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Environmental Science
A Study of Interrelationships
Eleventh Edition
Enger & Smith
Chapter 5
Interactions: Environments and Organisms
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Interactions: Environments and Organisms
Outline
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Ecological Concepts
Natural Selection and Evolution
Organism Interactions
Community and Ecosystem Interactions
Ecological Concepts
 Ecology is the study of ways organisms interact
with each other and with their nonliving
surroundings.
 Deals w things such as ways of adapting to
surroundings, how make use of surroundings, how
an area is altered by presence & activities of
organisms.
 Interaction require E and matter.
 Living things need E & matter to survive.
 Organism depend on each other for that, so
ecology can be divided in several special fields
 Environment means everything that
affects an organism during its lifetime.
• Biotic factors: Living components. All
forms of life with which the organism
interacts.
• Some categories include photosynthetic
plants, animals that eat others, bacteria &
fungi that cause decay, bacteria, viruses
and parasitic organisms cause disease.
• Abiotic factors: Nonliving things that
influence an organism, such as energy,
nonliving matter, living space, and
ecological processes.
• All livings need a source of E & sun is
the ultimate source of E for almost all
organisms.
• All forms of life require atoms of
elements such as C, N, P & molecules
such as water to construct & maintain
themselves.
• These elements r obtained from env & r
returned to env through respiration,
excretion, death & decay.
• Structure & location of the space
organisms inhabit r important abiotic
aspect of env. Some live at sea level some
in mountains & high levels, some env r
homogeneous others r jumble of rocks…
• Climate is another important factor. It’s the
result of interaction of matter & E & is
determined by factors such as amount of
solar radiation, proximity to equator,
patterns of wind .
Ecological Concepts
Levels of organization in ecology
Ecology deals w interaction between organisms & env & can take place at
different levels from ecosystem level through community interactions to
population studies & study of niche of individual organism. Ecology also
involves study of physical env & atoms & molecules that make up living &
nonliving parts.
Limiting Factors
 All organisms interact w their surroundings & certain
factors may be critical to particular species success.
 Limiting factors are any factors whose shortage or
absence restricts species success. They can be
biotic/abiotic, & can be quite different from one species
to another. Ex sunlight, water, nutrients for plants,
oxygen in water for fish
• Scarcity of water or specific nutrients (plants)
• Climate, availability of a specific food (animals).
Each species has a Range of tolerance indicates a range
of conditions in which an organism can survive.
– Temperature
– pH
Ecological Concepts
Limiting factors (connection bet T (limiting factor) and reptiles popn)
Ecological Concepts
Limiting factors
Habitat and Niche
 Env influences organism & organism influence env.
 The habitat of an organism is the space in which an
organism lives; it is defined by the biological
requirements of each particular organism (organism
address).
• Usually highlighted by prominent physical or biological
features. Ex mosses: small plants, must be covered w thin
layer of water to produce, if exposed to sunlight, wind,
drought they die. So typical habitat is cool, moist & shady
 The niche of an organism is the functional role
(profession) the organism has in its surroundings.
• This term includes all the ways an organism affects the
organisms with which it interacts as well as how it modifies
its physical surroundings.
Habitat and Niche
Moss habitat
Ecological Niche
Ecological niche is complex set of interactions between organism & its env &
includes all the ways an organism influences its surroundings as well as the ways it
is affected by env. A beavers ecological niche includes building dams, flooding
forested areas, providing protection for them, killing trees, providing habitat for
ducks & other animals, serving as food for predators & many other effects.
Habitat and Niche
Ecological niche of a beaver
Genes, Populations, and Species
 Genes are distinct pieces of DNA that
determine the characteristics an individual
displays. Such as leaf shape, feather color
 A population includes all organisms of the
same kind found within a specific geographic
region.
• A population contains more kinds of genes than
any single individual within the population. Genes
are passed from one generation to the next
through reproduction.
 A species is a population of all the
organisms potentially capable of
reproducing naturally among themselves
and having offspring that also reproduce. An
individual organism is a member of a
species.
• Working definition that only applies to
organisms that sexually reproduce.
–Some species are easy to recognize, while
others are more difficult. Humans vs
mosquitoes
–3 points in this definition of species:
– some individuals in any popn will never reproduce
& many pairs never meet one another but have
still potential to interbreed
– Ability to produce fertile offspring. 2 kinds of
organisms may interbreed & produce offspring but
offspring r sterile & never reproduce so they r not
same species. Ex horses & donkeys produce
mules that r sterile
– Some organisms reproduce primarily by asexual
reproduction, not mate just reproduce copies of
themselves. These not fit in this definition.
