Over the last 20 years, the Knysna warbler has experienced a widespread and severe population crash. The total number of individuals of this species is now estimated at only around 2,500 (2). In the Western Cape, the Knysna warbler population has halved since the 1980s (5), and this species has disappeared from around Durban. The most likely cause of this decline is habitat loss through the clearing of coastal forests and the burning of fire-breaks at the edges of forests (2).
The Knysna warbler is becoming increasingly rare in native woodlands and now more commonly occupies stands of non-native plants and suburban woodland (5) (6). It is possible that in native woodlands the suppression of natural fires, which are a frequent natural disturbance and help regenerate native habitats, is reducing the availability of nesting sites for this species, causing it to nest elsewhere (6).
Additional threats to the Knysna warbler include inbreeding depression in the tiny, fragmented Eastern Cape population. The removal of brambles, which are the subject of an eradication campaign, may also remove nesting sites for this species (2).