The garden snail, Helix aspersa, is a herbivore.

[Source: Sinan Önder /https://www.pexels.com/photo/animal-close-up-crawling-garden-243128/]

Explain how the population of snails in an ecosystem could be estimated by capture-mark-release-recapture, identifying the limitations in the method.

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 ✔

This longer response question was also well answered, and stronger candidates often scored full marks. Only very weak students were not able to provide some outline of the method used, although fewer students achieved the marks that rewarded knowledge about the assumptions of the method. Many felt that the time required was a drawback, and very few were able to achieve the marking point available for correct use of the Lincoln index. For all experimental skills in the subject guide, students should be taught to outline how the method is carried out and how the data collected can be processed; and to evaluate errors, limitations and assumptions.