FOOD WEBS

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FOOD WEBS

• READINGS
• FREEMAN
• Chapter 54

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What is a Biological Community (I)?

• An assemblage of many populations, each of
  different species, that have similar requirements
  or tolerances.
• All species that interact with each other in a
  local area (acres or 1,000s of square meters or
  smaller).

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What is a Biological Community (II)?

• Has a few species that are common (represented by
  many individuals), many more that are rare
  (represented by a few individuals) and most with
  intermediate population sizes.
• Named on the basis of vegetative type, prevalent
  species, moisture gradient, or geographical
  location.
• Characterized by productivity, key species,
  and/or species diversity.

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Milkweed Community A Model for Study of Species
Interactions

• Summer and fall insect visitors at milkweeds
  come to forage. The interactions that occur
  between species include herbivory, predation,
  parasitoidism and scavenging. These interactions
  can be summarized in a simple food web.

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Biotic Interactions in a Milkweed
Community (Summer)

• Common milkweeds1 attract a number of species of
  insect14 and arachnid1 species.
• Milkweeds act as resources for bees3, wasps2,
  ants2, butterflies2 and moths3.
• Crab spiders1 prey on visiting insects.
• Wasps and flies2 are parasitoids on eggs and
  larvae of insects and spiders.

Number of species in a particular milkweed
community.
Scientific American 253(1) 112-119
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Biotic Interactions in a Milkweed
Community (Summer)

• Pollinators/Nectar Feeders
• - Bumble Bees2
• - Moths3 (night)
• - Honeybees1
• Nectar Feeders (only)
• - Ants2

Scientific American 253(1) 112-119
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Biotic Interactions in a Milkweed
Community (Summer)

• Major Herbivores
• -Monarch Butterflies1
• Larvae eat leaves and young pods.
• Adults collect necter and pollen.

Scientific American 253(1) 112-119
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Biotic Interactions in a Milkweed
Community (Summer)

• Predator
• - Crab Spider1
• Parasitoids
• - Trachinid Flies2
• - Ichneumon Wasps1

Scientific American 253(1) 112-119.
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Biotic Interactions in a Milkweed Community (Fall)

• Milkweed1 community in fall has a different
  assemblage of species8.
• Nectar feeders are gone. Aphids1 ( mosquities1)
  feed on plant sap.
• Milkweed bugs1 suck on developing seeds.
• Milkweed beetles1 eat foliage

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Biotic Interactions in a Milkweed Community (Fall)

• Crab spiders1 continue to ambush prey.
• Ichneumon1 and mud-dauber1 wasps are parasitoids
  and predators.
• Daddy longlegs1 scavenge on insect remains,
  capture small prey, sip nectar from the few
  remaining flowers.

Number of species
Scientific American July 1985
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Biotic Interactions in a Milkweed Community (Fall)

• Primary Consumers (plant sap, seeds, leaves, etc)
• - Aphids1
• - Mosquitos1
• - Milkweed Bugs1
• - Milkweed Beetles1

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Biotic Interactions in a Milkweed Community (Fall)

• Predator
• - Crab Spider1
• - Mud-dauber wasp1
• Parasitoid
• - Ichneumon Wasp1
• Scavenger
• - Daddy Longlegs1

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FOOD WEB

• A food web provides a summary of important eating
  (trophic) relations between populations of
  different species in a community.
• A complex pathway along which matter and energy
  moves among many different species at different
  trophic levels.
• It is a network of interlinked food chains.
• It links primary producers with primary
  consumers, secondary consumers and higher level
  (3rd, 4th ) consumers.
• A given species can be included in several
  trophic levels in a food web.

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A Milkweed Community Food Web
Crab Spiders1
Ichneumon Wasps1
Trachinid Flies2
Honeybees1
Monarch Butterfly1 Larvae
Moths3
Bumblebees2
Ants2
(Leaves)
(Flowers)
Common Milkweed1
Producer Primary Consumer Secondary
Consumer
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Defining Community Structure

• Several approaches have been used to define
  community structure. They are
• - species composition
• - successional stage
• - species richness and species diversity
• - food web complexity.
• Each approach has strengths and weaknesses. Thus,
  community ecology is currently in a state of
  active investigation.

