1. Food Webs
Chapter 17

2. Arctic Food Web
Bear Island in High Arctic.

3. feeding activities of a few species may have a dominant influence on community structure.

4. Keystone Species
Keystone species – species that, despite low biomass, exert strong effects on the structure of the community they inhabit
If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities.

5. Figure 53.15 Sea otters as keystone predators in the North Pacific

6. Keystone Species

7. Food Web Structure and Species Diversity
Paine found as number of species in intertidal food webs increased, proportion of the foodweb represented by predators also increased.
higher proportion of predators produces higher predation pressure on prey populations, in turn promoting higher diversity.
Removal of starfish (top predator) caused decline in diversity from 15 to 8 species.

8. Fish as River Keystone Species

9. Effects of Predation by Birds on Herbivory
Atlegrim studied influence of birds on herbivorous insects and insect-induced plant damage.
Insectivorous birds may act as keystone species via effects on herbivorous insects.
Larval insect densities peak when many insectivorous birds are feeding their young.

10. Keystone Species: Summation
Power : Keystone species exert strong effects on their community structure, despite low biomass.

11. Exotic Predators
Exotic species have dramatic impacts on communities because they were outside the evolutionary experience of local prey populations.
Nile Perch (Lates nilotica) exotic fish predator in Lake Victoria.
Fish fauna dramatically reduced.

12. Exotic Predators

13. Exotic Predators

14. Exotic Predators
Kaufman pointed out changes in Lake Victoria fish community coincide with other ecosystem changes.
Dissolved oxygen concentrations significantly decreased.
Cultural eutrophication.

15. Primary Production and Energy Flow
Chapter 18
NEVER FORGET!!!
ENERGY FLOWS:
solar (or nuclear) input
heat output
NUTRIENTS RECYCLE:
mass conservation
inorganic ↔ organic

16. Focus Areas: Trophic Levels Terrestrial Production Controls:
Actual evapotranspiration (AET)
Precipitation
Soil fertility
Consumers
Aquatic Production Controls:
Nutrients
Light

17. Introduction
Trophic Level: Position in a food web determined by number of energy transfers from primary producers to current level:
Primary producers occupy first level.
Primary consumers occupy second level.
Secondary consumers occupy third level.
Tertiary consumers occupy fourth level.

21. Primary production: Fixation of energy by autotrophs in an ecosystem.
Rate of primary production: Amount of energy fixed over a given period of time.
Gross primary production: Total amount of energy fixed by autotrophs.
Net primary production: Amount of energy leftover after autotrophs have met their metabolic needs.

23. Primary Production Controls
Bottom-Up Controls
Influences of physical and chemical factors of an ecosystem.
Top-Down Controls
Influences of consumers.

25. Soil Fertility and Terrestrial Primary Production
Significant variation in terrestrial primary production can be explained by differences in soil fertility.
Shaver and Chapin found arctic net primary production was twice as high on fertilized plots as unfertilized plots.
Bowman suggested N is main nutrient limiting net primary production in a dry tundra meadow, and N and P jointly limit production in a wet meadow.

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27. Primary Production on the Serengeti
McNaughton estimated Serengeti grazers consume an average of 66% of annual primary production.
Rate of primary production in the Serengeti is positively correlated with rainfall quantity.

28. Primary Production in the Serengeti
Found grazers can increase primary production.
Increased growth rate.
Compensatory Growth
Lower respiration rate due to lower biomass.
Reduced self-shading.
Improved water balance due to reduced leaf area.

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30. Patterns of Aquatic Primary Production
Several studies have found quantitative relationship between phosphorus and freshwater phytoplankton biomass.
Several studies support generalization that nutrient availability controls rate of primary production in freshwater ecosystems.

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32. Global Patterns of Marine Primary Production

34. Biogeochemical cycles
Concepts of Ecology

36. Carbon cycle Is strongly tied to the energy flow
Carbon in the form of CO2 (and Carbonates in aquatic eco systems) is fixed into organic compounds in the process of photosysnthesis, passses through the food chain, and returned to the atmosphere through the process of respiration
the Carbon cycle exhibits both annual and diurnal fluctuations

38. Nitrogen cycle
Is characterized by the fixation of atmospheric nitrogen by mutualistic nitrogen fixing bacteria associated with roots of many plants, largely legumes and cyanobactria
Other processes are
Ammonification, the breakdown of amino acids by decomposer organisms to produce ammonia;
Nitrification, the bacterial oxidation of ammonia to nitrates and nitrites
Denitrification, the reduction of nitrates to gaseous nitrogen

40. Phosphorous cycle
Is wholly sedimentary, with reserves coming largely from phosphate rocks
Once in the soil, inorganic P is taken up by plants and incorporated into organic compounds
During the process of decomposition of living matter, it becomes again available through mineralization

41. In aquatic eco systems, P exists in three major states;
Particulate Organic Phosphorous (POP)
Dissolved Organic Phosphorous (DOP)
Dissolved Inorganic Phosphorous (DIP)
Transfer among theses states is controlled by the processes of uptake by primary producers, decomposition, and excretion of DIP by zooplankton.

