1. CHAPTER 53 COMMUNITY ECOLOGY Section A: What Is a Community?
1. Contrasting views of communities are rooted in the individualistic and interactive hypotheses
2. The debate continues with the rivet and redundancy models
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

2. Introduction What is a Community?
A community is defined as an assemblage of species living close enough together for potential interaction.
Communities differ in their species richness, the number of species they contain, and the relative abundance of different species.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

3. 1. Contrasting views of communities are rooted in the individualistic and interactive hypotheses
An individualistic hypothesis depicts a community as a chance assemblage of species found in the same area because they happen to have similar abiotic requirements.
Fig. 53.1a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

4. An interactive hypothesis depicts a community as an assemblage of closely linked species locked in by mandatory biotic interactions.
Fig. 53.1b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

5. These two very different hypotheses suggest different priorities in studying biological communities.
In most actual cases, the composition of communities does seem to change continuously.
Fig. 53.1c
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

6. CHAPTER 53 COMMUNITY ECOLOGY
Section B1: Interspecific Interactions and Community Structure
Populations maybe linked by competition, predation, mutualism, and commensalism
There are different interspecific interactions, relationships between the species of a community.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

7. 1. Populations may be linked by competition, predation, mutualism and commensalism
Possible interspecific interactions are introduced in Table 53.1, and are symbolized by the positive or negative affect of the inter- action on the individual populations.
Table 53.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

8. Competition.
Interspecific competition for resources can occur when resources are in short supply.
There is potential for competition between any two species that need the same limited resource.
The competitive exclusion principle: two species with similar needs for same limiting resources cannot coexist in the same place.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

9. An organism’s niche is its role in the environment.
The ecological niche is the sum total of an organism’s use of abiotic/biotic resources in the environment.
An organism’s niche is its role in the environment.
The competitive exclusion principle can be restated to say that two species cannot coexist in a community if their niches are identical.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

10. Classic experiments confirm this.
Fig. 53.2
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

11. Resource partitioning is the differentiation of niches that enables two similar species to coexist in a community.
Ex. African carnivores.
Fig. 53.2
Fig. 53.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

12. Hereditary changes evolve that bring about resource partitioning.
Character displacement is the tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species.
Hereditary changes evolve that bring about resource partitioning.
Fig. 53.4
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

13. Predation. A predator eats prey.
Herbivory, in which animals eat plants.
In parasitism, predators live on/in a host and depend on the host for nutrition.
Predator adaptations: many important feeding adaptations of predators are both obvious and familiar.
Claws, teeth, fangs, poison, heat-sensing organs, speed, and agility.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

14. Animal defenses against predators.
Plant defenses against herbivores include chemical compounds that are toxic.
Animal defenses against predators.
Behavioral defenses include fleeing, hiding, self- defense, noises, and mobbing.
Camouflage includes cryptic coloration, deceptive markings.
Fig. 53.5
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

15. Mechanical defenses include spines.
Chemical defenses include odors and toxins
Aposematic coloration is indicated by warning colors, and is sometimes associated with other defenses (toxins).
Fig. 53.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

16. Mimicry is when organisms resemble other species.
Batesian mimicry is where a harmless species mimics a harmful one.
Fig. 53.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

17. Müllerian mimicry is where two or more unpalatable species resemble each other.
Fig. 53.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

18. Parasites and pathogens as predators.
A parasite derives nourishment from a host, which is harmed in the process.
Endoparasites live inside the host and ectoparasites live on the surface of the host.
Parasitoidism is a special type of parasitism where the parasite eventually kills the host.
Pathogens are disease-causing organisms that can be considered predators.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

19. Mutualism is where two species benefit from their interaction.
Commensalism is where one species benefits from the interaction, but other is not affected.
An example would be barnacles that attach to a whale.
Fig. 53.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

20. Coevolution and interspecific interactions.
Coevolution refers to reciprocal evolutionary adaptations of two interacting species.
When one species evolves, it exerts selective pressure on the other to evolve to continue the interaction.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

21. CHAPTER 53 COMMUNITY ECOLOGY
Section B2: Interspecific Interactions and Community Structure
2. Trophic structure is a key factor in community dynamics
3. Dominant species and keystone species exert strong controls on community structure
4. The structure of a community may be controlled bottom-up by nutrients or top-down by predators
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

22. 2. Trophic structure is a key factor in community dynamics
The trophic structure of a community is determined by the feeding relationships between organisms.
The transfer of food energy from its source in photosynthetic organisms through herbivores and carnivores is called the food chain.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

23. Charles Elton first pointed out that the length of a food chain is usually four or five links, called trophic levels.
He also recognized that food chains are not isolated units but are hooked together into food webs.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

