Ecosystems Ecology Section V

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Ecosystems Ecology Section V

• Three links in this food chain.
• How much energy is passed from link to link?
• How many links are there?
• Ecosystem ecology focuses on the flow of energy
  nutrients through food chains.

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Trophic Structure

• Chapter 20

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What Is an Ecosystem?

• A. G. Tansley (1935) coined the term ecosystem.
• Includes the biotic community of organisms and
  the abiotic environment around the community.

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What Is an Ecosystem?

• Ecosystem ecology is concerned with the movement
  of energy and materials through communities.
• Can be applied at any scale.

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What Is an Ecosystem?

• Most ecosystems do not have clearly defined
  boundaries.

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What Is an Ecosystem?

• Advantage of ecosystem ecology is the common
  currency of energy or nutrients, which allows
  communities and populations to be compared
  between and within trophic levels.

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Food Web Complexity

• Few ecosystems can be characterized by a single
  unbranched food chain.
• More correct to draw relationships as an
  elaborate interwoven food web.

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Food Web Complexity

• Measures of food web complexity
• Chain length denotes the average number of
  links between trophic levels.
• Connectance actual number of links / potential
  number of links.
• Linkage density number of links per species.

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Food Web Patterns

• Connectance remains constant as the number of
  species in the food web increases.

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Food Web Patterns

• Imagine a bird that feeds on insects in two
  different communities.
• Community A has twice the number of species as
  community B.
• It seems reasonable that the bird will eat twice
  the number of different insect species in
  community A.
• If this applies to all species, connectance would
  remain the same.

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Food Web Patterns

• Mean chain length increases as the number of
  species in the web increases.

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Food Web Patterns

• Top predators tend to be rather large and
  sparsely distributed, whereas herbivores are
  smaller and more common (termed pyramid of
  numbers).

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Food Web Patterns

• Charles Elton (1927) described pattern.
• Small pond

Fish - one hundred
Beetles - one thousand
Daphnia - hundreds of thousands
Protozoa - millions
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Food Web Patterns

• Exceptions to the pyramid of numbers
• Oak tree (one producer) supports thousands of
  herbivorous beetles, caterpillars, and other
  primary consumers, which support tens of
  thousands of predators and parasites.

Parasites and predators - tens of thousands
Insect herbivores - thousands
Oak tree - one
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Food Web Patterns

• Reconcile this exception
• Weigh the organisms in each trophic level.
• Oak tree weighs 30,000 kg, all herbivores. in
  tree weigh 5 kg, and the predators sum about 100
  g.

Parasites and predators - 100g
Herbivores - 5 kg
Oak tree - 30 tonnes
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Food Web Patterns

• Inverted pyramids can still occur even when
  biomass is used.
• Biomass of phytoplankton supports a higher
  biomass of zooplankton in the English Channel.

Primary carnivores (pelagic fish) - 1.8
Herbivores (zooplankton) -1.5
Producers (phytoplankton) - 0.4
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Food Web Patterns

• May be because the production rate of
  phytoplankton is much higher than zooplankton,
  and the small standing crop of phytoplankton
  processes large amounts of energy.

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Food Web Patterns

• The most realistic pyramid is the energy pyramid,
  which is never inverted.

Primary carnivores - 0.0016
Herbivores - 0.15
Producers - 0.4
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Problems With Food Web Patterns

• Food web analysis has helped us understand
  ecosystems, but
• Some problems with data may invalidate the
  observed patterns in food webs.
• Predation on "minor" species is frequently
  omitted. Food webs are far more complex than is
  reported.

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Problems With Food Web Patterns

• Gary Polis (1991) conducted an intensive
  10-year study of the local desert ecosystem.
• Found 174 species of plants, 138 vertebrate
  species, 55 species of spiders and scorpions, an
  estimated 2,000-3,000 species of insects, and an
  unknown number of microorganisms and nematodes.

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Problems With Food Web Patterns

• Compared to literature, chains were longer,
  omnivory was common, and linkage density was much
  greater.

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Problems With Food Web Patterns

• Data on quantities of food consumed indicating
  the thickness of connecting links, are usually
  absent from published work.

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Problems With Food Web Patterns

• Robert Paine (1992) first example in which
  interaction strengths were calculated in the food
  web.
• Most connections were found to be weak, so the
  web was essentially very simple.

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Problems With Food Web Patterns

• Blake and Wallace (1997) showed how misleading
  a normal food web for a riverine system in
  Alabama is, because it implied the equivalence of
  all food resources.

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Problems With Food Web Patterns

• Great variation in the strengths of linkages.
• Perhaps we should give weight to strong linkages
  when calculating connectance.

