Elements of Ecology

1. CHAPTERS 17-20
PRESENTED BY: LEANDRO MICHAEL C. DE LOS SANTOS, JR.

2. CHAPTER 17
Community Structure

3. 1. Explain how ecologists define species
diversity.What data must be collected to
determine the diversity of a community?

4. Species diversity- a measurement that relates
number and relative abundance of species
within a community.This mix of species defines
the biological structure of a community.
Data needed to determine Species diversity:
1. Count of all species
2. Relative abundance of each species in a
community

6. 2. Use examples to illustrate the
difference between a dominant
species and a keystone species.

7. Dominant species- when a single of few species
predominate within a community
Keystone species- a species that has a
disproportionate impact on the community relative
to its abundance

8. Keystone species:
Source: www.uic.edu

9. 3. Draw a diagram of a simple food web
with three trophic levels, and describe
the difference between a food chain
and a food web

10. FoodWeb with 3 trophic levels

11. Food web- interlocking pattern formed by a
series of interconnecting food chain
Food chain- an abstract representation of
feeding relationships within a community

12. 4. Explain the difference between a
guild and a functional type, and discuss
why it may be useful for ecologists to
classify species in these ways.

13. Guilds- subdividing each trophic level into
groups of species that exploit a common
resource in a similar fashion.
Ex. Humming birds and other nectar-feeding
birds form a guild of species that exploit
flowering plants

14. Functional type- a group of species based on their
common response to the environment, life
history, characteristics, or role within the
community.
Ex. Plants categorized based on their
photosynthetic pathways, shade tolerance or
timing of reproductive effort

15. Importance of classification:
* Simplify the study of communities

16. 5. Discuss the physical structure of either a forest or a
lake, and explain how the physical structure affects
the biological structure

17. Physical structure of a lake
Stratification is determined by light penetration
& profiles of temperature and oxygen

18. Characteristic organisms inhabit each available
vertical layer in a community.
Various types of consumers and decomposers
occupy all levels in a community

19. 6. Explain why defining the boundaries of a
community can sometimes be very difficult. Do
you think this detracts from the usefulness of the
concept of community?

20. Since community is a spatial concept, defining
boundaries between communities can
sometimes be difficult because when the
species composition and patterns of
dominance shift gradually the distinction or
boundary between each community will be
unclear.

21. 7. Explain the difference between the organismic
concept of communities and the individualistic
continuum concept.Which of these concepts
best describes the majority of communities?

22. Organismic concept- clusters or groups of species
repeatedly associate together, with each species
representing an interacting, integrated
component of the whole.
Individualistic continuum- relationship among
coexisting species is due to the similarities in
their requirements and tolerances, not to strong
interactions or common evolutionary history.

23. 8. Give two reasons why it is difficult to accurately
determine the species diversity of any given
community, and suggest how you might go about
trying to do so.

24. 1. Quantifying the difference in the set of
species from one community to the other
2. How to determine where one community
ends and another one begins

26. 9. Choose either of the two most common
indexes of community similarity (Sorenson's
coefficient of similarity and percent similarity),
and explain how it is calculated. For what
purpose might such an index be used?

27. Sorensen’s coefficient of community (CC)
- useful when the intended focus is on the
presence or absence of species
Where: c, number of species common to both
communities
s1, number of species in community 1
s2, number of species in community 2

28. CHAPTER 18
Factors Influencing the Structure of
Communities

29. 1. Explain the differing roles of fundamental
niches and species interactions in controlling
community structure.

30. Fundamental niche- sometimes referred to as
physiological niche, which provides a
description of the set of environmental
conditions under which a species “can persist’’.

31. Species interactions

32. 2. Explain what an indirect interaction is, and give
an example of an indirect interaction that has
a strong effect on a specific species in a community.

33. Indirect Interactions- occurs when one species
does not interact with a second species directly,
but instead influences a third species that does
directly interact with the second species

34. 3. Explain the difference between bottom-up and
top-down control of community structure. Do
you think one or the other is most important, or
some combination of both?Why?

35. Bottom-up control- productivity and abundance
of population at any given trophic levelare
controlled by the productivity and abundance of
populations in the trophic level below them
Competition is strong, predation is weak
Top-down control- predator populations can
control the abundance of prey species.
Predation is strong, competition is weak

36. 4. Explain the potential relationship between the
physical structure of a plant community and the
diversity of an animal community in the same
habitat. Give a real or hypothetical example to
support your answer.

37. Physical structure of plant community
vs. animal species diversity

38. 5. Describe the response that plant communities
typically exhibit when nutrients are added, and
summarize the prevailing hypothesis to explain
this response.

39. Inversely proportional relationship between
soil fertility and species diversity

40. Plant species diversity in a control plot and a fertilized
plot in the Parkgrass experiment in Rothamstead,
England. Fertilized plots have lower species diversity.
http://cedarcreek.umn.edu

41. 6.What are the possible explanations for the
paradox that diversity in plant communities
decreases when nutrient availability increases?