43. Nutrient Cycling and Retention
Chapter 19
nitro/biggraph.asp

44. Focus Areas Nutrient Cycles Rates of Decomposition
Phosphorus
Nitrogen
Carbon
Rates of Decomposition
Terrestrial
Aquatic
Organisms and Nutrients
Disturbance and Nutrients

45. Phosphorus Cycle
Global phosphorus cycle does not include substantial atmospheric pool.
Largest quantities found in mineral deposits and marine sediments.
Much of this in forms not directly available to plants.
Slowly released in terrestrial and aquatic ecosystems via weathering of rocks.

46. Phosphorus Cycle

47. Nitrogen Cycle Includes major atmospheric pool - N2.
Nitrogen fixers can use atmospheric supply directly (only prokaryotes).
Energy-demanding process; reduces to N2 to ammonia (NH3).
Industrial N2- fixation for fertilizers equals the biological process annually.
Denitrifying bacteria release N2 in anaerobic respiration (they “breath” on nitrate).
Decomposer and consumers release waste N in form of urea or ammonia.
Ammonia is nitrified by bacteria to nitrate.

49. Nitrogen Cycle

50. Carbon Cycle
Moves between organisms and atmosphere as a consequence of photosynthesis and respiration.
In aquatic ecosystems, CO2 must first dissolve into water before being used by primary producers.
Although some C cycles rapidly, some remains sequestered in unavailable forms for long periods of time.

51. Carbon Cycle

52. Rates of Decomposition
Rate at which nutrients are made available to primary producers is determined largely by rate of mineralization.
Occurs primarily during decomposition.
Rate in terrestrial systems is significantly influenced by temperature, moisture, and organic chemical compositions (labile versus refractory).

53. Decomposition in Temperate Forest Ecosystems
Melillo et.al. used litter bags to study decomposition in temperate forests.
Found leaves with higher lignin:nitrogen ratios lost less mass.
Suggested higher N availability in soil might have contributed to higher decomposition rates.
Higher environmental temperatures may have also played a role.

54. Decomposition in Aquatic Ecosystems
Gessner and Chauvet found leaves with a higher lignin content decomposed at a slower rate.
Higher lignin inhibits fungi colonization of leaves.
Suberkropp and Chauvet found leaves degraded faster in streams with higher nitrate concentrations.

55. Decomposition in Aquatic Ecosystems

56. Nutrient Spiraling in Streams

57. Animals and Nutrient Cycling in Terrestrial Ecosystems
MacNaughton found a positive relationship between grazing intensity and rate of turnover in plant biomass in Serengeti Plain.
Without grazing, nutrient cycling occurs more slowly through decomposition and feeding of small herbivores.
Huntley and Inouye found pocket gophers altered N cycle by bringing N-poor subsoil to the surface.

58. Animals and Nutrient Cycling in Terrestrial Ecosystems

59. Human influences on nutrient cycling
Loss of nutrients via deforestation
Addition of nutrients
Fertilizers
Nitrogen in atmosphere from coal burning
Consequences
Reduced biodiversity
Reduction of mycorrhizal fungi
Eutrophication of lakes

60. (Peierls et al., 1991)

61. Succession and Stability
Chapter 20

62. SUCCESSION
Succession: Gradual change in plant and animal communities in an area following disturbance.
Primary succession on newly exposed geological substrates.
Secondary succession following disturbance that does not destroy soil.
Climax Community: Late successional community that remains stable until disrupted by disturbance.

65. Primary Succession at Glacier Bay

66. Secondary Succession in Temperate Forests
Oosting found number of woody plant species increased during secondary succession at Piedmont Plateau.
Johnston and Odum found increase in bird diversity across successional sequence closely paralleled increase in woody plant diversity observed by Oosting.

69. Succession in Stream Communities
Fisher studied rapid succession in Sycamore Creek, AZ.
Evaporation nearly equals precipitation - flows generally low and intermittent.
Subject to flash floods.
Observed rapid changes in diversity and composition of algae and invertebrates.
Invertebrates found refuge because many adults in aerial stage.
Re-colonized after flooding.

