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.
Types of agents of change
- Stress: A factor that reduces the growth or reproduction of individuals (e.g., reduced nutrients, competition).
- Disturbance: An event that injures or kills some individuals and creates opportunities for other individuals (e.g., sufficiently large so intermediate replacement by neighbors is not possible).
- Stressors and disturbances can be abiotic or biotic.
What is an agent of change?
During succession, interactions results in the replacement of one species with another.
Intermediate Disturbance Hypothesis
Suggests that species richness and diversity at the local scale peaks at intermediate: (3)
- values of disturbance frequency
- times after a disturbance
- spatial extents of disturbance
How do humans increase frequency of disturbances?
Can INCREASE the frequency, magnitude, and intensity of disturbance (e.g., farming)àareas are…
• always dominated by early successional species
• depleted of biomass, soil organic matter, and
nutrient pools.
How do humans decrease frequency of disturbances?
Can DECREASE the frequency, magnitude, and intensity of disturbance (e.g., fire suppression)àareas are…
• always dominated by late
successional species
• more susceptible to catastrophic
disturbance (e.g., recent fires in western U.S.).
Vegetation changes often lead to changes in…
fauna
Animals greatly influence the sequence and timing of succession by
• eating,
• dispersing,
• trampling, and • destroying
vegetation
