Transcript Disturbance - Iowa State University

Disturbance
Biology/Env S 204
Spring 2009
Disturbance
• Definition: any process or condition
external to the natural physiology of
living organisms that results in the
sudden mortality of biomass in a
community on a time scale
significantly shorter than the
accumulation of the biomass.
Disturbance
• Examples:
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Fires
Windstorms
Floods
Extreme cold temperatures
Treefalls
Epidemics
Bulldozers
Disturbance
• Might kill a few, many, or all of the
organisms in a community, or may kill
a portion of a single individual (as
often happens with plants)
• Not all mortality results from
disturbances (e.g., natural death of
individuals from aging)
Disturbance
• Five main factors:
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Intensity
Frequency
Timing
Area
Effect on resources
Disturbance
1) Intensity: proportion of total biomass
killed; often inversely related to
frequency
2) Frequency: number of disturbances in a
given time interval; determines how far
succession can proceed; annual patterns
(e.g., storms, temperatures, precip) can
give highly predictable frequencies for
certain types of disturbances such as
fires or floods
Disturbance
3) Timing: coincidence of the disturbance
with important cycles or events in the
ecosystem affected by disturbance; e.g.,
prairie fire in spring has different effects
on species composition and nutrient cycling
than a fire in the fall
4) Disturbance area: absolute and relative
size of disturbance area and the shape
have important effects on
recolonization/succession
Disturbance
5) Resource Availability: immediate
mortality is the most dramatic
effect, but most important usually
are the longer-term consequences for
resource availability (e.g., nutrients,
light)
Disturbance
• Some level of disturbance is normal
• Diversity tends to increase at intermediate
levels of disturbance
• Disturbance in ecological time (succession)
creates a mosaic called patchiness that
promotes heterogeneity on
community/landscape scales
• Heterogeneity also results from variation in
topography, geology, etc. but these are
mechanistically independent
Disturbance: Heterogeneity
• From a conservation perspective,
heterogeneity is desirable because the
greater it is, the more niches are
available
• Therefore important to understand
the natural disturbance regime
• Also important to understand effect
of disturbance on different species
Fragmentation
• Species-Area Curve in reverse
• Reduction in total amount of a
habitat, also quality of habitat
changes (smaller, more isolated
patches)
• End result is often a patchwork of
small, isolated natural areas in a sea
of developed landscape
Fragmentation
• Results in biotic impoverishment:
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Local extinctions or extirpations
Smaller populations
Loss of genetic diversity
Shifts in composition and abundance
patterns
• As populations become more isolated,
dispersal ability becomes critical
Fragmentation
If patchiness is good, why is humangenerated fragmentation bad?
Fragmentation
1) Naturally patchy landscape had rich
internal patch structure vs. simplified
patches in a fragmented landscape
2) Less contrast between patches in a
natural landscape vs. greater contrast in
a fragmented landscape
3) Certain features of fragmented
landscapes pose specific threats to
populations and species
Fragmentation
If patchiness is good, why is humangenerated fragmentation bad?
Less continuity
Less complexity
Higher contrast
in artificial/fragmented landscapes
Biological Consequences of
Fragmentation
1) Extirpation/Extinction
2) Barriers and Isolation
3) Exclusion
1) Extirpation/Extinction
• Extirpation = elimination of a species
in a given area but with populations of
that species remaining elsewhere
• Extinction = total elimination of a
species
1) Extirpation/Extinction
• Direct elimination of populations or
species in the areas where habitat is
destroyed
• Rare endemic species with narrow
distributions are most vulnerable, but
even common species may not be
immune
• Species exploited or persecuted by
humans (for food, medicine, fear)
1) Extirpation/Extinction
• What if a keystone species is
eliminated?
• Direct losses may result if destroyed
habitat contained specialized
resources (e.g., destruction of
wetlands resulting in population
crashes of wading birds, amphibians)
2) Barriers and Isolation
• Insufficient fragment size to sustain
populations (e.g., grazers)
• Restriction of movement often leads
to reduced genetic variability,
especially in species with poor
dispersal ability
• Restriction of movement also affects
species that need a mix of different
habitats at different life stages
2) Barriers and Isolation
• Not just a problem in terrestrial
habitats (dams may block access by
migratory fishes)
• The smaller and more isolated the
population, the greater the risk of
extinction
3) Exclusion
• Change in quality of habitat may exclude
species
• Fragmentation produces greater contrast
between patches leading to intensified edge
effects
• Some species require “interior” habitat
• Other species may be attracted to edges
(for food) but then suffer higher rates of
nest predation
3) Exclusion
• Some species are adapted to edge
conditions; management for these
species may negatively affect total
biodiversity (e.g., increase in deer
populations)
Fragmentation—Summary
• Fragmentation results from habitat
destruction and is a reality of the
modern world
• Species losses associated with
fragmentation occur for a variety of
reasons
• Loss of genetic diversity is a common
result
Fragmentation—Summary
• Diversity may increase through the
introduction of exotic species, which
tend to invade disturbed or
fragmented habitats more easily, but
competition from exotics may
eventually reduce overall biodiversity
• Threat of global warming and climate
change is especially ominous
Invasive Species
• Defined as species introduced from
somewhere else (non-native or exotic)
that compete aggressively with natives
• Continuing introduction of non-natives
has great implications for native
communities and conservation
• Long-distance dispersal can happen
naturally but is not all that common
Invasive Species
• Deliberate or accidental introduction of
species has been happening since humans
have been traveling extensively
• Many examples of disastrous invasions but
not all introductions result in disaster
• Effects of invasions depend a great deal on
which species and which communities are
involved
Invasive Species
• Successful invaders tend to be
opportunistic, have broad diet, high
genetic variability, good dispersal
ability, among other things
• Invadable communities tend to have a
low diversity of native species, be
disturbed, lack species similar to the
invader, among other things
Brown Tree Snake
On Guam
Evidence shows
that it drove
10 species to
extinction on the
island; 2 of these
were endemic.
Global warming/climate change
• Global warming—increase in
temperatures due to the build-up of
greenhouse gases (CO2 emissions) and
other carbon sources
• Effects—melting of glaciers and ice
caps, rise in sea levels, changes in
weather patterns, changes in weather
intensity, higher levels of
disturbance, loss of biodiversity, etc.
Global warming/climate change
• Recent Intergovernmental Panel on Climate
Change report indicates at least 90%
chance that most warming since 1950 from
continuing emissions of CO2
• CO2 is the #1 contributor (40%), but black
carbon is now recognized as a significant
contributor, responsible for ca. 18% of
current global warming
Global warming/climate change
Black carbon is soot, mainly from cooking stoves.
Global warming/climate change
• Examples of effects of global
warming/climate change on
biodiversity:
– Coral bleaching
– Sky islands in the southwestern U.S.
– Sundarbans in India and Bangladesh
Coral bleaching
• Generally attributed to higher surface
water temperatures
• Heat and increased UV cause the corals to
expel their symbiotic algae (zooxanthellae)
leaving the white coral skeleton
• Can recover after short periods of
bleaching but longer periods may cause
death or higher susceptibility to disease
Sky Islands
• Sky islands are forested mountain
ranges separated by
deserts/grasslands
• Perhaps the best known system is in
the southwestern U.S.
• High diversity, fairly high endemism
• Fire-maintained ecosystem
• Provided much early inspiration for
Aldo Leopold
Ganges delta
Sundarbans Forest
Originally ca. 16,700 km2
Now ca. 4,100 sq km
1 of 3 largest
mangrove forests