Biology 12

THINKING LAB

Sustainable Harvesting

Background

Theoretically, populations regulated by density-dependent

factors can be harvested. In other words, individuals can be

removed for commercial and/or recreational purposes in

such a way that the population is not depleted. This will be

the case if the number of individuals being harvested

(referred to as the yield of the population) is just equal to

the number being added as a result of reproduction. One of

the tasks of those who manage populations of organisms

that humans use as resources (for example, forests; fish

stocks; and deer, moose, and bear populations) is to try to

determine the Maximum Sustainable Yield (MSY). MSY is

the largest number of individuals that can be removed from

the population each year (or other harvest period) without

reducing the population’s growth over the long term. In this

lab, you will see how the MSY can be assessed — in theory.

Growth curve for a theoretical, density-dependent population

You Try It

1.The graph shows the growth curve for a theoretical,

density-dependent population living in an environment

in which the carrying capacity is 500 (K= 500 ). The

population started out with five individuals (N 0 = 5 ) and

grew at a rate of 0.2 offspring per individual per time

period (r=0.2). Using the curve, estimate the number

of individuals added to the population in the first five

time units. Estimate the size of the population in the

middle of this time interval (that is, at time = 2.5 units).

Follow the same procedure for intervals 5–10, 10–15,

15 – 20, and so on up to 45–50. Record your data in a

table, or use a spreadsheet program if available.

2.Plot the data you obtained in Step 1 for each of the

10 time intervals on a graph of “Number of individuals

added to the population” (y-axis) versus “Population

size” (x-axis). How big is the population during the

interval when the maximum number of individuals is

being added? Is Nat this time near K, far below it, or

halfway to K? Explain the shape of the curve you have

plotted. Why does the curve increase to a maximum

and then decrease?

3.Suppose you are the manager of this population

and you are trying to determine its MSY. At what size

would you want to maintain the population, and why?

How would an increase in the population’s size above

the MSY point affect yield? How would a decrease

affect yield?

4.Although MSY values have often been used to set

harvest quotas (such as the number of fish that can be

caught or deer that can be hunted), the system has not

worked well in some situations. Many populations have

been seriously depleted as a result of overfishing or

excessive hunting, in part because quotas were set

too high. Why do you think this might have happened?

What problems can you see occurring as a result of

using this method to set quotas?

0

100

200

300

10

400

500

20 30 40 50

Population size (

N

)

Time

N 0 =5,K=500,r=0.2

490 MHR • Unit 5 Population Dynamics

abiotic and biotic factors are part of the population’s

environment, the combination of their effects is

sometimes referred to as environmental resistance

to population growth.

The examples discussed under “Symbiotic

Relationships” might suggest that populations of

some kinds of organisms (such as insects) are

regulated by abiotic, density-independent factors,

whereas populations of other kinds of organisms

(including birds and mammals) are regulated by

biotic, density-dependent mechanisms. This

apparent contrast led early ecologists to argue

about which form of population regulation was

more important, or at least more common. These

discussions, along with more research, eventually

led to the understanding that for most populations,

both types of regulating factors play a role. For

example, most of the time a population of birds

may be regulated by density-dependent factors.

However, occasional catastrophes may reduce the

population to a point where density has little

influence on the growth rate. Similarly, severe

weather may regularly cause the size of an insect

population to decline. If the environment contains

a limited number of protective sites where members

of this population can hide to avoid the weather,