Transcript 21 Com Struc-Develop 2010

Fall 2010 IB Workshop Series
sponsored by IB academic advisors
Anatomy of a PreHealth Student
Wednesday, Nov. 3
4:00-5:00pm 217 Noyes Lab
Career Center staff will present a workshop designed to help anyone
who has decided to pursue a health profession. Come learn what
types of experiences, courses, characteristics, and skills are required
to be successful in gaining admission to various health professional
programs.
Ch 18: Community Structure
Objectives
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Spatial
Is the community a natural unit?
Are communities in equilibrium? stable? resilient?
Functional
How strong are connections among species?
How are food webs measured/compared?
Do complex food webs aid stability?
What controls abundance within trophic levels?
Do keystone species aid stability?
Do alternative stable states exist?
Temporal
How do communities change following
disturbance?
What is a community? group of interacting
species (populations) that co-occur.
‘Community’ can be studied in different
dimension:
• Spatially defined-->focus on constancy of
species
• Functionally defined --> focus on
interactions
• Temporally defined-->focus on recovery of
species following disturbance
Spatial: How many are here? How abrupt are
boundaries of these communities? Would
answer differ for coyote vs. insect?
Physical conditions that change sharply
may cause sharp community boundaries =
ecotone.
How many
communities
are here?
How are they
defined?
Is the community a natural unit of
ecological organization?
Closed vs. open community structure…
Holistic
(closed)
Individualistic
(open)
Gradient analysis (Whittaker) (UI!):
quantify each species’ abundance along a
physical gradient…
Continuum concept: Within broadly defined
habitats, species replace one another continuously
along gradients of physical conditions.
How are these species distributed along a
moisture gradient?
• Open or closed?
• Ecotones?
• Continuum?
• Most communities
are open…
• species distribute
independently of
other species.
Question: Do identical communities
develop in identical environments?
•If communities are holistic (closed)
• Experimental set-up?
•prediction: then identical plankton
communities will develop in all ponds.
•alternative prediction: Different plankton
communities will develop in different
ponds.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
2. Examine water samples;
3. ID each species present in each pond.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
What is
conclusion?
Are communities
predictable?
• What is the ‘nature’ of the community?
• How tightly connected are species?
• Are communities stable or transient?
OPEN
Later models
CLOSED
Non-equilibrium
Equilibrium
Redundancy
Rivet
Community: functionally defined
How strong are the connections among
species in a community?
• Rivet model
•
tight linkage in ‘web of life’
•
obligate associations of species
•
or obligate exclusion of species
• Redundancy model
•
loose ‘web of life’
•
most species have no impact on other
species
•
species and ecological processes are
redundant
Feeding relationships organize
communities in food webs.
Communities of similar species richness
can have very different food webs.
***What changes with increased food web
complexity? What doesn’t change?
# trophic levels
3
4
6
Variables that quantify food webs:
• # species
• # guilds (groups of species with different
feeding or foraging ecology)
• Total # feeding links
• # feeding links per species
• Connectance = # interactions/total possible
•
• Linkage density = # interactions/# species
Explain the three approaches to depicting
trophic relationships.
Seabirds transfer nutrient-rich feces -->
enhance plant productivity. What happens
when fox are absent (left) vs. present (right)?
Keystone species: non-redundant, key
species in maintaining community
stability and diversity
•
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•
•
Can be:
plants
herbivore
top predator
Critical to conserve in community
• If a beetle is the keystone species,
• then removal of this species will result in
lower species diversity.
What influences abundance within each trophic
level?
H1: predation (top-down control)
H2: production by plants (bottom-up control).
Trophic
cascade
links all
trophic
levels.
Bottom-up
Top-down
Trophic cascade: indirect effects of
consumer-resource interactions extended
through additional trophic levels of the
community.
• top-down effect
• consumers depress size of trophic
level immediately below them, which
indirectly increases populations two
tropic levels below.
How does the presence/absence of fish in
ponds influence dragonfly densities?
Prey of adult dragonflies include pollinators.
What is indirect effect of fish on pollinator
visits?
Concept map showing direct (solid) and
indirect (dashed) effects of fish on species
in and around ponds.
***Does greater complexity of food web
increase community stability?
Constancy/Resilience?
***Does food web complexity lead to
increased community stability?
• Pro:
• alternative resources--->less dependent on
fluctuations in any one resource
• Redundancy of species--->removal has
little effect
• energy can take many routes --> disruption
of one pathway shunts more energy to
another
• Con:
• more links may create pervasive,
destabilizing time lags in population
processes
Question: Which are more stable
(resilient) when disturbed: species-rich
or less-diverse communities?
• Hypothesis: Species richness determines
resilience to disturbance,
• Experimental Design?
• Prediction: Plots with more species before
drought are more resilient to change.
• Operational variable…
resilience = no change in plant biomass
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
What is the conclusion?
How does species diversity affect resilience
(recovery from disturbance) of community?
Community:
temporally defined
How does nutrient level change this
community between alternative stable states?
***Do these data support a stable
(equilbrium) or transient (non-equilbrium)
view of the community?
Disturbance is a natural occurrence in a
community ….Mt. St. Helens 1980
Succession: Sequence of community changes
initiated by exposure of new substrate or
habitat disturbance .
Primary succession on bare substrate;
no soil;
no seed bank
Retreating
glacial
debris
Lava flow
Primary succession begins in newly formed
habitat with no history of plants or soil.
IB 203 in 1905
Earliest studies of succession
Disturbance
initiates
secondary
succession
Change in land use
initiates ‘old field’
succession
Species replacement --> end in climax community