Food Webs and Ecological Networks

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Quiz 4.  Food Webs and Ecological Networks

Purpose of Quiz:is to give you hands-on experience in building a network model with a situation you know very well, namely, you and your food system, in order to study how both negative and positive feedback work in your nutrition.


  1. Do the reading for this section, visit the websites, view the videos, and watch the lecture. Now you are ready to begin building your network model and answering the questions.
  2. You willuse a signed digraph type of representation for your network that enables you to have 9 different qualitative relationships between each pair of nodes. Although your model will feature foods and feeding relationships, it will be more of an ecological network than a food web per se by using so many different types of links:(0/-), 2. (0/+), 3. (-/+), 4. (+/-), 5. (-,-), 6. (+, +), 7.(+/0), 8. (-/0), and 9.(no link to either node-null case-there is no interaction).Some of these link types are shown below, counting down the lines:






Line 1 (-,0) you reduce the food, but the food has no effect on you.

Line 2 (0,-) the food hurts you, but it is not changed by the interaction.

Line 3(-, +) your benefit from the food, which is reduced by your consumption

(typical predator-prey).

Line 4 (+, +) the food benefits you and you benefit the food (mutualism).

A small circle-head on a link indicates a negative effect on the node it touches, and an arrowhead indicates a positive effect on the node it touches).This is a signed digraph because there are + and – signs that can go in 2 directions (“Di”).  When there is zero effect like Line 2 above, there is no change in the food node via the unsigned linkthat touches the food node.

In addition, you should include self-damping loops of a node on itself with the link having one circle-head.  This is a density dependent effect.  As the node increases the negative effect on itself pushes against the growth reducing it in size.  In the sample model above, self damping is shown on food.

  1. Be sure to use as many of the 9node to node link types as possible and one or more self damping loops to draw your network.  Define each of your nodes.  Some may be non-trophicnon-ecological, and even social or economical. Number your links and explain them.  Explain your network.  (2 pts)


  1. Find 2 positive and 2 negative feedback loops in your diagram-write out the links they include in order. A feedback loop returns to the node it starts while only gong in and out of each node on the loop one time each(one in and one out per node).  To determine if a loop is positive or negative, multiple the algebraic signs around the loop(+ x + = +) (- x – = +), (- X + = -) etc.  Links with both ends with signs either arrowheads or circle-heads are two-node loops. Links with only one end signed are only links not loops. For example, in Line 1 in the diagram, you can travel from You to Food but not the other way around-this I a link not a loop, but Line 3 is a loop. (1 pt.)


  1. Compute the C=connectivity (L/N2) and B = link density (L/N)of your food web where N = the number of nodesand L = number of links or = the number of arrowheads and circleheads. How connected are you to your food? (1 pt.)


  1. Describe 2 new things you learned about your food web. (2 pts).

Note: For all quizzes, be sure to reference every source that you use, but this list is not counted in your word count and can be on the second sheet of paper.  You can also use sources not mentioned in the module for all quizzes.

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