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Community Ecology
Food webs & top-down vs. bottom-up controls
Outline:
1. History of the food web concept
A. Elton’s pyramid of numbers
B. Lindeman’s estimates of ecological efficiency
2. Structure, composition, and properties of food webs
A. Structure: nodes and links
B. Composition: basal, intermediate, and top predator species
C. Properties
i. cycles are rare
ii. link-scaling law: linkage density constant across webs
iii. connectance decreases as richness increases
a. implication: food web connectance is inversely related to system stability 
diversity/stability debate
iv. food chains are short
v. food chains are shorter in 2D than in 3D habitats
vi. greater environmental constancy leads to greater web connectance
3. Do predators regulate prey or vice versa?
A. HSS - an explanation for "why the world is green"
i. criticisms - Murdoch, Ehrlich and Birch
ii. reply - SSH
iii. what about aquatic systems? Wiegert and Owen
a. Fretwell - alternation of regulatory mechanisms
b. Hairston and Hairston - number of terrestrial vs. aquatic trophic levels
c. Oksanen - influence of primary productivity
4. Top-down vs. bottom-up regulation
Terms/people:
food web (cf. food chain)
link
"why the world is green" (HSS)
trophic cascade (trickle?)
R. Lindeman
Wiegert and Owen
top-down regulation
compartment (subweb)
linkage density
Ehrlich and Birch
Chas. Elton
node
Oksanen
Murdoch
energetic constraint hypothesis
ecological efficiency
pyramid of numbers
Hairston, Smith, Slobodkin (HSS)
Fretwell
donor control
bottom-up regulation
Stuart Pimm
connectance
link-scaling
interval
Joel Cohen
Food web: (see Fig.s 10.1-10.4 in Mittelbach text)
cf. food chain
A food web is the pattern of flows of energy and material among organisms that result when
some organisms eat other living organisms or their parts. Food webs provide a pattern of basic
ecological interactions among species and trophic levels. Food webs describe a pattern of
ecological relationships but do not in themselves provide evidence of ecological processes. Food
webs are useful as descriptions of ecological systems. They have been much-studied in
community ecology.
Charles Elton (1927, Animal Ecology)
"pyramid of numbers" (a.k.a. Eltonian pyramid)
But how do we explain the inverted pyramid seen in aquatic systems?
Lindeman (1942) - ecological efficiency  limits to chain length
Structure of a food web: nodes and links
Composition: basal spp. (producers), intermediate spp. (herbivores, lower predators), top
predators
Properties of food webs (Pimm 1982, Lawton and Warren 1988, Cohen et al. 1989): empirical
evidence limited and mechanisms unclear for many of these! But most are related to ecological
efficiency.
1) cycles (loops) are rare
2) link-scaling law: linkage density is constant across webs
3) connectance (ratio of actual interactions: possible interactions in a food web) decreases as
species richness increases
c=L/{[S(S-1)]/2}
where c=connectance, L=observed number of links, S=number of nodes (if S spp., then S-1 links
are possible)
0<c<1
Implications: food web connectance is inversely related to system stability
Species richness / stability debate
4) food chains are short due to energetic constraint hypothesis (Jenkins et al. 1992)
5) food chains are shorter in physiognomically simpler environments
6) effects of environmental variation
BUT...these properties/patterns were based on:
- mostly studies on vertebrates
- mostly simple webs (since they are easier to study, easier to observe connections): "artistic
convenience" (e.g. Paine’s keystone system of 7 nodes represented a community of over 300
species!)
And keep in mind: food webs can and do change over time.
Do predators regulate prey or vice versa?
bottom-up food web regulation vs. top-down food web regulation
Hairston, Smith, Slobodkin 1960 (HSS 1960) - "why the world is green"  generated
controversy
criticisms - Murdoch (1966), Ehrlich and Birch (1967): idea is untestable, world may be green
but is not entirely edible
reply - SSH (1967)
What about aquatic systems? They don't appear to be green (Wiegert and Owen 1971)
explanations for differences between terrestrial and aquatic systems:
Fretwell (1977) - alternation of regulatory mechanisms; Hairston and Hairston (1993)
applied this to terrestrial vs. aquatic trophic levels (different numbers of trophic levels)
And another thing: "it makes no sense to ask why the world is green while standing in the
middle of the Atacama desert or the northern shores of Greenland" (Oksanen 1988), i.e., effects
of predators or competitors varies along a gradient of primary productivity. In
unproductive habitats herbivores are rare because there is not enough forage to support them. At
the highest productivity there will only be few herbivores because herbivores are limited by
predators. At intermediate productivity, plants are limited by herbivores because there
insufficient herbivores to support large numbers of predators. Consequently, plants should
compete in the least and the most productive ecosystems. The most productive systems are
green but would not be so if predators were removed. Herbivores compete in ecosystems of
intermediate productivity, and predators compete in ecosystems of highest productivity.
Evidence to support the Oksanen model:
1. Relationship between biomass and productivity matches model predictions.
2. Herbivore removal experiments almost always lead to increases in plants.
3. Predictions of the model have been confirmed in experimental studies: Plants should
compete in habitats of low or high productivity only. Thirty-one plant competition
experiments in green worlds (forests, meadows) clearly demonstrated competition; 6 did
not. In intermediate productivity ecosystems (grasslands, arid areas), 4 demonstrated
competition, 25 did not. In low productivity systems (deserts), 4 demonstrated
competition and none failed to do so.
Ultimately, all food webs depend on producers: therefore, "plants have primacy" (Power 1992)
 called donor control (Strong 1992)
Bottom-up (donor control) -Organisms on each trophic level are limited by the resources available from the level below.
-Logically, plants have primacy in biotic systems because a system that lacks primary producers
lacks resources for consumers to comprise a trophic structure
-But, given sufficient resources to support a community, the relative effects of resources and
consumers should be critical in understanding the processes that structure communities.
-Bottom up forces include level of primary productivity and availability of light or nutrients.
Top-down (trophic cascade) -Top-down forces include predation and herbivory.
-are more like "trophic trickles" than cascades (Strong 1992), especially in speciose communities
because the "trophic tangle precludes cascades"
-Successive trophic levels alternate between consumer limitation and resource limitation.
-Primary produces will be resource limited in chains that contain odd numbers of trophic levels
(i.e., predators reduce herbivores[and herbivory] in a three-trophic level community)
-Primary producers will be consumer limited (i.e., competition among consumers) in habitats
that contain an even number of trophic levels.
-Terrestrially, trophic cascades with top-down dominance are restricted to systems of low species
diversity.
-But in many places, predators have been extirpated (Terborgh et al. 2001 call this an “ecological
meltdown”).
-The number of trophic levels may be constrained by the level of primary productivity in a
community.
-Oksanen et al. (1981) formalized this idea by arguing that effective top-down forces are only
likely to occur in habitats with high primary productivity, since low productivity habitats do not
have functional upper trophic levels.
There is no clear pattern with respect to the kinds of terrestrial systems likely to exhibit top-down
or bottom-up trophic cascades. Instead, they are scale-dependent (Gripenberg and Roslin 2007).
Something to think about: recall the lecture on the importance of biodiversity to community
health/stability. After learning about food webs today, is biodiversity a good assay of community
health or stability?
For MUCH more information on the complex topic of food webs, check out www.foodwebs.org.
But be prepared: Even Robert May (1999) called this “Heady stuff, verging on the Newtonian.”
Next time: scale
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