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IB DP Biology- B4.1 Adaptation to environment - IB Style Questions For SL Paper 2 - New Syllabus

IBDP Biology Paper 2 - All Chapters

Question

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 (Topt) and maximum temperature (Tmax) at which germination commonly occurs. The eight species were then divided into two groups according to the effects of temperature on germination.

(a) Identify the species in which Topt is lowest. [1]

(b) Calculate the range for Tmax among the eight species. [1]

(c) Analyse the graphs to find the key difference between species in Group 1 and Group 2. [1]

To investigate the seasonality of germination, soil temperature and soil moisture were monitored at sites in Korung National Park, which is part of the Northern Jarrah Forest.

The graph shows the results.

(d) (i) Distinguish between soil moisture in winter and in summer. [2]

(ii) Suggest reasons for the differences. [2]

(e) Predict, giving reasons, when germination would start in Kennedia prostrata, following a forest fire at the start of summer. [3]

(f) 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. [4]

Syllabus Topic Codes (IB DP Biology):

• B4.1 Adaptation to environment: Abiotic variables affecting species distribution and range of tolerance of limiting factors, including temperature — parts (a), (b), (c), (d), (e), (f)
• C4.1 Populations and communities: Competition for resources within and between populations — parts (c), (e)
• D4.1 Natural selection: Abiotic factors, including high and low temperatures, as selection pressures — parts (e), (f)
▶️ Answer/Explanation

(a)

The lowest Topt shown in the graphs is for Kennedia coccinea, with an optimum temperature of approximately 12°C.

(b)

The lowest Tmax among the eight species is approximately 20°C, while the highest Tmax is approximately 30°C.

\[ \text{Range}=30-20=10^\circ\text{C} \]

(c)

The key difference is the size of the gap between Topt and Tmax. Species in Group 2 generally have a larger gap between their optimum and maximum temperatures than species in Group 1.

This means that Group 2 species generally have a wider tolerance of higher temperatures. Group 2 species also generally show higher germination percentages over the warmer temperature range.

(d) (i)

Soil moisture fluctuates considerably during winter and is generally higher than in summer. In summer, soil moisture remains relatively low and shows much less fluctuation.

(d) (ii)

There is more rainfall during winter, with rainfall occurring on some days and not others. This causes fluctuations in soil moisture.

In summer, there is little or no rainfall and the higher soil temperatures increase evaporation. Water may also be lost from plants through transpiration, causing soil moisture to remain low.

(e)

A forest fire occurring at the start of summer would initially leave the soil temperature above the suitable range for Kennedia prostrata. Its Tmax is approximately 21–22°C, while summer soil temperatures rise well above this value.

Therefore, germination would not be expected to start immediately after the fire. It would most likely begin in autumn, around March or April, when soil temperature falls below Tmax and soil moisture begins to increase.

Two important conditions would therefore become more favourable: the temperature would fall towards the suitable range and the availability of water in the soil would increase.

(f)

Species in Group 1 are generally more likely to be adversely affected by increases in soil temperature caused by global warming.

The main reason is that Group 1 species generally have a smaller gap between Topt and Tmax. Therefore, they have a narrower range of temperatures over which germination can occur before reaching their maximum temperature.

In addition, Tmax is generally lower in Group 1 than in Group 2. An increase in soil temperature would therefore cause Group 1 seeds to reach their maximum temperature sooner, reducing germination.

However, some Group 2 species also have relatively low Tmax values, so these species could also be adversely affected. Increasing soil temperature may also reduce soil water availability through increased evaporation, which could adversely affect germination in species from either group.

Therefore, although Group 1 is generally more vulnerable to increased soil temperature, the effect will vary between species and may also depend on changes in soil moisture.

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