Sixteen guides, four sections, one question

The treeline tracks growing-season warmth rather than winter minimum, which is why it sits at wildly different altitudes at the same latitude.

A single hillside can cross three vegetation zones, and the transitions are sharper than any map suggests.

The intertidal is the only habitat boundary that is re-drawn every six hours, and the zonation on the rock records it exactly.

Between desert and savannah the change is continuous, and where you draw it depends entirely on the rainfall threshold you pick.

Savannah persists because burning and grazing prevent canopy closure; remove both and it becomes woodland within decades.

Kelp, coral and beaver each construct the physical structure other species live in, which makes them habitat rather than inhabitants.

Remove the predator, the urchins increase, the kelp is grazed off and the forest becomes barren rock — a documented sequence, not a metaphor.

Cold-water coral builds reef structure hundreds of metres down, with no algae and no sunlight involved.

Beaver ponds change the water table, the sediment and the species list of a whole reach of stream.

Aridity is defined by precipitation against evaporation, which is why the largest desert on the planet is Antarctica and why sand is beside the point.

The boreal forest wraps the northern hemisphere and holds a very small human population and a very large amount of carbon.

The definition is two years below zero, which means the map of it is drawn from boreholes rather than from the surface.

Same growth form, different rainfall regime, different fire return — and almost all of it now ploughed.

A biome boundary exists on a map because somebody walked a transect and recorded what was growing at each point.

Quadrat, auger, hand lens, tape, GPS and notebook — the instruments that produce most of what we know about where habitats stop.

A line samples a line; boundaries that are patchy at a hundred metres are recorded as sharp or missed entirely.