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OCR for page 47
Community Ecology
INTRODUCTION
Every species population is part of an assemblage of species-plants,
animals, and microorganisms-that share space and interact. We speak
of this group of interacting organisms as an-"ecological community." It
is difficult to define that term precisely, for two reasons. First, whether
communities are discrete entities with higher-order "emergent" properties
(not readily derivable from an analysis of constituent species) or simply
groups of species from the available pool is controversial (Krebs, 1985;
Simberloff, in press). Second, whether sympatric species (occupying the
same area) should be considered parts of a community if they do not
interact with many of the other species present is debatable. The debate,
in part, is over the strengths of interactions and how they contribute to
community structure. Many environmental problems arise because the
alteration of some of these interactions leads to new arrangements of
species populations that, from a human perspective, are less desirable than
the former ones.
Because the population dynamics of species depend on the kind and
intensity of their interactions with other species- species that prey on
them and compete with them and on which they prey a knowledge of
these interactions is often valuable in managing individual species. Chapter
2 discusses population interactions as though they are generally indepen-
dent of each other. Yet we know that each species interacts with many
others and that the interactions are often indirect. For example, grazers
47
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48 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR PLACATIONS
depend for food on the productivity of grasses, whose growth depends on
the activity of earthworms in the soil, which can be affected by the addition
of toxic materials to the soil. Human manipulations of the environment
can therefore affect individual species, not only directly, but also in in-
direct ways that can be understood only in the context of the functional
structure of the whole community (Davidson et al., 19841.
Ecological communities have numerous properties that transcend those
of their constituent species and that require study themselves trophic
structure, rates of energy and nutrient flow, growth form and physical
structure, number of species and their numerical distribution, stability
characteristics, and ecotones (zones of transition between two habitat
types), to name a few. The composition of a community changes in space
and time as a result of physical and biological processes. This variability
makes it difficult to detect changes caused by human intervention, even
if long-term data are available.
Communities are usually named for the commonest or "most impor-
tant" kinds of organisms found in them. Thus, we speak of "chaparral
communities" and "blue mussel communities." The organisms chosen
to identify communities are usually the ones that provide physical structure
for them, such as plants in terrestrial environments, or that are the bases
of food chains in systems lacking fixed structural elements, such as plank
ton in the ocean.
In addition to its value in managing individual species, a knowledge of
community ecology is essential when the object is to manage communities
themselves. For example, we often wish to maintain a diversity of or-
ganisms in an area, as mandated by law in the U.S. national forests
(Chapter 15), or to maintain a particular set of species together, as in
parks or agricultural areas. Restoring degraded habitats requires not only
a knowledge of the physical conditions that favor the growth of individual
species, but also an understanding of how the species interact under dif-
ferent conditions (Chapter 18~. We might need to know how the intro-
duction of a species can alter community makeup or how well a new
species can substitute for another in maintaining community stability. Or
we might wish to know how much disturbance a community can tolerate
before undergoing important compositional change.
SPECIES COMPOSITION
The species composition of a community the number and kinds of
species present is one of its most obvious features. A knowledgeable
ecologist can deduce a great deal about environmental conditions in an
area simply by looking at a site and inspecting a list of species present
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COMMUNITY ECOLOGY
49
(Dayton, in press). Systems under the influence of strong perturbations
typically show reductions in the number of species that are numerically
dominant. For example, continued eutrophication of lakes usually leads
to an excess of some nutrients, a deterioration in water clarity due to algal
proliferation, and a reduction in available oxygen due to increased rates
of decay. Species that depend on clear water for foraging or on high
oxygen content for respiration disappear from the lakes and are replaced
by species that can use the increase in nutrients under relatively anoxic
conditions. In Lake Washington (Chapter 20), continued addition of sew-
age led to the appearance of the blue-green alga, Oscillatoria rubescens,
now known to be characteristic of severe eutrophication; when the sewage
was reduced, Oscillatoria disappeared and later the zooplankton com-
munity changed.
