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OCR for page 68
Scales in Space and Time
When we alter the size, shape, and spatial distribution of patches of
particular ecological communities, we alter the population dynamics of
the species that live in them. The ability of a population to withstand
environmental fluctuations, for example, depends not only on the life
history of the species, but also on population size and the availability of
immigrants from other populations. Analogously, the temporal charac-
teristics of environmental manipulations can influence the kind and strength
of their effects. Perturbations of longer duration or greater frequency might
exceed the capacity of a community or species to absorb or recover from
them, whereas short or single disturbances might not. Focusing on the
consequences of single perturbations can lead to a failure to perceive the
patterns and cumulative effects of those perturbations over time and space.
If a population or community is repeatedly disturbed for long enough,
changes qualitatively different from and more serious than the effects of
single perturbations often occur. An appropriate choice of scale for think-
ing about, analyzing, and manipulating these processes is crucial.
PATCHINESS AND COMMUNITY COMPOSITION
Species-Area Relationship
Large areas tend to have more species than small areas of similar habitat
type (Cain, 1938; Connor and McCoy, 1979; Gleason, 1922; MacArthur
68
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SCALES IN SPACE AND TIME
69
and Wilson, 1967; Preston, 1960), partly because larger areas typically
have more habitat variation within the general habitat type. Each variant
contains species adapted specially to it. However, even when habitats in
a region are uniform, there is a relationship between area and number of
species. A second reason for this relationship, which is especially pro-
nounced in small areas, is that each species has a minimal viable population
size for a given probability of extinction (Shaffer, 19811. As the area of
a habitat decreases, local populations get smaller and extinction becomes
more likely. In addition, the species-area relationship is partly a result of
sampling (Connor and McCoy, 1979), i.e., large areas receive more im-
migrants than small ones and therefore obtain larger samples from the
species pool. These three reasons for the species-area relationship are not
mutually exclusive and can be difficult to distinguish (Connor and McCoy,
19791.
Extinction of Small Populations
At least five forces increase the probability of extinction of small pop-
ulations:
· Demographic stochasticity. Random fluctuations of demographic events
(birth, death, and determination of sex) endanger small populations. For
example, the probability that all individuals in a generation will be male
is much greater in a small than in a large population.
· Genetic stochasticity, consisting of inbreeding depression and pro-
duction of homozygotes for lethal or severely deleterious recessives. In-
breeding depression is the general decrease in traits that contribute to
fitness, such as fertility. It has been documented in many plant and animal
species and appears to be associated with the increased homozygosity (in
which the two copies of a particular gene are identical) that results when
near relatives mate. Because mating with relatives is more frequent in
small populations, inbreeding depression is greater. In addition, greater
homozygosity for the population as a whole might be associated with
reduced genetic variability and lead to reduced ability of the population
to adapt to environmental change (Soule, 19801. Mating with near relatives
also increases the likelihood of producing individuals homozygous for
recessive traits that are lethal or severely deleterious.
· Environmental stochasticity. Random variation in the physical or
biotic environment of a species affects demographic values, whether the
population is large or small. Such variation, even if not severe, can threaten
the very existence of a small population, however, because the probability
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70 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
that all individuals will be killed is much greater for a small than for a
large population.
· Disasters and catastrophes. The once-in-a-century flood or fire, for
instance, can destroy a local population.
· Social behavior. Some animal species have stylized forms of social
behavior (e.g., predation, defense, thermoregulation, and mating displays)
that break down if there are too few individuals. A breakdown can lead
to breeding failure and endanger the population.
Minimal viable population size varies widely among species, for a
number of reasons. In general, the minimal number of individuals nec
essary to support a population for a long period increases as average
population density decreases. The average area necessary to support an
individual animal is greater for predators than herbivores, and in general
it increases with body size within groups of similar species (McNab, 1963;
Schoener, 19681. Species differ in their ability to tolerate the increase in
inbreeding that occurs in small populations. In general, plant populations
appear to be able to survive longer on smaller sites than animal populations.
It is easier to conserve plants than animals by artificial means (e.g., cold
storage and seed banks).
Patch Geometry and Edge Effects
The shapes of habitat patches cause effects similar to those due to patch
size. As patches deviate from circular to linear, the proportion of their
area close to an edge increases, as it does when they decrease in size.
