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

...