The presence or absence of an indicator species (Chapter 7) is in itself,
however, not always a reliable sign of conditions resulting from human-
induced perturbations. Species "typical" of some communities are not
invariably present. Local populations can die out for various reasons,
including disease, high predation rates, and unusually harsh weather. A
species might have failed to colonize an area because of its isolation.
Consequently, reliable use of species as indicators of perturbations requires
knowledge of their distribution under "normal" conditions and knowledge
of past conditions at the site. In addition, knowledge of the composition
of an entire community can substantially improve the usefulness of the
indicator approach. Patrick et al. (1967), for example, related a variety
of stream pollutants to changes in the relative abundance of groups of
algal species that had different tolerances to the pollutants.
The number of species in a community (species richness) often changes
in response to disturbance, and species richness has been used as an
indicator of disturbance. Some types of stream pollution simplify the
stream environment and reduce the number of available niches; others kill
off many species outright (Patrick et al., 19671. But moderate disturbance
can produce less clear-cut effects, and some perturbations can alter the
relative abundance of species without changing the number of species
present (Dickman, 1968~.
More sophisticated measures and indexes of community composition
weight the number of species with the relative abundance or biomass of
each (Krebs, 19851. A diversity index increases both as the number of
species increases and as the numerical distribution of species becomes
more even. A large environmental change often leads to local extinction
of many sensitive species and to the predominance of a few "disturbance-
tolerant" organisms or organisms capable of using the new conditions for
increased growth, so diversity indexes have been used as measures of
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50 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
disturbance in a community. Such uses of diversity indexes have been
controversial, because they have been applied with little regard for the
functional changes that occur in disturbed ecosystems.
Many ecologists hoped that diversity measures would capture key changes
in fundamental community processes and thus obviate more detailed anal-
yses of species composition. These hopes have been largely unrealized,
and there is increasing recognition that the most important information is
often discarded in calculating these indexes (May, 19851. A diversity
index should be used with caution, for several reasons:
· Many factors other than the disturbance of concern can cause a change
in the index. This problem is particularly acute when communities in
different areas are compared only once. Site differences in physical and
biological factors nutrient availability, presence or absence of key spe-
cies, climatic differences, etc. can cause differences in diversity.
· The species present in a community can change substantially without
any significant change in diversity indexes.
· Some disturbances can increase diversity if they increase habitat
heterogeneity, reduce the influence of competitively dominant species, or
create opportunities for new species to invade (discussed below).
Most of the complexities of the processes that change diversity are not
captured in diversity indexes, which are appropriately used only when we
are confident that they reflect the behavior of the system being measured.
However, because comparison of long species lists from several com-
munities is cumbersome and trends can be difficult to communicate with
such lists, changes in diversity indexes within a community can be used
to capture the most salient features of that community. The complete lists
of species are still needed, but they need not always be presented in full.
FACTORS AFFECTING SPECIES DIVERSITY
Several processes that contribute to change in the number of species in
a community can be of concern when the goal of management is to preserve
or increase diversity. A primary determinant of species diversity is en-
vironmental heterogeneity. On a large scale, species diversity increases
as habitat types are added to the environment (Chapter 5~. More niches
are also added as structural complexity increases within a habitat. For
example, vertical complexity in the form of an increase in the number of
foliage layers is associated with an increase in bird species diversity,
because birds often partition a habitat by occupying different horizontal
strata (MacArthur, 19651. Plantings that change the number of foliage
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COMMUNITY ECOLOGY
51
layers can change the number of bird species that a park can support
(Gavereski, 19761.
Bird species diversity is correlated much less with plant species diversity
than with the structural heterogeneity of vegetation (Kerr and Roth, 197 1;
MacArthur and MacArthur, 1961; Orians, 1969; Recher, 19691. Birds
forage widely every day and visit many plants in their search for food,
responding differently to plant species primarily when those species are
structurally very different, as are coniferous and broad-leaved trees
(MacArthur and MacArthur, 1961) or spiny and nonspiny desert shrubs
(Orians and Solbrig, 19771. In contrast, many herbivorous insects spend
their entire foraging lives on one plant and choose their host plants on the
basis of chemical characteristics, which often vary substantially with spe-
cies (Caswell et al., 1973; Ehrlich and Raven, 1965; Fox, 1981; Rhoades,
1979; Westoby, 19781.
