ECOLOGICAL SUCCESSION: COMMUNITIES IN TRANSITION

1. ECOLOGICAL SUCCESSION: COMMUNITIES IN TRANSITION • New environmental conditions allow one group of species in a community to replace other groups. • Ecological succession: the gradual change in species composition of a given area • Primary succession: the gradual establishment of biotic communities in lifeless areas where there is no soil or sediment. • Secondary succession: series of communities develop in places containing soil or sediment.

2. Primary Succession: Starting from Scratch • Primary succession begins with an essentially lifeless area where there is no soil in a terrestrial ecosystem Figure 7-11

4. Primary Succession • Pioneer species • first ones on site • Ecological development • accumulation of organic debris allowing germination of seeds • orderly sequence of stages occurs as organisms modify the environment in ways that allows one species to replace another

6. Secondary Succession • Occurs due to disruption of community • Bare soil colonized by rapidly growing annual plants • Plants replace by perennial plants • Biomass accumulates and allows richer soil, better shelter, and allows a more complex community

7. Secondary Succession • Occurs due to disruption of community • Bare soil colonized by rapidly growing annual plants • Plants replace by perennial plants • Biomass accumulates and allows richer soil, better shelter, and allows a more complex community

9. Beginning of secondary succession Plant colonizing lava flow Climax woodland community

10. This is primary succession on a recent lava flow in the Galapagos Islands, Ecuador. The vegetation near the center of the photo is mangrove, on the slopes there are cactus and other desert plants.

11. This is the northern lower flank of Mt. Saint Helens taken about 24 years after the eruption. This area was buried under hundreds of feet of ash, so all the plant life you can see has come in from wind-blown seed. The plants are things like willows, cottonwoods, grasses and forbes that have fine, easily wind-transported seeds. In the thin ash area, most of the new vegetation arises from perennial plants whose roots survived the blast underground.

12. This fascinating photo shows plant succession following the retreat of the Emmons Glacier on Mt. Rainier. The glacier began retreating around the time of the Civil War and has moved up the valley several miles. The glacier itself is the dark mass (not the white area) that terminates in the lower left third of the picture. You can see plant succession marching up-valley toward the snout of the glacier. If you look closely you can see the White River emerging from the snout. This river flows all the way to Puget Sound at Tacoma. To the right is a beautiful lateral moraine.  Plant succession is visible on the right (north) slope of the moraine but is being impeded by hot dry conditions on the south (left) slope of the moraine. Most of the pioneering vegetation is alder and willow, with a few conifers gradually becoming established. The seeds are coming down from the forests above. The dark old growth forest to the right is mainly mountain hemlock and subalpine fir.

13. This is reforestation of the blast zone around Mt. Saint Helens. All of the trees were planted (by Weyerhaeuser) about 20 years ago.

14. Climax Community • Culmination of successional process; is a stable, complex mature form. • Equilibrium communities • Never reach a stable climax because they are characterized by and adapted to periodic disruption. • Example: fire-climax communities (Long-leaf pine forest)

15. Forest Ecosystem • The longleaf/wiregrass community is part of a fire ecosystem. • Fire is required to clear brush to allow pine seedlings to grow • Wire grass is most likely to bloom and set seed after a late spring or early summer burn.

16. Climax forest on Stone Mountain Beech-Maple Forest