Module #8: Disturbance

Question 1

Disturbance

Answer 1

Any relatively discrete event (has a start and end date) in time that disrupts an ecosystem, community, or population structure, and changes resource availability or the physical environment.

Examples include fire, wind, or pest outbreaks.

Question 2

Scale

Answer 2

The areal and temporal extent of a disturbance

*minimum size depends on the size and home range of the organism

Question 3

Endogenous disturbance

Answer 3

A disturbance driven by inherent biological properties of organisms/ a community (example: the fall of trees due to senescence).

*usually a local scale, does not alter the entire landscape vegetation.
*associated with uneven age distribution and negative exponential age curve.

Question 4

Exogenous disturbance

Answer 4

A disturbance driven by the external environment.

*usually broader in scale, can alter landscape vegetation
*associated with even age distribution and unimodal curve

Question 5

Frequency

Answer 5

The mean number of disturbance events per time period.

*often displayed as a decimal, for example, 0.25 is one hurricane event every 4 years

Question 6

Return interval / turnover time

Answer 6

The mean time between disturbances (opposite of frequency).

Predictability: variability in return interval

Question 7

Magnitude Intensity

Answer 7

Physical force or energy generated during a disturbance (example: heat released or wind speed)

Question 8

Magnitude Severity

Answer 8

Impact on the organism, community, or ecosystem

Question 9

Chronic disturbance

Answer 9

High frequency, low magnitude event

Question 10

Acute disturbance

Answer 10

Low frequency, high magnitude event

Question 11

Synergism

Answer 11

Two or more agents working together that produce an outcome not obtainable independently.

(ex: like, a hurricane and flood create xyz together)

Question 12

Ecological surprise

Answer 12

Substantial and unanticipated changes in the abundance of one or more species that result from previously unsuspected processes

Question 13

Effect of disturbance on organizational hierarchy

Answer 13

Individuals- can be killed or damaged

Population- may be eliminated or reduced in size. May respond by high reproduction or growth rates due to increased resources

Community/ecosystem- potential temporary loss in biodiversity and decreased productivity, but may result in higher levels of both during recovery.

Question 14

Disturbance and diversity

Answer 14

disturbance may help maintain a higher diversity, such as clearing off new areas for new organisms to colonize. R-strategist and invasive species favor disturbances

Question 15

Intermediate Disturbance Hypothesis (IHD)

Answer 15

Species diversity is low at low disturbance frequency because of competitive exclusion. Species diversity is higher at intermediate disturbance frequency due to a mix of good colonizer and good competitor species. Species diversity is low at high disturbance because only good colonizers or highly tolerant species can persist.

Question 16

How does the IDH model work to spatially and temporally influence diversity?

Answer 16

Temporally- “intermediate time scales” - traditional IDH concept focuses on frequency of disturbance. Does disturbance happen before recovering species are big enough to withstand the next event, colonize, etc?

Spatially- between patches- many disturbances create a mosaic of disturbed and undisturbed patches, so r-strategist in different areas, dispersal is important.

Spatially-within patches- many species, mostly plants, go through life stages that can withstand disturbance (ex: seeds)

Question 17

Fire

Answer 17

Rapid oxidation of a fire source (carbon-based), requires oxygen, fuel, and an ignition source

Question 18

Dead vs. living fuel

Answer 18

dead fuel burns better. So, the type of vegetation can determine how a fire burns.

Question 19

Fire production vs. decomposition

Answer 19

Fire is like decomposition, but much faster. The relationship of the rates of production and decomposition influence fuel availability.

Question 20

Influences on fire

Answer 20

-Both production and decomposition of fire are greater in warmer, wetter environments. In cold and dry environments, there might be slow decomposition but productivity might stay high.

-Herbaceous areas in the understory can cause frequent fire because they are highly productive and dry out quickly.

-surface area/volume of fuels

-presence of volatile chemicals, like found in chaparral.

-atmospheric conditions

-presence of ignition source

Question 21

Atmospheric conditions on fire

Answer 21

Dry and hot: dries fuel
Wind: helps dry fuel and spread fire
Ignition source: lightning or human made fire

Question 22

Fire types

Answer 22

1) Ground fire: Most common when the organic layer in the soil is dry. A wetland with a lot of organics that dries in drought (peat fire). Low intensity and low temperatures that smoke a lot.

2) Surface fire: consumes fuels on the soil surface and upward only a few meters at most, but not into canopy. More frequent than canopy fires, above ground parts of small plants will be consumed. Common in prairies and savannahs.

