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Unit 2 Ecology (Chapters 3, 4, 5 & 6)
Lesson Summary
Studying Our Living Planet Ecology is the scientific study of interactions among organisms
and between organisms and their environment.
Earth’s organisms live in the biosphere. The biosphere consists of the parts of the planet in
which all life exists.
Ecologists may study different levels of ecological organization:
 Individual organism
 An assemblage of individuals that belong to the same species and live in the same area is
called a population.
 An assemblage of different populations that live together in an area is referred to as a
community.
 An ecosystem includes all the organisms that live in a particular place, together with their
physical environment.
 A group of ecosystems that have similar climates and organisms is called a biome.
Biotic and Abiotic Factors Ecosystems include biotic and abiotic factors.
A biotic factor is any living part of an environment.
An abiotic factor is any nonliving part of an environment.
Ecological Methods Ecologists use three basic methods of research: observation,
experimentation, and modeling:
Observation often leads to questions and hypotheses.
Experiments can be used to test hypotheses.
Modeling helps ecologists understand complex processes.
Primary Producers Sunlight is the main energy source for life on Earth. Organisms that can
capture energy from sunlight or chemicals and use that energy to produce food are called
autotrophs, or primary producers.
The process in which autotrophs capture light energy and use it to convert carbon dioxide and
water into oxygen and sugars is called photosynthesis.
The process in which autotrophs use chemical energy to produce carbohydrates is called
chemosynthesis.
Consumers Organisms that rely on other organisms for their energy and food are called
heterotrophs. Heterotrophs are also referred to as consumers. There are many different types of
heterotrophs:
Herbivores, such as cows, obtain energy by eating only plants.
Carnivores, such as snakes, eat only animals.
Omnivores, such as humans, eat both plants and animals.
Detritivores, such as earthworms, feed on dead matter.
Decomposers, such as fungi, break down organic matter.
Scavengers, such as vultures, consume the carcasses of other animals.
Food Chains and Food Webs Energy flows through an ecosystem in one direction from
primary producers to various consumers.
A food chain is a series of steps in which organisms transfer energy by eating and being eaten.
Producers, such as floating algae called phytoplankton, are at the base of every food chain.
A food web is a network of all the food chains in an ecosystem. Food webs are very complex.
Small disturbances to one population can affect all populations in a food web. Changes in
populations of zooplankton, small marine animals that feed on algae, can affect all of the
animals in the marine food web.
Trophic Levels and Ecological Pyramids Each step in a food chain or food web is
called a trophic level. Producers make up the first trophic level. Consumers make up higher
trophic levels. Each consumer depends on the trophic level below it for energy.
An ecological pyramid is a diagram that shows the relative amounts of energy or matter contained
within each trophic level in a food chain or food web. Types of ecological pyramids are pyramids of
energy, pyramids of biomass, and pyramids of numbers:
Pyramids of energy show relative amounts of energy available at different trophic levels.
Pyramids of biomass show the total amount of living tissue at each trophic level.
A pyramid of numbers shows the relative numbers of organisms at different trophic levels.
Recycling in the Biosphere Matter, unlike energy, is recycled within and between
ecosystems. Elements pass from one organism to another and from one part of the biosphere to
another through biogeochemical cycles, which are closed loops powered by the flow of energy.
The Water Cycle Water moves between the ocean, the atmosphere, and land.
Evaporation is the process in which water changes from a liquid to a gas.
Transpiration is the process in which water evaporates from the leaves of plants.
Nutrient Cycles The chemical substances that an organism needs to survive are called
nutrients. Like water, nutrients pass through organisms and the environment.
Carbon Cycle: Carbon is a key ingredient of all organic compounds. Processes involved in the
carbon cycle include photosynthesis and human activities such as burning.
Nitrogen Cycle: Nitrogen is needed by all organisms to build proteins. Processes involved in
the nitrogen cycle include nitrogen fixation and denitrification.
 In nitrogen fixation, certain bacteria convert nitrogen gas into ammonia.
 In denitrification, other soil bacteria convert nitrogen compounds called nitrates back into
nitrogen gas.
Phosphorus Cycle: Phosphorus is needed for molecules such as DNA and RNA. Most of the
phosphorus in the biosphere is stored in rocks and ocean sediments. Stored phosphorus is
gradually released into water and soil, where it is used by organisms.
