IB Biology SL B4.1.4 Range of tolerance of limiting factor

[Maximum number: 8]

Fires are a natural phenomenon in some forest ecosystems, so plants evolve to become fire-adapted. The Northern Jarrah Forest in southwest Australia is an example. The seeds of many species germinate after forest fires, when there is less competition for resources from existing plants.

The effect of temperature on germination was measured in species that occur commonly in the Northern Jarrah Forest. The graphs show results for eight species across the range of soil temperatures expected in this ecosystem. Four replicates of 25 seeds were sown at each temperature. Statistical modelling was used to predict the overall relationship between temperature and germination, and to estimate the optimum temperature ( $\mathrm{T}_{\text {opt }}$ ) and maximum temperature ( $\mathrm{T}_{\max }$ ) at which germination commonly occurs. The eight species were then divided into two groups according to the effects of temperature on germination.

Group 1

Group 2

Figure for Question IB Biology SL B4.1.4 Range of Tolerance of Limiting Factor Questions question 1 — IB Biology SL

(a)

Identify the species in which $T_{\text {opt }}$ is lowest.

[ 1 ]

(b)

Calculate the range for $T_{\max }$ among the eight species.

[ 1 ]

$10 / 9.5^{\circ} \mathrm{C}$

OR

20 to $30^{\circ} \mathrm{C} / 20-30^{\circ} \mathrm{C}$

OR

30 to $20^{\circ} \mathrm{C} / 30-20^{\circ} \mathrm{C}$ ;

Marking guidance:

Accept answers in the range of 9 to $10^{\circ} \mathrm{C}$ .

Accept range of 20 to $29^{\circ} \mathrm{C}$ , 21 to $30^{\circ} \mathrm{C}$ Units required.

Working ( 30-20=10 ) not required because only one mark can be earned.

(c)

Analyse the graphs to find the key difference between species in Group 1 and Group 2.

[ 1 ]

larger gap/wider tolerance between $\mathrm{T}_{\text {opt }}$ and $\mathrm{T}_{\max }$ in Group 2 / vice versa / OWTTE OR

the species in group 2 show a higher germination rate than the species in group 1 OR

(generally)species in group 2 can tolerate warmer temperatures than group 1;

1

Marking guidance:

max

(d)

Predict, giving reasons, when germination would start in Kennedia prostrata, following a forest fire at the start of summer.

[ 3 ]

Prediction

a. in autumn/April/March;

Reasons 2 max

b. when temperature has dropped below $\mathrm{T}_{\text {max }}$ (for K. prostrata)

OR

when temperature has reached Topt (for K. prostrata);

c. when $\mathrm{T}_{\max }$ in K. prostrata is $21 / 22^{\circ} \mathrm{C}$ (after the forest fire);

d. when soil moisture has risen;

2 marks max for the reasons.

c: mark is earned for citing the $\mathrm{T}_{\max }$ temperature.

3

Marking guidance:

max

(e)

Using the data in the graphs, discuss whether species in Group 1 or Group 2 are more likely to be adversely affected by increases in soil temperature due to global warming.

[ 2 ]

a. group 1 because there is less gap between $\mathrm{T}_{\text {opt }}$ and $\mathrm{T}_{\text {max }}$
OR
group 1 is more likely to be adversely affected by increase in soil temperature because they have a narrow range of tolerance for higher temperatures than in group 2;
b. group 1 because $\mathrm{T}_{\max }$ is lower in most species;
c. if global warming heats up the soil temperature, the seeds will germinate earlier which may be adverse (in either group);
d. group 2 because $\mathrm{T}_{\text {opt }}$ is (generally) lower (than group 1);
e. however, some species in group 2 have as low a $\mathrm{T}_{\max }$ as group 1, therefore some species in group 2 will be adversely affected;
f. species (regardless of group) may be adversely affected because increasing soil temperature means less water available for germination;
2
max

IB Biology SL B4.1.4 Range of Tolerance of Limiting Factor Questions question 1

Question (a)

Question (b)

Question (c)

Question (d)

Question (e)