BIOS 3010: Ecology
Lecture 21: Food Webs:
•  Lecture summary:
–  Indirect effects:
•  Processes.
•  Keystone species.
–  Top-down & bottomup effects.
–  Community stability:
•  Classical view.
•  Modern view.
–  Properties of food
webs.
Fig 6-27, Miller (2002) Essentials of Ecology
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 1
2. Indirect effects in food webs:
•  In addition to direct effects such as competition,
predation, herbivory and parasitism, indirect effects
are also important in structuring communities.
–  Such as exploitation (scramble) competition mediated via the
resource, and consumer-mediated coexistence where predation
alters the competitive status of species in the next trophic level
down.
•  For example, experimental removal of a species from a
food web can reveal its role in structuring a
community.
•  But unexpected, indirect effects appear to occur in a
third of 100 experimental studies of predation.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 2
3. Indirect effects in food webs:
•  Keystone species:
–  A species whose removal would produce a significant
effect (extinction or a large change in density) in at least
one other species and significant changes that spread
throughout a food web.
–  The keystone locks other components together and its
removal leads to collapse of the structure producing a
community with a different species composition and
different physical appearance:
•  “keystone” vs “foundationstone”? at any trophic level.
–  Exclusion of bird predators in Fig. 20.3 showed their
complex direct and indirect effects on community
structure of a rocky shore.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 3
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4. Top-down or bottom-up control
of food webs?:
•  Top-down control:
–  Invokes the control of prey numbers by predators and consequent
benefits to plants.
•  Hairston, Smith & Slobodkin, 1960 - the world is green so
predators must control herbivores.
•  Bottom-up control:
–  Invokes energy and nutrient flux from lower to higher trophic levels.
•  Resource availability is the key process and so competition is more
important than predation.
•  Murdoch's view that the world is prickly and tastes bad.
•  In simpler communities bottom-up control may be most likely, but
both bottom-up and top-down control appear as trophic levels
increase (Figs. 22.2 & 20.2).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 4
5. Community stability and foodweb structure:
•  Resilience:
–  Speed with which a community returns to its former state after
perturbation and displacement from that state.
•  Resistance:
–  Ability to avoid displacement in the first place (Fig. 20.7).
•  Stability:
•  Local stability - tendency to return to original state after a small
perturbation.
•  Global stability - tendency to return to original state after a large
perturbation.
•  Fragility:
•  Dynamically fragile - stable over a narrow range of environmental
conditions.
•  Dynamically robust - stable over a wide range of environmental
conditions.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 5
6. Classical versus modern ideas
of community stability:
–  Classical idea:
•  Believed up to 1970 that increased complexity within a
community leads to increased stability:
–  Elton, 1958 see Table 22.1; MacArthur, 1955.
–  Modern idea:
•  Since 1970 theoretical work of May and Pimm indicated
that increase in number of species, increase in
connectance (fraction of all possible pairs of
species that interacted directly), and increase in
interaction strength all tend to decrease stability.
•  All measures of increased complexity that lead to
instability (reverse of classical idea).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 6
2
7. Classical versus modern ideas
of community stability:
•  The modern idea is corroborated in Fig. 22.6
(stability decreases with complexity) for all
species under top-down control (toppredators removed).
•  But the classical idea is upheld (stability
increases with complexity) under bottomup control (basal species removed).
–  Much as envisaged originally by MacArthur!
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 7
8. May s criterion for community
stability:
•  May (1972) established stability criterion of:
β(SC)1/2 < 1
•  where, β = average strength of interaction between
species; S = the number of species; C =
connectance of the web (fraction of all possible
pairs of species that interacted directly)
–  Thus increasing S leads to decreased stability
unless there is a decrease in C or β
•  Thus S x C should be approximately constant to retain
stability as in real communities Fig. 20.9a.
•  But see exceptions in 20.9b-d.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 8
9. May s criterion for community
stability:
•  Overall it does now appear that complex,
fragile communities in relatively constant
environments (e.g. tropical forests: more K-selected +
high resistance, but low resilience) are more
susceptible to outside disturbance than the
simpler, more robust communities of areas
with more frequent disturbance (e.g. temperate
regions: more r-selected + low resistance, but high
resilience).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 9
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10. Empirical properties of food webs:
•  Why are food chains generally short at
between 2 & 5 trophic levels? (Fig. 20.14)
–  Energy flow hypothesis:
•  Energy transfer inefficiency limits trophic levels (1-30%)
–  Dynamic fragility:
•  Extinction of top predators more likely in long food
chains and return times longer (Fig. 22.11).
–  Constraints on predator design & behavior:
•  Limit to predator size (is this why Tyrannosaurus rex or
Carcharodontosaurus went extinct?).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 10
11. Empirical properties of food webs:
•  Omnivory was at first thought to be
destabilizing.
•  But its prevalence is high and it may
stabilize
– (Fig. 20.17).
•  Predator:prey ratios also appear to
be constant in food webs
– (Fig. 22.13).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 11
BIOS 3010: Ecology
Lecture 21: slide 12
Figure 20.3:
Direct and
indirect
influences of
predatory
seabirds on
intertidal
community
structure
Dr. S. Malcolm
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Figure 22.2 (3rd ed.): Bottom-up (B) or top-down (T)
influences in communities with 1, 2, 3, or 4 trophic levels
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 13
BIOS 3010: Ecology
Lecture 21: slide 14
BIOS 3010: Ecology
Lecture 21: slide 15
Figure 20.2:
Effect of salinity on
Great Salt Lake
ecosystem structure
Dr. S. Malcolm
Figure 20.7:
Aspects of
community
stability
Dr. S. Malcolm
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Table 22.1
(3 ed.):
rd
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 16
Figure 22.6 (3rd ed.) : Species-deletion stability
against connectance for 6-species models.
predators
herbivores
plants
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 17
Figure 20.9: Connectance (C) against species richness (S) for (a) 40 mixed
webs, (b) 95 insect-dominated webs, (c) seasonal pond webs, (d)
food webs in swamps and streams in Costa Rica & Venezuela
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 18
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Figure 20.14: Food chain lengths in a community matrix
for an exposed intertidal rocky shore in Washington
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 19
Figure 22.11 (3rd ed.): Frequency (F) of return times
(RT) for 2-, 3-, & 4-trophic level models.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 20
Figure 20.17: Prevalence of omnivory in
glacial lakes greater than in Briand webs.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 21
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Figure 22.13 (3rd ed.): Numbers of prey species against numbers of
predator species in (a) freshwater invertebrate communities, (b)
model community with species subject to Lotka-Volterra dynamics.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 21: slide 22
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