Ground-dwelling vertebrates often partition habitats horizontally, and
species diversity increases with increase in horizontal heterogeneity (Pianka,
19661. Horizontal heterogeneity also contributes to bird species diversity:
patchier habitats support more species (Roth, 19761. Structural hetero-
geneity is positively associated with insect diversity, with many species
supported on a single plant, each specialized for foraging or hiding on a
different substrate, such as upper leaf surface, twig, and trunk (Heinrich,
1979; Ricklefs and O'Rourke, 1975; Schultz, 1983a). In addition, chem-
ical variations within a plant cause insects to move more often (Schultz,
1983b). Manipulation of physical and chemical structures of crop plants
and mixtures of plants can thus be an effective way of combating pests
(Crawley, 1983; Hare, 1983; Whitham et al., 19841.
Repeated perturbations in a community change its makeup as species
undergo local extinction and reinvade. The number of species is usually
relatively small in highly disturbed communities, because few populations
are able to re-establish themselves before they are reduced by later dis-
turbances. In contrast, a low rate of disturbance provides few opportunities
for pioneering species and might allow competitively dominant species to
usurp limiting resources, particularly in space-limited systems, such as
rocky intertidal zones or some terrestrial plant communities. Therefore,
the number of species in a community is often greater at intermediate rates
of disturbance (Cornell, 1978; Huston, 1979) than at either low or high
rates.
Some climax plant communities seem to require periodic disturbance
for long-term maintenance. For example, some California chaparral com-
munities (Biswell, 1974) and some grasslands (Wells, 1965) appear to be
maintained by periodic fires. When fire is controlled, these communities
are replaced by others.
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52 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR PLACATIONS
Predation
Predation and periodic disturbance of other types influence species
diversity in similar ways. By removing competitively dominant species,
predators can increase species diversity. For example, experimental re-
moval of starfish from a rocky intertidal zone allows competitively dom-
inant mussels to usurp space from other species; when present, starfish
open up space for other species by selectively removing mussels (Paine,
1966, 19741. However, although mussels in the absence of starfish pre-
dation can take over space and reduce the diversity of macroinvertebrates
in the rocky intertidal zone, the mussel beds provide vertical structure that
actually increases total diversity when microinvertebrates are also consid-
ered; this shows how the effects of spatial heterogeneity and predation
can interact (Suchanek, 19791.
Herbivores can exert similarly powerful effects on community structure
in terrestrial grazing systems. The effects of grazing on the diversity of
plants depends on whether herbivores selectively graze on the competi-
tively dominant species, which increases diversity, or on poorer compet-
itors, which decreases diversity (Harper, 19691. The selective grazing of
herbivores can lead to the replacement of naturally dominant but palatable
species with species that are spiny or toxic. The influence of predators
on species diversity seems to be most powerful in space-limited systems.
Freshwater predators often select their prey by size, and that can result
in large changes in the makeup of plankton communities (Zaret, 19801.
In some cases, the introduction of planktivorous fish into fish-free lakes
can reduce the numbers of larger, competitively dominant zooplankton
and lead to increases in smaller species (Brooks and Dodson, 1965),
although the changes can be complex and difficult to predict (DeMott and
Kerfoot, 19821.
Competition
Competition can influence community structure by causing the elimi-
nation of some species from local regions or habitats and by reducing the
abundances of species in the habitats in which they occur. There are
reasons for expecting competition to be strongest among closely related
species (Darwin, 1859; Lack, 1954), and many such cases of competition
have been documented. However, competition has been especially looked
for among closely related species, and the frequency of competition among
more distantly related organisms could be much higher than currently
believed. Competition among distantly related species is especially prev-
alent when space is the limiting resource. Plants of all taxonomic groups
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COMMUNITY ECOLOGY
53
compete strongly with one another and, in the rocky intertidal zone,
animals of different phyla compete with one another and with algae (Con-
nell, 1975; Paine, 1966; Underwood and Denley, 19841.