Species adapted to conditions found at the interfaces between patches of
different types can exploit an increasing fraction of the areas of small
patches. They can compete with species adapted to the interiors of patches,
parasitize them (Brittingham and Temple, 1983), or function as predators
against which interior species are not well adapted (Wilcove, 1985~. Hu-
man modifications of environments often create patches that are much
longer than they are wide (Godron and Forman, 1983), thereby exacer-
bating the effects of patchiness itself.
The shape and orientation of patches can have important ecological
consequences. Cutting of forests into strips causes less erosion if the strips
follow the contours of the terrain, rather than being oriented at right angles
to them (Hornbeck et al., 19751. At high latitudes, direct sunlight might
penetrate to the ground only at dawn and dusk in narrow clearcuts oriented
in an east-west direction, whereas in north-south patches direct sunlight
is present at ground level at midday the time of most intense solar
radiation.
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SCALES IN SPACE AND TIME
DISTRIBUTION OF PATCHES IN SPACE AND TIME
71
Spatial or temporal patchiness is sometimes obvious, but more often
difficult to detect. For example, the distribution of herb species in a field
might appear random when actually it is determined by the microspatial
heterogeneity of soil nutrient conditions (Tilman, 1982~. Patchy distri-
bution of organisms can result from variability in the physical environment
(such as soil types), physical disturbance, and patchiness in biological
interactions (see also Chapter 3~. Studies of intertidal and subtidal com-
munities have shown the importance of local heterogeneity generated by
physical and biological disturbances in both temperate areas (Dayton,
1971; Menge and Sutherland, 1976; Paine, 1966; Paine and Levin, 1981)
and tropical areas (Cornell, 1 978; Porter et al ., 1 98 1 ). The spatial and
temporal variability of lake and deep-sea benthos is well known (Berg,
1938; Brinkhurst, 1974; Grassle et al., 1975; Jonasson, 19721.
Because marine systems are dominated by species that do not derive
nutrients from the substrate (animals and nonvascular plants), substrate-
related variability is due primarily to the physical environment and stability
of the substrate and the type of anchorage it provides (e.g., Dayton, 1984,
19851. In terrestrial systems, however, soils differ primarily in their ability
to supply nutrients and water, so variability in distribution of plants,
burrowing animals, and species that depend on plants is related to these
properties. Soil scientists could be included in terrestrial research teams
more often than they usually are.
Spatial (::onsiderations
Population dynamics in patchy environments are determined pri-
marily by the rates of individual movements between patches and rates
of local population extinction. The properties that enable populations
to maintain themselves in patchy environments include high dispersal
rates, tendencies to cross unsuitable habitats, high growth rates, early
reproduction, and high reproductive rates (Baker and Stebbins, 1965;
MacArthur and Wilson, 1967~. These traits increase the probability that
isolated patches will be found, that reproduction will occur before the
patch becomes unsuitable, and that new colonists will be generated.
Species with the traits increase in abundance in environments that are
heavily modified by people.
Patches are not static, but change with time, primarily as a result of
the growth of and interactions among colonizing organisms. The succes-
sion of organisms over time is driven by several processes that are not
mutually exclusive, but that differ in their relative importance under
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72 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
different conditions (Horn, 1974, 1976; Shugart and West, 1980). Spe
cies are adapted to conditions at different stages before populations
become extinct as a result of within-patch changes. Early successional
stages typically are much shorter than later successional ones. There-
fore, even if rates of disturbance are low, populations of early succes-
sional species are more likely to become extinct. However, current rates
of human-caused disturbance are so high in most areas of the world
that species requiring late stages of succession, such as old-growth
forests, are in the most precarious positions (Chapter 171. In addition,
many organisms have complex life histories in which different stages
require distinct habitats. Not only must individuals find all the necessary
habitat types at the correct time, but populations are affected by fluc-
tuations in the availability of habitats for each stage; these fluctuations
can be independent of each other (Istock, 19671. The supply of appro-
priate habitat must be adequate at the correct time during the year; its
abundance at other times can be irrelevant.