3) Canopy fire: Fire that burns up into the canopy. Most intense and severe. Dominates boreal forrest and most forests in the mountain western US.

Question 23

Landscape-scale fire effects

Answer 23

Fire creates heterogeneity or patches in the landscape, resulting in:

-increased diversity in landscape mosaic
-decrease in disease spread and pest outbreaks
-fuel accumulation mosaic that affects future fire spread
-fire may help maintain prairie-forest ecotone or woodland-steppe ecotone

Question 24

Ecotone

Answer 24

Transition area between two different types of biomes

Question 25

Mangrove ecotone in Everglades

Answer 25

Ecotone begins in coastal marsh dominated by sawgrass and terminates in mixed mangrove forest. After a fire, they found that the ecotone shifted 20 meters towards the river.

Question 26

Ecological effects of fire

Answer 26

1) Nutrient availability: release or mineralization of nutrients locked in biomass. Also, nitrogen compounds suffer volatilization, which can bring in nitrogen fixers to make the nitrogen more usable by other organisms.

2) Light regime and microclimate: greater light availability creates warmer temps during the day, and colder temps at night because tree cover is gone or thinned, so microclimate is more extreme.

3) Soil stability: erosion will affect nutrient availability and water availability (ability of soil to hold water).

4) Moisture and water availability: water at the surface tends to dry out because of higher light, causing high evaporation rates

5) Dry matter accumulation: After the fire, the fuel build-up may set back to 0, carbon is recycled.

Question 27

Fire supression

Answer 27

The prevailing policy in the settled US in the early-late 20th century. A lot of changes were experienced in areas that usually burned on a daily basis.

Effects of fire supression:
-changes in composition caused late successional species
-nutrient availability slowed cycling
-fuel accumulation changed fire intensity and severity
-altered light regime
-increase in pest and disease threat
-change in scale of disturbance
-change in establishment ability and dispersal

Question 28

Cone serotiny

Answer 28

Fire stimulates seed release (such as in chaperral)

Question 29

Mineral seedbed from fire

Answer 29

Litter layer might be burned off, so plant can get seed and roots down faster, which many fire-adapted species need.

Question 30

Vegetative propagation from fire + apical dominance

Answer 30

Once fire kills or injures the above ground parts, underground parts such as roots and runners are stimulated to produce more underground shoots or new above ground shoots

Apical dominance: All effort is put in one trunk or shoot upward, and after a fire this tendency may change to multiple trunks or shoots.

Question 31

Wind

Answer 31

Wind damage results from a number of event types:

-tornadoes
-hurricanes
-gale
-wind storm

*gusting does the most damage, and wind not consistent with the prevailing wind direction also does the most damage.

Question 32

Types of damage from wind

Answer 32

-uprooting, blowdown, windthrow
-branch breakage
-defoliation
-drying of leaf tissue
-stem/trunk snapping, basal sheer
-indirect damage (domino effect of falling trees)

Question 33

Good impacts of wind disturbance

Answer 33

-branch breakage can stimulate growth due to more light on the forest floor
-uprooting of a tree may result in sprouting. Uprooting of canopy trees may be the principal way species richness is maintained. A canopy gap provides sunlight, and encourage the release of suppressed individuals.

Question 34

Pit and mound topography

Answer 34

Uprooting creates a pit where the roots were located, and a mound of topsoil dirt that was attached to the roots. This creates microclimates and exposes nutrients and affects soil moisture.

Question 35

Biotic influence on wind damage

Answer 35

-size of plant: smallest sizes are protected, young large trees are most damaged because they were not preconditioned. Older large trees will be less damaged because of preconditioning and previous thinning.

-deciduous vs. coniferous: higher percent of uprooting than snapping in pines and vise versa for hardwoods

-forest density: increased damage due to chain rxns in dense areas, or decreased damage in other areas where weak trees were already removed

-pathogens or pests: more vulnerable to the disturbance

Question 36

Abiotic influences on wind damage

Answer 36

-soil structure
-topography
-proximity to ocean
-type of storm

Question 37

Biological response to wind damage

Answer 37

-advance regeneration layer released: a midstory tree that was suppressed before disturbance starts to grow quickly after
-resprouting of damaged trees (most important for wind recovery)
-seed germination in changed understory (mostly important for fire)
-lateral growth of gap edge trees
-increased vulnerability of trees to subsequent disturbances if the tree is damaged or weakened`

Question 38

Pathogens: Chestnut blight

Answer 38

Introduced in 1904 on Asian chestnut, but ended up killing the entire American chestnut tree population in 40 years.