However most organisms reproduce asexually as
well as sexually & can be assigned to a species
based on time they mate
Natural Selection
 Each species of organism is specifically adapted to a
particular habitat in which it has a specific role. But how
they fit in a specific role in such a precise way?
 The process that fit organisms characteristics w the
demands of its env is known as:
 Natural selection is the process that determines which
individuals within a species will reproduce and pass their
genes to the next generation. Because genes r
determinant of structural, physiological & behavioral
characteristics of organisms.
 The changes seen in the genes and characteristics
displayed by successive generations of a population of
organisms over time is known as evolution.
Natural Selection

Several conditions and steps are involved in
the process of natural selection:
1. Individuals within a species show genetically
determined variation. Some r useful some not. Some
color of animals make them conspicuous others not
2. Organisms within a species typically produce more
offspring than are needed to replace the parents
when they die. Most of the offspring die.
3. The excess number of individuals results in a
shortage of specific resources. They compete for
food, space, mates or other things that r limited.
Natural Selection
4. Due to individual variation, some individuals
have a greater chance of obtaining needed
resources and therefore have a greater
likelihood of surviving and reproducing than
others.
5. As time passes, the percentage of individuals
showing favorable variations will increase while
the percentage showing unfavorable variations
will decrease.
Evolutionary Patterns
 When we look at the effects of natural selection over
time, we see changes in the characteristics of a
species & kinds of species present.
 Some changes take milin of years, others few years.
We have to remember as env change species
change, some species can adapt some not.
 Evolution - A change in the kinds of organisms that
exist and in their characteristics. Study of fossil
records show as new species come into being, other
species disappear. This is called
 Speciation - Production of new species from
previously existing species.
• Thought to occur as a result of a species dividing into two
reproductively isolated subpopulations.
• If 2 subpopulations have some genetic
differences & their env r different, natural
selection will work on them differently & they will
begin to diverge from each other.
• Eventually differences become so great that 2
subpopulation can not interbreed & become 2
different species.
• In plants polyploidy results in new species
happens when the # of sets of chromosomes in
the cells of plants is increased.
• Many organisms r diploid=2 set of chromosomes;
one from each parent, one in egg, one in sperm.
• Polyploid organism have several sets. Ex many
plants have extra sets of chromosomes & cant
produce w closely related species w different # of
sets of chromosomes
Evolutionary Patterns
 Env keep changing, species w lack of genetic
resources to cope w these changes go extinct.
 Extinction - Loss of entire species.
• Of estimated 500 million species believed to have ever
existed on earth, 98-99% have gone extinct.
• Studies of fossils & geological features show only thousands
years ago huge glaciers covered much Europe & N America.
• Humans coexisted w mammoths, saber-toothed tigers &
giant cave bears.
• As climate became warmer glaciers receded, humans
continued to prey on these animals causing mammoths,
saber-tooted tigers & giant cave bears to extinct. Humans,
horses & some plants that adapted survived.
• It is possible to have extinction of specific popn of
a species, ex small local popn can easily go
extinct this can result in loss of specific gene
combinations
• Natural selection work to shape organisms to fit a
changing env
• Human have a significant impact on extinction of
many kinds of species
• Modification of env by human for their purposes,
introduction of exotic species has caused
displacement of species from the area
• Humans r also subject to evolution & might go
extinct
 Co-Evolution - Two or more species can reciprocally
influence the evolutionary direction of the other. In
other words, organisms affect evolution of other
organisms.
• Ex Grazing animals and grass species. Grasses eaten by
animals grow from the base rather than from tips & have
hard materials in their cell walls making it difficult for
animals to crush the cell walls and digest them
• Animals have teeth that r very long or grow continuously
to compensate for the wear associated w grinding hard
materials
• Others have complicated digestive tracts allow
microorganisms to do digestion
Kinds of Organism Interactions
 Ecologists study how organisms interact w each other.
One common kind of interaction is
 Predation is a kind of interaction in which one
animal kills/eats another.
• Predator benefits from food. & prey is harmed. Ex lions &
zebras, robins & earthworms, venues flytrap & insects
• Prey adaptation is manifested in a higher
reproduction rate.