Territorial exclusion, a form of competition for space, can also occur
between species. Interspecific territoriality is most common among closely
related organisms, but does occur among more distantly related ones as
well, especially among fish (Ebersole, 1977; Myerberg and Thresher,
19741. Some cases of interspecific territoriality among birds also involve
distantly related species (Cody, 1969; Moore, 1978; Orians and Willson,
19641.
Competition occurs among distantly related grazing mammals, such as
moose and hares (Belovsky, 19841. In desert ecosystems, ants and rodents
compete strongly for seeds (Davidson et al., 1980, 1984; Kodric-Brown
and Brown, 19791. Many more cases of competition among distantly
related species are likely to be uncovered as ecologists devote more effort
to the study of such competition.
Competition seems to be rare among herbivorous arthropods (Strong,
1984~. This suggests that management practices that exert their effects
precisely on the target insect species are unlikely to result in unintended
side effects on many other species in the community. The use of toxic
substances that adversely affect many species has led to greatly magnified
influence on population dynamics of other species (Chapter 244. Careful
targeting of management toward the focal species decreases the likelihood
of side effects that undermine the goals of management.
Productivity
Field studies of more or less natural ecosystems have shown a positive
relationship between the number of species in an ecosystem and its pro-
ductivity (Cornell and Orias, 19641. The most common interpretation is
that in productive ecosystems more resources are above the minimal abun-
dance required to support users than in unproductive ecosystems (Cornell
and Orias, 1964; MacArthur, 1972) and that animals in productive eco-
systems can therefore specialize on resources that in less productive en-
vironments can be used only by generalists. Also, if productivity affects
the number of species of plants, then the richness of species of animals
that depend on the plants automatically increases as a consequence.
In contrast, it is commonly observed that eutrophication of lakes re-
duces, rather than increases, species richness (Rosenzweig, 19721. There
is no generally accepted explanation of this response. One model assumes
that increasing productivity makes predators more effective in eliminating
some of their prey or in inducing wide oscillations in their abundances,
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54 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
which are likely to lead to extinction from a number of causes (Rosen-
zweig, 19721. Another suggests that competitive exclusion is more im-
portant in enriched environments; enriched environments favor "weedy"
species that dominate typical members of less productive environments
(Huston, 19791. Managers must be alert to the possibility that changes in
ecosystem productivity, a common goal or by-product of human inter-
vention, will lead to unexpected changes in abundances and distributions
of many species in a system. Among the affected species are likely to be
some of aesthetic or commercial value.
Spatial Factors
Habitat patch size and isolation, two factors that can have strong effects
on diversity, are discussed in detail in Chapter 5. Species diversity in-
creases as the area of an " island" of habitat increases, and species diversity
in a given habitat patch generally decreases as the patch becomes more
isolated either by distance or by the unsuitability of intervening habitat.
How these factors affect design and management of ecological reserves
and biological control programs is important, and there is much discussion
over how to apply understanding of them to the long-term preservation
of species and communities (Franker and Soule, 1981; Simberloff and
Abele, 19761. For example, to conserve some communities of species, it
is necessary to maintain a mosaic of various successional stages (Pickett
and Thompson, 19801. The resulting habitat patchiness allows species
adapted to each stage to find suitable areas. A mosaic of stages can also
afford temporary refuge to prey or host species; they will eventually be
eliminated in any particular patch by predators or parasites, but by then
they will have colonized other patches (Dodd, 19591.
The spatial configuration of habitat patches also determines the extent
and nature of ecotones. Ecotones support many species that would not be
present in pure communities. Some forest management plans attempt to
maximize diversity by creating configurations of habitat patches with much
ecotone while retaining patch sizes and proximities that can support species
that rely on single community types (Thomas, 1979~. What constitutes
the most appropriate configuration of patch size, shape, and spacing de-
pends on the requirements of the species to be maintained.