Even within an apparently uniform patch, interactions among sub-
units can be complex. This complexity was not a factor in the case of
Lake Washington (Chapter 20), because the high inflow of water during
winter and early spring, when the lake is isothermal, causes free cir-
culation throughout the lake. Southern Indian Lake (Chapter 21), how-
ever, contains several subbasins, and the major outlet has been close
to the inlet since the diversion of the Churchill River. The lake is thus
not a well-mixed body, and its properties differ between regions, whether
or not they are affected by the flow. The high quality of the whitefish
fishery before impoundment was maintained by confining fishing to
areas where the stocks were virtually free from cestode infestation.
After impoundment and diversion of the normal river flow, the stocks
became redistributed, and the fish were concentrated in areas with high
infestation rates.
Local populations occasionally become extinct because of predation,
disease, or physical disturbance. The rate of recolonization is inversely
related to the distance from other occupied sites that are sources of im-
migrants. Increasing isolation of patches increases the probability that
locally extinct populations will not be replaced, and creation of com-
munities that would normally develop without help might need to be
managed because of a lack of immigrants (Chapter 181. In general, species
characteristic of later successional stages are poorer dispersers than "weedy"
species of earlier stages. In plants, late stages show a striking increase in
the average size of seeds (Harper etal., 1970; Salisbury, 19421; in animals,
the later stages show less tendency to disperse and greater reluctance to
cross stretches of unsuitable habitat.
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SCALES IN SPACE AND TIME
Temporal Considerations
73
Community dynamics are strongly affected by interactions among per-
turbation characteristics (intensity, duration, and frequency), succession,
and the rate at which a community recovers. Very different outcomes are
possible if relative rates of disturbance and recovery are altered. For
example, tropical slash-burn agriculture is compatible with long-term soil
fertility if plots are small, are farmed for only a few years, and are allowed
to remain fallow for several decades (Gomez-Pompa et al., 19721. How-
ever, if plots are made larger and are recut after shorter fallow periods,
soil fertility cannot recover within the period of a single cycle and rapidly
declines (Myers, 19841.
If pesticides are used infrequently, time might be available between
applications for susceptible genotypes of pests to replace resistant types
that are at a disadvantage in the absence of the pesticides. If pesticides
are used more often, this process does not go to completion, and many
resistant genotypes are still present when the pesticide is used again. As
a result, the pesticide becomes progressively less effective, applications
are increased in frequency and magnitude, and the evolution of resistance
among the pests is accelerated (Chapters 1 and 241.
Repeated or continuous perturbations can lead to qualitative changes in
community structure, because the ability of the system to remove or
recover from the disturbance is exceeded. Striking changes took place in
the plankton communities of Lake Washington when the addition of sew-
age at multiple points exceeded the flushing rate of the lake (Chapter 20~.
In addition to the deleterious genetic effects that often occur in small
populations (Franklin, 1980; Selander, 1983), there are long-term evo-
lutionary consequences of patch size and distribution. Evolution in re-
sponse to spatial or temporal change in the environment is retarded by
small population size, because genetic variability is reduced in small
populations (see Endler, 1977, for a discussion of the effect of migration
on evolution). For example, dividing agricultural plots into different sec-
tions and applying a different pesticide to each should retard the evolution
of resistance.
CONCLUSIONS
The great importance of size and spatial relationships in the working
of ecological processes points to the importance of dealing explicitly with
scales in space and time in all efforts to solve environmental problems.
The major changes in processes and products that accompany changes in
spatial and temporal scales can escape attention, if efforts are not directed
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74 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
specifically at them in all phases of environmental problem-solving. Well-
intentioned efforts can be undermined if they are planned for too short a
term or for areas that are too small (Soule and Wilcox, 19801. However,
key ecological processes might be obscured if inappropriately large tem-
poral and spatial scales are used. Averaging over large areas can mask
the importance of local patchiness for the survival of particular species.
Individual trees could be especially susceptible to attack by herbivores,
and the maintenance of large populations of trees could depend critically
on patches. Similarly, patches of high concentrations of nutrients resulting
from defecation of zooplankton might be essential to survival of algae,
and patches of high concentrations of plankton might be important to the
survival of marine fish larvae (Sissenwine, 19841.
Representative terms from entire chapter:
species adapted