• Predator uses several strategies:
– strong & speedy ones chase & overpower prey,
– others lie in wait & quickly strike,
– some use snares to catch
• prey tries to avoid predation:
– Some have keen senses to detect predators,
– others camouflage,
– many remain motionless
Prey adaptation is manifested in a higher reproduction rate.
Kinds of Organism Interactions
• Individual organisms that is killed & eaten is
harmed (old & slow ones) but not the prey
species
• Prey species benefits by eliminating non-adaptive
genes from the gene pool. Individuals that are
best adapted will survive & reproduce offsprings
w better survival characteristics
– Poorly adapted predators are less likely to obtain
food and thus pass on non-adaptive genes. They not
survive to reproduce
Competition
 Competition is a kind of interaction in which two
organisms strive to obtain the same limited
resource and both are harmed to some extend. Ex
trees growing close to each other, both get less
nutrients.
• Intraspecific competition is competition between
members of same species. Corn plants competing for
water & nutrients in a field
• Interspecific - Members of different species competing
for resources. Ex foxes & coyotes use same prey
species=mice/rabbit, if shortage of supply certain
predator may be more successful than others
 The more similar the competing species, the
more intense the competition.
 If one of two competing species is better
adapted to live in the area, less fit one must
• evolve into a slightly different niche,
• migrate to different geographic area,
• or become extinct. This is called Competitive
Exclusion Principle.
Competition
 The competitive exclusion principle holds
that no two species can occupy the same
ecological niche in the same place at the
same time.
• Less-fit species must evolve into a slightly different
niche.
• Examining niche requirements of two
similar species show significant differences
between their niches which reduces
intensity of the competition between them
• Ex many birds eat insects, but may obtain
them in different ways: woodpecker
excavates openings to obtain insects in
rotting wood, warblers flit about in the
foliage capturing insects.
Symbiotic Relationships
 Symbiosis is a close, long-lasting, physical
relationship between two different species. At least
one species derives benefit from the interaction.
 There are three categories of symbiotic
relationships:
• Parasitism
• Commensalism
• Mutualism
Symbiotic Relationships
 Parasitism is a relationship in which one
organism (parasite) lives in or on another
organism (host), from which it derives
nourishment. Host is not killed immediately.
• Some parasites more destructive, both evolve in such
a way that they can accommodate one another. Host
evolves defenses against parasite to reduce the
harm,
• Many parasites have complex life that involves more
than one host for different stages in its life cycle. Ex
many worm parasites have adult, reproductive stage
in a carnivore but immature stage in another animal
that carnival uses as food (dog tapeworm)
• Some parasite life cycles involve animals that carry
the parasite from one host to another.
• These carriers known as vectors. Ex biting insect &
mites can transmit parasites when they obtain blood
meal
• Malaria, lyme disease, sleeping sickness are
transmitted by vectors
• Ectoparasites live on the host’s surface.
– Fleas, lice, molds, mildews
• Endoparasites live inside the body of the host.
– Tapeworms, malaria parasites, bacteria, fungi
How about plants? Mistletoe is parasite of some trees,
birds transfer the seed to tree
Symbiotic Relationships
 Commensalism is a relationship in which one
organism benefits while the other is not affected.
A parasitism relationship may evolve into a commensal
one if the host is not harm at all.
• Remoras and sharks
• Many commensal relationships r rather opportunistic & may not
involve long-term physical contact. Ex birds use trees to build
nest but not use the same tree year after year
 Mutualism is a relationship in which both
species benefit. The relationship is obligatory in
many cases, as neither can exist without the
other. Or even if they can, it is more successful when
together
• Mycorrhizae
• many kinds of fungi form an association w roots of plants
called Mycorrhizae. Fungi obtain organic molecules form
roots and fungus assists plant in obtaining nutrients such
as phosphates & nitrates
• Some relationships r hard to identify & categorize,
mosquito/tick=temporary parasites/blood predators
• Difficult to fit in other category: relationship between
certain birds such as cowbirds & European cuckoos that
not build nest & lay egg in nests of others to care for in the
expense of their own nestlings who generally die=nest
parasitism
Symbiotic Relationships
Examples of symbiotic relationships
Community and Ecosystem Interactions
 Humans have complicated relationship w other
organisms which can be placed in same
categories: predator, herbivore, scavenger,
commensalism (using shade of tree),
parasitism (Africa blood of cow drawn mixed w
milk), mutualism (domesticated plants),
competition (w all organism)
 Two concepts that focuses on relationships that
involve many different kinds of interactions r
community & ecosystem
Community and Ecosystem Interactions
 A community is an assemblage of all
interacting species of organisms in an
area. Ex in prairie, grasses have major
role since they do photosynthesis &
provide food & shelter for others;
grasshoppers, prairie dogs & bison's r imp
consumers; meadowlarks consume
insects but have minor role; bacteria &
fungi provide nutrients
 An ecosystem is a defined space in which interactions
take place between a community, with all its complex
interrelationships, and the physical environment.
