1. READINGS: FREEMAN Chapter 54
FOOD WEBS
READINGS:
FREEMAN
Chapter 54

2. 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 1,000’s of square meters or smaller).

3. 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.

4. 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.

5. 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):

6. Biotic Interactions in a Milkweed Community (Summer)
Pollinators/Nectar Feeders
- Bumble Bees2
- Moths3 (night)
- Honeybees1
Nectar Feeders (only)
- Ants2
Scientific American 253(1):

7. 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):

8. Biotic Interactions in a Milkweed Community (Summer)
Predator
- Crab Spider1
Parasitoids
- Trachinid Flies2
- Ichneumon Wasps1
Scientific American 253(1):

9. 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

10. 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

11. Biotic Interactions in a Milkweed Community (Fall)
Primary Consumers (plant sap, seeds, leaves, etc)
- Aphids1
- Mosquitos1
- Milkweed Bugs1
- Milkweed Beetles1

12. Biotic Interactions in a Milkweed Community (Fall)
Predator
- Crab Spider1
- Mud-dauber wasp1
Parasitoid
- Ichneumon Wasp1
Scavenger
- Daddy Longlegs1

13. 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.

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

15. 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.

16. 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.

17. 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.

18. Early Stages of Secondary Succession

19. Late Stages of Secondary Succession

20. Simple Model of Primary Succession

21. More Complex Model of Primary Succession

22. 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.

23. 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.

24. 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

25. Species Richness and Community Stability (I)

26. Species Richness and Community Stability (II)

27. 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 1,000’s of species, only a few have populations that account for the majority of matter and energy transactions.

28. 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

29. 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.

30. 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.

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

32. 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.

33. 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.

34. Brown Food Web in Hubbard Brook
Red-tailed Hawk
Short-tailed Shrew
Eastern Chipmunk
Spiders
Red-backed
Salamander
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.

35. Another View of the Brown (Decomposer) Food Web(I)

36. Another View of the Brown (Decomposer) Food Web(II)

37. Food Webs Consist of Short Food Chains

38. READINGS: FREEMAN Chapter 54
FOOD WEBS
READINGS:
FREEMAN
Chapter 54