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33.1 The Scope of Ecology
Ecology is the study of the interactions of organisms with each other and with the physical
environment. A population is defined as all the organisms of the same species interacting with
the environment at a particular locale. A community consists of all the various populations at a
particular locale. An ecosystem encompasses a community of populations, as well as the
nonliving environment. The biosphere is the portion of the entire Earth’s surface where living
things exist.
33.2 Patterns of Population Growth
The population size can stay the same, increase, or decrease. The highest possible per capita rate
of increase for a population is called its biotic potential. A J-shaped growth curve has a lag phase
and an exponential growth phase. An S-shaped pattern has a lag phase, an exponential growth
phase, a deceleration phase, and a stable equilibrium phase. The stable equilibrium phase is said
to occur at the carrying capacity of the environment.
Three types of idealized survivorship curves are recognized.
Human Population Growth
Growth in less-developed countries is still in the exponential phase. Growth in moredeveloped countries has leveled off. The rapid growth of the human population can be
appreciated by considering the doubling time. Currently, the doubling time is estimated
to be 56 years.
More-Developed versus Less-Developed Countries
The more-developed countries are those in which population growth is low and
people enjoy a good standard of living. The less-developed countries are those in
which population growth is expanding rapidly and the majority of people live in
Age Distributions
An age-structure diagram divides the population into three age groups:
prereproductive, reproductive, and postreproductive.
33.3 Regulation of Population Growth
Members of opportunistic populations are small in size, mature early, and have a short life span.
Equilibrium pattern organisms are fairly large, slow to mature, and have a fairly long life span.
Abiotic factors, such as weather and natural diseases, are density-independent. Biotic factors,
such as competition, predation, and parasitism, are called density-dependent.
Competition occurs when members of different species try to utilize a resource that is in
limited supply. The ecological niche is the role an organism plays in the community,
including its habitat or where the organism lives. Competition may result in resource
Predation occurs when one organism, called the predator, feeds on another, called the
Predator-Prey Population Dynamics
Predators reduce the population density of prey. Mathematical formulas predict
that predator and prey populations may sometimes cycle instead of maintaining a
steady state.
Antipredator Defenses
Prey organisms have evolved strategies to escape predation.
Mimicry occurs when one species resembles another species that has
evolved to defend against predators or resembles an object in the
environment to deceive prey.
Symbiosis refers to close interactions between members of two species. Three types of
symbiotic relationships have traditionally been defined—parasitism, commensalism, and
Parasitism is a symbiotic relationship in which the parasite derives nourishment
from another organism, called the host.
Commensalism is a symbiotic relationship between two species in which one
species is benefited and the other is neither benefited nor harmed.
Mutualism is a symbiotic relationship in which both members of the association
Ecological Succession
Ecological succession is a change in a community’s composition that is directional and
follows a continuous pattern of extinction and colonization by new species.
Models of Succession
The climax-pattern model of successions says that particular areas will always
lead to the same type of community in a particular area, called a climax
community. How are the various stages of succession related? The facilitation
model, inhibition model, and tolerance model propose different explanations.