The salt marsh intertidal zones of eastern North America are typically dominated

Question

The salt marsh intertidal zones of eastern North America are typically dominated by salt marsh cordgrass, Spartina alterniflora. In recent years, many marshes have experienced overfishing of blue crabs, a top predator, and subsequent outbreaks (rapid population growth) of the marsh periwinkle, a small grazing snail. A number of different marshes were surveyed for abundances of periwinkles and percent cover of Spartina and mud. Consider the following figure:

1) What does each open circle on the graph represent?

a) percent mud cover

b) percent Spartina cover

c) a natural marsh

d) an overfished marsh

2) What does each solid circle on the graph represent?

a) percent mud cover

b) percent Spartina cover

c) a natural marsh

d) an overfished marsh

3) What is/are the independent variable(s)?

a) percent mud cover

b) percent Spartina cover

c) percent mud and Spartina cover

d) location of sampling (i.e., marsh)

4) What is/are the dependent variable(s)?

a) percent mud cover

b) percent Spartina cover

c) percent mud and Spartina cover

d) location of sampling (i.e., marsh)

5) In general, overfishing of blue crabs would be expected to result in __________ percent Spartina cover?

a) increased

b) decreased

c) no change in

d) more variable

6) What biological mechanisms might produce the observed pattern?

a) decrease in predation on snails

b) increase in herbivory on Spartina

c) decrease in predation on snails followed by a increase in herbivory on Spartina

d) increase in predation on snails followed by a decrease in herbivory on Spartina

Explanation / Answer

1)   b) percent Spartina cover

2)   a) percent mud cover

3)   a) percent mud cover

4)   a) percent Spartina cover

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