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Course Description (50 minutes)


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Course Description
(50 minutes)


Students play a game that simulates changes in a stream when an environmental stressor, such as a pollutant, is introduced. They are shown a playing field and its boundaries.

A student volunteer is selected to become an "environmental stressor (e.g., sedimentation, sewage, or fertilizer). The group discusses ways that a stream can become polluted and how this can alter stream conditions (large groups will require more students to become stressors).



The class is divided into seven groups to play the game. Each group represents one type of macroinvertebrate species listed in Macroinvertebrate Groups. The number of members in each group is recorded, using a table as follows:

Organism

Tolerance

Numbers (at start/after each round)

Start

Round One

Round
Two


Round Three

Caddisfly larva

Intolerant

5

2

2

2

Mayfly nymph

Intolerant

5

4

1

0

Stonefly nymph

Intolerant

4

4

4

2

Dragonfly nymph

Facultative

5

5

4

4

Damselfly nymph

Facultative

4

4

4

3

Midge larva

Tolerant

4

6

7

9

Rat-tailed maggot

Tolerant

4

6

9

11

TOTAL




31

31

31

31

Note: Have at least four students in each group; for smaller classes, reduce the number of groups. For example, eliminate the stonefly nymphs and the damselfly nymph groups.

Group members receive appropriate identification labels. The picture of each group's macroinvertebrate should face outward when labels are attached.

Students are informed that some macroinvertebrates have hindrances to crossing the field (see the following Intolerant Macroinvertebrates and Hindrances):

Intolerant Macroinvertebrates and Hindrances

Organism

Hindrance

Rational For Hindrance

Caddisfly

Must place both feet in a bag* and hop across field, stopping to gasp for breath every five hops.

Caddisflies are intolerant of low oxygen levels.

Stonefly

Must do a push-up every ten steps.

When oxygen levels drop, stoneflies undulate their abdomens to increase the flow of water over their bodies.

Mayfly

Must flap arms and spin in circles when crossing field.

Mayflies often increase oxygen absorption by moving gills.

*Caddisfly larvae build cases and attach themselves to rocks for protection and stabilization.

These obstacles symbolize sensitive organisms' intolerance to pollutants. Students then practice their motions.

Macroinvertebrate groups are assembled at one end of the playing field and the environmental stressor(s) at midfield. When a round starts, macroinvertebrates move toward the opposite end of the field and the stressor will try to tag them. To "survive," the macroinvertebrates must reach the opposite end of the field without being tagged by the environmental stressor. The environmental stressor can try to tag any of the macroinvertebrates, but will find it easier to catch those with hindered movements.

Begin the first round of the game. Tagged macroinvertebrates must go to the sideline and flip their identification labels to display the more tolerant species (i.e., rat-tailed maggot or midge larva). Tagged players who are already in a tolerant species group do not flip their labels.

The round ends when all of the macroinvertebrates have either been tagged or have reached the opposite end of the playing field. The new number of members in each species is then recorded.

Students complete two more rounds, with all tagged players rejoining the macroinvertebrates who successfully survived the previous round. The numbers of members in each species of macroinvertebrates at the conclusion of each round is recorded. Because some players will have flipped their identification labels, there will be a larger number of tolerant species in each successive round.

The game is completed after three rounds. Discuss the outcome with students. Emphasize the changes in the distribution of organisms among groups. Have students compare population sizes of groups at the beginning and end of the game and provide reasons for the changes. Review why some organisms are more tolerant of poor environmental conditions than others. Have students compare the stream environment at the beginning of the game to the environment at the end.


Background Information


Macroinvertebrates (organisms that lack an internal skeleton and are large enough to be seen with the naked eye) are an integral part of wetland and stream ecosystems. Examples of macroinvertebrates include mayflies, stoneflies, dragonflies, rat-tailed maggots (maggot is the term used for the larva of some flies), scuds, snails, and leeches. These organisms may spend all or part of their lives in water; usually their immature phases (larvae and nymphs) are spent entirely in water. Larvae do not show wing buds and are usually very different in appearance from the adult versions of the insects. Nymphs generally resemble adults, but have no developed wings and are usually smaller.

