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
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
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 ✔