 Physical world has major impact on kinds of plant &
animal in an area as well as the kind of soil &
amount of moisture
 At the same time organism also impact their
physical surroundings
 While concepts of community & ecosystem r closely
related but ecosystem is a broader concept
because involves physical as well as biological
processes
 Ecosystem has parts that must be
organized in specific ways or the
systems will not operate, so we will
look at ecosystems from 3 points of
view:
• major role played by organisms,
• the way E is utilized w/in ecosystems,
• the way atoms r cycled from one organism
to another
Major Roles of Organisms in Ecosystems

Ecologists have divided organisms’ roles in
ecosystems into three broad categories:
1. Producers: Organisms that are able to use sources of
energy to make complex organic molecules from
simple inorganic substances in their environment.
Plants, algae, phytoplankton (aquatic organism)
2. Consumers: Organisms that require organic matter as
a source of food. They consume organic matter to
provide themselves with energy and organic molecules
necessary for growth and survival.

An important part of their role is the process of
respiration= braking organic matter to inorganic
one
Major Roles of Organisms in Ecosystems
•
Consumers can be further divided into categories based
on the things they eat and the way they obtain food.
– Primary consumers, or herbivores, eat plants as a source
of food.
– Secondary consumers, or carnivores, are animals that eat
other animals.
– Omnivores consume both plants and animals.
3. Decomposers use nonliving organic matter
as a source of energy and raw materials to
build their bodies. Many small animals,
bacteria, and fungi fill this niche.
– Since decomposers carry on respiration, they r
extremely important in recycling matter by
converting organic matter to inorganic
material.
Keystone Species
 Although there r many species interacting w each
other and their env, some r more important.
 A keystone species plays a critical role in the
maintenance of specific ecosystems.
• When bison are present in American tallgrass prairie
ecosystems, they increase the biodiversity of the site.
– Bison in American Tall Grass Prairie: grazing animal r
extremely important in maintaining the mix of species of a
grassland.
– w/o them nature of prairie changes; present of bison will
increase the biodiversity of the site, by eating tall grasses and
allowing smaller plant species grow.
– Bison dig holes in soil that support many species of plants
that typically live in disturbed areas.
– Their urine important source of nitrogen for plants.
So not all species can be treated equally, some have pivotal
role and their elimination or severe reduction can
significantly alter ecosystem.
– In some cases lost of K.S results in permanent modification
of ecosystem to something totally different from original one.
Energy Flow Through Ecosystems
 Ecosystem is a stable, self-regulating unit, it
changes & need E for stability.
 Sunlight is significant E source trapped by
producers and then become available to
ecosystem and is stored in the form of chemical
bonds in large organic molecules
(carbohydrates, fat, protein) & is transferred to
other organisms when eat producers.
 Each step in the flow of energy through an
ecosystem is known as a trophic level.
 As energy moves from one trophic level to the
next, most of the useful energy (90%) is lost as
heat (second law of thermodynamics).
• When E is passed from one trophic level to next,
some useful E is lost, which dissipate to the
surroundings and warms the air, water, or soil.
• Since organisms must expend E to maintain life
processes, amount of E contained in higher trophic
levels is considerably less that that at lower levels.
 Because it is difficult to measure the amount of
energy contained in each trophic level, biomass
(weight of living material) is often used as a
proxy to approximate the relationship between
amounts of E at each level.
Energy Flow Through Ecosystems
E passes through several levels known as trophic levels. Each level has
certain amount of E, each time E flow to another level, about 90% of useful
E is lost, usually as heat to surroundings. So higher trophic levels contain
less E & fewer organisms.
Categories of organisms within an ecosystem.
Food Chains and Food Webs
 A food chain is a series of organisms occupying
different trophic levels through which energy passes as a
result of one organism consuming another.