Nutrient Limitation A nutrient that, in short supply, can limit the productivity of an ecosystem
is called a limiting nutrient.
Weather and Climate Weather is the condition of Earth’s atmosphere at a particular time and
place. Climate is the average condition of temperature and precipitation in a region over long periods.
Climate can vary over short distances.
These variations produce microclimates.
Factors That Affect Climate Climate is affected by solar energy trapped in the biosphere, by
latitude, and by the transport of heat by winds and ocean currents.
Temperature on Earth stays within a range suitable for life due to the greenhouse effect. The
greenhouse effect is the trapping of heat by gases in the atmosphere.
Earth’s curvature causes different latitudes to receive less or more intense solar energy. The
unequal distribution of the sun’s heat on Earth’s surface results in three main climate zones:
polar, temperate, and tropical.
Unequal heating of Earth’s surface also causes winds and ocean currents. Winds and currents
move heat and moisture through the biosphere.
The Niche Every species has its own tolerance, or a range of conditions under which it can grow
and reproduce. A species’ tolerance determines its habitat, the place where it lives.
A niche consists of all the physical and biological conditions in which a species lives and the way
the species obtains what it needs to survive and reproduce.
An organism’s niche must contain all of the resources an organism needs to survive. A resource
is any necessity of life, such as water, nutrients, light, food, or space.
Competition Competition occurs when organisms try to use the same limited resources.
Direct competition between species often results in one species dying out. This is the basis of the
competitive exclusion principle. This principle states that no two species can occupy exactly the
same niche in exactly the same habitat at the same time.
Competition helps to determine the number and type of species in a community.
Predation, Herbivory, and Keystone Species Predator-prey and herbivore-plant
interactions help shape communities.
Predation occurs when one organism (the predator) captures and eats another (the prey).
Herbivory is an interaction that occurs when an animal (the herbivore) feeds on producers (such
as plants).
Sometimes changes in the population of a single species, often called a keystone species, can
cause dramatic changes in the structure of a community.
Symbioses Symbiosis occurs when two species live closely together in one of three ways:
mutualism, commensalism, or parasitism.
In mutualism, both species benefit from the relation ship.
In parasitism, one species benefits by living in or on the other and the other is harmed.
In commensalism, one species benefits and the other is neither helped nor harmed.
Primary and Secondary Succession The series of predictable changes that occurs in a
community over time is called ecological succession. Over the course of succession, the number of
different species usually increases.
Primary succession begins in areas with no remnants of an older community. It occurs on bare
rock surfaces where no soil exists. The first species to live in an area of primary succession are
called pioneer species.
Secondary succession occurs when a disturbance changes a community without completely
destroying it.
Climax Communities A climax community is a mature, relatively stable ecosystem.
Secondary succession in healthy ecosystems following natural disturbances often reproduces the
original climax community.
Ecosystems may or may not recover from extensive human-caused disturbances.
The Major Biomes A biome is a group of terrestrial regional climate communities that covers a
large area and is characterized by soil type, climate, and plant and animal life.
In tropical rain forests, the tops of tall trees form a covering called the canopy. Shorter trees and
vines form another layer called the understory. It is hot and wet all year.
Tropical dry forests are found in areas with alternating wet and dry seasons. The trees in these
forests may be deciduous, meaning they shed their leaves during a particular season.
In a tropical grassland, grassy areas are spotted with isolated trees.
Deserts have less than 25 centimeters of precipitation annually.
Temperate grasslands have warm summers, cold winters, and deep soil.
Temperate woodlands and shrublands are large areas of grasses and wildflowers such as poppies
interspersed with trees or shrubs.
Temperate forests are made up of deciduous and evergreen coniferous trees. Coniferous trees
produce seed-bearing cones and most have waxy needles. Temperate forests have soils rich in
humus, which forms from decaying leaves and makes soil fertile.
Northwestern coniferous forests have mild temperatures with cool, dry summers and abundant
precipitation in fall, winter, and spring.
Boreal forests, or taiga, are dense forests of coniferous evergreens.
Tundra is characterized by permafrost, a layer of permanently frozen subsoil.
