| Date | November 2009 | Marks available | 1 | Reference code | 09N.2.SL.TZ0.1 |
| Level | Standard level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | State | Question number | 1 | Adapted from | N/A |
Question
Limpets are small animals that feed on the green algae which grow on rocks on seashores. Oystercatchers (Haematopus bachmani) are birds that feed on limpets.
In a study on the north-west coast of the USA, where three limpet species are common (Lottia digitalis, Lottia pelta and Lottia strigatella), the limpets were protected from the oystercatchers by large wire cages. After two years the number of limpets in this area was compared with the number of limpets in an area without cages, where oystercatchers were present.
There is evidence to show that both air and water temperatures have increased over a period of time. An investigation was undertaken to determine the effect this change in climate had on the populations of another species of limpet, Patella depressa, around south-west England. The population of the limpet was recorded in many locations and around 30 years later, this study was repeated. The chart below compares the population in each of the locations.
State the effect that the exclusion of the oystercatchers had on the total number of limpets per m2.
Suggest reasons for the difference in numbers of L. strigatella between the areas where oystercatchers were present and where oystercatchers were excluded.
On the map above label one location,
(i) with the letter X, where the limpet population was abundant in the 1950s and occasional in the 1980s (1980–1984).
(ii) with the letter Y, where the limpet population was abundant in both the 1950s and in the 1980s (1980–1984).
Outline, using the data, the overall trend in the limpet population from the 1950s to the 1980s (1980–1984).
Suggest two reasons for the change in limpet population between the 1950s and the 1980s (1980–1984).
Markscheme
total number of limpets increased
L. strigatella decrease when oystercatchers excluded;
could be due to increase in number of L. digitalis;
increased competition/predation (for L. strigatella);
less food/green algae (for L. strigatella);
less habitat available (for L. strigatella);
(i) X at 4th location going anticlockwise from top right
(ii) Y at 8th, 9th or 11th location going anticlockwise from top right
(overall) decline in limpet population;
in no location has the population increased;
greatest decrease in numbers at most northern/western locations;
in many/some/quantitative value locations the population has not changed;
more predation of the limpets than before;
less food available;
less habitat available;
more competition from other species;
water/air too warm for their metabolic activities/other specific reason;
water pollution / toxins;
Examiners report
Weaker candidates often missed the reference to “total”. This oversight resulted in answers about changes in individual limpet species rather than a single answer describing the effect on all the species added together.
Very few candidates noted the decline in the population of L. strigatella when oystercatchers were excluded. Answers frequently mentioned the increase in the number of L. digitalis with reference to more predation and less food. Less habitat for L. strigatella was rarely, if ever, described.
Almost everybody correctly placed the X but much fewer the Y.
This question was answered well by most candidates. The popular answers were that the (overall) population declined and that in many locations the population had not changed. Only a few candidates mentioned that the population never increased or that the greatest decrease was in the most northern/western locations.
Again, this question was generally answered well. Most candidates mentioned greater predation of limpets, less availability of food or water pollution. Some talked about warmer air/water but then dropped the idea by not including any specific effect on limpets. In contrast, some candidates suggested that warmer water might inhibit reproductive behaviour, thus gaining a mark. A few other candidates gained a mark by suggesting loss of habitat.