• Some chains rely on detritus (small pieces of nonliving
organic material). Ex bottoms of the deep lakes & oceans r
2 dark for photosynthesis, so organisms rely on a steady
rain of small bits of organic matter from upper layers of the
water where there is photosynthesis activity.
 A food web is a series of multiple, overlapping food
chains.
• More stable. A single predator can have multiple prey species at
the same time. so if one is short it will switch to other ones.
Food Chains and Food Webs
Food chain
Food web
Food Chains and Food Webs
Food chain
Food web
Nutrient Cycles in Ecosystems—
Biogeochemical Cycles
 Organisms are composed of molecules and atoms that
are cycled between living and non-living portions of an
ecosystem.
 These nutrient cycles are called biogeochemical cycles.
Some atoms (C, N, O, H, P) r more important. They r found in
protein, DNA, carbohydrates, fats.
Organic molecules contain large # of carbon atoms. These
molecules r manufactured from inorganic molecules by producers
& r transferred from one living organism to another in food chains.
Processes of respiration & decay break down the complex organic
molecules to simpler inorganic ones.
We will look at flow of 3 kinds of atoms w/in communities & between
biotic & abiotic part of ecosystem.
Carbon Cycle

All living things r composed of organic mol which r composed of
C.
1.
Carbon and oxygen combine to form carbon dioxide.
2.
Plants use carbon dioxide during photosynthesis to produce
sugars (algae & bacteria also perform photo). E for photo is
provided by sunlight. Oxygen is released. So light E is converted
to chemical-bond E in organic molecules.
3.
Plants use sugars for growth & to provide E for other necessary
process.
4.
Herbivores eat plants, break down the complex organic
molecules into simpler molecular building blocks, and incorporate
those molecules into their structure.
5.
Respiration breaks down organic molecules into CO2 and water
and releases those compounds back into the atmosphere.
–Almost all organisms perform Respiration which
breaks down sugars releasing CO2 and water
back into the atmosphere.
»When an herbivore is eaten by a carnivore,
some of the carbon-containing molecules
get incorporated into their body and
remaining organic molecules r broken down
in process of respiration.
In carbon cycle all organisms require organic
molecules for their survival and must either
manufacture them or consume them.
In C cycle the same C atoms r used over &
over. Same C cycle operates in aquatic
systems.
Carbon Cycle
6. The decay process of decomposers
involves respiration and therefore
recycles naturally occurring organic
molecules.
7. Burning fossil fuels takes carbon atoms
that were removed temporarily from the
active, short-term carbon cycle and
reintroduces them into the active cycle.
Carbon Cycle
Carbon atoms r cycled through ecosystems. Plants can incorporate carbon
atoms from carbon dioxide into organic molecules when they carry on
photosynthesis. The carbon containing organic molecules r passed to animals
when they eat plants or other animals. Organic wastes or dead organisms r
consumed by decay organisms. All organisms, plants, animals and
decomposers return carbon atoms to the atmosphere when they carry on
respiration. Oxygen atoms r being cycled at the same time that carbon atoms r
being cycled.
Carbon cycle
Nitrogen Cycle
 78% of the gas in the air we breath is made up of N
& N cycle involve Cycling of nitrogen atoms between
abiotic and biotic ecosystem components.
• Producers are unable to use atmospheric N. They must
make new N-containing molecules such as proteins &
DNA.
– Must get nitrate (–NO3) or ammonia (NH3.) so N containing
compounds r often in short supply & its availability is a
factor that limits the growth of plants.
• Nitrogen-fixing bacteria convert nitrogen gas N2 into
ammonia that plants use.
• Some live freely in soil & r called free-living N-fixing
bacteria, others r known as symbiotic N-fixing
bacteria, have a mutualistic relationship w certain
plants & live in nodules in roots of plants known as
legumes & certain trees.
• Some grasses & evergreen have similar relationship
w certain root fungi.
– Plants construct organic molecules (protein, DNA).
– Eaten by animals. Plant protein molecules r broken down
to smaller building blocks called amino acids which form
proteins typical for herbivore.
• Decomposers also break down nitrogen-containing
molecules releasing ammonia.
Nitrogen Cycle
 Nitrifying bacteria are able to convert ammonia
to nitrite, which can be converted to nitrate &
plants use as a source of N for synthesis of
N-containing organic molecules.
