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What questions do ecologists
ask about communities?
Structure
How many species? How do they
compare in abundance? Who eats
who?
Dynamics
How do changes in abundance of
one species translate into changes
in other species?
Function
How does energy flow through
trophic levels? How are nutrients
cycled and retained?
What are communities?
●
●
Set of all populations in an enclosed area
Movement of plants and animals and
multiple scales of organization
complicate definition
Measures of community structure
●
●
●
Trophic structures
Relative abundances
Species numbers
Trophic Structure
●
Trophic structure
—
—
Is the feeding relationships between
organisms in a community
Is a key factor in community dynamics
●
Food chains
Quaternary
consumers
Carnivore
Carnivore
—
Link the trophic
levels from
producers to top
carnivores
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Zooplankton
Herbivore
Primary
producers
Plant
Figure 53.12
A terrestrial food chain
Phytoplankton
A marine food chain
Food Webs
●
Smaller toothed
whales
Baleen
whales
A food web
—
Humans
Is a branching
food chain with
complex
trophic
interactions
Crab-eater seals
Birds
Sperm
whales
Elephant
seals
Leopard
seals
Fishes
Squids
Carnivorous
plankton
Copepods
Euphausids
(krill)
Phytoplankton
Figure 53.13
●
Food webs can be simplified
—
By isolating a portion of a community that
interacts very little with the rest of the
community
Juvenile striped bass
Sea nettle
Fish larvae
Figure 53.14
Fish eggs
Zooplankton
Limits on Food Chain Length
●
Each food chain in a food web
—
●
Is usually only a few links long
There are two hypotheses
—
That attempt to explain food chain length
●
The energetic hypothesis suggests that
the length of a food chain
—
●
Is limited by the inefficiency of energy
transfer along the chain
The dynamic stability hypothesis
—
Proposes that long food chains are less
stable than short ones
Most of the available data
—
Support the energetic hypothesis
6
Number of species
6
No. of species
5
No. of trophic
links
4
4
3
3
2
2
1
1
0
0
High
(control)
Medium
Productivity
Figure 53.15
5
Low
Number of trophic links
●
Keystone Species
●
Keystone species
—
—
Are not necessarily abundant in a
community
Exert strong control on a community by their
ecological roles
●
Field studies of sea stars
Exhibit their role as a keystone species in
intertidal communities
Number of species
present
—
20
With Pisaster (control)
15
10
Without Pisaster (experimental)
5
0
1963 ´64 ´65 ´66 ´67 ´68 ´69 ´70 ´71 ´72 ´73
(a) The sea star Pisaster ochraceous feeds
preferentially on mussels but will
consume other invertebrates.
Figure 53.16a,b
(b) When Pisaster was removed from an intertidal zone,
mussels eventually took over the rock face and eliminated
most other invertebrates and algae. In a control area from
which Pisaster was not removed, there was little change
in species diversity.
●
Observation of sea otter populations and
their predation
80
60
40
20
0
(a) Sea otter abundance
400
Grams per
0.25 m2
Shows the
effect the otters
have
on ocean
communities
300
200
100
0
(b) Sea urchin biomass
Number per
0.25 m2
—
Otter number
(% max. count)
100
10
8
6
4
2
0
1972
1985
1989
1993
1997
Year
Figure 53.17
Food chain before
killer whale involvement in chain
(c) Total kelp density
Food chain after killer
whales started preying
on otters
Bottom-Up and Top-Down
Controls
●
The bottom-up model of community
organization
—
●
Proposes a unidirectional influence from lower to
higher trophic levels
In this case, the presence or absence of
abiotic nutrients
—
Determines community structure, including the
abundance of primary producers
●
The top-down model of community
organization
—
●
Proposes that control comes from the trophic
level above
In this case, predators control herbivores
—
Which in turn control primary producers
●
Long-term experiment studies have
shown
—
That communities can shift periodically from
bottom-up to top-down
Percentage of
herbaceous plant cover
100
75
50
25
0
0
Figure 53.20
100
200
Rainfall (mm)
300
400
Species in communities vary widely in
abundance
One or a few common
species with many many
rare species
Important concept:
Rare species can be
important in communities:
many weak interactions can
lend stability
Important concept:
Some species there by
accident
Patterns of Rarity
●
Most species
common somewhere
—
●
Source-sink
dynamics lead to
“spill-over” into
nearby habitats and
communities
Some species rare in
all environments
—
Low growth rate or
highly specialized life
history
Species numbers
●
The species number of a community
—
—
Is the variety of different kinds of organisms
that make up the community
Has two components
Species numbers vary widely across
communities
Forest birds
Vascular plants in deciduous forests
Vascular plants in fir forests
●
Species richness
—
●
Is the total number of different species in the
community
Species diversity
—
Is the total number of different species weighted
by their relative abundance
●
Two different communities
—
Can have the same species richness, but
a different species diversity
A
B
C
D
Figure 53.11
A: 25%
Community 1
B: 25%
C: 25%
D: 25%
A: 80%
Community 2
B: 5%
C: 5%
D: 10%
●
A community with an even species
abundance
—
Is more diverse than one in which one or
two species are abundant and the
remainder rare