Life between the tides
328 words over 28 numbered lines, then 6 parts worth 12 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice unless a source is named.
Life between the tides
Twice a day the tide comes in and goes out. A rocky shore is covered
by sea water for part of each day and exposed to the air for the rest.
Organisms at the top of the shore may be out of the water for most of
the day, while those near the low-tide mark are almost always covered.
A class of students studied the seaweeds on a rocky shore. They laid a
tape measure from the low-tide mark straight up the shore to the
high-tide mark, a distance of 40 metres. Every 5 metres along the tape
they placed a square quadrat and estimated the percentage of the
quadrat covered by each type of seaweed. This method is called a belt
transect.
The students found two main seaweeds. Serrated wrack grows near the
low-tide mark. Channelled wrack grows near the top of the shore, where
it can lose much of its water during a hot day and still recover when
the tide returns. Their results are shown in Figure 1.
Several abiotic factors change up the shore. Near the top, organisms
are exposed to the air for longer, so they are more likely to dry out.
They also face larger changes in temperature, and rain can lower the
salt concentration of rock pools. Biotic factors matter too. Where
conditions are easier, many species compete for space and light, and
grazing animals such as limpets eat young seaweeds.
The students also counted limpets on a flat area of rock measuring 120
square metres. They placed ten quadrats at random, each with an area
of 0.25 square metres. The mean number of limpets in each quadrat was
6.4. Limpets cling tightly to the rock with a muscular foot, and move
over it at high tide to graze on a thin film of algae.
Rocky shores can be damaged by oil spills and by visitors who turn
over rocks and leave them upside down, killing the animals that live
underneath.
Figure 1 Percentage cover of two seaweeds along a transect up a rocky shore
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term population.
(1)Describe the distribution of channelled wrack shown in Figure 1.
(2)Explain why serrated wrack is not found near the top of the shore.
(2)Calculate an estimate of the number of limpets on the area of rock.
(3)Explain why the students placed the quadrats at random.
(2)Suggest two biotic factors that could explain why serrated wrack covers less of the rock at 10 m than at 0 m.
(2)Total for this question: 12 marks
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