Adapted to Survive: Investigating an Australian Habitat

An adaptation is a feature that makes an organism better able to survive and reproduce in the place it actually lives. The last six words are the whole idea, and they are the part people drop.

Take the thorny devil from the hook. Grooves between its scales draw water across its skin to its mouth, so it can drink from damp sand or overnight dew and never needs to find a puddle. In the Great Sandy Desert, where rain may not fall for a year, that is close to the best possible feature an animal could have. Now put the same lizard in a Tasmanian rainforest. The grooves still work, and they are now worth nothing, because water is everywhere and drinking was never the problem there. Worse, the thorny devil's pale sandy colouring hides it beautifully against red sand and makes it obvious against wet green moss, and an animal that cannot hide is an animal that gets eaten.

So a feature is not "good" or "advanced". It is suited to a set of conditions, and outside those conditions the same feature can be useless or actively dangerous. That is why "is a koala better adapted than a kangaroo?" has no answer while "what is the koala adapted to?" does. Whenever you describe an adaptation, name the conditions it is a solution to, or you have not explained anything.

Adaptations are sorted into two kinds, and the test is simple: could you photograph it, or would you have to watch for it?

The difference is not just a labelling exercise, because the two kinds work on completely different timescales. A structural adaptation is fixed. A thorny devil wakes up with the same grooves it had yesterday and cannot put them away when it rains. A behavioural adaptation can be switched on and off within minutes: the same lizard shelters under a bush at midday and comes out at four o'clock, and it makes that choice fresh every day according to the conditions.

That is why so many Australian animals rely heavily on behaviour. The problem here is often heat and water loss, and both of those change enormously between noon and midnight in the same place. A structural solution has to be a compromise that works at both. A behavioural one does not: a red kangaroo lies in the shade and licks its forearms while it is hot, and grazes once it cools, so it never has to solve the midday problem with its body at all. Being able to change what you do is the cheapest adaptation there is.

The method for describing any habitat is the same every time. First name the conditions, then look for the features that answer them. Here it is run twice, on two Australian habitats that could hardly be less alike.

Habitat one: mallee scrub, north-western Victoria. The conditions are the problem statement: about 300 mm of rain a year, falling unpredictably, summer days over 40 °C, thin sandy soil, and fire every few decades. Now read the plants. A mallee eucalypt holds its leaves hanging vertically rather than flat, so at midday the sun strikes them edge-on and they heat far less than a horizontal leaf would. The leaves are thick, with a waxy surface and their breathing pores sunk into small pits, which slows water loss. And underground the tree has a swollen woody lignotuber, a bulge full of stored buds, so when fire burns the whole plant to the ground it resprouts from the lignotuber within weeks instead of having to start again from seed. Every one of those three features answers a condition you can name.

Habitat two: a wet sclerophyll gully in the Dandenongs. Change the conditions and every answer changes with them: 1,400 mm of rain, deep shade under a tall canopy, damp soil, and rarely more than 25 °C at ground level. Water is not the problem here; light is. So the tree ferns on the gully floor do the opposite of the mallee at every point. Their fronds are broad, thin and held out flat, presenting the largest possible area to what little light gets through, and they can afford to be thin because there is no drought to survive. They have no lignotuber, because a gully this wet almost never burns.

Now compare the two. A thin flat leaf and a thick vertical leaf are opposite solutions and neither is better. Move the tree fern to the mallee and it dies within a week from water loss through all that thin surface. Move the mallee eucalypt into the gully and its small waxy vertical leaves collect a fraction of the light its neighbours get, and it is shaded out. Each is suited to the conditions it is actually in, and demonstrating that is what a habitat report is for.

In Lesson 11 every investigation had an independent variable you changed on purpose. A habitat survey does not, and that trips people up, so it is worth settling before you go outside.

In a survey you change nothing. You go to a place, observe what is already living there, and record it. There is no cup you moved closer to the lamp, so there is nothing to put on an x-axis and no "if … then …" hypothesis about a variable you manipulated. That does not make it unscientific; it makes it a different kind of investigation, and it is the kind ecologists spend most of their working lives doing, because you cannot run a fair test on a national park.

What replaces the hypothesis is a prediction, and it still has to be able to be wrong. "There will be plants and animals" cannot fail, so it is worthless. "Plants on the exposed northern side of the ridge will have smaller, thicker leaves than plants of the same species in the shaded gully" can fail, and you will know within an hour whether it did.

The other thing that replaces the fair-test rules is a sampling rule you decide before you start. Wander around photographing whatever catches your eye and you will find the biggest, brightest and strangest organisms and miss everything small and brown, so your results describe your attention rather than the habitat. Ecologists fix this with a quadrat, a square frame of a set size dropped at set intervals along a measured line, recording everything inside it whether it is interesting or not. Fixing that rule in advance is what makes a survey valid, in the same way that controlling variables is what makes a fair test valid.

A scientific report is not an essay about your day out. It is a fixed set of sections, in a fixed order, and the order exists so that another person can find any part of it without reading the whole thing. These are the same six steps as Lesson 11, written up rather than planned.

The line that causes the most trouble is the one between Results and Discussion, and it is worth being strict about. "The ridge wattles had leaves averaging 22 mm" is a Result: you measured it, and nobody can argue with it. "The ridge wattles had small leaves to reduce water loss" is a Discussion: the first half is measured, the second half is your explanation, and someone could reasonably disagree with it. Keeping them apart is what lets a reader accept your data while questioning your interpretation, which is exactly what scientists do to each other's work.

Method is the section students most often write too briefly. Apply the test in the table: could someone else repeat this? "We looked at plants along the track" fails, because the next person does not know where you started, how far apart your samples were, or what counted as a sample. "We placed a 50 cm quadrat every 10 m along a 60 m track, starting at the gate, and recorded every plant species rooted inside it" passes, and it is barely longer.