Module #8: Disturbance Flashcards
Disturbance
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.
Scale
The areal and temporal extent of a disturbance
*minimum size depends on the size and home range of the organism
Endogenous disturbance
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.
Exogenous disturbance
A disturbance driven by the external environment.
*usually broader in scale, can alter landscape vegetation
*associated with even age distribution and unimodal curve
Frequency
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
Return interval / turnover time
The mean time between disturbances (opposite of frequency).
Predictability: variability in return interval
Magnitude Intensity
Physical force or energy generated during a disturbance (example: heat released or wind speed)
Magnitude Severity
Impact on the organism, community, or ecosystem
Chronic disturbance
High frequency, low magnitude event
Acute disturbance
Low frequency, high magnitude event
Synergism
Two or more agents working together that produce an outcome not obtainable independently.
(ex: like, a hurricane and flood create xyz together)
Ecological surprise
Substantial and unanticipated changes in the abundance of one or more species that result from previously unsuspected processes
Effect of disturbance on organizational hierarchy
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.
Disturbance and diversity
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
Intermediate Disturbance Hypothesis (IHD)
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.
How does the IDH model work to spatially and temporally influence diversity?
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)
Fire
Rapid oxidation of a fire source (carbon-based), requires oxygen, fuel, and an ignition source
Dead vs. living fuel
dead fuel burns better. So, the type of vegetation can determine how a fire burns.
Fire production vs. decomposition
Fire is like decomposition, but much faster. The relationship of the rates of production and decomposition influence fuel availability.
Influences on fire
-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
Atmospheric conditions on fire
Dry and hot: dries fuel
Wind: helps dry fuel and spread fire
Ignition source: lightning or human made fire
Fire types
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.
Landscape-scale fire effects
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
Ecotone
Transition area between two different types of biomes
Mangrove ecotone in Everglades
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.
Ecological effects of fire
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.
Fire supression
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
Cone serotiny
Fire stimulates seed release (such as in chaperral)
Mineral seedbed from fire
Litter layer might be burned off, so plant can get seed and roots down faster, which many fire-adapted species need.
Vegetative propagation from fire + apical dominance
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.
Wind
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.
Types of damage from wind
-uprooting, blowdown, windthrow
-branch breakage
-defoliation
-drying of leaf tissue
-stem/trunk snapping, basal sheer
-indirect damage (domino effect of falling trees)
Good impacts of wind disturbance
-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.
Pit and mound topography
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.
Biotic influence on wind damage
-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
Abiotic influences on wind damage
-soil structure
-topography
-proximity to ocean
-type of storm
Biological response to wind damage
-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`
Pathogens: Chestnut blight
Introduced in 1904 on Asian chestnut, but ended up killing the entire American chestnut tree population in 40 years.
