Lecture 15: Change in communities Flashcards
Blow down zone
trees knocked down
Scorch zone
trees dead but stems still intact
Mount st. Helens
- new habitats devoid of living organisms
- enormous debris avalanche
- Some survived in burrows, or under ice-covered lakes, or were plants with underground parts
- Gophers survived in their tunnels and their preferred habitat, expanded after the eruption
Facilitation by dwarf lupines on the Pumice Plain
they trap seeds and detritus, and have nitrogen-fixing bacteria
Agents of change
act on communities across all temporal and spatial scales
Coral bleaching
- Temp rises, or stresses
- Algae gets kicked out of coral, lose important symbiosis
- Gradual, large scale change
Tsunami
- Went through and stripped coral reefs
- Dramatic effect
Abiotic Factors
- waves, currents, wind, water supply chemical composition, temperature, volcanic activity
Biotic Factors
- negative interactions
- Ex. Competition, predation, herbivory, disease, parasitism
Succession
- the directional change in species composition over time as a result of abiotic and biotic agents of change.
- often focus on vegetative change, but the roles of animals, fungi, bacteria, and other microbes are equally important
Primary Succession
- involves the colonization of habitats devoid of life
- Low frequency events with massive change (volcano, hurricane, etc.)
- can be very slow and Initial conditions are very inhospitable
Secondary succession
- Involves re-establishment of a community in which some, but not all, organisms have been destroyed (forest fire)
- high disturbance but leaves organisms that can rebuild
- The legacy of the pre-existing species and their interactions with colonizing species play larger roles than in primary succession.
Climax stage
- final stable stage a community can reach (controversial, not sure if its real)
superorganisms
- groups of species working together toward some deterministic end. Thus, succession is similar to the development of an organism
climax community
composed of dominant species that persist over many years and provide stability that can be maintained indefinitely.
Facilitation model
- Early species modify the environment in ways that benefit later species. The sequence of species facilitations leads to a climax community
Tolerance model
- also assumes the earliest species modify the environment, but in neutral ways that neither benefit nor inhibit later species
Inhibition model
assumes early species modify conditions in negative ways that hinder later successional species
- something needs to be removed for succession
Glacier Bay, Alaska example
- primary succession
- Melting glaciers have led to a sequence of communities that reflect succession over many centuries
- Get young vegetation close to the glacier and much older vegetation (spruce forests) further from glacier
Intact spruce system, shady, seed predators
very competitive but have a lot of germination as they drop a lot of seeds
New England Salt Marsh
- Salt marshes have different species compositions and physical conditions at different tidal elevations.
- A common disturbance is tidal deposition of wrack (dead plant material) that smothers and kills plants, leaving patches where secondary succession occurs
- Spike grass, Distichlis spicata, colonizes the patches first. It is eventually outcompeted by both Spartina and Juncus in their respective zones
Rocky intertidal zone
- Disturbance is created mostly by storms— waves and debris rip out the organisms
- Low tides expose organisms to high or low temperatures which can kill them or cause them to detach.
What happens as succession progresses?
- larger, slow growing and long-lived species begin to dominate
- Competition probably plays a more dominant role later in succession
- In mid- to late successional stages, an array of both positive and negative interactions are operating
Alternative stable states
- different communities follow different successional paths develop in the same area under similar environmental conditions
- can be depicted as a topographic surface.
valley/ball analogy
- Valleys represent different community types; a ball represents a community.
- The ball can move from one valley to another, depending on presence or absence of strongly interacting species
- If stable, push ball it’ll go back to where it’s meant to be
- If big enough change, can roll over edge, if it is the same, wont be able to get back to old species composition
Are human activities shifting communities to alternative states?
Yes
- These regime shifts are caused by the removal or addition of strong interactors that maintain a community type.
