General Ecology: Lecture 15
November 4, 2005
I.
What factors contribute to the impact of a predator on prey abundance?
A.
The responses of the predator to prey
1.
Functional response of individual predators. Three models [Fig. 15.7]
a)
Type 1: Predator responds linearly to prey concentration until a
maximum is reached
b)
Type 2: The rate of prey capture decreases with more prey present

Caused by a finite “handling time” for each prey. This is the
time it takes to catch, kill and eat it.
c)
Type 3: At low prey levels, the predator is ineffective. This might be
due to the fact that prey are more difficult to find when in low
numbers. At high prey levels, the predator is satiated (as for type 2).

The type 3 curve may also be seen due to prey-switching by
generalist predators [Fig. 15.10]

In this case, the relatively low rate of capture is due to
the predator feeding on a different, more abundant prey.

Can have effect of stabilizing prey populations.

Might also reduce impact of competition in similar
species that both serve as prey.
2.
Numerical response of the predator
a)
Short term

Aggregative response

Example: Feeding aggregations of birds on fish.
b)
Long term

Immigration into an area with abundant prey

Increased reproductive rates (think about original LotkaVolterra equation)
B.
The population size of the prey
1.
For prey already at high densities, loss due to predation may be insignificant
II.
Optimal foraging theory
A.
Basic concept: A predator’s foraging strategy will be optimal: maximizing energy
intake/time.
Profitability = energy value/handling time = E/h
B.
C.
Assumption made:
1.
It is energy that must be maximized.
a)
Other measures of food quality may be important

Example: Adelie penguins require krill to reproduce:
proportion of lipid is important

Example: Importance of particular nutrients and vitamins:
Anemia with iron deficiency, etc…
Simple model for choosing between two foods:
1.
E1/h1 > E2/h2. This means that Food Type 1 is the better food.
a)
So, when should the predator eat Food Type 2, and when not? It
essentially makes a “choice” between eating Food Type 2 or
searching for/eating Food Type 1.
2.
3.
Example 1: Pisaster ochraceus (ochre star common in tidepools) encounters
an urchin that it can eat, but it gets more energy more quickly out of eating
mussels. The choice, according to optimal foraging theory, depends upon
how easy it will be for the animal to find mussels
a)
Prediction: If mussels are scarce, it should eat the urchin. Whereas if
mussels are abundant, it should leave the urchin to seek out mussels.
This knowledge of “scarce” vs. “abundant” is based on its recent
experience of density.
b)
Mathematically, this means that the predator should eat the less
preferred prey if:

E2/h2 > E1/(S1 + h1)
Example 2: Bluegill sunfish feeding on Daphnia (water fleas) [Fig. 15.16]
a)
Note the lack of selectivity at low prey densities vs. a relatively high
selectivity at high prey densities
b)
Note the confounding factor: the ratio encountered was skewed
towards larger Daphnia at the higher prey densities. Why important?
Study questions:
1.
Sketch the three types of functional responses and provide a basic explanation as to what each
curve means in terms of the ability of a predator to find and eat its prey.
2.
Which of these functional response curves (could be more than one):
a.
Takes predator satiation into account?
b.
Would help a rare prey item from going extinct?
3.
What makes small prey populations more vulnerable to predator limitation than larger ones?
4.
What is meant by the “numerical response” by predators? Give examples of both short and longterm numerical response.
5.
Is generalist or specialist predator more likely to create stability in its prey populations? Explain.
6.
Is a generalist or specialist predator more likely to wipe out/create oscillations in its prey
populations? Explain.
7.
What is meant by “optimal foraging”?
8.
What key assumption was made in the simple optimal foraging model introduced in the text and
lecture? Provide an example where this assumption doesn’t hold true.
9.
Mathematically, what constitutes a “superior” vs. “inferior” prey item?
10.
Under what conditions would the predator be expected to eat the inferior prey? State both in
words and mathematically.
11.
Explain graph 15.16 (foraging of bluegill sunfish on Daphnia) focusing on how this data supports
the concept of optimal foraging strategy. Also explain what confounds the interpretation of these
graphs.
12.
What is meant by foraging efficiency? What are the predictions made by optimal foraging theory
for animals foraging on patchy food resources?