food webs readings: freeman chapter 54. what is a biological community (i)? an assemblage of many...

FOOD WEBS

READINGS:

FREEMAN

Chapter 54

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).

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.

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.

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


• Pollinators/Nectar Feeders

- Bumble Bees2

- Moths3 (night)

- Honeybees1

• Nectar Feeders (only)

- Ants2 Scientific American 253(1): 112-119


• Major Herbivores

-Monarch Butterflies1

Larvae eat leaves and young pods.

Adults collect necter and pollen.

Scientific American 253(1): 112-119


• Predator

- Crab Spider1

• Parasitoids

- Trachinid Flies2

- Ichneumon Wasps1

Scientific American 253(1): 112-119.

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


• 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


• Primary Consumers (plant sap, seeds, leaves, etc)

- Aphids1

- Mosquitos1

- Milkweed Bugs1

- Milkweed Beetles1


• Predator - Crab Spider1

- Mud-dauber wasp1

• Parasitoid - Ichneumon Wasp1

• Scavenger - Daddy Longlegs1

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.

A Milkweed Community Food Web

Common Milkweed1

(Leaves) (Flowers)

MonarchButterfly1

Larvae

Trachinid Flies2

Ichneumon Wasps1

CrabSpiders1

Bumblebees2 Ants2

Honeybees1

Moths3

Producer Primary Consumer Secondary Consumer

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.

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.

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.

Early Stages of Secondary Succession

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Late Stages of Secondary Succession



Simple Model of Primary Succession



More Complex Model of Primary Succession



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.

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.

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.

0

10

20

30

40

50

60

1 3 5 7 9 11 13 15 17 19 21 23

NUMBER OF PLANT SPECIES PER PLOT (S)

TOTAL PLANT COVER (%)

Species Richness and Community Stability (I)



Species Richness and Community Stability (II)



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.

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

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.


• 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.


Maple, Beech, Ash, Cherry, Blackberry (Leaves, Fruits, Seeds)

White-tailed Deer

Saddled ProminentMoth (Caterpillar)

EasternChipmunk

Scarlet Tanager

Red-tailed Hawk

A food chain from producers to highest level consumers.

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.


• 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.


Dead plant parts and the remains and droppings of animals areAt the base of brown or decomposer food webs.

Litter(Dead parts of Maple, Beech, etc.)

Wood boring, Stag, & Scarab Beetles

Sow Bugs Bacteria

Fungi

Earthworms

Red-backedSalamander

Ground BeetlesTiger Beetles

Spiders

Short-tailed Shrew

Scarlet Tanager

Eastern Chipmunk

Red-tailed Hawk

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



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



Food Webs Consist of Short Food Chains



FOOD WEBS

READINGS:

FREEMAN

Chapter 54

food webs readings: freeman chapter 54. what is a biological community (i)? an assemblage of many...

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