A variety of environmental stressors can impact macroinvertebrate populations. Urban and/or agricultural runoff can produce conditions that some macroinvertebrates cannot tolerate. Sewage and fertilizers added to streams induce the growth of algae and bacteria that consume oxygen and make it unavailable for macroinvertebrates. Changes in land use from natural vegetation to a construction site or to poorly protected cropland may add sediment to the water. Sedimentation destroys habitats by smothering the rocky areas of the stream where macroinvertebrates live. The removal of trees along the banks of a river and alternation of stream velocity can both alter normal water temperature patterns in the stream. Some organisms depend on certain temperature patterns to regulate changes in their life cycles. Other stressors include the introduction of alien species and stream channelization.

Some macroinvertebrates, such as the mayfly and stonefly nymphs and caddisfly larvae, are sensitive (intolerant) to changes in stream conditions brought about by pollutants. Some of these organisms will leave to find more favorable habitats, but others will be killed or will be unable to reproduce.

Macroinvertebrates (e.g. , rat-tailed maggots and midge larvae) that may thrive in polluted conditions are called tolerant organisms. Other organisms, called facultative organisms (e.g., dragonfly and damselfly nymphs) prefer good stream quality but can survive polluted conditions.

Water quality researchers often sample macroinvertebrate populations to monitor changes in stream conditions over time and to assess the cumulative effects of environmental stressors. Environmental degradation will likely decrease the diversity of a community by eliminating intolerant organisms and increasing the number of tolerant organisms. If the environmental stress is severe enough, species of intolerant macroinvertebrates may disappear altogether. For example, if a sample of macroinvertebrates in a stream consists of rat-tailed maggots, snails, and dragonfly nymphs, the water-quality conditions of that stream are probably poor (i.e., low oxygen level, increased sediment, contaminants). If, on the other hand, the sample contains a diversity of organisms, the stream conditions are likely good. However, baseline data is essential because some healthy streams may contain only a few macroinvertebrate species. A variety of good sources, adequate oxygen levels, and temperatures conducive to growth all characterize a healthy stream.

Macroinvertebrate Tolerance to Pollution

Artwork: Gould League of WA and Waterwatch SA

Sensitive Macroinvertebrates

STONEFLY LARVAE


Stonefly larvae have two long tails, tubes of thread-like gills on their undersides, wing pads, antennae, and two claws on each foot. They are found among stones or plants in clear, cool, well oxygenated streams.



MAYFLY LARVAE
Mayfly larvae have three long filaments at the end of their abdomen, with wing pads and lateral gills along the abdomen. They have short antennae, and a single claw on each foot. They are found under stones in fast flowing water or among plants in slow flowing water.



CADDISFLY LARVAE
These are worm-like insect larvae with three pairs of legs on the first three body segments. They are usually found in cases made from rolled leaves or hollow twigs, with only their head and legs protruding when they move. "worm-like" appearance- 6 legs near head






DRAGONFLY LARVAE
Dragonfly larvae are short, chunky predators with wing pads and internal gills. They are found on plants, among stones and leaf litter, or on the bottom.



DAMSELFLY LARVAE
Damselfly larvae are more slender than dragonflies, have a distinct head section, and three gills on the tail tip. They are also found on plants, among stones and leaf litter, or on the bottom.

 

Tolerant Macroinvertebrates

BEETLE LARVAE
Beetle larvae may be confused with other animals. They are segmented, have three distinct pairs of legs, are never found in cases, but have a wide variety of forms. They are very active, aggressive predators with large mouth parts, and are found in all habitats.

BEETLES (COLEOPTERA)
Beetles have hard front wings folded side by side along the center of the back. From above, they have a more oval shape than bugs. Beetles have biting mouth parts They are found on plants, or swimming in or on the water at all levels.



BUGS (HEMIPTERA)
Bugs tend to be shield shaped when viewed from above. Their soft front wings are folded and overlap to leave a small triangle on the back, and they have sucking mouth parts They are found among the aquatic plants on the water's surface, or swimming freely at all levels of slowly flowing water. Water boatmen and backstriders are bugs.



Very Tolerant Macroinvertebrates

FLY LARVAE


There are many types of fly larvae. They are worm-like creatures with no legs, or stumpy unjointed legs, and may have a sucker on the abdomen and a brush on the head. They occur in all sorts of aquatic habitats; swimming, on rocks, or on the bottom.


MIDGE LARVAE
Midge larvae are slender worm-like creatures, sometimes red, with no legs, or stumpy unjointed legs, and bristles. They are found in all sorts of aquatic habitats; swimming, on rocks, or on the bottom.




 


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