A. Role of Predators in Regulating Prey Populations
The control of predators for nature conservation purposes is becoming an increasingly important issue. The growing populations of predator species in some areas and the introduction of predators in other areas have led to concerns about their impact on vulnerable bird species and to the implementation of predator control in some cases. This is set against a background of increasingly fragmented semi-natural habitats and declining populations for many species. To assess the efficiency of predator removal as a conservation measure, the results of 20 published studies of predator removal programs were meta-analyzed. Removing predators had a large, positive effect on hatching success of the target bird species, with removal areas showing higher hatching success, on average, than 75% of the control areas. Similarly, predator removal increased significantly post-breeding population sizes (i.e. autumn densities) of the target bird species. The effect of predator removal on breeding population sizes was not significant, however, with studies differing widely in their reported effects. We conclude that predator removal often fulfills the goal of game management, which is to enhance harvestable post-breeding populations, but that it is much less consistent in achieving the usual aim of conservation managers, which is to maintain and, where appropriate, increase bird breeding population size. This may be due to inherent characteristics of avian population regulation, but also to ineffective predator removal and inadequate subsequent monitoring of the prey populations
1. Predation - Definition
- Eating of one species by another
- Herbivory
- Carnivory
- Parasitism
- Cannibilism
2. Examples
3. Primary components
a. Prey population growth in absence of predators
b. Prey consumption per predator-functional response
c. Predator population growth-numerical response
4. Lotka-Volterra formulation
Lotka-Volterra Model Assumptions
- Neither prey nor predator inhibit their own growth.
- Environment is homogeneous.
- Every prey has an equal probability of being attacked.
- Predators have an unlimited capacity for increas.
- Prey density has no effect on the probability of eing eaten.
- Predator density has no effect on the probability of a predator capturing prey.
- There are no time lags.
5. Logistic growth of prey
dH/dt = rH (1 - H/K)
6. Functional response
a. Ivlev's studies (Ivlev's fish)
Combined response:
dH/dt = rH (1 - H/K) - aHP/ (1 + aHh)
b. Holling's type 1, 2 and 3 responses (type 2 is very common)
dH/dt 1/P = -aH/(1 + aHh)
c. Handling time models
Functional response model looking at influence of handling time:
EQUATION
Example of how the functional response curve changes as handling time changes:
A) Say a fisher is feeding on snowshoe hare.
search rate ( a ) = 10 acre / day or 300 acre per month
prey density ( H ) changes from 1 / acre to 100 / acre
total time ( T ) is a month
handling time per prey ( h ) for hare is 3 hours or .0042 month.
B) When hunting porcupine the fisher has a higher handling time but the search rate etc. stays the same.
So handling time per prey for porcupine is 8 hours or .011 month
Let’s look at functional response under these two prey types.
Prey Type A Prey Type B Prey Density per acre Number Captured per month Prey Density per acre Number Captured per month 1 132 1 69 2 170 2 79 3 188 3 82 4 198 4 84 5 205 5 86 6 210 6 87 7 213 7 87 8 216 8 87 9 218 9 88 10 220 10 88 15 226 15 89 20 229 20 89
7. Numerical response
a. Holling's types a, b and c
b. Examples
8. Combined response
a. Total prey removed
b. Percent mortality on prey
9. Stability of predator-prey interaction
Tanner's Model (1975) provides a nice framework for describing the complex interactions of a predator and prey which determine the stability or instability of the relationship. Stable equilibria, stable limit cycles, or unstable interactions may result from combinations of the following factors (See Predation Lab for more details of Tanner's results):
a. Effect of combined response
b. Prey self limitation
c. Relative growth rates of predator and prey
d. Refuges for prey
e. Habitat modification and other effects on predators search rate
f. Differential susceptibility of prey
g. Compensatory nature of predator mortality
Define and distinguish between predation, herbivory, carnivory, parasitism and cannibalism.
Identify the primary components of the predator-prey interaction and their significance.
Be able to recognize the Lotka-Volterra formulation of the predator-prey interaction and to discuss its significance and behavior.
What assumptions of the Lotka-Volterra model are unreasonable?
How would you improve them?
Describe the types of functional responses identified by Holling, their cause and their significance.
Be able to recognize, understand, and use the disc model and other handling time models of the functional response.
Describe the types of numerical responses and their causes.
What types of total response result from combinations of the various functional and numerical responses?
What is the importance of the various types of total response?
What determines the stability of the predator-prey interaction?
Why are there sometimes stable limit cycles and other times a stable equilibrium in predator and prey numbers?
Is predator mortality compensatory?
What difference does it make if it is compensatory?
Be able to apply those ideas to 2 or 3 real world predator-prey interactions with which you are familiar.
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Revised: 25 August 2011