Lecture 19: Succession and Disturbance Flashcards
Succession
the change in species composition in communities over time, resulting from both biotic (living) and abiotic (non-living) factors.
Succession: Change in species
composition in communities over time
(introduced as “___________”)
directional
“Superorganism” Hypothesis
- Plant communities are groups of species working together toward some deterministic end.
- Succession is directional, always tending toward a “climax” community in any given area.
- “Climax communities” are stable, resistant to disturbance, and determined by climate.
“Individualist” Hypothesis
- Communities are the result of fluctuating environmental conditions acting on individual species.
- Community composition changes gradually.
- Communities are not a predictable and repeatable result of groups of species species working together.
Interactions
_________ results in the replacement of one species with another.
Primary Succession
• Occurs when bare land is settled for the first time by
living organisms.
• Development of biotic community is slow.
• Examples: bare rock, shifting sand dunes, cooled
lava, retreating glaciers, etc.
ie -Volcanoes, glaciers, shifting sand dunes
Secondary Succession:
• Occurs in an area where life existed earlier but many
species destroyed (soil remains intact) by some
catastrophic event.
• Development of biotic community is fast.
• Examples: abandoned farmland, burnt/cutover
forest, windstorm, insect outbreak, etc.
ie - Hurricanes and severe weather, Fire, Anthropogenic Influences
“Pioneer Stage”
- Early-stage species
- 1 year after abandonment: grasses and weeds dominate
“Intermediate Stage”
-5 years after abandonment: perennial weeds dominate
-10 years after abandonment: tree saplings establish
-20 years after abandonment:
trees and shrubs replace herbaceous plant species
-30 years after abandonment: saplings dominate, canopy starts to close
“Climax Stage”
- late-stage species
- 100 years after abandonment: “Climax Community”
Characteristics of a Climax community
- “Final”, late stage of succession (“stable” community)
- Continues to change in “small” ways
- May remain “the same” through time if not disturbed
Stochastic events
Unpredictable, random
the first to colonize a bare substrate in primary succession or first to colonize a destroyed habitat in secondary succession.
Pioneer Species
ie- lichen,
- Dryas genus: small seeds, young age at first reproduction, short life span, nitrogen-fixing
- Fireweed genus: seeds germinate readily, wind dispersed, prolific seed producer, can reproduce vegetatively via rhizomes
Characteristics of pioneer species
• Hardy (tolerate harsh environments)
• Seeds are easy to germinate
• Short life cycle
• Pollination and seed dispersal both by wind
• High rates of seed production and dispersal
• Adapt to a wide range of ecological & geographical
environments
• Asexual (e.g., cloning) life cycle more common
• Often replaced by species that colonize later
Provide examples of pioneer species
ie- lichen,
- Dryas genus: small seeds, young age at first reproduction, short life span, nitrogen-fixing
- Fireweed genus: seeds germinate readily, wind dispersed, prolific seed producer, can reproduce vegetatively via rhizomes
Early-arriving species and later-arriving species may be linked in one of three processes:
• Facilitate the colonization for subsequent species (most
often occurring model) by making the habitat MORE
suitable (e.g., fix nitrogen)
• Inhibit the colonization for subsequent species by making
the habitat LESS suitable (e.g., casts too much shade)
• Tolerate the colonization of subsequent species but have
NO IMPACT on their establishment
Discuss example of facilitation
1) Softwoods (e.g., pines) are often replaced by hardwoods over time during succession.
• Many softwood species prefer sunlight (e.g., do not tolerate low light levels) and establish quickly in open sites under harsh conditions.
• Hardwoods tolerate low light levels as seedlings (e.g., shaded understory) but not open sites with harsh sunlight.
• Softwood colonization and development facilitate the colonization of hardwoods in the understory (e.g., it becomes more suitable and shady).
2) Nutrient content in soil substrate influences species composition.
• In early primary succession, there is little organic matter and nitrogen (N).
• N availability limits primary production.
• As N in soil organic matter increases, primary production increases.
• N-fixing species (e.g., Dryas, alder) facilitate colonization by other species.
Dunes are among the _________ landforms on Earth; changes happen fast and plants must adapt quickly to new environments.
least stable
How do we study succession?
Most straight-forward method:
1. Establish permanently
marked plots in an area recently affected by disturbance.
2. Record community variables (e.g., species abundance, diversity, biomass).
3. Re-measure variables in exact same plots over time.
4. Compare variables between these different plots.
_______ plays a more dominant role later in succession.
Competition
In mid- to late-successional stages, an array of both ____________ interactions are operating.
positive and negative
As succession progresses, ________________ species begin to dominate.
larger, slow- growing, and long-lived
__________are often important drivers of early succession, especially when physical conditions are stressful.
Facilitative interactions
Succession is driven by _____________
many mechanisms.
