1. BIOL 4131 Lecture 10
Species diversity
Ecological communities differ in species number and composition
tropics > temperate
remote islands < large islands
continents > islands

2. Species diversity Comprised of
species richness: number of species present
heterogeneity of species
equitability or evenness
relative abundance of each species present in the community

3. Measurement of species diversity
Species richness
number of species present in community
first and oldest concept of diversity
simplest estimate of diversity
only residents are counted
treats common and rare species with the same weight

4. Measurement of species diversity
Heterogeneity of species
uses relative abundance to give more weight to common species
possibilities in a 2-species community:
Comm 1 Comm 2
Species A
Species B

5. Measurement of species diversity
Shannon-Wiener diversity function
H' = - (pi) [ln(pi)] 
H’ = Shannon-Wiener index of species diversity
s = number of species in community
pi = proportion of total abundance represented by ith species s

6. Shannon-Wiener diversity index
Community 1
Species N pi
ln(pi)
pi[(ln(pi)] A 99 B 1
Community 2 50

7. Shannon-Wiener diversity index
Community 1
Species N pi
ln(pi)
pi[(ln(pi)] A 99
0.99
-0.010 B 1
0.01
-4.605
-0.046
100
1.00
-0.056 H’
0.056
Community 2 50
0.50
-0.693
-0.347
-0.694
0.694

8. Measurement of species diversity
Shannon-Wiener diversity function
values range from near zero to ???
increased values indicate increased diversity
index has no units; value only as comparison between at least two communities

9. Species diversity What increases species diversity (H’)?
increasing the number of species in the community (s)
increasing the equitability of the abundances of each species in the community

10. E = H’ / Hmax E = Pielou evenness Evenness
Measurement of equitability among species in the community
Pielou evenness
E = H’ / Hmax
E = Pielou evenness
H’ = calculated Shannon-Wiener diversity
Hmax = ln(s) [species diversity under maximum equitability conditions]
values range from near zero to 1

11. Diversity and evenness
Community 1
Community 2 s 2 H’
0.056
0.694
Hmax
0.693 E
0.081
1.000

12. Practice problem Community 1 Species N pi ln(pi) pi[(ln(pi)] A 62 B 97
110 D 84 E 16

13. Practice problem Community 2 Species N pi ln(pi) pi[(ln(pi)] A 88 B 10D
211 E 27

14. Practice problem
Community 1
Community 2 s H’
Hmax E

15. Species diversity indices

16. Commonness, rarity and dominance
Preston’s log normal distribution model
a few common species with high abundances
many rare species with low abundances

17. Commonness, rarity and dominance
MacArthur’s broken stick model
random breaks in a stick  log normal distribution of pieces
results in a few large pieces and many small pieces

18. Commonness, rarity and dominance
Community organization
model 1
a few very common species
many rare species
model 2
a few very common and very rare species
most species of intermediate abundance

19. Fig. 22.1, p. 435: Relative abundance of Lepidoptera captured in a light trap in England (6814 individuals representing 197 species).

20. Biogeography Observations of relationships between Island biogeography
area and number of species
distance from source
Island biogeography
E.O. Wilson and Robert MacArthur

21. Island biogeography Island communities: well-defined, captive
Variables
size
degree of remoteness
elevation
Simple community structure
Increase in area  increase in number of species

22. Island biogeography
Habitats considered as “insular” because they are isolated from other communities
caves
mountain tops
some peninsulas
wildlife or game preserves

23. Fig , p. 502: Number of land-plant species on the Galapagos Islands in relation to the area of the island.

24. Fig. 24.15, p. 503: Species-area curve for amphibians and reptiles of the West Indies.

25. Island biogeography
Relationship between remoteness and number of species
increase distance from mainland  decrease number of species
number of species present is dependent on immigration from mainland
rate is a function of the number of species already present on the island
number of species present = balance between immigration and extinction

26. Fig. 24.17, p. 504: Equilibrium model for biota on a single island.

27. Fig. 24.18, p. 504: Equilibrium model for biota on several islands of different size and remoteness.

28. Island biogeography
Small species are found on more islands than are large species
Number of herbivore species > carnivores
Number of generalist herbivore species > specialist herbivores

29. Island biogeography Species:area relationship log : log relationship
10-fold decrease in area  50% decrease in number of species

30. Island biogeography
Species:area relationship

31. Latitudinal diversity gradients
Abundance and diversity patterns
latitude
elevation
mountainsides
peninsulas

32. Fig. 22.5, p. 438: Number of tree species in Canada and U.S.

33. Fig. 22.6, p. 439: Number of species of land birds in North and Central America.

34. Fig. 22.7, p. 440: Number of species of calanoid copepods in top 50 m of transect from tropical Pacific to Arctic Ocean.

