A dichotomous key is a series of steps, each involving a decision, that can be used to identify unknown organisms. The key prompts us to decide, through careful observation, whether or not a specimen displays particular visible features, and allows us to distinguish between specimens on this basis.
When constructing a key to identify organisms such as those shown in Figure 5.27, fi rst examine each specimen in the set carefully, and choose a characteristic that is present in about half of the individuals and absent in the others. For example, the presence of wings could be this fi rst distinguishing characteristic, which eff ectively divides the specimens into two smaller groups.
Now for each group, another diagnostic feature must be chosen whose presence or absence divides the specimens into two further groups. A branching tree diagram can be constructed, as shown in Figure 5.27, progressively dividing the specimens into smaller and smaller groups, until at the end of each branch a single individual is identifi ed.
Finally, the tree diagram is ‘translated’ into a written key, in which the branch points are expressed as alternative statements. Each alternative either names the identifi ed specimen or leads the user to a subsequent pair of statements, until an identifi cation is reached. A well-written key is composed of a series of questions or steps, such that an organism that is being studied can only be placed in one of two groups. The style of the questions is therefore very important in the design of a good key.
So, for example, the dichotomous key arising from the tree diagram in Figure 5.27 would be as follows.
1 Wings present go to 2
No wings go to 5
2 Two pairs of wings go to 3
One pair of wings fl y
3 Legs all approximately the same length go to 4 Hind pair of legs much longer than front two pairs locust
4 Wings covered in scales butterfl y
Wings transparent, not covered in scales dragonfl y
5 Four pairs of legs go to 6
More than four pairs of legs go to 7
6 Pair of claws present crab
No claws spider
7 Body clearly divided into equal-sized segments centipede Body in two regions, segments only clear on hind region prawn
33 If you were making a dichotomous key to identify leaves, explain why the question ‘Is the leaf large?’ would not be useful.
5 ECOLOGY AND EVOLUTION 125 Figure 5.27 A dichotomous tree diagram distinguishing eight organisms.
Start
fly
locust
crab
prawn centipede
butterfly dragonfly
spider wings present
2 pairs of wings 1 pair of wings
legs not all same length legs all about same length claws present
wings transparent wings with scales equal-sized body segmentation segmentation in hind region only claws absent
more than 4 pairs of legs 4 pairs of legs wings absent
End-of-chapter questions
1 Which of the following statements is correct?
A A community is the place where several diff erent species live.
B Heterotrophs are organisms that feed off organic matter.
C Decomposer bacteria are detritivores.
D A habitat is a community and its abiotic environment. (1)
2 Which of the following statements is correct?
A The precautionary principle states that if the eff ects of a human-induced change would be very large, those responsible for the change must prove that it will not be harmful before proceeding.
B The greenhouse gases methane and oxides of nitrogen have increased global temperatures by converting short-wave radiation to long-wave radiation.
C In the carbon cycle, the process of complete decomposition of plant remains stores carbon in the form of fossil fuels.
D A dichotomous key can be used for the identifi cation of organisms because all
organisms have a binomial classifi cation. (1)
3 Which of the following is the correct sequence for the hierarchy of taxa?
A kingdom, phylum, family, order, species B kingdom, order, class, genus, species C kingdom, class, phylum, order, species
D kingdom, phylum, family, genus, species (1)
4 If two organisms are members of the same order, then they are also members of the same
A genus
B class
C family
D species. (1)
5 The total energy in an area of grassland was analysed and found to be 400 kJ m−2 y−1. Construct a labelled pyramid of energy for this grassland for the fi rst three trophic levels, assuming an energy loss of 90% at each
level. It is not necessary to draw it to scale. (3)
6 Outline the role of variation in evolution. (3)
7 Outline three factors that cause the transitional phase in the growth of a population. (3)
8 Phenologists are biologists who study the timing of seasonal activities in animals and plants, such as the opening of tree leaves and the laying of eggs by birds. Data such as these can provide evidence of climate changes, including global warming.
5 ECOLOGY AND EVOLUTION 127 The date in the spring when new leaves open on horse chestnut trees (Aesculus hippocastaneum) has been
recorded in Germany every year since 1951. The graph below shows the diff erence between each year’s date of leaf opening and the mean date of leaf opening between 1970 and 2000. Negative values indicate that the date of leaf opening was earlier than the mean. The graph also shows the diff erence between each year’s mean temperature during March and April and the overall mean temperature for these two months. The data for temperature were obtained from the records of thirty-fi ve German climate stations.
