Page 107 - Environment: The Science Behind the Stories

such as aspens and poplars begin to grow. As time passes, pine If we disturb a community, won’t it return
trees rise above the pioneer trees and shrubs, forming a pine- FaQ to its original state if we just leave the
dominated forest. This pine forest develops an understory of area alone?
hardwood trees, because pine seedlings do not grow well under
a canopy but some hardwood seedlings do. Eventually the hard- Probably not, if the disturbance has been substantial. For
woods outgrow the pines, creating a hardwood forest. example, if soil has become compacted, or water sources
Processes of succession occur in a diversity of ecologi- have dried up, then the plant species that grew at the site orig-
cal systems. For instance, ponds may undergo succession as inally may no longer be able to grow. Different plant species
algae, microbes, plants, and zooplankton grow, reproduce, may take their place—and among them, a different suite of
and die, gradually filling the water body with organic mat- animal species may find habitat. Because species interact and
ter. The pond acquires further organic matter and sediments because they rely on particular habitat conditions, a change
from streams and surface runoff, and eventually it can fill in, in one aspect of a community can lead to a cascade of other
becoming a bog (p. 413) or even a terrestrial system. changes. Sometimes a whole new community may arise. For
In the traditional view of succession described here, the instance, in some grasslands, livestock grazing and fire sup-
process leads to a climax community, which remains in place pression have led shrubs and trees to invade, changing the
until some disturbance restarts succession. Early ecologists grasslands to shrublands. And we have seen (p. 102) how
felt that each region had its own characteristic climax com- removing sea otters can lead to the loss of kelp forest commu-
munity, determined by climate. nities. In the past, people didn’t realize how permanent such
changes could be, because we tended to view natural sys-
tems as static, predictable, and liable to return to equilibrium.
Communities may undergo shifts Today ecologists recognize that systems are highly dynamic
and can sometimes undergo rapid, extreme, and long-lasting
Today, ecologists recognize that community change is far change.
more variable and less predictable than early models of suc-
cession suggested. Conditions at one stage may promote
progression to another stage, or organisms may, through com-
petition, inhibit a community’s progression to another stage. Invasive species pose new threats
The trajectory of change can vary greatly according to chance to community stability
factors, such as which particular species happen to gain an
early foothold. And climax communities are not determined Traditional concepts of communities involve species under-
solely by climate, but vary with soil conditions and other fac- stood to be native to an area. But what if a species not native
tors from one time or place to another. Ecologists came to to the area (a non-native, alien, or exotic species) arrives from
modify their views about how communities respond to dis- elsewhere? In our age of global mobility and trade, people
turbance after observing changes in long-term field studies at have moved countless organisms from place to place, inten-
locations such as Mount St. Helens following its eruption (see tionally or by accident, such that today most non-native arriv-
The Science behind The STory, pp. 108–109). als in a community are introduced species, species introduced
Once a community is disturbed and changes are set in by people.
motion, there is no guarantee that the community will ever Most introduced species fail to establish populations
return to its original state. Sometimes communities may in the places they arrive, but some turn invasive, spreading
undergo a phase shift, or regime shift, in which the charac- widely and coming to dominate communities. Such invasive
ter of the community fundamentally changes. This can occur species often thrive in disturbed communities, and in turn dis-
if some crucial climatic threshold is passed, a keystone spe- turb them further. By altering communities, invasive species
cies is lost, or a non-native species invades. In recent years are one of the central ecological forces in today’s world.
many coral reef communities have undergone a phase shift Introduced species may become invasive when limiting
and become dominated by algae, after people had overhar- factors (p. 85) that regulate their population growth are absent.
vested populations of fish or turtles that eat algae. Once algae Plants and animals brought to a new area may leave behind
overgrow a reef, the community may never shift back to its the predators, parasites, herbivores, and competitors that had
original coral-dominated state. Phase shifts make clear that exploited them in their native land. If few organisms in the
we cannot count on being able to reverse damage caused by new environment eat, parasitize, or compete with the intro-
human disturbance. Instead, some of the changes we set in duced species, then it may thrive and spread. As the species
motion may become permanent. proliferates, it may exert diverse influences on other commu-
Moreover, many ecologists now think that human distur- nity members (Figure 4.15).
bance is creating communities that are wholly new and have Zebra and quagga mussels spread with global trade,
not previously occurred on Earth. These novel communities, or inadvertently transported in the ballast water of cargo ships.
no-analog communities, are composed of novel mixtures of (To maintain stability at sea, ships take water into their hulls
plants and animals and have no analog or precedent. As we as they begin their voyage and then discharge that water at
enter more deeply into an age of fast-changing climate, habi- their destination.) Decades of unregulated exchange of bal-
tat alteration, species extinctions, and species invasions, many last water have ferried countless species across the oceans.
scientists predict that we can expect to see more and more Other types of freshwater invaders include predatory fishes
106 novel communities. released from aquaria, bait buckets, stocking, or aquaculture;

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