Reassessment of the invasion history of two species of Cakile (Brassicaceae) in Australia.

Abstract

Abstract:In this paper we revisit the invasion history of two species of Cakile in Australia. Cakile edentula subsp. edentula arrived in the mid 19 th Century and spread into coastal strandline habitat from the southeast towards the west and to the north; Cakile maritima arrived in the late 19 th Century and has replaced Cakile edentula over much of the range. While Cakile edentula is morphologically quite uniform, the great variation within Cakile maritima has confused field ecologists. Using herbarium records we update previous accounts of the spread of the species and report on field surveys that determined their current geographic overlap in Tasmania and in northern New South Wales/southern Queensland. We examine regional morphological variation within Cakile maritima using the national herbaria collections and variation within new population samples. We support previous interpretations that Cakile maritima has been introduced on more than one occasion from morphologically distinct races, resulting in regional variation within Australia and high variability within populations in the south-east. Western Australian populations appear distinct and probably did not initiate those in the east; we consider that eastern populations are likely to be a mix of Cakile maritima subsp. maritima from the Mediterranean and Cakile maritima subsp. integrifolia from Atlantic Europe. Although introgression from Cakile edentula into Cakile maritima cannot be discounted from our results, it is not required to explain the levels of variation in the latter species observed in Australia. Cakile maritima continues to spread southwards in Tasmania and northwards in NSW; in Queenland, a recent occurrence has proliferated in Moreton Bay, spreading slowly to the north but not appreciably southwards.

Genetic structure along the strandline: Unravelling invasion history in a one‐dimensional system

“…appeared in Australia first and spread widely. It was first collected in Victoria, eastern Australia, in 1863 and was soon found in the neighbouring states; it was found in Queensland in 1922 (Rodman, 1986;Cousens et al, 2013). It appears only to have reached the eastern-most extremities of Western Australia.…”

“…Two-dimensional geographical arenas, often accompanied by sporadic geographical leaps and back-filling (Cousens et al, 2008) and with poor historical information, mean that invasion histories are very difficult to reconstruct with confidence. Inferences made from spatial surveys after the invasion has progressed for many years (e.g.…”

“…Morphometric analysis of fruit shape (Cousens et al, 2013) has since indicated that Western Australian populations of C. maritima are distinct and more than one introduction into Australia is likely. Rodman (1976) also noted that glucosinolate types in Australian C. maritima suggest both Atlantic coast and Mediterranean origins, while he attributed numerous herbarium specimens to the Baltic on the basis of morphology.…”

Hybridization can facilitate species invasions, even without enhancing local adaptation

“…Moreover, we show in a case study using a simplified version of the model that the parameter values required for the demographic mechanism to be as effective as genetic changes in fitness are fulfilled by the sequential invaders Cakile edentula and Cakile maritima. Our hypothesis therefore provides a possible explanation for the rapid replacement of C. edentula over a large part of its invasive range by C. maritima in the west of North America, New Zealand, and Australia (12)(13)(14)(15).…”

Replacement of Cakile edentula with Cakile maritima in New South Wales and on Lord Howe Island.

“…Rodman 1986;Heyligers 2007). Cousens et al (2013) reviewed the history of invasion of Cakile in Australia and found that Cakile maritima was introduced into Australia from morphologically distinct races. This explains the regional variation within Australia and the high variability in the appearance of this taxon in the southeast, without the need for introgression of Cakile edentula into Cakile maritima.…”