Trophic Dynamics Ecosystem Ecology

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Trophic DynamicsEcosystem Ecology

• Part II Mechanisms
• Chapin, Matson, Mooney
• Principles of Terrestrial Ecosystem Ecology

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Trophic dynamics

• Variable role in energy and nutrient flow
• Modest role in terrestrial ecosystems
• Dominant pathway in aquatic systems
• Essential in understanding the dynamics of
  organisms and ecosystems
• Physical properties of the environment
• Carbon and nutrient cycling
• Disturbance and succession

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Two major trophic systems
Plant-based system Detritus-based system
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Food webs in real ecosystems are complex No
simple trophic levels
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Draw the trophic relationships for

• People in this room
• Your favorite animal
• A caribou (or a cow)
• A dog
• An aphid
• What determines trophic specificity?

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Major controls

• Bottom-up controls
• Food
• Top-down controls
• Predation
• Interactions and feedbacks
• How do these work?
• Probably most common situation in nature

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Bottom-up controls A fundamental
constraint Savannas mainly above the line (more
consumed/NPP) Forests mainly below the line
(less consumed/NPP)
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Top-down controls Predation and other factors
also influence energy flow
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Biome differences in herbivory

• Constrained by NPP
• What constrains NPP?
• Depends on plant allocation to structure
• Forest vs. grassland
• Aquatic vs. terrestrial
• All plant matter is not equally edible
• Often subsidized from other ecosystems
• Streams receive inputs of terrestrial litter
• Secondary production can exceed aquatic primary
  production
• Crops receive inputs of fertilizers and fossil
  fuel energy

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Plant defense reduces herbivory

• Types of defense
• Structural
• Carbon-based
• Nitrogen-based
• Depends on
• Genetic potential
• Environment
• Seasonal changes in allocation
• Most strongly expressed in low-resource conditions

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Plant toxicity to snowshoe hares Prefer birch
to alder Generalist herbivores have limit to
quantity of toxins ingested Preferentially eat
rare plants Tends to reduce plant diversity
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Plant toxicity to snowshoe hares Prefer birch
to alder Feeding trials in different
vegetation types
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Evolutionary impact of herbivores on plant
defense Context-dependent depends on fire
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Plant defense influences growth rate and herbivory
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Herbivores magnify differences among ecosystems
in NPP and energy flow
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Very little NPP becomes animal biomass
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Inefficiencies of food chains result in energy
pyramids
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Factors governing consumption efficiency

• 1. Plant quality
• Depends on resource supply and species
• Plant allocation to structure
• Plant defense
• Herbivores vs. carnivores

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Factors governing consumption efficiency

• 1. Plant quality
• 2. Activity budget of animal
• Selection of habitat
• Time spent eating
• Animals do many other things (avoid predators,
  reproduction, etc.)
• Selectivity of plants and plant parts

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Factors governing consumption efficiency

• 1. Plant quality
• 2. Activity budget of animal
• 3. Abundance of consumers (and predators)
  relative to plants
• Effects on search time
• Effects on food quality consumed

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Assimilation efficiency defines relationship
between ingestion and assimilated energy
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Assimilation efficiency depends on

• Food quality
• (e.g., summer vs. winter diet of hares)
• Physiology of consumer
• (e.g., homeotherm vs. heterotherm)

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Production efficiency defines relationship
between assimilated energy and production
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What is production?

• Growth
• Reproduction

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Production efficiency depends mainly on
the energy expenditure of the animal
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Trophic efficiency is net effect of component
efficiencies
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There is a narrow window of opportunity for many
insects Climate unfavorable too early Plants
inedible too late What if climate change shifts
timing of insects and plants?
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Trophic cascades Predation exerts a powerful
effect on trophic interactions and ecosystem
structure (especially in aquatic ecosystems)
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Nutrient cycling through food webs

• Pathway same as carbon and energy
• Except sodium (salt licks)
• Herbivores recycle a larger proportion of
  nutrients than of energy
• Choose nutrient-rich tissues
• Eat before plants senesce
• Act as rapid decomposers

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Food chains define element ratios of ecosystems
in aquatic ecosystems

• NP ratio of zooplankton determines NP ratio of
  recycled nutrients
• Excrete excess N, if P-limited
• Maintains P-limitation in the habitat
• NP ratio of recycled nutrients determines which
  nutrient limits NPP
• Contributes to P limitation of lakes

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Ecological stoichiometry Aquatic invertebrates
have NP ratio similar to plants Less
consistency in terrestrial systems
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Detritus-based trophic systems are more efficient
in converting available energy into production
than are plant-based systems Recycle unused
organic matter
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Plant-based system Detritus-based system
0.1-50 5-20 1-40
gt100 100 40-60
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Food webs in real ecosystems are complex
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Kenai bark beetle outbreak
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The future Aspen parkland??
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With a drier climate, new issues arise

• Current subsistence patterns (e.g., caribou
  hunting) may be less viable
• New animals adapted to grasslands are essential
  to maintain new habitat
• Bison are one of the logical animals that could
  maintain habitat and subsistence traditions

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Efforts to establish a grazing-dominated system
in northeast Siberia

• Grazers and browsers introduced to re-establish
  grasslands similar to those during the Pleistocene

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20,000 years ago the mammoth-steppe occupied all
unglaciated areas
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Pleistocene animals included bison. Present until
recent centuries.
Frozen sediments
Wooly rhino
Mammoth
Muskox
Horses
Wolf
Tiger lion
Horses
Bison
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Siberian grasslands are currently maintained by
natural disturbances
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Reindeer disturbance
Disturbance by tractor
Disturbance causes conversion from moss to grass
Forest fire
Human disturbance
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How can the steppe grazing system be restored?

• Requirements
• Location and climate
• High density of herbivores
• Diversity of herbivore species
• Predators

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Pleistocene Park
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Disturbance experiment with wild Yakutian horses
shrubland
grassland
Nitrogen in soil increased 10 fold
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Biome differences in herbivory reflect
differences in NPP and plant allocation to
structure
NPP constrains the energy flow through
plant-based food webs Less biomass is consumed
in forests than you would expect Most biomass is
consumed in aquatic ecosystems