| Date | May 2016 | Marks available | 2 | Reference code | 16M.3.HL.TZ0.15 |
| Level | Higher level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Compare and contrast | Question number | 15 | Adapted from | N/A |
Question
The figure shows the distribution of two species of freshwater flatworms, Planaria gonocephala and Planaria montenegrina, over a range of stream temperatures. Graph A and graph B show the distributions when each species is separate from the other. Graph C shows the distribution when they are found living together.
Using graph A and graph B, compare and contrast the temperature ranges of the two species when they are found separately.
Explain, with respect to the example of P. montenegrina, what is meant by realized niche.
Markscheme
P. gonocephala is found over a greater range of temperatures (Note: do not accept just numbers (T) of ranges without comparing/contrasting clearly).
P. gonocephala is found between 16.5 degrees and 23.0 degrees whereas P. montenegrina is not
OR
P. gonocephala is found at a higher temperature
Both are found in temperatures of 6.5 degrees to 16.5 degrees
Do not accept “both show a greater range” alone as this comes from graph C not A and B as the question asks.
Realized niche is that which organism actually occupies
Presence of another species/P. gonocephala narrows the niche
Limited by competition
OR
competitive exclusion
The realized niche is colder/smaller range in the presence of P. gonocephala
Examiners report
This question appeared to be fairly well understood but not always well answered. Some candidates simply listed the separate ranges without comparing them thus not getting the marks. As this was a ‘compare and contrast’ question, purely numerical descriptions do not score marks.
Again candidates often gave very poorly worded and confused answers and struggled to explain what a realized niche was. However, many were able to receive one mark as they understood that competition between species narrowed the niche.
