invertebrates. • Invertebrates found refuge because many adults in aerial stage. • Re-colonized after flooding.

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

16. Ecosystem Changes During Succession

17. Four Million Years of Ecosystem Change • Chronosequences (seq. of ages represented at study sites) 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. • Hedin et.al. found differing patterns of nutrient distribution across the chronosequence.

18. Recovery of Nutrient Retention Following Disturbance • Bormann and Likens found felling trees in Hubbard Brook substantially increased nutrient losses. • Herbicide used to suppress regrowth. • When application stopped, succession proceeded, nutrient losses decreased,and primary production increased.

19. Mechanisms of Succession • Facilitation • Tolerance • Inhibition

20. Facilitation • Proposes many species may attempt to colonize newly available space. • 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.

21. Tolerance • Initial stages of colonization are not limited to pioneer species. • Early successional species do not facilitate later successional species.

22. Inhibition • 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.

23. Mechanisms of Succession

25. Successional Mechanisms in Rocky Intertidal Zone • Scientist 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. • Results showed early successional species had lowest survivorship and were more vulnerable to herbivores.

26. 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.

27. Park Grass Experiment • Hertfordshire, England • Studied effects of fertilizer treatments. • Continued for 150 years. • Scientist investigated ecosystem stability. • Used community composition variability as measure of stability. • Represented composition as proportion of community consisting of each plant form.

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

29. Disclimax • Disclimax or "disturbance climax" describes a community that is held at an earlier successional stage by repeated but unpredictable disturbances that prevent succession from reaching the climax community that might be expected for the climate of the area.

30. The original prairies of Illinois are examples of disclimax communities. • The early successional grass and perennial plants are fire tolerant because of their underground roots and stems. • Repeated fires destroy shrubs, young trees, and other plants that would change the environment and result in further successional changes that would eventually result in the establishment of a deciduous forest.

31. Agricultural practices are essentially an artificial form of maintaining disclimax. • Crops like corn and soybeans as well as the common weeds found in agricultural fields have the characteristics of pioneer species and require repeated soil disturbance.