Summary
The exact roles of the factors that influence biogeographic patterns of
species are controversial (Brown, 1984), but the factors reviewed above
are all known to affect diversity. An understanding of these factors can
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COMMUNITY ECOLOGY
55
provide an environmental manager with a powerful set of tools for ma-
nipulating the environment to bring about or limit changes in diversity.
Because management is usually targeted to particular species or groups
of species and not toward diversity itself, however, additional knowledge
of the natural history and population dynamics of the species of interest
.
is required.
Competition, predation, and mutualistic interactions combine with com-
ponents of the environment to influence species richness in various ways.
Plant species richness is often high in the presence of low soil fertility
and periodic disturbance, both of which interact to slow down the takeover
of a site by competitively dominant species (Huston, 1979; Tilman, 1982;
Chapter 181. A similar phenomenon occurs in rocky intertidal habitats,
where animals are the dominant competitors for space (Menge and Suth-
erland, 1976; Paine, 1966~.
COMMUNITY ORGANIZATION
Species richness is only one property of a community that influences
its structure and dynamics. Abiotic factors, such as moisture and tem-
perature (Holdridge, 1967), and the biological processes of competition
(Strong et al., 1984), predation (Paine, 1980), evolution (Orians, 1975),
and trophic structure (May, 1983; Paine, 1980; Pimm, 1982, 1984) act
to influence the structure of a community by determining its makeup and
the constraints under which its constituent species live. Because the link-
ages between species can be at once complex, indirect, and strong (Paine,
1984), investigating the effects of perturbations by studying single species
can be misleading (Kimball and Levin, 19851.
Nonetheless, a small number of factors often dominate the organization
of a given community and determine its response to particular stresses.
For example, soil nutrients in many tropical forests are primarily tied up
in the vegetation and superficial soil layers. When all the vegetation of
these forests is removed for cultivation and the soils are unprotected from
nutrient leaching, the soil can lose its capacity for regeneration for a long
period (Gomez-Pompa et al., 19721. Particular species often dominate the
visual appearance and structure of communities, providing physical struc-
ture for the existence of many other species. For example, coral reef
communities depend critically on the reef-building activities of living
corals. In terrestrial communities, vascular plants provide the dominant
substrate on which most biological interactions are carried out.
A single species can be critical to the maintenance of a community in
its "normal" state, such as starfish in some rocky intertidal communities.
Those "keystone predators" exert an influence on community makeup
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56 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
out of proportion to their numbers or biomass. The idea of keystone species
was first applied to predators that have sessile prey, especially when the
preferred prey is a dominant competitor for space in the absence of pre-
dation. For example, the elimination of the sea otter along the Pacific
coast of North America contributed to a large increase in the number of
sea urchins, a major food item of otters; and the proliferation of urchins
resulted in the decline of kelp beds through excessive urchin grazing
(Duggins, 1980; Estes et al., 19821. Lobsters can function similarly as
keystone species by preying on urchins off the Atlantic coast (Mann and
Breen, 19721. The African elephant exerts a large effect on the landscape
by destroying shrubs and trees; that results in the proliferation of grasses,
an increase in the frequency of fires, and the conversion of woodland to
grassland (Krebs, 19851.
STABILITY AND RESILIENCE OF
ECOLOGICAL COMMUNITIES
Ecologists hold diverse opinions about the relationships between the
numbers of species in an ecosystem and the complexity of their interactions
and about the system's responses to perturbations. Some have asserted
that simple ecosystems are much less stable than complicated ecosystems
(Elton, 1958; Hutchinson, 1959; MacArthur, 1955; Watt, 1964), and
others have asserted the opposite (Gilpin, 1975; Goodman, 1975; Horn,
1974; May, 1973; Pimm, 19791. This diversity of opinion reflects inad-
equacies in information and the use of different definitions of stability and
different types of communities and perturbations. "Stability" has been
used to refer to lack of fluctuations (constancy), resistance to being changed
by external perturbations (inertia), speed of recovery from perturbations
(resilience), and other ideas (Goodman, 1975; Holling, 1973; Orians,
19751. Frank (1968) has pointed out that a community of long-lived species
can appear to have some aspects of stability merely because the component
species live a long time.
