Chapter 50 An Introduction to Ecology and the Biosphere
Overview: The Scope of Ecology
• Ecology is the scientific study of the interactions between organisms
and their environment.
Concept 50.1 Ecology is the study of interactions between organisms
and the environment
• Ecologists ask questions about factors affecting the distribution and
abundance of organisms.
• Ecologists might study how interactions between organisms and the
environment affect the number of species living in an area, the
cycling of nutrients, or the growth of populations.
Ecology and evolutionary biology are closely related sciences.
• Ecology has a long history as a descriptive science.
• Modern ecology is also a rigorous experimental science.
• Ecology and evolutionary biology are closely related sciences.
• Events that occur over ecological time (minutes to years) translate
into effects over evolutionary time (decades to millennia).
• For example, hawks feeding on field mice kill certain individuals
(over ecological time), reducing population size (an ecological
effect), altering the gene pool (an evolutionary effect), and
selecting for mice with fur color that camouflages them in their
environment (over evolutionary time).
Ecological research ranges from the adaptations of individual
organisms to the dynamics of the biosphere.
• The environment of any organism includes the following components:
• Abiotic components: nonliving chemical and physical factors
such as temperature, light, water, and nutrients.
• Biotic components: all living organisms in the individual’s
environment. • Ecology can be divided into a number of areas of study.
• Organismal ecology is concerned with the behavioral, physiological,
and morphological ways individuals interact with the environment.
• A population is a group of individuals of the same species living in a
particular geographic area. Population ecology examines factors that
affect population size and composition.
• A community consists of all the organisms of all the species that
inhabit a particular area. Community ecology examines the
interactions between species and considers how factors such as
predation, competition, disease, and disturbance affect community
structure and organization.
• An ecosystem consists of all the abiotic factors in addition to the
entire community of species that exist in a certain area. Ecosystem
ecology studies energy flow and cycling of chemicals among the
various abiotic and biotic components.
• A landscape or seascape consists of several different ecosystems
linked by exchanges of energy, materials, and organisms. Landscape
ecology deals with arrays of ecosystems and their arrangement in a
• Each landscape or seascape consists of a mosaic of different
types of patches, an environmental characteristic ecologists
refer to as patchiness. Landscape ecological research focuses
on the factors controlling exchanges of energy, materials, and
organisms among ecosystem patches.
• The biosphere is the global ecosystem, the sum of all of the planet’s
ecosystems. The biosphere includes the entire portion of Earth
inhabited by life, ranging from the atmosphere to a height of several
kilometers to the oceans and water bearing rocks to a depth of
Ecology provides a scientific context for evaluating environmental
• It is important to clarify the difference between ecology, the scientific
study of the distribution and abundance of organisms, and environmentalism, advocacy for the protection or preservation of the
• To address environmental problems, we need to understand the
interactions of organisms and the environment.
• The science of ecology provides that understanding.
• In 1962, Rachel Carson’s book Silent Spring warned that the use of
pesticides such as DDT was causing population declines in many
• Today, acid precipitation, land misuse, toxic wastes, habitat
destruction, and the growing list of endangered or extinct species are
just a few of the problems that threaten the Earth.
• Many influential ecologists feel a responsibility to educate legislators
and the general public about decisions that affect the environment.
• It is important to communicate the scientific complexity of
• Our ecological information is always incomplete. The precautionary
principle (essentially “an ounce of prevention is worth a pound of
cure”) can guide decision making on environmental issues.
Concept 50.2 Interactions between organisms and the environment
limit the distribution of species
• Ecologists have long recognized distinct global and regional patterns
in the distribution of organisms.
• Biogeography is the study of past and present distributions of
individual species in the context of evolutionary theory.
• Ecologists ask a series of questions to determine what limits the
geographical distribution of any species.
Species dispersal contributes to the distribution of organisms.
• The movement of individuals away from centers of high population
density or from their area of origin is called dispersal. • The dispersal of organisms is crucial to understanding geographic
isolation in evolution and the broad patterns of geographic distribution
• One way to determine if dispersal is a key factor limiting distribution is
to observe the results when humans have accidentally or intentionally
transplanted a species to areas where it was previously absent.
• For the transplant to be considered successful, the organisms
must not only survive in the new area, but also reproduce there.
• If the transplant is successful, then the potential range of the species
is larger than its actual range.
• Species introduced to new geographic locations may disrupt the
communities and ecosystems to which they are introduced.
• Consequently, ecologists rarely conduct transplant experiments
• Instead, they study the outcome when a species has been
transplanted accidentally or for another purpose.
Behavior and habitat selection contribute to the distribution of
• Sometimes organisms do not occupy all of their potential range but
select particular habitats.
• Does behavior play a role in limiting distribution in such cases?
• Habitat selection is one of the least-understood ecological processes,
but it appears to play an important role in limiting the distribution of
Biotic factors affect the distribution of organisms.
• Do biotic factors limit the distribution of species?
• Negative interactions with other organisms in the form of
predation, parasitism, disease, or competition may limit the
ability of organisms to survive and reproduce. • Predator-removal experiments can provide information
about how predators limit distribution of prey species.
• Absence of other species may also limit distribution of a
• For example, the absence of a specific pollinator or prey
species may limit distribution of an organism.
