Ecosystem Change

1. Ecosystem Change If there is no struggle, there is no progress. Frederick Douglass

2. Ecological Succession • Progressive change in species composition, ecosystem function and structure following a disturbance • Minor changes in structure and function accumulate over time • Ecological function of ecosystem changes • Initiated by a disturbance • Directional change in structure • Follows a predictable pattern

3. Successional Classifications (seres) • Primary succession: establishment of plant communities on newly formed habitats lacking plants • Lava flows, sand dunes, landslides, etc. • Secondary succession: return to vegetation following a disturbance • There is some blurring of the classifications

4. Some examples • Tornado levels a strip of forest • Trees are all gone, but seed bank is still there. • Secondary succession will follow • Severe fire burns through organic layer of soil and destroys the seed bank and nutrients • Primary succession would proceed even though plants were there before • Disturbances vary along with their impact

5. Succession as a Deterministic Process • Frederick Clements (1916) • Distinct steps…ends in a climax community

6. Clementsian Succession • Established species alter the environment • Allow new species to move in and establish • Directional change in composition – aka. Community structure – is maintained by the continuous alteration of the environment • Within the community (specified): • Succession always follow the same pattern as it develops to climax community • If interrupted, it will follow the path again. • This is why considered deterministic

7. Stability and Climax Community • In Clement’s view, • The concept of climax community assumes: • The species colonizing and establishing themselves in a given region can achieve stable equilibrium • Stable equilibrium – forces to change system = forces to keep system the same…no change results • Therefore, the climate community = stable equilibrium

8. Clementsian Example • Hardwood trees, a climax community, has a disturbance in the form of deforestation. • The community will always proceed this way • The transition will occur in predictable manner. • Barring any further disturbances, the hardwood community will be reached and will be stable.

9. Is deterministic pathway the only way? • Idea of stable community fell out of favor • Individualistic Perspective (Gleason, 1926) • The relationship between coexisting species (communities) as the result of similarities in their requirements and tolerance to the environment. • Partly result of chance • Succession is not deterministic

10. The challenge • Gleason argued that Clements explanation of succession could not explain retrogressive successions • Plant community simplifies and loses biomass over time

11. Clements vs. Gleason Clements Gleason Assume environment can deteriorate over time Random No relationship between species – together because of similar env. requirements • Assume long term stability • Deterministic • Interdependence among species

12. Stability Refuted • Succession viewed as a phenomenon that rarely attains equilibrium • Equilibrium related to nature of disturbance • Disturbance acts at variety of scales • Magnitude of disturbance varies • Many disturbances remove only part of the previous plant community

13. Disturbance • Variations in the definition (general agreement): • Any relative discrete event in space and time that disrupts an ecosystem, community, or population structure and changes resources, substrate, or the physical environment – Pickett and White, 1985 • Discrete in time (as opposed to chronic stress or background environmental variability) • Cause a notable change (perturbation) in the state of the system

14. Consequences of Disturbance • Total habitat destruction • Creation of new habitat (transformation) • Fragmentation (loss of certain habitat, isolation of habitats) • Increase patch number, isolation, edge • Decrease patch size, connectivity, interior • Alter local climate/microclimate, hydrology, biota diversity, behavior, health, persistence)

15. The good in disturbances • Dependent on temporal scale of analysis • Example: • Forest fire • Short term = disturbance • Long term = required to maintain seed bank and regeneration of intermediate tree growth (pines) • Intermediate disturbance hypothesis • Highest diversity when disturbances occur at certain time intervals.

16. Succession Related to Biodiversity

17. An example • Mt. St. Helens eruption – May 1980 • Good place to see the different aspects of succession and disturbance at work.

18. The disturbance

19. Eruption Survival • Survival of organisms was strongly influenced by characteristics of disturbance processes, local site conditions, and biological factors • Pyroclastic flow and avalanche debris: • Almost no organisms survived the blast • Those that survived: • Plants with underground buds, burrowing animals, and organisms protected by snow, topography, or other features • Having diverse refuges facilitated survival of some organisms

21. Eruption Survival • Life history attributes: • Many organisms not present yet • Anadromous fish – still at sea • Migratory birds • Larger, local populations suffered higher mortality than smaller species or migratory species • Surviving groups included all of the primary trophic levels – herbivores, predators, scavengers and decomposers • Complex food webs quickly developed in the emerging ecosystems • Surviving species established new interactions and began to process the dead organisms from the pre-eruption system.

22. Timing • Eruption in early morning • Allowed nocturnal animals to be protected in subterranean burrows • Eruption in early spring • Snow and ice created refuges and many plants had not broken out of winter dormancy at higher elevations • Early successional stage of many recently harvested forest sites • Profusion of wind dispersed seeds of pioneer plant species