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Populations and Communities in Ecology
Overview
Ecology is the study of how living things interact—with each other and with their physical
environment. One way of studying these interactions is to focus on populations, meaning
all the members of a single species living in one area. Another way of studying these
interactions is to look at communities, meaning all the populations of all the species
living in one area.
The Study of Ecology
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Ecology is the study of the interactions living things have with each other and
with their environment.
Ecology is not the same thing as environmentalism. The function of ecology is to
describe interactions that affect the living world, not to work on behalf of
environmentalism.
There are five scales of life that concern ecology: physiology, populations,
communities, ecosystems, and the biosphere. A population is all the members of
a single species that live together in a specified geographical area; a community
is all the species living in a single area; an ecosystem is a community and all the
nonliving elements that interact with it; the biosphere is the interactive collection
of all the Earth’s ecosystems.
Populations: Size and Dynamics
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Ecologists employ several means to estimate the size of populations of living
things, among them counting animal droppings or surveying bird populations as
they migrate.
An arithmetical increase occurs when, over a given interval of time, an unvarying
number of new units is added to a population. An exponential increase occurs
when the number of new units added to a population is proportional to the
number of units that exists. Populations of living things are capable of increasing
exponentially, because living things are capable of giving rise to more living
things.
The rapid growth that sometimes characterizes living populations is referred to as
exponential growth, or as the J-shaped growth curve. Populations that initially
grow, but whose growth later levels out, have experienced logistic growth,
sometimes referred to as the S-shaped growth curve.
The size of living populations is kept in check by environmental resistance,
defined as all the forces of the environment that act to limit population growth.
Exponential growth in living populations can be calculated by subtracting a
population’s death rate from its birth rate, which yields the population’s growth
rate. Denoted as r, this rate is also known as the population’s intrinsic rate of
increase; it can be thought of as the population’s potential for growth.
Carrying capacity, denoted as K, is the maximum population density of a given
species that can be sustained within a defined geographical area over an
extended period of time.
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r-Selected and K-Selected Species
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Different species have different reproductive strategies, meaning characteristics
that have the effect of increasing the number of fertile offspring they bear.
Some species are said to be K-selected, or equilibrium, species. These species
tend to be large, to experience their environment as relatively stable, and to
lavish a good deal of attention on relatively few offspring. The pressures on Kselected species tend to be density dependent, meaning that as a population’s
density goes up, factors that limit the population’s growth assert themselves ever
more strongly.
Other species are said to be r-selected or opportunist species. These species
tend to be small, to experience their environment as relatively unstable, and to
give little or no attention to the numerous offspring they produce. The pressures
on r-selected species tend to be density independent, meaning pressures that
are unrelated to the population’s density.
Survivorship curves describe how soon species members tend to die within the
species’ life span. There are three idealized types of survivorship curves: late
loss, constant loss, and early loss.
Thinking about Human Populations
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Survivorship curves are created from life tables, which set forth the probabilities
of a member of a species being alive after given intervals of time.
An important step in calculating the future growth of human populations is to
learn what proportion of the population is at or under reproductive age. A
population pyramid displays this proportion. Populations whose pyramids are
heavily weighted toward younger age groups are likely to experience relatively
large growth.
Following centuries of explosive increase, the world’s human population is
projected to stabilize in the coming decades, going from about 6.3 billion now to
a maximum of about 8.9 billion at mid-century. This stabilization is being brought
about by a decrease in female fertility, defined as the number of children born, on
average, to each woman in a population.
The global reduction in female fertility masks enormous, ongoing differences
between fertility in more-developed and less-developed countries. Fertility in
less-developed countries tends to be much higher than that in more-developed
countries. The fertility in most European nations is now so low that the
continent’s population stands to shrink significantly by mid-century. The
population of the United States, however, is projected to grow significantly during
this same period. The primary factors bringing the U.S. increase about are
immigration and relatively high female fertility.