Abiotic factors affect the distribution of organisms.
• The global distribution of organisms broadly reflects the influence of
abiotic factors such as temperature, water, and sunlight.
• The environment is characterized by spatial and temporal
• Environmental temperature is an important factor in the distribution of
organisms because of its effect on biological processes.
• Very few organisms can maintain an active metabolism at very
high or very low temperatures.
• Some organisms have extraordinary adaptations to allow them
to live outside the temperature range habitable for most other
• The variation in water availability among habitats is an important
factor in species distribution.
• Most aquatic organisms are restricted to either freshwater or
• Terrestrial organisms face a nearly constant threat of
desiccation and have adaptations to allow them to obtain and
• Sunlight provides the energy that drives nearly all ecosystems.
• Intensity of light is not the most important factor limiting plant
growth in most terrestrial environments, although shading by a
forest canopy makes competition for light in the understory
intense. • In aquatic environments, light intensity limits distribution of
• Every meter of water depth selectively absorbs 45% of
red light and 2% of blue light passing through it.
• As a result, most photosynthesis in aquatic environments
occurs near the surface.
• Photoperiod, the relative length of daytime and nighttime, is a
reliable indicator of seasonal events and is an important cue for
the development or behavior of many organisms.
• Wind amplifies the effects of temperature by increasing heat loss due
to evaporation and convection. It also increases water loss by
increasing the rate of evaporative cooling in animals and transpiration
• The physical structure, pH, and mineral composition of soils and
rocks limit distribution of plants and, thus, of the animals that feed
upon them, contributing to the patchiness of terrestrial ecosystems.
• In streams and rivers, substrate composition can affect water
chemistry, affecting distribution of organisms.
• In marine environments, the structure of substrates in the intertidal
areas or seafloor limits the organisms that can attach to or burrow in
Four abiotic factors are the major components of climate.
• Climate is the prevailing weather conditions in an area.
• Four abiotic factors—temperature, water, sunlight, and wind—
are the major components of climate.
• Climatic factors, especially temperature and water, have a
major influence on the distribution of organisms.
• Climate patterns can be described on two scales. Macroclimate
patterns are on global, regional, or local levels, and microclimate
patterns are very fine patterns such as the conditions experienced by
a community of organisms under a fallen log. • Climate determines the makeup of biomes, the major types of
• Annual means for temperature and rainfall are reasonably well
correlated with the biomes found in different regions.
• Global climate patterns are determined by sunlight and Earth’s
movement in space.
• The sun’s warming effect on the atmosphere, land, and water
establishes the temperature variations, cycles of air movement,
and evaporation of water that are responsible for latitudinal
variations in climate.
• Bodies of water and topographic features such as mountain ranges
create regional climatic variations, while smaller features of the
landscape affect local climates.
• Ocean currents influence climate along the coast by heating or
cooling overlying air masses, which may pass over land.
• Coastal regions are generally moister than inland areas at the
• In general, oceans and large lakes moderate the climate of
nearby terrestrial environments.
• In certain regions, cool, dry ocean breezes are warmed
when they move over land, absorbing moisture and
creating a hot, rainless climate slightly inland.
• This Mediterranean climate pattern occurs inland from the
• Ocean currents also influence climate in coastal areas.
• Mountains have a significant effect on the amount of sunlight
reaching an area, as well as on local temperature and rainfall.
• In the Northern Hemisphere, south-facing slopes receive more
sunlight than north-facing slopes, and are therefore warmer and
• These environmental differences affect species distribution. • At any given latitude, air temperature declines 6°C with every 1,000-
m increase in elevation.
• This temperature change is equivalent to that caused by an
880-km increase in latitude.
• As moist, warm air approaches a mountain, it rises and cools,
releasing moisture on the windward side of the peak.
• On the leeward side of the mountain, cool, dry air descends,
absorbing moisture and producing a rain shadow.
• Deserts commonly occur on the leeward side of mountain
• The changing angle of the sun over the course of a year affects local
• Belts of wet and dry air on either side of the equator shift with
the changing angle of the sun, producing marked wet and dry
seasons around 20° latitude.
• Seasonal changes in wind patterns produce variations in ocean
currents, occasionally causing the upwelling of nutrient-rich,
cold water from deep ocean layers.
• Lakes are also sensitive to seasonal temperature changes.
• During the summer and winter, many temperate lakes are
thermally stratified or layered vertically according to
• These lakes undergo a semiannual mixing, or turnover, of their
waters in spring and fall. Turnover brings oxygenated water to
the bottom and nutrient-rich water to the surface.
• Many features in the environment influence microclimates.
• Forest trees moderate the microclimate beneath them.
• Cleared areas experience greater temperature extremes
than the forest interior. • A log or large stone shelters organisms, buffering them from
temperature and moisture fluctuations.
• Every environment on Earth is characterized by a mosaic of
small-scale differences in abiotic factors that influence the local
distribution of organisms.
• Long-term climate changes profoundly affect the biosphere.
• One way to predict possible effects of current climate changes is to
consider the changes that have occurred in temperate regions since
the end of the last Ice Age.
• Until about 16,000 years ago, continental glaciers covered much of
North America and Eurasia.
• As the climate warmed and the glaciers melted, tree distribution
• A detailed record of these migrations is captured in fossil po