| Date | November 2016 | Marks available | 2 | Reference code | 16N.2.SL.TZ0.1 |
| Level | Standard level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Draw | Question number | 1 | Adapted from | N/A |
Question
Beavers are large rodents that live in waterways throughout the northern hemisphere. Dams made by beavers change the temperature of the streams and affect the mayfly, Baetis bicaudatus. In the summer of 2008, beaver ponds in West Brush Creek and Cement Creek, Colorado, were studied to evaluate their impacts on mayflies. The study sites included streams flowing into (upstream) and out of (downstream) each beaver pond.
Mayflies, including the species B. bicaudatus, are aquatic insects that hatch and spend their larval stages in water emerging from the water as adults. Larger females produce an increased number of better quality eggs.
The table shows the mean temperature differences (downstream – upstream) and mean dry mass for female and male mayflies.
The bat, Pipistrellus nathusii, feeds on insects including mayflies. A study was undertaken in Poland to see the effect of European beavers (Castor fiber) on the activity of bats. Beaver activity can affect forests that are covered by trees and meadows that are covered by grasses and have no trees. The following habitats were studied:
- forest (F)
- flooded forest with canopy gaps created by beavers and flooding due to the presence of beaver dams (FFG)
- forest with canopy gaps created by beavers but no flooding (FG)
- meadow (M)
- meadow with flooding due to the presence of beaver dams (MF).
As bats feed they fly through the air catching insects. The number of feeding passes made by bats was counted. The graph shows differences in the bat activity between particular habitats.
Calculate the difference in the mean dry mass of mayflies upstream and downstream of Cement Creek pond 9 for female and male mayflies.
Describe the effect dams have on water temperature.
The graph shows the mean dry mass of mayflies relative to the water temperature in their habitats.
Using the graph, discuss evidence for the hypothesis that mayflies grow to greater dry mass in cooler water.
Analyse the data to find the effect of flooding and tree felling by beavers on the activity of bats.
The trout, Oncorhynchus mykiss, that live in West Brush Creek and Cement Creek also feed on the mayflies. Fishermen come to Colorado to catch and eat trout. Draw a diagram of part of a food web for the creeks in Colorado, including mayflies, humans, trout and bats.
Identify an example of competition between organisms in this food web.
The North American beaver (Castor canadensis) was introduced to islands adjacent to Argentina and Chile where they have become an invasive species. Discuss one ecological criterion (a basis for deciding) whether beavers are harmful or helpful to the ecosystems there.
Markscheme
I. female: «+» 0.04 «mg»
AND
II. male: «–» 0.12 «mg»
Both needed.
a. height of dam affects the temperature
b. high dams tend to cool the water
c. low dams tend to warm the water
d. pond 5/one pond shows no change/stays the same
a. trend lines support «the hypothesis»
OR
trend shows a negative correlation shown «between increased temperature and size»
Do not credit answers with just numbers.
Accept “line of best fit” wtte.
b. the trend line is shallow / small slope
OR
there is a large amount of scatter at higher temperatures (reducing the certainty)
OR
wide/overlapping ranges so no significant difference «(at» 9oC )
Note that it is only the trend line that indicates support.
c. (hypothesis not supported because) females in water over 10oC have the highest «mean dry» mass
Words other than “hypothesis not supported” may be used: “as opposed to”, “whereas”, to express deviation from support.
a. both flooding and tree felling increases bat passes/activity / WTTE
b. flooding has greater/increase on bat passes/activity / WTTE
OR
flooding has the greater impact than tree felling on bat passes
c. supporting argument from the data
a. arrow pointing from trout to human
b. arrows pointing from mayflies to trout and bats
Award [1 max] if answer does NOT show all 4 organisms.
bats and trout compete for mayflies
a. criterion
b. reason that beavers damage
c. reason that beavers help
eg,
a. biodiversity
b. if indigenous species are eliminated biodiversity is reduced, then the beavers would be seen as damaging
c. if biodiversity increases (due to the engineering of waterways), then beavers could be a benefit
Consider criteria something that may be dealt with from a range of perspectives.
Other possible criteria: abiotic disturbance changes to food webs diversity
