It is not rare. The assumption comes from poor searching. People look at the forest floor. They see leaf litter. They see nothing else. The Nikau Bonnet grows on nikau. Not on soil. Not on wood. On the dead fronds of the only palm native to New Zealand. Find a nikau palm. Find the collapsed skirt of old leaves at its base. Kneel. Part the fronds. The mushrooms are tiny. Pale brown. Translucent at the margins. The caps are bell-shaped, grooved, no wider than a fingernail. They grow in troops. Twenty. Fifty. A hundred. Each one on a fallen frond. Each one feeding on palm tissue. Diet is saprotrophic and specialised. Mycena nikauensis decomposes the lignin and cellulose in nikau fronds. No other palm grows in New Zealand. No other plant produces the same chemistry. The fungus does not eat wood. It does not eat broadleaf litter. It follows the palm. When the frond falls, the fungus arrives within weeks. The mycelium spreads through the fibrous tissue. The frond softens. It darkens. Within months, the mushrooms appear. They fruit in autumn and winter. By spring, the frond is gone. Collapsed into humus. The fungus moves to the next frond. Habitat is the floor of nikau-dominated forest and forest margins. Lowland broadleaf forest. Coastal gullies. The steep slopes of the Coromandel and the Waitakere Ranges. Any place with mature nikau and enough moisture to keep the fronds damp. The fungus does not tolerate drying. A single week without rain stops fruiting. The mycelium survives. The mushrooms wait. When rain returns, they emerge within days. The relationship with nikau is absolute. Where the palm stops, the fungus stops. The northern limit of nikau is near Te Paki. The southern limit is Banks Peninsula and the West Coast south to Hokitika. The fungus matches both boundaries. Threats are indirect. Removal of nikau for garden planting removes substrate. Garden nikau are usually solitary. They drop few fronds. The fungus needs density. It needs many palms dropping many leaves. Forest clearance for pasture removes both palm and fungus. Urban expansion removes nikau from entire suburbs. Climate change may shift the southern limit of nikau. The palm is frost-sensitive. Warmer winters allow it to spread south. The fungus cannot spread faster than its host. The palm may arrive first. The fungus may follow decades later. The lag period is a gap. The gap is vulnerability. Range across New Zealand parallels the natural distribution of nikau. North Island: widespread from Northland to Wellington, including the Coromandel, Bay of Plenty, Taranaki, Manawatu, Wairarapa, and the Kapiti Coast. South Island: limited to Nelson, Marlborough Sounds, Banks Peninsula, and the West Coast as far south as Hokitika. Absent from Canterbury plains, Otago, Southland, Stewart Island, and the Chatham Islands. The southern South Island is too cold. The fungus does not occur where nikau does not. The two species share a range. They share a fate.