The Biogeography of Drosera Stricticaulis (Droseraceae) in Australia: a Disjunct ‘Island’ Refugee?

Abstract

SummaryAlthough previously regarded as a Western Australian endemic from east of Perth, Drosera stricticaulis (Diels) O.H. Sarg, it is now also known from several populations in South Australia on the lower Eyre Peninsula and one in the southern Flinders Ranges, approximately 2000 km east of its Western Australian range. The Western Australian and South Australian populations were found to be morphologically and cytologically identical, making D. stricticaulis genuinely disjunct between Western Australia and South Australia and not a trans-Nullarbor pair of closely related species. BIOLINK predictive distribution mapping using both total collection data and just data for Western Australian populations found climatic matches for the Eyre Peninsula and Flinders Ranges, as well as parts of Kangaroo Island and the upper Murrayland of South Australia. However, those sites where D. stricticaulis is known to occur are mainly erosional or plains landforms with sodosolic brown or red duplex or red loam soils over Precambrian rocks, whereas Kangaroo Island and the upper Murrayland (where the species is absent) are depositional landforms with calcareous, alkaline and often saline soils. These patterns suggest that the species is relictually vicariant in areas that are climatically similar (moderate winter rainfall, hot dry summers) on Eocene-derived infertile sodosolic (duplex) soils with clay subsoils over Precambrian rocks. These areas in the main have also not been subject to past extensive marine incursion or inundation events, or to extensive drying during times of lower sea level.