24. Food webs. Who eats whom in a community?
Trophic relation- ships can be diagrammed in a community.
What transforms food chains into food webs?
A given species may weave into the web at more than one trophic level.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

25. What limits the length of a food chain?
The energetic hypothesis suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain.
The dynamic stability hypothesis states that long food chains are less stable than short chains.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

26. 3. Dominant species and keystone species exert strong controls on community structure
Dominant species are those in a community that have the highest abundance or highest biomass (the sum weight of all individuals in a population).
If we remove a dominant species from a community, it can change the entire community structure.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

27. Keystone species exert an important regulating effect on other species in a community.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

28. If they are removed, community structure is greatly affected.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

29. 4. The structure of a community may be controlled bottom-up by nutrients or top-down by predators
Simplified models based on relationships between adjacent trophic levels are useful for discussing how communities might be organized.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

30. Other models go between the bottom-up and top-down extreme models.
Consider three possible relationships between plants (V for vegetation) and herbivores (H).
V  H V  H V  H
Arrows indicate that a change in biomass of one trophic level causes a change in the other trophic level.
The bottom-up model postulates V  H linkages, where nutrients and vegetation control community organization.
The top-down model postulates that it is mainly predation that controls community organization V  H.
Other models go between the bottom-up and top-down extreme models.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

31. CHAPTER 53 COMMUNITY ECOLOGY
Section C1: Disturbance and Community Structure
1. Most communities are in a state of nonequilibrium owing to disturbances
2. Humans are the most widespread agents of disturbance
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

32. Introduction Disturbances affect community structure and stability.
Stability is the ability of a community to persist in the face of disturbance.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

33. 1. Most communities are in a state of nonequilibrium owing to disturbances
Disturbances are events like fire, weather, or human activities that can alter communities.
Some are routine.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

34. Marine communities are subject to disturbance by tropical storms.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

35. We usually think that disturbances have a negative impact on communities, but in many cases they are necessary for community development and survival.
Ex. Many
forests &
grasslands
require
episodes
of fire.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

36. 2. Humans are the most widespread agents of disturbance
Human activities cause more disturbance than natural events and usually reduce species diversity in communities.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

37. CHAPTER 53 COMMUNITY ECOLOGY
Section C2: Disturbance and Community Structure
3. Ecological succession is the sequence of community changes after a disturbance
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

38. 3. Ecological succession is the sequence of community changes after a disturbance
Ecological succession is the transition in species composition over ecological time.
Primary succession begins in a lifeless area where soil has not yet formed.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

39. Mosses and lichens colonize first and cause the development of soil.
An example would be after a glacier has retreated.
Table 53.2
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

40. Secondary succession occurs where an existing community has been cleared by some event, but the soil is left intact.
Grasses grow first, then trees and other organisms.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

41. Soil concentrations of nutrients show changes over time.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

42. CHAPTER 53 COMMUNITY ECOLOGY
Section D: Biogeographic Factors Affecting the Biodiversity of Communities
1. Community biodiversity measures the number of species and their relative abundance
2. Species richness generally declines along an equatorial-polar gradient
3. Species richness is related to a community’s geographic size
4. Species richness on an island depends on island size and distance from the mainland
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

43. Introduction
Two key factors correlated with a community’s biodiversity (species diversity) are its size and biogeography.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

44. 1. Community biodiversity measures the number of species and their relative abundance
The variety of different kinds of organisms that make up a community has two components.
Species richness, the total number of species in the community.
Relative abundance of the different species.
Imagine two small forest communities with 100 individuals distributed among four different tree species.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

45. Species richness may be equal, but relative abundance may be different.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

46. Counting species in a community to determine their abundance is difficult, especially for insects and smaller organisms
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

47. 2. Species richness generally declines along an equatorial-polar gradient
Tropical habitats support much larger numbers of species of organisms than do temperate and polar regions.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

48. Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

49. What causes these gradients?
The two key factors are probably evolutionary history and climate.
Organisms have a history in an area where they are adapted to the climate.
Energy and water may factor into this phenomenon.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

50. Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

51. 3. Species richness is related to a community’s geographic size
The species-area curve quantifies what may seem obvious: the larger the geographic area, the greater the number of species.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

52. 4. Species richness on islands depends on island size and distance from the mainland
Because of their size and isolation, islands provide great opportunities for studying some of the biogeographic factors that affect the species diversity of communities.
Imagine a newly formed island some distance from the mainland.
Robert MacArthur and E. O. Wilson developed a hypothesis of island biogeography to identify the determinants of species diversity on an island.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

53. Two factors will determine the number of species that eventually inhabit the island.
The rate at which new species immigrate to the island.
The rate at which species become extinct.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

54. Studies of plants on many island chains confirm their hypothesis.
Fig
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings