1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 53 (Campbell) Chapter 47 (Starr/Taggart) Community Ecology

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: What Is a Community? A biological community – Is an assemblage of populations of various species living close enough for potential interaction

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The various animals and plants surrounding this watering hole – Are all members of a savanna community in southern Africa Figure 53.1

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and disease Populations are linked by interspecific interactions – That affect the survival and reproduction of the species engaged in the interaction

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Interspecific interactions – Can have differing effects on the populations involved Table 53.1

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Competition Interspecific competition – Occurs when species compete for a particular resource that is in short supply Strong competition can lead to competitive exclusion – The local elimination of one of the two competing species

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Competitive Exclusion Principle The competitive exclusion principle – States that two species competing for the same limiting resources cannot coexist in the same place

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ecological Niches The ecological niche – Is the total of an organism’s use of the biotic and abiotic resources in its environment – If an organism’s “profession” and the habitat it its “address”. – An organism’s niche is its ecological role– how it “fits into” an ecosystem

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The niche concept allows restatement of the competitive exclusion principle – Two species cannot coexist in a community if their niches are identical

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings However, ecologically similar species can coexist in a community – If there are one or more significant difference in their niches When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area. The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of Chthamalus much smaller than its fundamental niche. RESULTS CONCLUSION Ocean Ecologist Joseph Connell studied two barnacle species  Balanus balanoides and Chthamalus stellatus  that have a stratified distribution on rocks along the coast of Scotland. EXPERIMENT In nature, Balanus fails to survive high on the rocks because it is unable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata. Low tide High tide Chthamalus fundamental niche Chthamalus realized niche Low tide High tide Chthamalus Balanus realized niche Balanus Ocean Figure 53.2

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings As a result of competition – A species’ fundamental niche may be different from its realized niche – Fundamental Niche: is the niche potentially occupied by that species – Realized Niche: the niche it actually occipies in a particular environment

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A. insolitus usually perches on shady branches. A. distichus perches on fence posts and other sunny surfaces. A. distichus A. ricordii A. insolitus A. christophei A. cybotes A. etheridgei A. alinigar Figure 53.3 Resource Partitioning Resource partitioning is the differentiation of niches – That enables similar species to coexist in a community Seven species of Anolis lizard live in close proximity, and feed on insects and other small arthropods. However, competition for food is reduced because each has a different perch, thus occupying a distinct niche

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G. fortis Beak depth (mm) G. fuliginosa Beak depth Los Hermanos Daphne Santa María, San Cristóbal Sympatric populations G. fuliginosa, allopatric G. fortis, allopatric Percentages of individuals in each size class 40 20 0 40 20 0 40 20 0 810121416 Figure 53.4 Character Displacement In character displacement – There is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species An example is the variation in beak size between different population of the Galapagos finches.

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Predation Predation refers to an interaction – Where one species, the predator, kills and eats the other, the prey

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Feeding adaptations of predators include – Claws, teeth, fangs, stingers, and poison Animals also display – A great variety of defensive adaptations

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cryptic coloration, or camouflage – Makes prey difficult to spot Figure 53.5

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aposematic coloration – Warns predators to stay away from prey Figure 53.6

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In some cases, one prey species – May gain significant protection by mimicking the appearance of another

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In Batesian mimicry – A palatable or harmless species mimics an unpalatable or harmful model (a) Hawkmoth larva (b) Green parrot snake Figure 53.7a, b Larva weaves head back and forth and hisses like a snake.

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In Müllerian mimicry – Two or more unpalatable species resemble each other (a) Cuckoo bee (b) Yellow jacket Figure 53.8a, b

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Herbivory Herbivory, the process in which an herbivore eats parts of a plant – Has led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parasitism In parasitism, one organism, the parasite – Derives its nourishment from another organism, its host, which is harmed in the process

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parasitism exerts substantial influence on populations – And the structure of communities

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Three types of parasitism Endoparasites: parasites that live within the body of their host, such as tapeworms and malarial parasites Ectoparasites: parasites that feed on the external surface of host, such as ticks and lice Parasitoidism: Insects – usually small wasp-lay eggs on or in living host. The larvae then feed on the body of the host, eventually killing it.

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Disease The effects of disease on populations and communities – Is similar to that of parasites

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pathogens, disease-causing agents – Are typically bacteria, viruses, or protists

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutualism Mutualistic symbiosis, or mutualism – Is an interspecific interaction that benefits both species Figure 53.9

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Commensalism In commensalism – One species benefits and the other is not affected Figure 53.10

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Commensal interactions have been difficult to document in nature – Because any close association between species likely affects both species

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Interspecific Interactions and Adaptation Evidence for coevolution – Which involves reciprocal genetic change by interacting populations, is scarce

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings However, generalized adaptation of organisms to other organisms in their environment – Is a fundamental feature of life

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 53.2: Dominant and keystone species exert strong controls on community structure In general, a small number of species in a community – Exert strong control on that community’s structure, particularly on the composition, relative abundance, and diversity of its species.

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Species Diversity The species diversity of a community – Is the variety of different kinds of organisms that make up the community – Has two components

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Species richness – Is the total number of different species in the community Relative abundance – Is the proportion each species represents of the total individuals in the community

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Two different communities – Can have the same species richness, but a different relative abundance Community 1 A: 25%B: 25%C: 25%D: 25% Community 2 A: 80%B: 5%C: 5%D: 10% D C B A Figure 53.11 Ecologist would say that community 1 has greater species diversity, a measure that includes both species richness and relative abundance

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A community with an even species abundance – Is more diverse than one in which one or two species are abundant and the remainder rare

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Trophic Structure Trophic structure – Is the feeding relationships between organisms in a community – The transfer of food energy up the trophic levels from its source in plants and other photosynthetic organism (primary producers) through herbivores (primary consumers) to carnivores (secondary and tertiary consumers) – Is a key factor in community dynamics

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Food chains Quaternary consumers Tertiary consumers Secondary consumers Primary consumers Primary producers Carnivore Herbivore Plant Carnivore Zooplankton Phytoplankton A terrestrial food chainA marine food chain Figure 53.12 – Link the trophic levels from producers to top carnivores

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Food Webs A food web Humans Baleen whales Crab-eater seals Birds Fishes Squids Leopard seals Elephant seals Smaller toothed whales Sperm whales Carnivorous plankton Euphausids (krill) Copepods Phyto- plankton Figure 53.13 – Is a branching food chain with complex trophic interactions

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Food webs can be simplified – By isolating a portion of a community that interacts very little with the rest of the community Sea nettle Fish larvae Zooplankton Fish eggs Juvenile striped bass Figure 53.14

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Limits on Food Chain Length Each food chain in a food web – Is usually only a few links long (about 5 trophic levels) There are two hypotheses – That attempt to explain food chain length

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The energetic hypothesis suggests that the length of a food chain – Is limited by the inefficiency of energy transfer along the chain – Hypothesis predicts that food chains should be relatively longer in habitats of higher photosynthetic productivity – Generally known as the 10% Law: That states that only 10% of energy will be transferred from one trophic level to the next.

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The dynamic stability hypothesis – Proposes that long food chains are less stable than short ones – Population fluctuations at lower trophic levels are magnified at higher levels potentially causing the local extinction of top predators. – In variable environment, top predators must be able to recover from environmental shocks (such as extreme winters) that can reduce the food supply all the way up the food chain.

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Most of the available data – Support the energetic hypothesis High (control) Medium Low Productivity No. of species No. of trophic links Number of species Number of trophic links 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Figure 53.15

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Species with a Large Impact Certain species have an especially large impact on the structure of entire communities – Either because they are highly abundant or because they play a pivotal role in community dynamics

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominant Species Dominant species – Are those species in a community that are most abundant or have the highest biomass (the total mass of all individuals in a population) – Exert powerful control over the occurrence and distribution of other species – Example: Sugar maples in North American forest communities has major impact on abiotic factors such as shading and soil, which in turn affect which other species live there

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings There is no single explanation for why a species becomes dominant in a community. One hypothesis suggests that dominant species – Are most competitive in exploiting limited resources Another hypothesis for dominant species success – Is that they are most successful at avoiding predators

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Keystone Species Keystone species – Are not necessarily abundant in a community – Exert strong control on a community by their ecological roles, or niches

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Field studies of sea stars – Exhibit their role as a keystone species in intertidal communities Figure 53.16a,b (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. With Pisaster (control) Without Pisaster (experimental) Number of species present 0 5 10 15 20 1963 ´64´65 ´66 ´67 ´68 ´69 ´70´71 ´72 ´73 (b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity. A good way to identify the keystone species is to removal experiment (like the experiment here)

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Observation of sea otter populations and their predation Figure 53.17 Food chain before killer whale involvement in chain (a) Sea otter abundance (b) Sea urchin biomass (c) Total kelp density Number per 0.25 m 2 19721985198919931997 0 2 4 6 8 10 0 100 200 300 400 Grams per 0.25 m 2 Otter number (% max. count) 0 40 20 60 80 100 Year Food chain after killer whales started preying on otters – Shows the effect the otters have on ocean communities

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ecosystem “Engineers” (Foundation Species) Some organisms exert their influence – By causing physical changes in the environment that affect community structure

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Beaver dams – Can transform landscapes on a very large scale Figure 53.18

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some foundation species act as facilitators – That have positive effects on the survival and reproduction of some of the other species in the community Figure 53.19 Salt marsh with Juncus (foreground) With Juncus Without Juncus Number of plant species 0 2 4 6 8 Conditions Juncus helps prevents the salt marsh soils form becoming anoxic (decrease in oxygen level) as it transports oxygen to its below ground tissue

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bottom-Up and Top-Down Controls The bottom-up model of community organization – Proposes a unidirectional influence from lower to higher trophic levels In this case, the presence or absence of abiotic nutrients – Determines community structure, including the abundance of primary producers

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The top-down model of community organization – Proposes that control comes from the trophic level above In this case, predators control herbivores – Which in turn control primary producers

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Long-term experiment studies have shown – That communities can shift periodically from bottom-up to top-down Figure 53.20 0 100200300400 Rainfall (mm) 0 25 50 75 100 Percentage of herbaceous plant cover Moisture limitation on a plant growth during dry non-El-Nino years (red points) creates strong bottom up control on this community, whereas the abundant moisture during El Nino years (blue points) stimulates increased plant and animal abundance, inducing strong top-down control

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pollution – Can affect community dynamics But through biomanipulation – Polluted communities can be restored Fish Zooplankton Algae Abundant Rare Abundant Rare Polluted State Restored State

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 53.3: Disturbance influences species diversity and composition Decades ago, most ecologists favored the traditional view – That communities are in a state of equilibrium

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings However, a recent emphasis on change has led to a nonequilibrium model – Which describes communities as constantly changing after being buffeted by disturbances

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings What Is Disturbance? A disturbance – Is an event that changes a community – Removes organisms from a community – Alters resource availability

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fire – Is a significant disturbance in most terrestrial ecosystems – Is often a necessity in some communities (a) Before a controlled burn. A prairie that has not burned for several years has a high propor- tion of detritus (dead grass). (b) During the burn. The detritus serves as fuel for fires. (c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living. Figure 53.21a–c Sometimes enhances a community like the following example

62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The intermediate disturbance hypothesis – Suggests that moderate levels of disturbance can foster higher species diversity than low levels of disturbance

63. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The large-scale fire in Yellowstone National Park in 1988 – Demonstrated that communities can often respond very rapidly to a massive disturbance Figure 53.22a, b (a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance. (b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Human Disturbance Humans – Are the most widespread agents of disturbance – Examples: Agricultural development, Logging and clearing for an urban area

65. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Human disturbance to communities – Usually reduces species diversity Humans also prevent some naturally occurring disturbances – Which can be important to community structure

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ecological Succession Ecological succession – Is the sequence of community and ecosystem changes after a disturbance

67. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Primary succession – Occurs where no soil exists when succession begins – No Soil has ever existed!!! – Example: Volcanic Island that has just formed.

68. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Secondary succession – Begins in an area where soil remains after a disturbance – Soil Exist!! – Example: Logging, Abandoned farm

69. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Early-arriving species – May facilitate the appearance of later species by making the environment more favorable – May inhibit establishment of later species – May tolerate later species but have no impact on their establishment

70. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings McBride glacier retreating 0510 Miles Glacier Bay Pleasant Is. Johns Hopkins Gl. Reid Gl. Grand Pacific Gl. Canada Alaska 1940 1912 1899 1879 1949 1879 1935 1760 1780 1830 1860 1913 1911 1892 1900 1879 1907 1948 1931 1941 1948 Casement Gl. McBride Gl. Plateau Gl. Muir Gl. Riggs Gl. Retreating glaciers – Provide a valuable field-research opportunity on succession Figure 53.23

71. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Succession on the moraines in Glacier Bay, Alaska – Follows a predictable pattern of change in vegetation and soil characteristics Figure 53.24a–d (b) Dryas stage (c) Spruce stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. Soil nitrogen (g/m 2 ) Successional stage Pioneer Dryas Alder Spruce 0 10 20 30 40 50 60 (a) Pioneer stage, with fireweed dominant

72. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 53.4: Biogeographic factors affect community diversity Two key factors correlated with a community’s species diversity – Are its geographic location and its size

73. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Equatorial-Polar Gradients The two key factors in equatorial-polar gradients of species richness – Are probably evolutionary history and climate

74. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Species richness generally declines along an equatorial-polar gradient – And is especially great in the tropics The greater age of tropical environments – May account for the greater species richness

75. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Climate—Weather History Climate – Is likely the primary cause of the latitudinal gradient in biodiversity – These factors can be considered together by measuring community rate of evaportranspiration, the evaporation of water from soil plus the transpiration

76. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The two main climatic factors correlated with biodiversity – Are solar energy input and water availability (b) Vertebrates 500 1,000 1,500 2,000 Potential evapotranspiration (mm/yr) 10 50 100 200 Vertebrate species richness (log scale) 1 100 300 500700 900 1,100 180 160 140 120 100 80 60 40 20 0 Tree species richness (a) Trees Actual evapotranspiration (mm/yr) Figure 53.25a, b

77. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Area Effects The species-area curve quantifies the idea that – All other factors being equal, the larger the geographic area of a community, the greater the number of species

78. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A species-area curve of North American breeding birds – Supports this idea Area (acres) 11010010 3 10 4 10 5 10 6 10 7 10 8 10 910 Number of species (log scale) 1 10 100 1,000 Figure 53.26

79. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Island Equilibrium Model Species richness on islands – Depends on island size, distance from the mainland, immigration, and extinction

80. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 53.27a–c The equilibrium model of island biogeography maintains that – Species richness on an ecological island levels off at some dynamic equilibrium point Number of species on island (a) Immigration and extinction rates. The equilibrium number of species on an island represents a balance between the immigration of new species and the extinction of species already there. (b) Effect of island size. Large islands may ultimately have a larger equilibrium num- ber of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands. Number of species on island (c) Effect of distance from mainland. Near islands tend to have larger equilibrium numbers of species than far islands because immigration rates to near islands are higher and extinction rates lower. Equilibrium number Small island Large islandFar island Near island Immigration Extinction Immigration Extinction Immigration (small island) (large island) (small island) Immigration Extinction Immigration (far island) (near island) (far island) Extinction Rate of immigration or extinction

81. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Studies of species richness on the Galápagos Islands – Support the prediction that species richness increases with island size The results of the study showed that plant species richness increased with island size, supporting the species-area theory. FIELD STUDY RESULTS Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island. CONCLUSION 200 100 50 25 10 0 Area of island (mi 2 ) (log scale) Number of plant species (log scale) 0.1 1 10100 1,000 5 400 Figure 53.28

82. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 53.5: Contrasting views of community structure are the subject of continuing debate Two different views on community structure – Emerged among ecologists in the 1920s and 1930s, based primarily on observations of plant distribution.

83. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Integrated and Individualistic Hypotheses The integrated hypothesis of community structure – Describes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactions that cause the community to function as an integrated view includes the observation that certain species of plants are consistently found together as a group.

84. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The individualistic hypothesis of community structure – Proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements

85. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The integrated hypothesis – Predicts that the presence or absence of particular species depends on the presence or absence of other species Population densities of individual species Environmental gradient (such as temperature or moisture) (a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together. Figure 53.29a

86. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The individualistic hypothesis – Predicts that each species is distributed according to its tolerance ranges for abiotic factors Population densities of individual species Environmental gradient (such as temperature or moisture) (b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar abiotic needs. Figure 53.29b

87. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In most actual cases the composition of communities – Seems to change continuously, with each species more or less independently distributed Number of plants per hectare Wet Moisture gradientDry (c) Trees in the Santa Catalina Mountains. The distribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities. 0 200 400 600 Figure 53.29c

88. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rivet and Redundancy Models The rivet model of communities – Suggests that all species in a community are linked together in a tight web of interactions – Also states that the loss of even a single species has strong repercussions for the community

89. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The redundancy model of communities – Proposes that if a species is lost from a community, other species will fill the gap

90. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings It is important to keep in mind that community hypotheses and models – Represent extremes, and that most communities probably lie somewhere in the middle