▶️ Answer/Explanation
a.
- The fundamental niche is the full range of environmental conditions and resources a species could use.
- The realized niche is the actual conditions and resources the species does use, limited by interactions such as competition and predation.
b.
- Competition reduces the available niche space.
- Resources are limited due to competition.
- Removal of competitors may allow a species to expand its niche.
- Competitive exclusion may occur, where one species is eliminated from a niche due to being outcompeted.
c.
- Keystone species have a disproportionate impact on the structure and functioning of an ecosystem.
- Their removal can cause significant changes, potentially leading to the collapse or dramatic alteration of the ecosystem or food web.
Markscheme:
a. Fundamental niche is the potential niche and realized niche is the actual niche.
OWTTE.
b.
• Competition can limit the niche
• Competition can limit resources
• When competition is removed the niche can expand
• Can result in exclusion or removal of a species from a niche / only one species can occupy a niche
c.
• Keystone species has a strong/disproportionate effect on a community/food web/ecosystem
• Absence of keystone species would completely alter the ecosystem
▶️ Answer/Explanation
a.
- A: Decomposition (Decay) — Transfer of nutrients from litter to soil.
B: Leaching (or erosion/run-off/weathering) — Loss of nutrients from soil, typically due to water movement.
b.
Ecosystem I is representative of a tropical rainforest.
Reason:
- Small litter store due to rapid decomposition (warm, moist conditions promote decay).
- Large biomass store reflects high primary productivity.
- High leaching from the soil is consistent with heavy rainfall in tropical regions.
Markscheme:
a.
Process A: decomposition/decay
Process B: leaching/erosion/run-off/weathering
b.
Ecosystem I
Low levels of litter due to high rates of decomposition OR high amounts of biomass related to high rates of productivity OR weathering/leaching due to high rates of precipitation
▶️ Answer/Explanation
a.
Answer: 32
The diploid number is 64, so the haploid number is half of that:
b.
- Mules have 63 chromosomes, which is an odd number.
- During meiosis, chromosomes must pair up to divide evenly.
- With an odd number, not all chromosomes can pair, preventing proper gamete formation.
- Therefore, mules are sterile and cannot produce viable sperm or egg cells.
c.
Not the same species, because:
- Species are defined by the ability to interbreed and produce fertile offspring.
- Although horses and donkeys can mate, their offspring (mules) are not fertile.
- They also have different chromosome numbers (horses: 64, donkeys: 62), suggesting genetic incompatibility.
- Therefore, they are classified as different species.
d.
Non-disjunction
- This is an error in meiosis where chromosomes fail to separate properly.
- If non-disjunction occurred in either the horse’s or donkey’s gametes, the resulting zygote could end up with an extra chromosome.
- For example, if the donkey sperm carried an extra chromosome (31 instead of 30), and the horse egg had 32, the resulting mule would have 64 chromosomes.
Markscheme:
Answer to part (a): 32
Answer to part (b):
a. Because the chromosome number is not an even number (63).
b. (So) cannot divide by two during meiosis / chromosomes cannot pair up during meiosis.
c. One chromosome has no homologue.
d. Because unlikely to/cannot produce viable gametes (sperm/egg cells).
Answer to part (c):
a. To be in the same species, two organisms must have the same genes arranged on the same chromosomes OR must have the same number of chromosomes.
b. Members of the same species produce fertile offspring, and a mule is not fertile.
Answer to part (d): Non-disjunction
Accept description of non-disjunction.