35. Fig. 22.9, p. 440: Number of species of mammals in continental North America.

36. Fig. 22.10, p. 440: Species richness of mammals in North and South America in relation to latitude.

37. Latitudinal diversity gradients
Tree species
Malaysia (4 acres): 227
Michigan (4 acres): <15
Ant species
Brazil: 222
Trinidad: 134
Cuba: 101
Utah: 63
Alaska: 7

38. Latitudinal diversity gradients
Snake species
Mexico: 293
U.S.: 126
Canada: 22
Fish species
Amazon R: >1000
Central American rivers: 450
Great Lakes: 172

39. Latitudinal gradient hypotheses
History (time)
Spatial heterogeneity
Competition
Predation
Productivity
Environmental stability (climate)
Disturbance

40. Latitudinal gradient hypotheses
History (time) hypothesis
tropical habitats older, more stable
support for
geological past of temperate less constant than tropics due to glaciation
all communities diversify with time
argument against
as glaciers moved in, species moved south to escape
history hypothesis can not be tested

41. Latitudinal gradient hypotheses
Spatial heterogeneity hypothesis
higher diversity in tropics due to increase in number of potential habitats
 environmental complexity moving away from equator
macro level: e.g., topographic features
micro level: e.g., particle size, vegetation complexity

42. Latitudinal gradient hypotheses
Spatial heterogeneity hypothesis
Hutchinson’s n-dimensional niche   specialization
types of diversity defined by spatial heterogeneity
within-habitats ( diversity)
between-habitats ( diversity)

43. Diversity defined by spatial heterogeneity
Between habitat diversity ()
Temperate
Tropical
No. species per habitat 10
No. different habitats 50
Within-habitat diversity ()

44. Latitudinal gradient hypotheses
Competition hypothesis
less competition in temperate and polar environments compared to tropics because these populations are more regulated by extreme environmental conditions than by biological factors
populations maintained <K due to weather, etc. and major sources of mortality are abiotic
since population sizes small, decreased competition for resources

45. Latitudinal gradient hypotheses
Competition hypothesis
no weather extremes in tropics,  populations can increase to densities at which competition for resources is necessary
promotes species diversity through specialization  resource partitioning
 and  diversity higher in tropics due to organisms being more specialized to habitats

46. Fig a, p Niche breadth versus niche overlap determined by competition within the community.

47. Latitudinal gradient hypotheses
Predation hypothesis
increased species diversity in tropics is function of increased number of predators that regulate the prey species at low densities
decreases competition among prey species
allows coexistence of prey species and potential for new additions

48. Fig , p Janzen-Connell model for increased diversity of tropical rainforest trees: seed predation versus distance of seed from tree versus seed survival.

49. Latitudinal gradient hypotheses
Predation hypothesis
there is more selective pressure on prey evolving avoidance mechanisms than in becoming better competitors
cropping principle
remove predators and prey start competing
predation increases diversity by reducing intraspecific competition among prey species

50. Community anchored by keystone starfish Heliaster in northern Gulf of California.

51. Latitudinal gradient hypotheses
Predation hypothesis
cropping principle in lakes
top predators (fish) feed on zooplankton
if fish are removed  community diversity decreases, becomes dominated by a few species of large, grazing zooplankton
add fish  diversity of small zooplankton and their invertebrate predators increases

52. Latitudinal gradient hypotheses
Productivity hypothesis
tropics support a greater number of species because more resources are available, allowing for more specialization
in general:  production   diversity
exceptions
marshes: high production, relatively low diversity
deserts: low production, high diversity

53. Latitudinal gradient hypotheses
Environmental stability (climatic) hypothesis
annual climate in tropics more stable than temperate or polar climates
constant climate  finer specializations and adaptations, shallower niches
tropical species  number of broods / year   potential for evolutionary change   rate of speciation

54. Latitudinal gradient hypotheses
Environmental stability (climatic) hypothesis
high diversity habitats generally found in stable climates; low diversity habitats associated with severe and/or unpredictable climates

55. Latitudinal gradient hypotheses
Disturbance hypothesis
if community disturbance frequency is very high  local extinction of species   species diversity
if community disturbance frequency is very low  competitive exclusion by dominant species   species diversity

56. Latitudinal gradient hypotheses
Disturbance hypothesis
intermediate disturbance hypothesis
moderate disturbance maximizes diversity
leads to patches at local level
intermediate disturbance  high species diversity in some communities (not all)

