#ThreatenedSpecies of the Week: Bancrofts Symonanthus (Symonanthus bancroftii)

Bancrofts Symonanthus (Symonanthus bancroftii) is listed as Endangered under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act), and Critically Endangered under the WA Biodiversity Conservation Act 2016.
Symonanthus bancroftii. Photo credit: Florabase, DBCA.
Symonanthus bancroftii. Photo credit: Florabase, DBCA.

Symonanthus bancroftii is a low, many-stemmed herbaceous undershrub, which grows up to 25cm. It has stalkless, egg-shaped to narrow leaves which are hairy, somewhat warty and rolled over at the margins. These plants are dioecious (having the male and female reproductive organs in separate individuals), and its flowers are white in colour, small, hairy and streaked with violet inside. The fruit is a nearly globular capsule, 3-4mm long, 2.5-4mm wide, with 3-5 seeds.

Historically, the species has been found over a range of 80 km between Bruce Rock, Muntadgin, Ardath and Bendering, however, it is currently known only from the Ardath area – where just two plants have been located. The species, or the species habitat, may be found as far north as Carnamah, Three Springs and Mingenew.

Symonanthus bancroftii is found in areas of disturbed shallow granitic soil, which is prone to hard-setting. It is thought that the plant prefers a residual/colluvial soil derived from granitic rocks. Beyond this, the clues are conflicting. The plant may grow near granitic rock that outcrops as a sheet or dome, or it may grow close to drainage lines lower down in the landscape.

The surrounding vegetation at known sites is woodland consisting of Eucalyptus erythronema subsp. erythronema, Eucalyptus salmonophloia, Eucalyptus sheathiana and Eucalyptus salubris over dwarf scrub of Acacia orbifolia.

There are two species in the Symonanthus genus: Symonanthus aromaticus and Symonanthus bancroftii. Because of the limited collections, and just two known plants, little is known about the biology of Symonanthus bancroftii. However, as both plants occur in disturbed sites it is presumed to be a disturbance opportunist.

It is presumed that Symonanthus bancroftii is insect pollinated – as a varied range of insects, including native bees, have been observed on flowering plants. A further indicator that Symonanthus bancroftii is insect pollinated is the small tubular corolla of the flower.

Symonanthus bancroftii’s population has been reduced by approximately 80 per cent over the last three generations based on a decline in area and quality of habitat, and the population size is now estimated to be less than 50 mature individuals. The entire population is currently known from just two mature plants (one male and one female) that occur approximately 500 metres apart.

Threats to the population include senescence; accidental destruction during firebreak, rail and road maintenance; herbicide spraying; and weed invasion. If no further plants are found or few seeds are stored in the soil seed bank, lack of genetic diversity may also be a threat to the long-term conservation of the species.

 

Information Source: Government of Australia, Department of Environment and Energy and Department of Biodiversity, Conservation and Attractions, and Florabase.

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