 Denitrifying bacteria are able (under anaerobic
conditions) to covert nitrite to nitrogen gas (N2)
which is ultimately released into the atmosphere &
enters the cycle by N-fixing bacteria.
 N-cycle has 2 significant differences from Ccycle:
 Most of the difficult chemical conversions r made by
bacteria & other microorganisms,
• 2nd although N enters organisms by way of N-fixing
bacteria & returns to atmosphere through action of
denitrifying bacteria, there is a 2ndary loop that recycles
N compounds directly from dead organisms & wastes
directly back to producers.
 In naturally occurring soil, N is limiting factor for plant
growth & has to be supplied to soil.
 Farmers use alternative methods to supply N; ex alternate
N yielding crops such as soybeans w N-demanding ones
such as corn, or plants alternating strips of crops, or grow
a N-fixing crop for a short period of time then plow crop
into soil and let organic matter decompose them (green
Manure).
 Also can spread manure from animal & rely on bacteria to
decompose them and release N.
Nitrogen Cycle
Nitrogen cycle
Phosphorus Cycle

P is another important element in the structure of living
things. It is present in many biological molecules such
as DNA, cell membrane, bone, teeth. Phosphorus is
not present in the atmosphere as a gas (important diff
from N & C cycle).

The ultimate source of phosphorus atoms is rock.
1. Phosphorus compounds are released by erosion and become
dissolved in water.
2. Plants use phosphorus to construct necessary molecules.
3. Animals gain necessary phosphorus when they consume
plants or other animals.
4. Decomposers recycle phosphorus compounds back into the
soil after animal die or excrete waste.
P dissolved in water r precipitated as deposits &
geological processes elevate them & expose them
to erosion, making them available to organism
again.
»Waste of animals have lots of P and places with
tick layer of animal dropping, is a major source of
P.
»Many soils and aquatic ecosystems are short in P.
»Fertilizers usually contain N, P, and K.
»In agriculture ecosystems these elements are
removed by harvesting crops and should be
supplied by fertilizers.
»Aquatic ecosystem are sensitive to nutrients level.
»High level results in rapid growth.
Phosphorus Cycle
Rock is the
source of P
and when is
dissolved
provide it
for plants &
animals.
Phosphorus cycle
Human Impact on Nutrient Cycles
 Humans have significantly altered level of these nutrients.
 Two activities caused significant changes in carbon cycle:
 Burning of fossil fuels releases large amounts of carbon dioxide
into the atmosphere.
 Converting forests (long-term carbon storage) to agricultural land (shortterm carbon storage) has increased the amount of carbon dioxide in the
atmosphere.
 Converting forests to agricultural land causes less C being stored
in the bodies of large, long-lived plants.
 Fossil fuel burning also increases the amount of nitrogen available to
plants.
 When fossil fuels are burned, O & C are heated to high
temperature and different kinds of N-containing compounds are
produced which can be used by plants as nutrients. These have
doubled level of N available.
Human Impact on Nutrient Cycles
 If too much nitrogen or phosphorus is applied as
fertilizer, or if it is applied at the wrong time, much of the
fertilizer is carried into aquatic ecosystems.
– Lowered oxygen concentrations. When organisms die,
decomposers use O from water to break down dead organic
molecules which lowers O.
– This create a “dead zone” with few fish and bottom-dwelling
organisms.
Agricultural activities in US, causes death of fish in Gulf of
Mexico.
• The presence of these nutrients increases the growth rate of
bacteria, algae, and aquatic plants.
– Toxic algae can kill fish and poison humans.
– An increase in the number of plants and algae results in lowered
oxygen concentrations, creating “dead zones.”
Summary
 An organism’s environment can be divided into biotic
(living) and abiotic (nonliving) components.
 The space an organism occupies is its habitat, and
the role it plays is its niche.
 Organisms interact with one another in a variety of
ways. Symbiotic relationships are those in which two
species live in physical contact and at least one
species derives benefit from the relationship.
 In an ecosystem, energy is trapped by producers
and flows from producers through various trophic
levels of consumers.
Summary
 The sequence of organisms through which
energy flows is called a food chain.
 Multiple interconnecting food chains constitute a
food web.
 The flow of atoms through an ecosystem
involves all the organisms in a community. The
carbon, nitrogen, and phosphorus cycles are
examples of how these materials are cycled in
ecosystems.