A general relationship between stability in any general sense and species
richness is unlikely. Many natural ecosystems are species-poor, but none-
theless stable by some definition mentioned above (e.g., Arctic tundra).
And some species-rich systems are sensitive to disturbance, because of
the intricacies of the connections among their component species (e.g.,
tropical rain forests). Moreover, human-induced perturbations not only
change species richness, but also create new patterns of interactions (e.g.,
Cairns, 19801. Until the species have adjusted through evolution to those
new patterns, the systems might behave in ways that reflect not simply
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COMMUNITY ECOLOGY
57
their altered richness, but the evolutionary novelty of the interactions
(May, 19731.
For management purposes, it is important that the meaning of "stabil-
ity" most appropriate for the problem at hand be clearly specified. In
some cases, such as preservation of valued species, it could be most
important to prevent the system from being changed very much by the
planned actions (Chapter 161. In other cases, such as control of erosion,
it could be more important to quicken the return to a former condition of
the community, because the problem depends primarily on the duration
of a disturbance. Ecological knowledge probably will never be able to
provide answers that are general and yet precise enough to replace the
need for understanding specific systems and perturbations. Such knowl-
edge can be expected, however, to help in focusing research more narrowly
on the most important interactions.
INVADAB IL IT Y
An early survey of invasion by plants and animals was carried out by
Elton (1958~. He concluded that invaders were more likely to establish
populations in cultivated and otherwise disturbed environments than in
pristine environments, and he noted that islands were more susceptible to
invasion than mainland areas. This general perspective has been supported
by recent research, although the reasons for the relationships are not much
clearer than they were 30 years ago. Determining whether a species might
invade new areas requires knowledge about its life history, relationships
with other species, and responses to various agents that perturb
ecosystems.
Herbaceous plants have been among the most successful invaders of
new environments. The flora of California now contains nearly 1,000
exotic plants, and much of the intermountain west is dominated by Eu-
ropean and Asian annuals (Mack, in press; Mack and Thompson, 19821.
Communities of freshwater fish also appear to be unusually susceptible
to invasion by exotic species (Courtenay and Stauffer, 19841. Birds do
not invade new areas as easily. Only three natural invasions of North
America by birds have occurred during the last century: those of the cattle
egret and two gulls. All three exploit food resources that have greatly
expanded in recent decades (Orians, in press). Deliberately introduced
species, such as starlings and house sparrows, primarily exploit human-
modified environments. Many of the escaped captive birds that have es-
tablished feral populations in North America also exploit new food re-
sources, particularly those provided by extensive plantings of ornamental
trees and shrubs in southern cities.
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58 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
invasions by herbivorous insects are complex, but most species feed
on plants that are closely related to the species on which they feed in their
native range (Furniss and Carolin, 19801. Many insects colonize intro-
duced plants in all parts of the world, but their natural food plants are
generally unknown (Strong et al., 19841.
SUBSTITUTABILITY
When a species is removed from an ecological community, its roles are
sometimes taken up entirely or in part by other species. The degree to
which this occurs is referred to as substitutability. Because all species are
involved in many different interactions, substitutability probably varies
with the particular role being considered. For example, the blight-caused
loss of the American chestnut in Appalachian forests resulted in only a
temporary reduction in rates of photosynthesis in those forests, because
other trees replaced chestnuts in the canopy. However, species that are
specialists on the tissues of chestnut trees (folivores, frugivores) must have
suffered major losses that will continue as long as chestnuts are rare.
The likely effects of species losses on community dynamics depend on
the details of current interactions, so an important part of project planning
is a survey of competitive, predator-prey, and mutualistic interactions of
an obligate and specialized nature. Such information can help in predicting
which species losses are most likely to affect other species in the system.
The significance of the potential effects can be evaluated, and steps to
reduce the likelihood of their occurrence can be included in the project
plan.
