BSC 314 Lecture Notes - Lecture 62: Primary Succession, Secondary Succession, Pioneer Species
28 Jun 2018
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Ecosystem Structure: Plant Communities
The plant community usually is the largest visible part of an ecosystem, and often both
the community and the ecosystem are named for the dominantplants present—that is,
the plants that, by virtue of their size or numbers, modify and control the environment.
The community is not a haphazard collection of organisms, but consists
of populations of individuals whose tolerance ranges—the range of environmental
conditions in which individuals of a particular species will grow—match those of the site.
Succession
Even the most stable ecosystems are in constant, normal flux. One of the easiest
ecological processes to observe is succession, the change in the composition of the
vegetation of a particular site over time. Two kinds occur. Primary succession takes
place on newly exposed surfaces such as might appear after a volcanic eruption or
following a rockslide in the mountains. Secondary succession occurs when vegetation
is removed from land and new kinds of plants return to colonize the bare ground. On
sites undergoing primary succession, no soil is present, and vegetation and soil develop
concurrently. In secondary succession, vegetation develops on soil already in place, but
the soil changes over time as the new colonizers develop new communities above, and
below ground.
Weedy annuals are the usual pioneer species that colonize bare ground. They are
“generalist” species with broad tolerance ranges and by their growth change conditions
at the site, making possible the development of communities of other species.
Productivity in the early stages is high, but as species richness and total biomass
increase, productivity decreases (the reason why agricultural ecosystems are kept in an
early successional stage).
Disturbance
Disturbances are necessary to sustain ecosystems and are the mechanisms by which
diversity in kinds and ages of species and habitats is maintained. Some disturbances
are large scale and rare (a tornado or hurricane, for example), but most are small and
frequent (the blowdown of a few trees opening a hole in the forest canopy or rodent
burrowing that brings underlying soil to the surface and destroys existing ground cover).
Ecosystem Functions
Three levels of organisms regulate the flow of energy in ecosystems: the producers,
the consumers, and the decomposers. They are organized in complex food
webs. Autotrophs—plants, algae, and some bacteria—are the primary producers of
an ecosystem. Heterotrophs—animals, fungi, most protists and bacteria, and a few
non green plants—are the‐ consumers in ecosystems. They obtain their energy and
carbon from the organic material produced by the autotrophs. Four trophic (feeding)
levels are recognized: The primary producers constitute the first level, followed by three
levels of consumers. Primary consumers are the herbivores (plant eaters) that feed
directly on the primary producers. The next level includes flesh eaters, the‐ primary
carnivores that consume the herbivores. The top or fourth level is that of
the secondary carnivoresthat dine on the primary carnivores. At each level, some of
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Document Summary
The community is not a haphazard collection of organisms, but consists of populations of individuals whose tolerance ranges the range of environmental conditions in which individuals of a particular species will grow match those of the site. Even the most stable ecosystems are in constant, normal flux. One of the easiest ecological processes to observe is succession, the change in the composition of the vegetation of a particular site over time. Primary succession takes place on newly exposed surfaces such as might appear after a volcanic eruption or following a rockslide in the mountains. Secondary succession occurs when vegetation is removed from land and new kinds of plants return to colonize the bare ground. On sites undergoing primary succession, no soil is present, and vegetation and soil develop concurrently. In secondary succession, vegetation develops on soil already in place, but the soil changes over time as the new colonizers develop new communities above, and below ground.
