Botkin Environmental Science Earth as Living Planet 8th txtbk

96 CHAPTER 5 Ecosystems: Concepts and Fundamentals
(a)
FIGURE 5.14 Primary succession. (a) Forests developing on new lava flows in Hawaii and (b) at the
edge of a retreating glacier.
(b)
recovers, is an example of secondary succession. 15 Forests
that develop on abandoned pastures or after hurricanes,
floods, or fires are also examples of secondary succession.
Succession is one of the most important ecological
processes, and the patterns of succession have many management
implications (discussed in detail in Chapter 10).
We see examples of succession all around us. When a
house lot is abandoned in a city, weeds begin to grow. After
a few years, shrubs and trees can be found; secondary
succession is taking place. A farmer weeding a crop and a
homeowner weeding a lawn are both fighting against the
natural processes of secondary succession.
Patterns in Succession
Succession follows certain general patterns. When ecologists
first began to study succession, they focused on three
cases involving forests: (1) on dry sand dunes along the
shores of the Great Lakes in North America; (2) in a
northern freshwater bog; and (3) in an abandoned farm
field. These were particularly interesting because each
demonstrated a repeatable pattern of recovery, and each
tended to produce a late stage that was similar to the late
stages of the others.
Dune Succession
Sand dunes are continually being formed along sandy
shores and then breached and destroyed by storms. In any
of the Great Lakes states, soon after a dune is formed on
the shores of one of the Great Lakes, dune grass invades.
This grass has special adaptations to the unstable dune.
Just under the surface, it puts out runners with sharp
ends (if you step on one, it will hurt). The dune grass rapidly
forms a complex network of underground runners,
crisscrossing almost like a coarsely woven mat. Above the
ground, the green stems carry out photosynthesis, and the
grasses grow. Once the dune grass is established, its runners
stabilize the sand, and seeds of other plants have a
better chance of germinating. The seeds germinate, the
new plants grow, and an ecological community of many
species begins to develop. The plants of this early stage
tend to be small, grow well in bright light, and withstand
the harsh environment—high temperatures in summer,
low temperatures in winter, and intense storms.
Slowly, larger plants, such as eastern red cedar and eastern
white pine, are able to grow on the dunes. Eventually,
a forest develops, which may include species such as beech
and maple. A forest of this type can persist for many years,
but at some point a severe storm breaches even these heavily
vegetated dunes, and the process begins again (Figure 5.15).
FIGURE 5.15 Dune succession on the shores of Lake
Michigan. Dune-grass shoots appear scattered on the slope, where
they emerge from underground runners.