Page 104 - Environment: The Science Behind the Stories

large and severe as a hurricane, tornado, or volcanic eruption.
Some disturbances are sudden, such as landslides or floods,
while others are more gradual, such as climate change. Some
disturbances regularly reoccur and are considered normal
aspects of a system (such as periodic fire, seasonal storms, or
cyclic insect outbreaks). Organisms may, in fact, adapt to regu-
lar and predictable types of disturbance. For instance, many
plants that grow in fire-prone regions have evolved ways of
surviving fire and have seeds that depend on fire to germinate
in the nutrient-rich soil that fire leaves behind. Today, human
impacts are major sources of disturbance for ecological com-
munities worldwide—from habitat alteration to pollution to the
introduction of non-native species such as the zebra mussel.
Communities are dynamic systems and may respond
to disturbance in several ways. A community that resists
change and remains stable despite disturbance is said to show
resistance to the disturbance. Alternatively, a community may
show resilience, meaning that it changes in response to distur-
bance but later returns to its original state. Or, a community
may be modified by disturbance permanently and never return Figure 4.13 Primary succession begins as organisms
to its original state. colonize a lifeless new surface. Here, plants establish on a
volcanic lava flow.

Succession follows severe disturbance landscape (Figure 4.13). Species that arrive first and colonize
the new substrate are referred to as pioneer species. Pioneer
If a disturbance is severe enough to eliminate all or most species are well adapted for colonization, having traits such as
of the species in a community, the affected site may then spores or seeds that can travel long distances.
undergo a somewhat predictable series of changes that ecolo- The pioneers best suited to colonizing bare rock are the
gists have traditionally called succession. In the conventional mutualistic aggregates of fungi and algae known as lichens.
view of this process, there are two types of succession. In lichens, the algal component provides food and energy via
Primary succession follows a disturbance so severe that no photosynthesis while the fungal component grips the rock and
vegetation or soil life remains from the community that had captures the moisture that both organisms need. As lichens
occupied the site. In primary succession, a biotic commu- grow, they secrete acids that break down the rock surface,
nity is built essentially from scratch. In contrast, secondary beginning the formation of soil. Small plants and insects
succession begins when a disturbance dramatically alters an arrive, providing more nutrients and habitat. As time passes,
existing community but does not destroy all living things larger plants and animals establish themselves, vegetation
or all organic matter in the soil. In secondary succession, increases, and species diversity rises.
vestiges of the previous community remain, and these build- Secondary succession begins when a fire, a hurricane, log-
ing blocks help shape the process. ging, or farming removes much of the biotic community. Consider
At terrestrial sites, primary succession takes place after a farmed field in eastern North America that has been abandoned
a bare expanse of rock, sand, or sediment becomes newly (Figure 4.14). After farming ends, the site will be colonized by
exposed to the atmosphere. This can occur when glaciers pioneer species of grasses, herbs, and forbs that disperse well or
retreat, lakes dry up, or volcanic lava or ash spreads across the were already in the vicinity. Soon, shrubs and fast-growing trees CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology

Hardwoods
Pines
Saplings,
poplar trees
Figure 4.14 Secondary
succession occurs after a
Shrubs, disturbance (such as a fire,
seedlings landslide, or farming) removes
Grasses,
herbs, forbs most vegetation from an
area. Shown is a typical series of
changes in a plant community of
eastern North America following
Time abandonment of a farmed field. 103

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