ECOLOGICAL SUCCESSION
As long as physical conditions do not change greatly, more or less
distinct communities tend to replace others after disturbance in a pre-
dictable way. Although ecological succession was originally thought of
as a community process, examination of particular successions has shown
that abrupt, wholesale extinction of the constituent species of one com-
munity with concurrent colonization by the species of another is rare
(Drury and Nisbet, 19731. The fates of some pairs or groups of species
are inextricably intertwined, as are the fates of some mutualists, but these
linkages are in a minority. Typically, the times of appearance and dis-
appearance of most species in a succession are generally independent of
those of others, and some species that seem late are present early, but in
. · ~
an inconspicuous form.
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COMMUNITY ECOLOGY
59
The nature of the interactions among species that determine their turn-
over during a succession and the relative stability of the climax stage are
poorly understood for many successions, partly because no community is
exactly like any other. Recent research has suggested that processes and
patterns of succession differ among communities and depend on which
species are present at the start and are available to colonize later and on
their life histories (Horn, 19761. Some early species modify the environ-
ment to facilitate growth and recruitment of other species, as colonizers
of sand dunes stabilize the soil and so allow others to become established
(Olson, 1958~. Many pioneering plant species are so intolerant of shade
that the shade they create inhibits growth of their own seedlings. Some
species inhibit others chemically (Rice, 19741. Some late successional
species persist because they are more tolerant of potential sources of
mortality, such as fire or grazing (Harper, 1969; Sousa, 19841.
Although successions are highly variable in detail, most have some
characteristics in common. Odum (1969) listed many patterns of change
in energy flow, biomass, and physical structure that are predictable. Some
of these patterns, such as the ratio of gross production to respiration
(PG/R) for the community as a whole, can indicate the stage of a succession
and how long a given stage is likely to persist without intervention.
Early successional stages typically have relatively high PER, whereas
later stages have ratios approaching 1:1. Part of the reason is that early
species are usually herbaceous, with most of each plant's resources devoted
to growth and reproduction; many later species, which persist longer,
support more woody tissue and devote more resources to competition than
to reproduction. Thus, early stages do not lose in respiration most of the
matter produced by photosynthesis, as do later stages, and usable (net)
production is relatively high. The high net production of early succession
is harvestable for human use, and this is taken advantage of in agriculture
and forestry.
Human societies usually try to maintain early successional stages pre-
cisely because they are more productive, but maintaining them in the face
of the natural tendency for change requires large expenditures of energy,
effort, and materials. Prolonging normally short-lived early successional
stages by calculated disturbances (such as plowing and weeding) or the
use of chemicals (such as herbicides and pesticides) entails environmental
and health problems (e.g., see Chapters 14, 23, and 241. Simply harvesting
in the same site for a long period can result in slow degradation of the
soil by erosion and leaching of nutrients. As the soil loses its capacity for
production, the economic and environmental costs of maintenance grow
with the use of fertilizers.
The challenge for ecologists is to help to identify ways of minimizing
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60 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
the large expenditures and environmental costs of maintaining early
successional communities. Integrated pest-management programs aim at
reducing the role of pesticides by integrating the use of pesticides with
other modes of control (e.g., crop rotation and biological control) on the
basis of detailed studies of pest life history and ecology. Soil erosion can
be reduced by such techniques as reducing tillage and selecting optional
contours (Greenland and Lal, 19771. Herbicide applications to powerline
rights of way can be reduced by planting shrubs that impede succession
(Niering and Egler, 19551.
CONCLUSIONS
All human-induced environmental disturbances alter interactions among
species in some way that leads to direct and indirect affects on the com-
position of ecological communities and their dynamics. Generally, the
direct effects on species of concern are more readily identified and antic-
ipated than are the indirect effects, especially the effects that influence
community properties that are the summation of activities of many species.
The major question is sometimes how long a community will remain in
an altered state. At other times, the main question is how seriously a
community is changed. Major changes might be intolerable, even if the
community eventually returns to its predisturbance state. The problems
described in this chapter are among the most difficult to deal with and
are accordingly those for which careful planning and monitoring of a
project are especially important, if unexpected and undesired ecological
changes are to be avoided or reduced.
Representative terms from entire chapter:
community ecology
