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IB DP Biology- D4.2 Stability and change- IB Style Questions For HL Paper 1A -FA 2025

IBDP Biology HL Paper 1 -FA 2025- All Chapters

Question

During the process of primary succession, how does the structure of an ecosystem typically change over time?

A. Saprotrophs are the first organisms to inhabit bare rock.
B. The level of primary productivity remains constant.
C. Species diversity tends to decrease as succession progresses.
D. The complexity and interconnections of food webs increase over time.

▶️ Answer/Explanation
In primary succession, pioneer species such as lichens and mosses first colonize bare rock and gradually create soil, allowing more complex plant and animal species to establish. As succession continues, both species diversity and the complexity of food webs increase, forming a stable climax community over time.
✅ Answer: (D) There is an increase in the complexity of food webs.

Question

In a stable natural ecosystem, how is the supply of nutrients and energy maintained?

A. Nutrients and energy are efficiently cycled within the ecosystem.
 
B. Energy from the Sun is cycled through the food chains, while nutrients are constantly lost.
 
C. Nutrients are efficiently cycled within the ecosystem, while there is a constant flow of energy.
 
D. Energy is cycled through photosynthesis and respiration, while food chains ensure that nutrients flow through the ecosystem.
▶️Answer/Explanation
Answer: C. Nutrients are efficiently cycled within the ecosystem, while there is a constant flow of energy.
Explanation:
In ecosystems, nutrients (like carbon, nitrogen, and phosphorus) are recycled through biogeochemical cycles. These cycles ensure that elements are reused in the ecosystem without being lost.

Energy, on the other hand, flows through the ecosystem. It enters primarily through solar radiation, is converted into chemical energy by producers (plants), and flows through the food chain as organisms consume each other. Energy is not recycled but is lost as heat at each trophic level (according to the second law of thermodynamics).

Option Breakdown:

A. Incorrect – Energy is not cycled, it flows through the ecosystem and is ultimately lost as heat.

B. Incorrect – While nutrients are cycled, energy is not cycled but flows through the system.

C. Correct – This statement accurately describes how nutrients are cycled, and energy flows through the ecosystem.

D. Incorrect – Energy is not cycled through photosynthesis and respiration, but rather flows, and while food chains do help with nutrient flow, this option doesn’t fully capture the roles of energy and nutrients.

Question

Which processes occur in an ecosystem?
I. Biomass increases in each successive trophic level.
II. Inorganic nutrients are recycled.
III. Chemical energy is stored in carbon compounds.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

▶️Answer/Explanation

Answer: C. II and III only

Explanation:

I. Biomass increases in each successive trophic level – This statement is incorrect. Typically, biomass decreases as you move up trophic levels due to energy loss at each step in the food chain (about 90% of energy is lost at each trophic level as heat, and only about 10% is transferred to the next level).

II. Inorganic nutrients are recycled – This statement is correct. Nutrients like nitrogen and phosphorus are recycled through the ecosystem via the biogeochemical cycles.

III. Chemical energy is stored in carbon compounds – This statement is correct. Producers (like plants) store chemical energy in carbon compounds (such as glucose) during photosynthesis, and this energy is passed through the food chain.

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