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Species Composition

• Species lists serve as the simplest versions of
  species composition in most biological
  communities.
• Only crude estimates of population sizes of
  species within communities have been attempted.

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Successional Stages

• Succession is a non-seasonal, directional and
  continuous change due to extinction and
  colonization of a site by species populations.
• Ecologists distinguish between primary succession
  and secondary succession.

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Early Stages of Secondary Succession
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Late Stages of Secondary Succession
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Simple Model of Primary Succession
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More Complex Model of Primary Succession
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Species Richness and Species Diversity

• Species richness is a simple count of how many
  species are present in a given area.
• Species diversity is a measure of the relative
  abundance of species.
• These two aspects of community structure will be
  studied in more detail later on in BioS 101.

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Species Richness in a Biological Community

• Over 10,000 species have been documented in the
  Great Smoky Mountains National Park (520,004
  acres). Scientists believe an additional 90,000
  species may live here.
• Wolf Road Prairie Preserve (80 acres) has a plant
  species list that contains 327 scientific names
  of native plants.

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Species Richness and Productivity

• Increasing species richness has been shown to
  increase community productivity.
• Note productivity is measured as of plot
  covered by plants rather than biomass.
• Freeman (2005) describes this experiment and
  reports the results in Figure 53.24.

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Species Richness and Community Stability (I)
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Species Richness and Community Stability (II)
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FOOD WEB COMPLEXITY

• A food web is one of the most basic and revealing
  description of community structure.
• In most communities, a few keystone species
  control population dynamics within the community.
• Thus, even in communities that contain 1,000s of
  species, only a few have populations that account
  for the majority of matter and energy
  transactions.

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Some Food Webs in Hubbard Brook

• The Hubbard Brook Experimental Forest, a 7,800
  acre New Hampshire reserve, has been a study site
  for matter and energy studies for over 50 years.
• The food web studies of the reserve serve as a
  model for the study of community structure.

An Ariel View of 3 Study Sites
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Green Food Web in Hubbard Brook

• Food webs that begin with primary producers are
  often called GREEN or GRAZING food webs.
• In Hubbard Brook, deciduous northern hardwood
  trees including sugar maple, beech, yellow birch
  and some white ash are the dominant producers.

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Green Food Web in Hubbard Brook

• White-tailed deer and saddled prominent moth
  (caterpillar) are two herbivores.
• Eastern chipmunk and scarlet tanager feed on both
  plant and insects.
• Red-tailed hawks are top level consumers.

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Green Food Web in Hubbard Brook
Red-tailed Hawk
Scarlet Tanager
Eastern Chipmunk
Saddled Prominent Moth (Caterpillar)
White-tailed Deer
Maple, Beech, Ash, Cherry, Blackberry
(Leaves, Fruits, Seeds)
A food chain from producers to highest level
consumers.
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Brown Food Web in Hubbard Brook

• A BROWN or DECOMPOSER food web begins with dead
  plant and animal remains (detritus).
• Leaf litter (detritus) provides an important food
  source in a deciduous forest.

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Brown Food Web in Hubbard Brook

• Bacteria and fungi are important primary
  decomposers in forest litter.
• Earthworms,sow bugs and beetles play a primary
  role in decomposition.
• Other species of beetles, spiders, salamanders
  and shrews feed on the larger invertebrates on
  the forest floor.

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Brown Food Web in Hubbard Brook
Red-tailed Hawk
Short-tailed Shrew
Eastern Chipmunk
Red-backed Salamander
Spiders
Scarlet Tanager
Ground Beetles Tiger Beetles
Earthworms
Fungi
Wood boring, Stag, Scarab Beetles
Sow Bugs
Bacteria
Litter (Dead parts of
Maple, Beech, etc.)
Dead plant parts and the remains and droppings of
animals are At the base of brown or decomposer
food webs.
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Another View of the Brown (Decomposer) Food Web(I)
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Another View of the Brown (Decomposer) Food
Web(II)
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Food Webs Consist of Short Food Chains
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FOOD WEBS

• READINGS
• FREEMAN
• Chapter 54