Other Land Areas Some areas, such as mountains and polar ice caps, do not fall neatly into the
major biomes.
Conditions Underwater Aquatic ecosystems are determined mainly by the depth, flow,
temperature, and amount of dissolved nutrients of the water.
The photic zone is the sunlit upper layer of water where photosynthesis can occur.
The aphotic zone is the dark lower layer where photosynthesis cannot occur.
The benthic zone is found on the bottoms of lakes, streams, and oceans. The organisms that live
on the floor of a body of water are called benthos.
Freshwater Ecosystems Freshwater ecosystems include flowing-water ecosystems,
standing-water ecosystems, and freshwater wetlands. Plankton are common. They form the base of
many aquatic food webs.
Estuaries Estuaries are wetlands formed where rivers meet the sea. They contain a mixture of
fresh and salt water. Most of the food produced in estuaries enters food webs as tiny pieces of
organic matter, or detritus.
Marine Ecosystems Marine ecosystems are found in the ocean.
The intertidal zone is the shallowest and closest to land. It is exposed to the rise and fall of tides
each day.
The coastal ocean is the relatively shallow border of water that surrounds the continents.
The open ocean begins at the continental shelf and extends outward. The open ocean can be
divided into the photic zone and the aphotic zone.
Describing Populations Researchers study five important characteristics of a population:
Geographic range is the area in which a population lives.
Population density is the number of individuals per unit area.
Population distribution is how individuals are spaced out in their range.
Growth rate determines whether a population grows, shrinks, or stays the same size .
Age structure is the number of males and females of each age in a population.
Population Growth Populations can grow, shrink, or stay the same size.
Factors that increase population size include births and immigration, which is the movement of
individuals into an area.
Factors that decrease population size include deaths and emigration, which is the movement of
individuals out of an area.
Exponential Growth When conditions are ideal, the larger a population gets, the faster it
grows. When a population’s numbers grow larger with each generation, exponential growth is
occurring. Ideal conditions include unlimited resources and absence of predation and disease .
Logistic Growth Resources become less available as a population grows.
Logistic growth occurs when population growth slows and then stops after a period of
exponential growth has occurred.
Population size stabilizes at the carrying capacity, the maximum number of individuals of a
given species that an environment can support.
Limiting Factors A limiting factor is a factor that controls the growth of a population.
Some factors depend on the density of the population. Others do not.
Acting separately or together, limiting factors determine an environment’s carrying capacity.
Limiting factors produce the pressures of natural selection.
Density-Dependent Limiting Factors
Density-dependent limiting factors operate strongly when the number of individuals per unit
area reaches a certain point.
Examples include:
 competition
 predation and herbivory
 parasitism and disease
 stress from overcrowding
Density-Independent Limiting Factors Some limiting factors do not necessarily depend
on population size.
Density-independent limiting factors depend on population density, or the number of
organisms per unit area.
Examples include severe weather, natural disasters, and human activities.
Some of these factors may have more severe effects when population density is high.
Historical Overview The size of the human population has increased over time.
For most of human existence, limiting factors such as the scarcity of food kept death rates high.
As civilization advanced, agriculture, industry, improved nutrition, sanitation, and medicine
reduced death rates. Birthrates stayed high in most places. This led to exponential growth.
Today, the human population continues to grow exponentially, although the doubling time has
slowed.
Patterns of Human Population Growth Demography is the scientific study of human
populations. Demographers try to predict how human populations will change over time.
Over the past century, population growth in developed countries slowed. As death rates dropped,
birthrates dropped also. Demographers call this shift the demographic transition. Most people
live in countries that have not undergone the demographic transition.
An age-structure graph shows how many people of each gender are in each age group in a
population. Demographers use such graphs to predict how a population will change. More people
of reproductive age usually means faster growth.
Many factors, including disease, will affect human population growth in the twenty-first century.
Current data suggest the human population will grow more slowly over the next 50 years than it
did for the last 50 years.
The Effect of Human Activity Humans and other organisms change the environment
when they obtain food, eliminate wastes, and prepare places to live.
Because Earth is like an island, life is limited to the resources that are here.
