Species dependence (a life is a bundle of needs)

Ask why any creature lives where it does and the honest answer is never simply “because it can.” A species is not a free-floating thing that turns up wherever it likes. It is a bundle of requirements, a list it carries everywhere it goes. Something to eat, very often a particular something. Somewhere to shelter. The precise conditions under which it can breed. And, more often than we notice, other species it cannot do without: a pollinator, a fungus, a prey animal, a tree old enough to have rotted itself a hollow. To read an animal or a plant is to hold that list up against the ground in front of you and go down it, item by item, asking whether the place in fact supplies each one.

Where it supplies them all, the species is present, and for as long as that holds, secure. Where a single item is missing, the species is gone, however green and unbroken the rest of the country looks. This is the part that catches people out. A forest can be tall and healthy, loud with birdsong, and still be uninhabitable for one particular animal, because the one thing that animal cannot do without has quietly dropped off the list. A creature tracks not the average quality of a place but its scarcest necessity. The weakest item on the list is the one that decides.

The length of the list is a rough measure of risk. A specialist lives on a few narrow, hard-won things and buys a niche almost nothing else can reach — but a key cut to one lock has nowhere to go when the lock is changed. The koala’s whole existence rides on the eucalypt leaf and on trees standing close enough together that it can move between them without coming to ground. The glossy black-cockatoo runs a double dependence, she-oak seed and a large old hollow at once, and so turns up only where a woodland is still whole enough to supply both, a living audit of intact country. The wallum’s acid frogs depend on a chemistry rather than a food or a tree: sour blackwater down around pH 3.5, too acidic for most of their predators to follow them into — and the moment it sweetens, the drawbridge is lowered.

A generalist carries the opposite kind of list: short, cheap, and met almost anywhere. The brush-turkey asks for leaf litter, some shade and moisture, and ground it can rake, and thrives in the very broken, half-cleared country that is starving the specialists out. So do the cane toad, the fox, the cat, the mosquitofish. None of them leans on a particular tree or hollow or chemistry, which is why a simplified landscape is a generalist’s paradise.

So the reading cuts two ways. It explains most of the coast’s declines: the animals people most want to see are, again and again, the ones that gambled on the narrowest bundle and are now missing a piece of it. It explains the winners just as cleanly, the animals that asked little of a landscape we have spent two centuries simplifying. The same reading tells you why the koala and the cockatoo and the acid frog keep appearing on the lists of the declining, and why the brush-turkey and the cane toad do not.

It also tells you what to do. You rarely save a species by managing the species. You save it by keeping, or mending, the particular items its list requires, which nearly always means looking after the whole system it is woven into: the feed trees and the old hollows, the connections it needs to move between them, the strange chemistry it breeds in. The Richmond birdwing, one of the largest butterflies in the country and tied to a single native vine, was nearly lost with that vine as the lowland scrub was cleared. Nobody brought it back by breeding butterflies. People replanted the vine, garden by garden, until the insect could find its one food again. Read the bundle, find the missing item, put it back. On this coast, saving an animal almost always turns out to mean saving a place.