Question
The garden snail, Helix aspersa, is a herbivore.
Explain how the population of snails in an ecosystem could be estimated by capture-mark-release-recapture, identifying the limitations in the method.
▶️Answer/Explanation
Explanation:
- A sample of snails is captured and marked harmlessly.
- Marked snails are released back into the habitat and allowed time to mix evenly with the population.
- A second sample of snails is captured, and the number of marked individuals in this sample is counted.
- The total population is estimated using the formula:
\[
\frac{\text{Total population}}{\text{Number marked in first sample}} = \frac{\text{Size of second sample}}{\text{Number of marked recaptured}}
\]
Limitations:
- Assumes the marked snails mix fully with the population before recapture.
- Assumes no immigration, emigration, births, or deaths occur between samples.
- Marks must not affect snail survival or behavior.
- Marks must remain visible and not be lost.
- Sample sizes must be large enough for accuracy.
- Assumes no misidentification of marked snails or species.
———————————————————————————————————-Markscheme—————————————————————————————————–
a. snails in the ecosystem are captured and marked ✔
b. they are released back in to the ecosystem and allowed to mix ✔
c. a second sample of snails are captured in the ecosystem and those that are marked are counted ✔
d. sufficient time given between first and second sample to allow for mixing ✔
e. the ratio of the first sample size to those recaptured marked = the ratio of the number in the second sample to the population/formula ✔
f. area of habitat determined ✔
g. assumes sample size is large enough to be significant ✔
h. assumes there is no emigration/immigration/death of snails ✔
i. assumes the marking of the snail does not affect their survival ✔
j. assumes no misidentification of species ✔
k. assumes marked snails do not lose their marks ✔
Question
a. Explain the factors that can contribute to the exponential growth phase in a sigmoid population curve. [3]
b. Outline the effect of carrying capacity on the growth of a population. [2]
▶️Answer/Explanation
(a)
- The environment is ideal with unlimited resources such as food, space, and water.
- There is low disease incidence and few predators, so mortality is low.
- The birth rate (natality) and immigration exceed the death rate (mortality) and emigration, leading to rapid population increase.
(b)
- The carrying capacity is the maximum number of individuals that an environment can sustainably support.
- As the population approaches this limit, growth slows down or fluctuates due to limited resources and increased competition.
——————————————————————————————————-Markscheme——————–
a. ideal environment/unlimited resources/below carrying capacity ✔
b. little disease/few predators ✔
c. high natality/birth rate AND immigration ✔ Both needed.
d. natality and immigration greater than mortality and emigration ✔
a. carrying capacity is maximum population size/number of individuals that environment can support
OR
carrying capacity varies with abundance of limiting resources ✔
b. population growth slows/fluctuates as the carrying capacity of environment reached ✔