Humans affect regional and global environments through three major activities:
 agriculture, particularly monoculture, which is the cultivation of a single crop
 development of cities and suburbs, including conversion of farmland and destruction of
habitats for other organisms
 industrial growth, which consumes energy and emits pollutants
Sustainable Development In economic terms, ecosystems are providers of goods and
services (natural resources).
Healthy ecosystems produce or replace renewable resources.
Humans must be careful about the use of nonrenewable resources, such as fossil fuels,
which cannot be replaced.
Sustainable development provides for human needs while preserving the ecosystems that
provide renewable resources.
Soil Resources Soil is a renewable resource, but it must be managed properly.
Soil erosion is the wearing away of surface soil by water and wind.
In dry climates, farming and overgrazing change farmland into deserts, a process called
desertification.
Deforestation is loss of forests. Because healthy forests hold soil in place, deforestation increases
erosion.
Sustainable uses include leaving stems and roots of previous crops in place, crop rotation, contour
plowing, terracing, selectively harvesting mature trees, and tree farms.
Freshwater Resources The amount of fresh water is limited, and some sources cannot be
replaced.
A pollutant is a harmful material that can enter the biosphere. Water pollutants come from
industrial chemicals, residential sewage, and other sources.
Many chemical pollutants become concentrated in organisms at higher trophic levels of the food
chain through biological magnification.
Sustainable uses include conservation, pollution control, and watershed protection.
Atmospheric Resources Clean air is important to human health and Earth’s climate.
Pollution reduces air quality.
Smog is a mixture of chemicals formed from emissions from cars and industry.
Burning fossil fuels releases compounds that join with water in air, forming acid rain.
Greenhouse gases, such as carbon dioxide and methane, can cause global warming.
Particulates are microscopic particles that cause health problems.
One way of sustaining air quality is controlling automobile emissions.
The Value of Biodiversity The sum of all the genetic diversity among all the organisms in the
biosphere is called biodiversity. There are three general types of biodiversity:
Ecosystem diversity is the variety of habitats, communities, and ecological processes in the
biosphere.
Species diversity is the number of different species in an area or in the biosphere.
Genetic diversity is the total of all genetic information carried in living things.
Biodiversity benefits humans through its contributions to medicine and agriculture and through
the provision of ecological goods and services.
Threats to Biodiversity Human activities threaten biodiversity.
Development splits ecosystems into pieces, resulting in habitat fragmentation. The smaller the
pieces of a habitat, the less likely that species in the habitat can survive.
Other threats to biodiversity include hunting, introduced species, pollution, and climate change.
Conserving Biodiversity Conservation efforts are focused on three things:
Protecting single species is the focus of groups such as the Association of Zoos and Aquariums
(AZA), which oversees species survival plans (SSPs).
Protecting habitats and ecosystems is the main thrust of global efforts. Biologists are particularly
concerned about ecological hot spots, which are places where significant numbers of habitats
and species are in immediate danger of extinction.
Considering local interests is part of developing plans to replace harmful activities with ones that
conserve environments and biodiversity.
Ecological Footprints The ecological footprint of an individual or a population is the amount
of land and water needed to provide resources, absorb wastes, and render the wastes harmless.
Ecology in Action Three case studies illustrate the three steps of ecology in action: (1)
recognize a change in the environment, (2) determine the cause of that change, and (3) change
behavior to have a positive impact.
Case Study 1: Atmospheric Ozone This gas blocks ultraviolet (UV) radiation.
 Ozone gas blocks ultraviolet (UV) rediation.
 The ozone layer is an area of relatively high concentration of ozone in the atmosphere, between
20 and 50 kilometers above Earth’s surface. In the 1970s, a hole in the layer was observed.
 Regulations reduced CFC use, and the hole may be slowly disappearing.
Case Study 2: North Atlantic Fisheries
 Commercial fish catches have declined in recent years.
 The cause is overfishing.
 Regulations closed some fishing grounds to allow fish stocks to replenish. In the mean time,
aquaculture, or fish farming, also can provide food for people.
Case Study 3: Climate Change
 Global warming, the rise in the biosphere’s average temperature, and climate change, a shift in
Earth’s overall weather patterns, has occurred.
 Physical evidence includes rising sea levels due to melting ice. Biological evidence includes
temperature-related behavior changes in organisms.
 Using less fossil fuel will reduce greenhouse gases in the atmosphere.