456 MHR • Unit 5 Population Dynamics
In 1966, one of the valleys in the forest area was
logged (as in Figure 13.30B). All the trees were cut
down and left to decay, and the area was sprayed
with herbicides to prevent new plant growth.
The inflow and outflow of nutrients in this valley
was then compared with that of an untouched
experimental forest. The amount of water that ran
through and out of the logged valley increased by
35 percent, presumably because there were no
plants to absorb the water from the soil. Substantial
amounts of nutrients were lost from the ecosystem,
with most flowing into the stream that ran through
the valley. The amounts of calcium and potassium
in the stream increased by 4 and 15 times
respectively, and the nitrite concentration was
60 times higher after logging. Not only were these
nutrients being lost from the forest ecosystem, but
in some cases they were making the water from the
stream unsafe to drink.
These results, as well as other data collected
over the last 35 years, have demonstrated that plants
control the amount of nutrients leaving a forest
ecosystem. When plants are removed, nutrients are
lost. The loss begins immediately and continues for
as long as the plants are absent. After succession
starts, nutrient loss is reduced. It may take a long
time to achieve the balance of loss and gain seen in
an intact ecosystem — if it ever does return.Environmental factors can have a significant
influence on the variety, distribution, and
productivity of autotrophs in an ecosystem. To
enable you to understand the potential effect these
factors can have, you will design and test your own
model ecosystem in the investigation on page 458.
Using controlled testing, you will observe and
make comparisons between a control ecosystem
set-up and one in which you have altered the
environmental factors.
In this chapter, you recognized the challenge of
learning about the complexity of the interactions
among organisms so they can be better understood.
You also learned about trophic structure, including
various ecological principles involving abiotic and
biotic components; the complex relationships that
exist among individuals, populations, commu-
nities, and ecosystems; ecological succession; and
the differentiation between the habitat, range, and
niche of a population. You also learned about
trophic structure, including the ecosystem role of
producers, consumers, and decomposers, and saw
how energy flows and nutrients cycle through an
ecosystem. In Chapter 14, you will learn about
population ecology, and examine features of
populations, including their size and density. You
will also investigate the factors that influence the
growth and decline of populations.Figure 13.30Hubbard Brook Experimental Forest(A)
Dams built along streams running through the area allowed
researchers to collect data on the usual outflow of nutrients.
Rain and snow samples were collected and analyzed tomeasure nutrient inflow. (B) After this area had been logged,
the loss of nutrients increased dramatically. Do you think the
loss would have stopped if secondary succession had been
allowed to occur naturally? Defend your answer.