Lecture 19: Succession and Disturbance

Question 1

Succession

Answer 1

the change in species composition in communities over time, resulting from both biotic (living) and abiotic (non-living) factors.

Question 2

Succession: Change in species
composition in communities over time
(introduced as “___________”)

Answer 2

directional

Question 3

“Superorganism” Hypothesis

Answer 3

• 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.

Question 4

“Individualist” Hypothesis

Answer 4

• 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.

Question 5

Interactions

Answer 5

_________ results in the replacement of one species with another.

Question 6

Primary Succession

Answer 6

• 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

Question 7

Secondary Succession:

Answer 7

• 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

Question 8

“Pioneer Stage”

Answer 8

• Early-stage species

- 1 year after abandonment: grasses and weeds dominate

Question 9

“Intermediate Stage”

Answer 9

-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

Question 10

“Climax Stage”

Answer 10

• late-stage species

- 100 years after abandonment: “Climax Community”

Question 11

Characteristics of a Climax community

Answer 11

• “Final”, late stage of succession (“stable” community)
• Continues to change in “small” ways
• May remain “the same” through time if not disturbed

Question 12

Stochastic events

Answer 12

Unpredictable, random

Question 13

the first to colonize a bare substrate in primary succession or first to colonize a destroyed habitat in secondary succession.

Answer 13

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

Question 14

Characteristics of pioneer species

Answer 14

• 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

Question 15

Provide examples of pioneer species

Answer 15

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

Question 16

Early-arriving species and later-arriving species may be linked in one of three processes:

Answer 16

• 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

Question 17

Discuss example of facilitation

Answer 17

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.

Question 18

Dunes are among the _________ landforms on Earth; changes happen fast and plants must adapt quickly to new environments.

Answer 18

least stable

Question 19

How do we study succession?

Answer 19

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.

Question 20

_______ plays a more dominant role later in succession.

Answer 20

Competition

Question 21

In mid- to late-successional stages, an array of both ____________ interactions are operating.

Answer 21

positive and negative

Question 22

As succession progresses, ________________ species begin to dominate.

Answer 22

larger, slow- growing, and long-lived

Question 23

__________are often important drivers of early succession, especially when physical conditions are stressful.

Answer 23

Facilitative interactions

Question 24

Succession is driven by _____________

Answer 24

many mechanisms.

Question 25

Types of agents of change

Answer 25

• 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.

Question 26

What is an agent of change?

Answer 26

During succession, interactions results in the replacement of one species with another.

Question 27

Intermediate Disturbance Hypothesis

Suggests that species richness and diversity at the local scale peaks at intermediate: (3)

Answer 27

• values of disturbance frequency
• times after a disturbance
• spatial extents of disturbance

Question 28

How do humans increase frequency of disturbances?

Answer 28

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.

Question 29

How do humans decrease frequency of disturbances?

Answer 29

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.).

Question 30

Vegetation changes often lead to changes in…

Answer 30

fauna

Question 31

Animals greatly influence the sequence and timing of succession by

Answer 31

• eating,
• dispersing,
• trampling, and • destroying
vegetation