Indicator Species ID

Indicator Species ID

You do not always need a machine to spot pollution. Some living things, called indicator species, reveal the state of the environment just by being there, or by being absent. The old version of this topic was a list to memorise. This one asks the better question at every step: why can this organism live here when that one cannot? Answer that and you never have to remember which way round any of them go. (Higher tier.)

The words for it

Five terms. The principle underneath them all is short: a species tells you about the conditions it can tolerate, so finding a fussy organism means conditions are good, and finding only a tough one means they are not:

Reading the water

What lives in a river depends mostly on how much dissolved oxygen it holds, and pollution such as sewage lowers that sharply as decomposers use the oxygen up:

Why can the worm survive?

A polluted stretch of river holds plenty of bloodworms but no stonefly larvae. What best explains the difference?

• Bloodworms contain haemoglobin, so they can still take up enough oxygen where very little is dissolved, while stonefly larvae have a high oxygen demand and cannot
• Bloodworms feed on the pollution itself, so they do well in it while other species starve
• The pollution is poisonous to stonefly larvae but harmless to bloodworms
• Bloodworms reproduce faster, so they replace the stonefly larvae over time

What does each survey show?

Reading the air

Two organisms indicate air quality, and both are sensitive to sulfur dioxide, so their presence means clean air. Lichens are the classic case, and the reason is structural. A lichen has no roots and no waxy cuticle: it absorbs water and dissolved minerals straight from the atmosphere over its whole surface. It has no way to exclude anything, so whatever is in the air goes in, and sulfur dioxide accumulates until it kills the lichen. Bare trees mean polluted air. Blackspot fungus on rose leaves works the same way and catches people out for a different reason: it is a plant disease, so finding it feels like bad news. For air quality it is the opposite.

The blackspot trap

A student writes: "We found blackspot fungus on the roses, so the air here must be polluted, because fungus is a disease and disease means something is wrong." What is wrong with the reasoning?

• They have run two different things together. The fungus is bad for the rose, but it is itself killed by sulfur dioxide, so its presence is evidence that the AIR is clean
• Blackspot fungus is not an indicator species at all, so it tells you nothing about the air
• They should have looked at the roses rather than the fungus, since the plant is the indicator
• They did not count how much fungus there was, and only the quantity matters

The lichens came back

For most of the twentieth century, British city centres were nearly lichen-free. Move outwards from the middle of an industrial city and you could watch them reappear: first the flat crusty kinds, then leafy ones further out, and only well beyond the built-up area the bushy species that tolerate almost no sulfur dioxide at all. That gradient was itself the measurement. Nobody needed a monitoring station to know the air got cleaner as you left town, because the lichens were already reporting it. Then sulfur dioxide emissions fell steeply, as coal burning declined and clean-air legislation took effect, and the lichens moved back in. Species absent from many British cities for decades are now found on urban trees again, which is about as direct a piece of evidence as environmental policy ever produces.

On a field survey

You survey a stream and the trees along its bank. Reason out what your findings actually support.

• In the water you find many bloodworms and sludgeworms and no shrimp or stonefly larvae. What does that support?
• The trees along the same bank are thick with leafy lichen. Does that contradict your first finding?
• A classmate suggests the stream is polluted because a factory two miles downstream discharges into it. What is wrong with that?
• How should the biological evidence be reported?

Spot the overclaim

Here is a student's conclusion after the survey. Tap the ONE sentence that claims more than indicator species can support.

• We found bloodworms and sludgeworms but no stonefly larvae.
• This suggests the water is low in dissolved oxygen.
• The dissolved oxygen must therefore be 4.2 milligrams per litre.
• We would want to confirm this with a dissolved-oxygen probe.
• The lichen on the bankside trees suggests the air here is clean.

Which are true?

Select ALL THREE statements that are TRUE.

• The presence of a pollution-sensitive species is evidence of good conditions, and the presence of only tolerant species is evidence of poor ones
• Indicator species reflect conditions over a period of time, whereas a single instrument reading captures only the moment it was taken
• Organic pollution such as sewage kills river invertebrates mainly by lowering the dissolved oxygen rather than by poisoning them directly
• Bloodworms thrive on pollution, which is why they are found in dirty water
• Lichen grows best where sulfur dioxide levels are high
• Indicator species give an exact numerical measurement of pollution

Indicator summary

An indicator species reveals conditions by its presence or _____. Bloodworms contain _____, so they survive the low oxygen that organic pollution causes, while stonefly larvae and freshwater shrimp need clean, well-oxygenated water. Lichen and blackspot fungus are both killed by _____, so finding them means the air is _____. Biological monitoring is cheap and reflects conditions over time, but it gives an indication rather than an exact figure.

Compare two rivers

Exam practice. In about 60 words, explain how you could use indicator species to compare the water quality of two rivers, and say what the method can and cannot tell you. Include:

• which organisms you would look for, and what finding each one would suggest
• why those organisms differ in where they can survive
• one limitation of the method, and what you would do about it