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Problems With Food Web Patterns

• In food web theory, species are often aggregated
  into "trophic species.
• Ex. various types of insects are often lumped
  together.
• This grouping disguises important biology.

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Problems With Food Web Patterns

• Data on the importance of chemical nutrients are
  sparse.
• Hard to define web boundaries.
• Food web links may obscure positive effects of
  higher trophic levels on their food production.
• Ex. Pollinators have a net positive effect on
  their hosts.

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Guilds

• Guilds a group of functionally similar organisms
  within a trophic level.

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Guilds

• Coined by Dick Root (1967)
• In his studies of birds, he used the term guild
  to describe a group of species that fed on the
  same resources and in the same way.

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Guilds

• Ex. In an insect community feeding on plants, we
  may have leaf chewing guild, sap sucking guild,
  leaf miners, stem borers, stem gallers, root
  feeders, and flower feeders.

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• The stem boring guild of a salt marsh grass.

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Guilds

• Interest in Guilds
• Guilds represent arenas of the most intense
  competition.
• Guilds represent the basic building block of
  communities.

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Guilds

• Main problem with guild theory
• How much overlap in diet does there have to be
  for species to belong to the same guild?
• Answer may depend on which resources one selects
  for analysis.
• Ex. the fox is an insectivore if you base diet
  overlap on species richness, but not if you base
  it on biomass of prey.

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Guilds

• Patterns found in guild analysis
• Density compensation within guilds could maintain
  overall guild abundance at or near carrying
  capacity.
• Fortunes of different species within the guild
  vary individually in response to factors other
  than resource variability.

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Guilds

• Evidence John Lawton and colleagues (1984,
  1993)
• Herbivore community of bracken fern in England,
  Brazil, South Africa, New Mexico, and United
  States, Borneo, and Hawaii.
• Insects were arranged into four guilds (chewers,
  suckers, miners, and gallers) and further divided
  based on where they fed on the plant (main stem,
  leaves, and leaf veins).

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Sap Sucker
Leaf Chewer
Leaf Miner
Stem Galler
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• Results failed to support the guild theory.

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Guilds

• Evidence Ashbourne and Putnam (1987)
• Herbivores feeding on red oak in Canada
  England.
• Insects arranged into four basic guilds (chewers,
  suckers, miners, and gallers), and included three
  more (species that folded one leaf in half and
  fed inside species that tied two leaves together
  and fed inside and species that rolled leaves up
  and fed inside).

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• Their result was opposite to Lawton. Striking
  similarities were seen between the guild
  composition of the two regions.

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Keystone Species

• Keystone species a species that has an effect on
  an ecosystem out of proportion to its abundance
  or biomass.
• Ex. the beaver can completely alter an ecosystem
  by building dams.

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Keystone Species

• Difference between a keystone species and a
  dominant species
• A dominant species has a large effect in a
  community because it is very common.
• Ex. Spartina is a dominant species in a salt
  marsh due to its large biomass and role in energy
  flow.

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Keystone Species

• Keystone enemies or predators
• Ex. Pisaster starfish and predatory whelks in the
  rocky intertidal zone.
• Removal of either results in the community being
  dominated by mussels.

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Keystone Species

• Keystone prey
• Ex. Palm nuts, figs and nectar.
• They are critical to tropical forest fruit-eating
  guilds.
• Without the fruit trees, wholesale extinction of
  frugivores would occur.

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Keystone Species

• Keystone habitat modifiers
• Ex. Gopher tortoises.
• Burrows provide homes for an array of mice,
  possums, frogs, snakes, and insects.
• Without the burrows, many of these creatures
  would be unable to survive.

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Keystone Species

• Both gopher tortoises and beavers are considered
  ecosystem engineers, because they modify the
  habitat and cause ecological changes.

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Summary

• Many food webs can be compared by simple
  properties such as connectance and linkage
  density.

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Summary

• The analysis of food webs has revealed some
  common patterns.
• Two are that, as the number of species in a food
  web increases, chain length tends to increase,
  but connectance remains constant. Another is
  that there is a pyramid of numbers, with fewer
  species on higher trophic levels than on lower
  trophic levels.

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Summary

• Because food webs are usually imperfectly known,
  the generalizations that have resulted from them
  may be incorrect.
• The inadvertent omission of minor or mobile
  species, lack of knowledge about the strength,
  importance, or beneficial effects of connecting
  links between species, and the taxonomic lumping
  of species into groups or guilds can all obscure
  true food web patterns.

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Summary

• Guilds are a valuable analytical tool because
  they focus attention on groups of species most
  likely to be competing for resources.

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Summary

• Keystone species have a large influence on
  ecosystems out of proportion to their abundance.