How relationships between species keep an ecosystem stable

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. 1.

   Ari reads the graph. The pattern keeps repeating. In which year will the lynx count reach its third peak?

   • Year 26: lynx peaks are 10 years apart, starting at year 6
   • Year 22: 2 years before the third hare peak, with lynx first
   • Year 24: the year of the third hare peak
   • Year 36: three cycles of 10 years after the lynx peak at year 6
2. 2.

   Bo reads the graph. The hare count falls from its peak of 80 to 20 by year 6. Which statement fits the graph?

   • The hare count fell 25 percent, and the lynx peak came 2 years after it
   • The hare count fell 75 percent, and the lynx peak came 2 years after it
   • The hare count fell 60 percent, and the lynx peak came 2 years after it
   • The hare count fell 75 percent, and the lynx peak came 2 years before it

3. Read this first

   A model stream after the banded tetras are absent (model data)

   year	banded tetras	phosphorus level (model units)	stream plants (percent cover)
   1	30	60	80
   2	0	15	78
   3	0	15	60
   4	0	15	40

4. 3.

   Jun reads the model data. The banded tetras are absent from year 2. Which sequence fits the table?

   • Plant cover changes little, because 30 tetras are a small count
   • Plant cover falls first, and the phosphorus level falls after it
   • Plant cover stays near 80, because plants do not interact with the tetras
   • Phosphorus falls first, and plant cover falls in the years after
5. 4.

   Kira reads the graph. Which explanation fits why the two lines differ?

   • Paramecium aurelia uses the same limited resource and outcompetes it in the tube
   • Paramecium caudatum needs a partner species to grow, so its count levels off alone
   • The test tube itself holds Paramecium caudatum back, so its count falls
   • Both species share one niche, so both counts should fall to 0 in the shared tube
6. 5.

   Luz reads the graph. Which explanation fits why the host count with the parasite stays above 0?

   • The parasite helps its hosts, so the count with it should rise
   • The parasite needs time to spread, so its hosts are weakened, not removed
   • A parasite removes its host quickly, so the graph must show another relationship
   • The parasite takes nothing from its hosts, so the count fell by chance

7. Counts of two model species that use the same resource (model data)

   year	species X	species Y
   1	60	60
   2	50	45
   3	40	30
   4	44	36
   5	48	42
   6	52	48

8. 6.

   Milo reads the model data. Species Y uses a different area of the habitat from year 4. Which explanation fits the counts?

   • A different area lessens the overlap in resources, so both counts began to rise
   • The two species began to help each other as partners, so both counts began to rise
   • The move changed nothing for species X, so its rise was by chance
   • The two species now use the very same area, so both counts began to rise again

9. Notebook

   Notes on four pairs of living things (classmate's notebook), 4 lines. Columns: line, entry. Line 1: A fungus and an alga in a lichen both gain, so it is mutualism. Line 2: A bird and the tree that holds its nest both gain, so it is commensalism. Line 3: A model parasite gains and its host plant is harmed, so it is parasitism. Line 4: Two plant species that share a limited supply of water both lose some growth, so it is competition.

10. 7.

    Esme reads a classmate's notebook about four pairs of living things. Which line has the mistake?

    • Line 1: a lichen is commensalism, because both partners gain
    • Line 4: plants that share a supply of water help each other to grow
    • Line 2: in commensalism the tree is unchanged, so the bird alone gains
    • Line 3: a host that is harmed means the pair is in competition, not parasitism

11. Notebook

    Notes on a model stream (classmate's notebook), 4 lines. Columns: line, entry. Line 1: The 20 banded tetras supply 90 percent of the stream's phosphorus. Line 2: The 800 stream snails supply the other 10 percent. Line 3: So the stream snail is the keystone species, because it has the most individuals. Line 4: Without the tetras, the phosphorus supply would change greatly.

12. 8.

    Ren reads a classmate's notebook about a model stream. Which line has the mistake?

    • Line 2: 800 individuals must supply more than 20 individuals
    • Line 1: 90 percent and 10 percent do not add up to a whole of 100 percent
    • Line 3: a keystone species is picked by its effect, not by its count
    • Line 4: removing one species cannot change a whole community

13. A bird and a tree, measured over a year (model data)

    partner	measure	without the other	with the other
    bird	safe nest sites	0	3
    tree	leaves on a branch	400	400

14. 9.

    Tala reads the model data. Which claim goes with the right reason?

    • Claim: commensalism. Reason: the bird gains 3 sites and the tree's 400 leaves stay the same
    • Claim: mutualism. Reason: the tree is not harmed by the nest, so the tree gains too
    • Claim: commensalism. Reason: the bird and the tree both gain something from the nest
    • Claim: parasitism. Reason: the bird gains a nest site and the tree has a nest on its branch

15. Three species in a model stream of 12 species (model data)

    species	individuals	share of the phosphorus supplied (percent)	species left if it is removed
    banded tetra	20	90	5
    stream moss	700	6	11
    stream snail	300	4	11

16. 10.

    Zee reads the model data. Which claim goes with the right reason?

    • Claim: no species is keystone. Reason: removing moss or removing snail still leaves 11 of the 12 species
    • Claim: the banded tetra is not keystone. Reason: 20 individuals is too small a count to matter much
    • Claim: the banded tetra is keystone. Reason: removing it leaves 5 species, though it has the fewest individuals
    • Claim: stream moss is keystone. Reason: it has the most individuals, 700, so it matters most to the stream