Some scientists believe that there is no greater single threat to the environment
than the continued growth of the human population. Others argue that a more
important concern is the use of natural resources per person.
Communities: Looking at the Interactions of Many Populations
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An ecological community is all the populations that inhabit a given area, though
the term can be used to mean a collection of populations in a given area that
potentially interact with one another.
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Most communities tend to be dominated by only a few species; the few species
that are abundant in a given area are called ecological dominants.
A keystone species is a species whose impact on the composition of a
community is disproportionately large relative to its abundance within that
community. Some keystone species are top predators in a community, meaning
species that prey upon other species but that are not preyed upon themselves.
Keystone species need not be top predators, however, nor does every
community have a keystone species.
Biodiversity is variety among living things. It takes three primary forms: a diversity
of species in a given area; a distribution of species across the Earth; and genetic
diversity within a species.
Recent research indicates that species diversity tends to enhance a community’s
productivity, defined as the amount of solar energy photosynthesizing organisms
are able to capture and transform into living material or biomass. There is
disagreement about whether species diversity also enhances community
stability, meaning the ability of a community to retain its characteristics in the
face of environmental disruption.
Types of Interaction among Community Members
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There are three primary types of interaction among community members:
competition; predation and parasitism; and mutualism and commensalism.
Habitat is the physical surroundings in which a species can normally be found.
Niche can be defined metaphorically as an organism’s occupation, meaning what
the organism does to obtain the resources it needs to live.
There are numerous instances in nature in which two related species use the
same kinds of resources from the same habitat over an extended period of time,
but will divide the resources up, such that neither of species undergoes local
extinction. This phenomenon is called coexistence through resource partitioning.
Predation is defined as one free-standing organism feeding on parts or all of a
second organism. Parasitism is a variety of predation in which the predator feeds
on prey, but does not kill it immediately, if ever.
The population dynamics of a predator and its prey can be linked, but predatorprey interaction generally is only one of several factors controlling the population
level of either group.
The prey of a parasite is known as the host. A parasite can use a host not only
as a food source but as a vehicle to facilitate its reproduction.
Mimicry is a phenomenon by which one species has evolved to assume the
appearance of another. A mimic species evolves to match the appearance of a
model species. In general, the value of mimicry is that the mimic species suffers
less predation as a result of its resemblance to the model species. Batesian
mimicry is the evolution of one species to resemble a species that has a superior
protective capability. In Müllerian mimicry, several species that have protection
against predators come to resemble each other.
Mutualism is an interaction between individuals of two species that is beneficial
to both individuals. Commensalism is an interaction in which an individual from
one species benefits while an individual from another species is neither harmed
nor helped.
Coevolution is the interdependent evolution of two or more species. Flowers
have evolved colors and fragrances that attract bees, for example, while bees
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have evolved vision that is most sensitive to the colors in the flowers they
pollinate.
Succession in Communities
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Parcels of land or water that have been abandoned by humans or devastated by
physical forces will almost always be reclaimed by nature to some degree. The
process by which this takes place is called succession: a series of replacements
of community members at a given location until a relatively stable final state is
reached.
Primary succession proceeds from an original state of little or no life and soil that
lacks nutrients. Secondary succession occurs when a final state of habitat is first
disturbed by some outside force, but life remains and the soil has nutrients. The
final community in any process of succession is known as the climax community.
A common set of developments occurs in most instances of primary succession,
including the arrival of “pioneer” photosynthesizers, facilitation of the growth of
some later species through the actions of earlier species, and the competitive
driving out of some species by the actions of later species.
The rejuvenation of the Mount St. Helens area that has occurred since 1980 has
provided ecologists with a wealth of information regarding both primary and
secondary succession. One of the chief lessons learned concerns the importance
of biological legacies, meaning living things, or products of living things, that
survive a major ecological disturbance. Biological legacies have been critical to
the rapid renewal of habitat at Mount St. Helens.
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