This chapter discusses key ecological concepts including the environment, habitat, niche, populations, limiting factors, and interactions between organisms such as competition, predation, and symbiotic relationships. It also addresses energy flow through ecosystems in food chains and webs, and nutrient cycling, with a focus on the carbon and nitrogen cycles. Human impacts like fossil fuel emissions and fertilizer runoff are disrupting these natural cycles.

1. Environmental Science
A Study of Interrelationships
Thirteenth Edition
Enger & Smith
Chapter 5
Interactions: Environments and Organisms
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Interactions: Environments and
Organisms

3. 5.1 Ecological Concepts
 Environment everything that affects an
organism during its lifetime.
• Abiotic factors: Nonliving things that influence an
organism
– energy, nonliving matter, living space, and ecological
processes
• Biotic factors: All forms of life with which the
organism interacts.

4. 5.1 Ecological Concepts
Levels of organization in ecology

5. Limiting Factors
 Limiting factors - factors whose shortage or
absence restricts species success.
• Scarcity of water or specific nutrients (plants).
• Climate, availability of a specific food (animals).

6. Limiting Factors
 Range of tolerance - indicates a range of
conditions in which an organism can survive.
• Some species have a broad range of tolerance, while
others have a narrow range of tolerance.

7. 5.1 Ecological Concepts
Limiting factors

8. Habitat—place
 The habitat
• space in which an organism lives
• defined by the biological requirements of each particular
organism
• Usually highlighted by prominent physical or biological
features.

9. Habitat and Niche
Moss habitat: cool, moist, and shady

10. Niche--role
 The niche
• functional role (profession) the organism has in its
surroundings
• Includes:
– all the ways it affects other organisms
– how it modifies its physical surroundings

11. Habitat and Niche
Ecological niche of a beaver

12. Populations
 A population
• all organisms of the same kind found within a specific
geographic region.

13.  A species
Species
• population of all the
organisms
• potentially capable of
reproducing naturally
among themselves
• and having offspring that
also reproduce.

14. 5.2 The Role of Natural Selection and
Evolution
 Natural selection
• process that determines which individuals within a
species will reproduce and pass their genes to the
next generation.
• Mechanism that causes Evolution
 Evolution
• Changes in genes and characteristics within
successive generations of a population over time

15. Natural Selection

Steps in natural selection:
Excess number of individuals
Results in a shortage of specific resources
Some individuals have a greater chance of obtaining
needed resources and
have a greater likelihood of surviving and reproducing than
others.

16. Results of
Natural
Selection
Percentage of
individuals showing
favorable variations
will increase
Percentage showing
unfavorable
variations will
decrease

17. Observations
Overproduction
of offspring
Individual
variation
Conclusion
Natural selection:
unequal reproductive success
Figure 13.UN3

18. Natural Selection in Action
• Examples of natural selection include:
– Pesticide-resistant insects
– Antibiotic-resistant bacteria
– Drug-resistant strains of HIV
– http://www.pbs.org/wgbh/evolution/educators/teachstuds/svide

19. 5.3 Kinds of Organism Interactions
 Predation
• interaction in which
one animal kills/eats
another.
• Predator benefits from
food.
• Prey have higher
reproduction rate
– (field mice 10 to 20
offspring/year)

20. Competition
Competition
interaction in which two
organisms strive to obtain
the same limited resource.
Intraspecific competitionbetween members of same
species.
Interspecific competitionbetween members of different
species.

21. Symbiotic Relationships
 Symbiosis
• close, long-lasting, physical relationship between two
different species
• At least one species derives benefit from the interaction.

22. Symbiotic Relationships
 There are three categories of symbiotic
relationships:
• Parasitism
• Commensalism
• Mutualism

23. Symbiotic Relationships
 Parasitism
• relationship in which one organism (parasite) lives in
or on another organism (host)
• which it derives nourishment

24. Symbiotic Relationships
 Commensalism
• relationship in which one organism benefits while the
other is not affected.
• Remoras and sharks

25. Symbiotic Relationships
 Mutualism
• relationship in which both species benefit.
• The relationship is obligatory in many cases, as
neither can exist without the other.
• Mycorrhizae

26. 5.4 Ecosystem Interactions
 An ecosystem
• defined space in which interactions take place
between a community and the physical environment.
• Ecologists have divided organisms’ roles in
ecosystems into three broad categories:
– 1. Producers
– 2. Consumers
– 3. Decomposers

27. 5.4 Ecosystem Interactions
 Producers:
 Organisms that are able to use sources of energy to make
complex organic molecules from simple inorganic
substances in their environment.

28. Major Roles of Organisms in Ecosystems
1. Consumers:
Organisms that require organic matter as a
source of food.
They consume organic matter to provide
energy, growth and survival.

29. 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.

30. Major Roles of
Organisms in
Ecosystems
 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.

31. Keystone Species
 A keystone species plays a critical role in the
maintenance of specific ecosystems.

32. Keystone Species: Prairie dogs

33. Energy Flow Through Ecosystems
 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).

34. Energy Flow Through Ecosystems
Categories of organisms within an ecosystem.

35. 10%
1%
biomass (weight of living material) is
often used as a proxy.

36. Discussion
 See handout

37. Food Chains
A food chain
series of organisms occupying
different trophic levels
through which energy passes as
a result of one organism
consuming another
Some chains rely on detritus.

38. Food Web
 A food web
series of multiple,
overlapping food
chains.
• A single predator can
have multiple prey
species at the same
time.

39. Nutrient Cycles in Ecosystems—
Biogeochemical Cycles
 Organisms are composed of molecules and
atoms that are cycled between living and nonliving portions of an ecosystem.
 These nutrient cycles are called biogeochemical
cycles.

40. Carbon Cycle
1. Producers: Plants use carbon dioxide during
photosynthesis to produce sugars.
Oxygen is produced as a by-product.
1. Consumers: Herbivores eat plants
break down the complex organic molecules into simpler molecular
building blocks
incorporate those molecules into their structure.
Respiration produces CO2 and water and releases
those compounds back into the atmosphere.

41. Carbon Cycle
3. Decomposers: The decay process of
decomposers involves respiration
Release of carbon dioxide and water
Dead organisms are recycled
4. Carbon sinks
processes or situations that remove atoms
from active, short-term nutrient cycles
Ex: long-lived trees, fossil fuels

42. Carbon Cycle
Carbon cycle

43. Human Impact on the Carbon Cycle
 Burning fossil fuels takes carbon atoms that were
removed temporarily from the active, short-term
carbon cycle and reintroduces them into the active
cycle.
 Converting forests (long-term carbon storage) to
agricultural land (short-term carbon storage) has
increased the amount of carbon dioxide in the
atmosphere.

45. Nitrogen Cycle
 The nitrogen cycle involves the cycling of nitrogen
atoms between abiotic and biotic ecosystem
components.
• Producers are unable to use atmospheric N.
– Must get nitrate (–NO3) or ammonia (NH3.)
• Nitrogen-fixing bacteria convert nitrogen gas N2 into
ammonia.
– Plants construct organic molecules.
– Eaten by animals.

46. Nitrogen Cycle
 Decomposers also break down nitrogen-containing
molecules, releasing ammonia.
 Primary sink for nitrogen is the atmosphere

47. Nitrogen Cycle
Nitrogen cycle

48. Human Impact on the Nitrogen Cycle
 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.
• 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.”

49. 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.

50. 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.