70. Hypothetical Periphyton Community Succession

71. Mirror Lake, Yosemite National Park

72. El Capitan Meadow, Yosemite National Park © Dan Baumbach 2002

73. Mechanisms of Succession
Clements, 1916
Facilitation
Connell and Slayter, 1977
Tolerance
Inhibition

74. Facilitation
Proposes many species may attempt to colonize newly available space.
Only certain species will establish.
Colonizers “Pioneer Species” modify environment so it becomes less suitable for themselves and more suitable for species of later successional stages.

75. Tolerance
Initial stages of colonization are not limited to pioneer species.
Early successional species do not facilitate later successional species, but do change the environment.
Species remain, leave, or establish based on tolerance to environment.
Long lived species are most broadly tolerant to a range of environmental change.

76. Inhibition
Early occupants of an area modify the environment in a way that makes it less suitable for both early and late successional species.
Early arrivals inhibit colonization by later arrivals.
Assures late successional species dominate an area because they live a long time and resist damage by physical and biological factors.

77. Mechanisms of Succession

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79. 20_25b.jpg

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Why Alder?

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82. Successional Mechanisms in Rocky Intertidal Zone
Sousa investigated mechanisms behind succession of algae and barnacles in intertidal boulder fields.
If the inhibition model is in effect, early successional species should be more vulnerable to mortality.
Results showed early successional species had lowest survivorship and were more vulnerable to herbivores.

83. Community and Ecosystem Stability
Stability: Absence of change.
Ability to resist change from original state and/or return to original state once disturbed

84. Community and Ecosystem Stability
Resistance: Ability to maintain structure and function in face of potential disturbance.

85. Community and Ecosystem Stability
Resilience: Ability to recover from disturbance. X X

86. – Large Scale Ecology and Global Change
Where have we been and where are we going?
Introduction to Landscape Ecology
Landscape =
Landscape Ecology =
Landscape
element =

87. What do landscape ecologists study?
Landscape structure – scale and configuration
Landscape processes – between patches
Ho do these landscapes differ structurally?

88. Quantifying landscape structure
Patch density

89. Quantifying landscape structure
Patch shape – area/perimeter
patch perimeter/circle perimeter
Concept of “the Edge” – why are edges important?

90. Landscape Processes
Metapopulation dynamics
-habitat patches
and dispersal
occur within the
context of a
landscape

91. Patch size and metapopulations

92. Patch size and population density

93. Landscape position
Position of lakes affects
proportion of groundwater and
surface water inputs

94. Ecosystem Interactions – e.g. riparian zones
RoSS
Stream
Permanent Groundwater
Bedrock
Hillslope
High Water
Table
Floodplain
Baseflow
Water Table
Constrained
Unconstrained
Interface
Baker et al. 2000

95. Landscapes and Change
How does landscape structure change?
Geological processes and climate

96. Different aged soils have different properties (which affects
the kinds of plants that can live there).

98. END

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100. Consumers’ Effects on Local Diversity
Lubchenko studied influence of intertidal snail (Littorina littorea) on structure of an algal community.
Snails fed on green (Enteromorpha spp.) and red (Chondrus crispus) algae.
Under normal conditions, Enteromorpha out-competes Chondrus in tide pools, and Littornia prefers Enteromorpha.
In the absence of snails, Chondrus is competitively displaced.

101. Consumers’ Effects on Local Diversity

102. Consumers’ Effects on Local Diversity
When snails are present in high densities, Littorina grazes down Enteromorpha, releasing Chondrus from competition.
Green crabs (Carcinus maenus) prey on young snails, preventing juveniles from colonizing tide pools.
Populations of Carcinus are controlled by seagulls.

103. Consumers’ Effects on Local Diversity
Low snail density - Enteromorpha dominates tide pool.
Medium snail density - Competitive exclusion eliminated, and algal diversity increased.
High snail density - Feeding requirements are high enough that snails eat preferred algae and less-preferred algae.
Algal diversity decreased.

104. Strong Interactions and Food Web Structure
Tscharntke studied food webs associated with wetland reeds (Phragmites australis).
Attacked by fly Giraudiella inclusa.
Attacked by 14 species of parasitoid wasps.
Predator specialization
Distinguished weak and strong interactions.
Determination of keystone species.

105. Fish as River Keystone Species
California roach Hsperoleucas symmetricus and steelhead trout Oncorhhyncus mykiss significantly influence food web structure.
Predatory fish decrease algal densities.
Low predator density increased midge production.
Increased feeding pressure on algal populations.
Thus, fish act as Keystone Species.