Year
temperature Key
leaf opening
1970 1980 1990
–15 –10 –5 0
5 10 15 4
3 2 1 0 –1 –2 –3
–4 2000
source: Walther et al., Nature (2002), 416, pages 389–395
Difference in mean temperature /°C Difference in date of leaf opening/days
Year 45
35
25
15
75
65
55
45
35 5
1972 1976 1980 1984 1988 1992 1996 Year
1972 1976 1980 1984 1988 1992 1996
source: Visser, Noordwijk, Tinbergen and Lessells, Proceedings of the Royal Society of London, (1998), 265, pages 1867–1870
Mean date of egg laying/ number of days after 31 March
Mean estimated date of maximum caterpillar biomass / number of days after 31 March
Year 45
35
25
15
75
65
55
45
35 5
1972 1976 1980 1984 1988 1992 1996 Year
1972 1976 1980 1984 1988 1992 1996
source: Visser, Noordwijk, Tinbergen and Lessells, Proceedings of the Royal Society of London, (1998), 265, pages 1867–1870
Mean date of egg laying/ number of days after 31 March
Mean estimated date of maximum caterpillar biomass / number of days after 31 March
a Identify the year in which there was the:
i earliest opening of horse chestnut leaves (1)
ii lowest mean temperature in March and April (1)
b Use the data in the graph to deduce the following:
i the relationship between temperatures in March and April and the date of opening of leaves on
horse chestnut trees (1)
ii whether there is evidence of global warming towards the end of the 20th century (2) From 1973 onwards, phenologists in the Netherlands have been studying a population of great tits (Parus
major) in a forest on the Hoge Veluwe. Nest boxes are checked every week to fi nd out when the great tits lay their eggs and how many eggs they lay. Young birds are ringed when they are seven days old, to allow the reproductive success of their parents to be monitored. Great tits feed on arthropods, especially caterpillars. The phenologists found that the date of maximum caterpillar biomass each year in the forest could be estimated accurately using temperature records. The graphs below show the mean date of egg laying and the estimated date of maximum caterpillar biomass for each year from 1973 to 1995.
c i Compare the date of egg laying with the date of maximum caterpillar biomass. (1) ii Suggest an advantage to great tits of the diff erence in dates. (1) d State the trend, shown in the graph, for the date of maximum caterpillar biomass. (1) There was no statistically signifi cant change in the date of egg laying between 1973 and 1995, but the
phenologists found evidence that natural selection will eventually cause a change in the date of egg laying.
e Explain how natural selection could cause a change in the date of egg laying in the population of
great tits in the forest on the Hoge Veluwe. (2)
(total 10 marks)
© IB Organization 2009
9 Ecosystems require an input of energy, water and nutrients to maintain themselves. Nutrients may be re-used through recycling within ecosystems.
Nutrient cycling within an ecosystem has been studied in many types of region. One factor studied is the mean residence time (MRT), which is the amount of time needed for one cycle of decomposition (from absorption by organism to release after death). The table below gives the mean residence time for certain nutrients in four diff erent types of region. In addition, the plant productivity is also shown. (Plant productivity gives an indication of the quantity of plant material potentially available to consumers.)
Mean residence time / years
Type of region Carbon Nitrogen Phosphorus Potassium Calcium Magnesium Plant productivity/
g cm−2 y−1
sub-arctic forest 353.0 230.0 324.0 94.0 149.0 455.0 360
temperate forest 4.0 5.5 5.8 1.3 3.0 3.4 540
chaparral 3.8 4.2 3.6 1.4 5.0 2.8 270
tropical rainforest 0.4 2.0 1.6 0.7 1.5 1.1 900
source: W H Schlesinger (1991), in M Bush, Ecology of a Changing Planet (1997), Prentice Hall, page 67
a i State which nutrient shows the shortest mean residence time in a temperate forest. (1) ii Identify the type of region in which potassium has the longest mean residence time. (1) b Compare the mean residence time for nutrients in the temperate forest and chaparral. (2) c Evaluate the relationship between the mean residence time and plant productivity for the diff erent types
of habitat. (2)
d Suggest one reason for the diff erence in mean residence time of nutrients in the tropical rainforest and
the sub-arctic forest. (1)
(total 7 marks)
© IB Organization 2009
5 ECOLOGY AND EVOLUTION 129 10 The graph below shows the variation in the concentration of atmospheric carbon dioxide since 1970.
320 325 330 335 340 345 350 355 360 365 370 375
1970 1975 1980 1985
Year
1990 1995 2000
CO2 concentration/ppm
source: C D Keeling and T P Whorf, Atmosphere CO2 concentrations (ppm) derived from in situ air samples, collected at Mauna Loa Observatory, Hawaii
The annual fl uctuation is mainly the result of changes in the levels of photosynthesis associated with the seasons in Northern Hemisphere forests.
a i Describe the overall trend shown in the graph. (1)
ii Suggest a cause for the overall trend throughout the period 1970–1999. (1) b i Using a clear label, identify any one point on the graph which shows the CO2 level in mid-summer. (1) ii Explain why the concentration of CO2 varies with the seasons. (2) c Identify one gas, other than CO2 , which is contributing to the enhanced greenhouse eff ect. (1) (total 6 marks)
© IB Organization 2009