Food Webs

Question 1

What is a food web?

Answer 1

Representation of feeding relationships in a community. Includes trophic levels, links and omnivory.

Question 2

What are direct effects within food webs?

Answer 2

Species occupying the same trophic level are direct competitors e.g. lions and hyenas
Members of adjacent trophic levels are predator and prey e.g. giraffes and acacia trees

Question 3

What are indirect effects within food webs?

Answer 3

Competitor species removal leads to decrease in other competitor species abundance
Predator removal leads to increase in prey abundance

Question 4

What are trophic cascades?

Answer 4

Food web interactions where the effects of top predators on their prey cascade down to lower trophic levels. 2 types:

3 level cascade - predator - herbivore prey - plants
4 level cascade - top predator - mesopredator - herbivore prey - plant

Question 5

What is top down and bottom up control?

Answer 5

Top-down - Predation keeps herbivores in check by allowing green plants to accumulate biomass.

Bottom-up - Herbivores are controlled by the availability of plant resources which are in turn controlled by chemical defences, physical defences and nutrients. Predators compete for scarce herbivores.

Question 6

What is a keystone species?

Answer 6

Effect on ecosystem large relative to abundance. Strong interactions whose removal leads to changes spreading throughout the food web. Huge conservation importance.

Question 7

What is the stability debate?

Answer 7

MacArthur (1955) and Elton (1958) - increased community complexity leads to increased stability.

May (1972) vs Cottingham (2001)

β(SC)^1/2 < 1 where,
β = interaction strength
S = number of species
C = connectance

Question 8

How do real food webs function?

Answer 8

Communities we observe must be stable otherwise we wouldn’t be able to.

If, β(SC)^1/2 < 1 then increases in S will lead to decreased stability unless there are compensatory decreases in C or β.

Since β is hard to measure, ecologists treat it as constant.

Question 9

What is compartmentation?

Answer 9

Food web having subunits within which interactions are strong but between which interactions are weak.

Map onto habitats

Spatial Subsidies - Crucial flows of energy and materials link habitats within a larger food web.

Predicts that communities will have increased stability if compartmentalised:
- Disturbance restricted within a single compartment
- Spatial subsidies buffer compartments against perturbation