Community Ecology

Community Ecology

Food webs & top-down vs. bottom-up control

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 connectedness is inversely related to system stability

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) Chas. Elton node

link Oksanen Murdoch

"why the world is green" (HSS) energetic constraint hypothesis

trophic cascade (trickle?) ecological efficiency pyramid of numbers

R. Lindeman Hairston, Smith, Slobodkin (HSS)

Wiegert and Owen Fretwell donor control

top-down regulation bottom-up regulation Stuart Pimm

compartment (subweb) connectance link-scaling

linkage density interval Joel Cohen

Ehrlich and Birch

Food web:

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 CE.

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 connectedness is inversely related to system stability

4) food chains are short due to energetic constraint hypothesis

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

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