42. Low nutrient concentration - species
competing below ground (symmetric) - more
equal ability of small and large plants to
access resources.
High nutrient concentration - species
competing above-ground (asymmetric) -
less equal ability to compete for light

43. 7. Explain the differences between the rivet
model and the redundancy model of
ecosystems.Which do you think more
accurately described ecosystems?

44. Rivet model- an idea that losing a species in an
ecosystem is analogous to losing a rivet of an
airplane.When loss reaches a certain threshold,
major catastrophic effects happen
Redundancy model- if a species is lost, other
species will fill the gap (“driver” and “passenger”)

45. CHAPTER 19
Community Dynamics

46. 1. Using a specific example, discuss the
ecological differences between early
successional species and late successional
species within a sere.

47. What is succession?

48. In a timber harvested area:
Early succession species or pioneer species-
shade-intolerant and fast growing plant species
predominate the area such as raspberry thickets,
pin cherry and pine birch
Late succession species- after many years the
area will be replaced with shade-tolerant species
and species of trees that are larger and longer
lived

49. 2. Using specific examples for each, explain
the differences between primary and
secondary succession

50. Primary succession- newly exposed substrates
that have never supported a community
Ex. Rock outcrops, cliffs, sand dunes,
newly exposed glacial till

51. Secondary succession- occurs after
disturbances such as old-field succession

52. 3. Explain how a chronosequence can be used
to understand succession.

53. Chronosequence- a group of sites within the same
area that are in different stages of succession

54. 4. Using specific examples for each, describe
the differences between autogenic and
allogenic environmental changes that drive
succession.

55. Autogenic environmental change- direct
result of the presence and activities of
organisms within the community
Allogenic environmental change- changes in
the environment governed by physical
environment rather than biological processes

56. Autogenic change
Source: www.physicalgeography.net

57. Allogenic change

58. Allogenic change
• Elevation on a mountain
• Decrease in temperature with depth in a
lake or ocean
• Change in salinity and water depth

59. CHAPTER 20
Landscape Dynamics

60. 1. Using an example, explain how human and
natural forces can combine to create habitat
patches

61. Patch- an area of habitat that differs from its
surroundings and has sufficient resources to
allow a population o persist.
Factors that create to combine habitat patches:
1. Human activities such as fragmentation of
land tracts due to development creates
patches of forest, grassland and shrubland
2. Fire- aftermath of forest fires create patches
3. Grazing by herbivores
4. Variations in geology and soil condition

62. 2. Discuss the major ecological differences
between edge species and interior species.

63. Edge effect- response of organisms, animals
in particular, to environmental conditions
created by the edge due to the blending
elements of adjacent patches

64. EDGE SPECIES INTERIOR SPECIES
Species restricted
exclusively to the edge or
border environment
Shade-intolerant
Can tolerate dry
conditions
Organisms that require
large areas of habitat,
even though their home
ranges may be small
Require environmental
conditions characteristic
of interior habitats
Stays away from abrupt
changes in environments

66. 3. Discuss the differences in diversity between
large and small habitat patches.What are the
reasons for these differences?

67. Large habitat Small habitat
High diversity of
species
Low diversity of
species
Interior species Edge species

68. 4. Using a graph, illustrate how the number of
species on an island can be affected by its size and
distance from a mainland.What are the two
important demographic variables that vary with
the size of an island and its distance from a
mainland?

69. Theory of Biogeography

70. 2 important demographic variables: extinction
& immigration

71. 5. Explain the potential value of corridors to
large-bodied species such as carnivores.

72. Corridors- strips of vegetation similar to the
patches they connect but different from the
surrounding matrix in which they are set.
Source: http://nac.unl.edu

73. Characteristics of mammalian carnivores:
• Sensitive to changes in landscape structure
• Susceptible to habitat fragmentation
• Rarely remain within the boundaries of
protected areas that are often too small
Potential benefit of corridors to mammalian
carnivores:
* Help in the facilitation of movement of these
animals between protected areas without
experiencing high mortality rates due to
conflicts with humans thus aiding in their
Source: http://esrd.alberta.ca

74. 6. Give two examples of disturbance, and explain
how disturbance can alter a community.

75. Disturbance- any relatively discrete event- such
as fire, windstorm, flood, extremely cold
temperatures, drought or epidemic- that disrupts
community structure and function

76. Spatial &Temporal characteristics of disturbance
• Small scale- makes gaps in the substrate creating
patches
• Large scale- favor opportunistic species
• Severe disturbance- can replace the community
altogether

77. How disturbance alters an ecosystem:
• Results in death of organisms and loss of
biomass
• Changes geomorphology of an ecosystem such
as breaking of barrier dunes by high tides and
winds, cutting of banks and course of rivers and
streams by floods
• Either reduce (eliminates organisms) or foster
(favor survival of some) species diversity