57. Fig. 22.20, p. 453. Model for intermediate disturbance hypothesis.

58. Fig. 22.21, p. 453. Effect of periwinkle grazing on algae diversity.

59. Fig. 22.21, p. 453. Effect of periwinkle grazing on algae diversity.
Community dominated by one algal species
Predator limits number of possible algal species

60. Basic concepts related to energy flow and trophic structure
Energy moves through community and is lost as heat
Nutrients move through the community in cycles and are retained

61. Basic concepts related to energy flow and trophic structure
Niche
sum of all parameters that enable an organism to live in its biotic and abiotic environments
competition, food gathering, predator escape, mate location, reproduction, etc.
temperature, moisture, nutrients, soil structure, salinity, etc.
Hutchinsonian niche: n-dimensional hypervolume

62. Basic concepts related to energy flow and trophic structure
Trophic level
Lindeman (1942)
classification of animals according to location in lake
lake trophic groups
benthic
demersal
plankton
nekton

63. Basic concepts related to energy flow and trophic structure
Trophic level
Lindeman (1942)
described food chain with primary producers at base and other trophic levels of animals based on feeding relationships
more accurately described as food web, since few organisms other than plants occupy only one feeding level

64. Food webs and energy flow
Trophic levels
ecosystem feeding levels
biomass and usable energy  as  level
most systems support only four trophic levels
aquatic communities have slightly longer food chains than terrestrial communities
ultimate food chain length limited by inefficiency of energy transfer from one trophic level to the next

65. Food webs and energy flow
Food chains
sequence of organisms where each is the food source for the next
Food webs
represent energy flow through ecosystem

66. Trophic levels Tertiary consumers (top carnivores)
Secondary consumers (carnivores)
Primary consumers (herbivores)
Primary producers (plants)

67. Food chain model

68. Figure 23.6, p. 465. Hypothetical food web model.

69. Food web terminology
Top predators: species eaten by nothing else in the food web
Basal species: species that feed on nothing within the food web
Intermediate species: species that have both predators and prey within the food web
Trophic species: groups of organisms that have identical sets of predators and prey
Cycles within food web: which species eat which other species
Interaction: any feeding relationship within food web
Connectance: number of actual interactions in food web divided by number of possible interactions
Linkage density: average number of interactions per species in the food web
Omnivores: species that feed on more than one trophic level
Compartments: groups of species with strong linkages among group members but weak linkages to other groups of species

70. 5518 food chain lengths counted
Figure 23.8, p Distribution of food chain lengths in the Ythan Estuary, NE Scotland.
95 species
5518 food chain lengths counted

71. Producers Producers Producers Producers
Food web of a rocky intertidal community, northern Gulf of California (after Paine 1966).
Producers Producers Producers Producers

72. Rocky intertidal community food web (Paine’s 1966 study)
(Producer level omitted from original figure)
Level 1
herbivorous gastropods and chitons
filter feeding bivalves
suspension feeding barnacles and brachiopods
Levels 2-4: carnivorous gastropods
Level 5: top carnivore
Heliaster starfish

73. Keystone species Usually the top carnivore
Presence or absence determines community structure and composition

74. Producers Producers Producers Producers
Food web of a rocky intertidal community, northern Gulf of California (after Paine 1966).
Top carnivore
Producers Producers Producers Producers

75. Food web of a rocky intertidal community, northern Gulf of California (after Paine 1966).
Top carnivore X
Species outcompeted in absence of keystone species X X X X X
Space competitor
Producers Producers Producers Producers

76. Keystone species Paine (1974): Pacific rocky intertidal community
dominated by Pisaster starfish
remove starfish → mussel Mytilus californiensis ↑ → excludes all other invertebrate species
Mytilus becomes numerically dominant
Pisaster feeds on Mytilus → prevents Mytilus domination of community → ↑ community diversity

77. Figure 23.3, p. 462. Simplified Antarctic marine food web.

78. Fig. 23.4, p. 464. Food web of boreal forest of northwest Canada.

79. Generalizations about food webs
Size of animal increases with increase in trophic level
Abundance decreases with increase in trophic level
Large animals can not exist on small animals as prey
Small carnivores are limited to prey that can fit into their mouths

80. Which trophic level is most important?
Studies by Charles Elton in two square miles of Wytham Woods
Which species could be removed without changing the community?
top carnivore, except keystone species
lower levels are food source for higher levels
importance of top carnivores <<< herbivores

81. Which trophic level is most important?
Dependent on complexity of community
increased number of interconnections in community → increased complexity of food web → increased stability of community structure → alternate food sources should one be removed
redundancy model versus rivet model

82. Which trophic level is most important?
Determining species importance
species with highest biomass
where nutrients accumulate
where energy accumulates