S

R. caucasicum x R. sochadzeae - outliers

__if Orientation

Fig. 4.12 Distribution of populations of the hybrid Rhododendron X sochadzeae and its parent species on Tiryal Dag in eastern Turkey. (Reproduced with permission from Milne et al, 2003.)

4.5.3 Spartina anglica - common cord grass

One of the most striking examples in Europe of hybridization leading to the creation of a new species with an ecological amplitude that is greater than either parent is that which arose between European cord grass (Spartina maritima) and the American species (S. alterniflora) producing the hybrid S. townsendii which was first recorded in Southampton Water in 1870. Chromosome doubling subsequently restored sexual fertility to the autotetraploid, which is now named S. anglica, and this facilitated a massive expansion into many estuaries and salt marshes (Fig. 4.13). Similar evolutionary scenarios have also taken place elsewhere. In the Basque region of south-west France a hybrid has arisen from the same parents that created S. anglica but is named differently as S. X neyrautii (Salmon et al., 2005).

The vigorous growth of S. anglica and its physiological tolerance of salt flooding, anoxia, and the presence ofhigh sulphide contents in estuarine soils has allowed this species to inhabit regions of the foreshore where previously no flowering plants other than the sea grasses (Zostera spp.) had ever survived. The perceived benefits of this species for coastal protection and land reclamation caused it to be introduced widely in Europe, North and South America, South Africa, Australia, and New Zealand. The increase in sediment stabilization caused by S. anglica growth raises the level of the foreshore and leads to the development of new salt marsh areas. The high productivity of S. anglica also results in a large amount of energy and organic matter entering the ecosystem and provides a food source for grazers as well as being an opportune species for coastal protection against rising sea levels. Despite these benefits S. anglica is often regarded as an undesirable invasive species in that it leads to the loss of feeding areas for wildfowl and waders in estuary margins and can threaten economic interests such as commercial oyster fisheries. Unfortunately for coastal protection, S. anglica has been nominated as among the 100 of the 'world's worst' invaders (Table 4.1).

Despite the many remarkable properties that S. anglica possesses in relation to its ability to colonize coastal mudflats there are limits to both its reproductive capacity and its long-term viability. Flowering takes place relatively late in the growing season (July to September) and imposes a northern phenological limit to the spread of the fertile autotetraploid S. anglica in Europe; little seed is set in the northern regions of the British Isles. The current range of S. anglica is from 48° N to 57.5° N in Europe, from 21° N to 41° N in China and from 35° S to 46° S in Australia and New Zealand (Gray & Raybould in Patten, 1997). However, it remains to be seen whether or not climatic warming will encourage the northward spread of this species. Even within the geographical limits of successful establishment for S. anglica fertile seed production is variable both temporally and spatially. Seed does not set in most years, resulting in periods of spread by clonal expansion. Even when sexual reproduction is taking place, less than 5% of the spikelets are likely to produce viable seed. Seeds that fail to germinate in

Fig. 4.13 Common cord-grass (Spartina anglica) colonizing mudflats at Bosham (Sussex). Bosham is one of the possible places for the origin of the Legend of the Waves and King Canute the Great (1017—1035). The King, having tired of the flattery of his courtiers, one of whom suggested that the King could even command the obedience of the sea, was provoked to prove him wrong by practical demonstration. It was either here or at Southampton or near his palace at Westminster, that he showed (contrary to the popular version of this legend) that even a king's powers have limits. Where Canute failed, the common salt-grass has succeeded in trapping mud and silt, raising the level of the shore and pushing back the waves.

Fig. 4.13 Common cord-grass (Spartina anglica) colonizing mudflats at Bosham (Sussex). Bosham is one of the possible places for the origin of the Legend of the Waves and King Canute the Great (1017—1035). The King, having tired of the flattery of his courtiers, one of whom suggested that the King could even command the obedience of the sea, was provoked to prove him wrong by practical demonstration. It was either here or at Southampton or near his palace at Westminster, that he showed (contrary to the popular version of this legend) that even a king's powers have limits. Where Canute failed, the common salt-grass has succeeded in trapping mud and silt, raising the level of the shore and pushing back the waves.

their first season do not remain viable and consequently there is no soil seed bank (Gray et al., 1991). Furthermore, as salt marshes mature there is a gradual decline in vigour with dieback being a common phenomenon, particularly in shores with fine sediments. This phenomenon is similar to the dieback that is also found in the common reed (Phragmites australis; see Chapter 8) and is possibly induced by prolonged anaerobiosis and toxic sulphide levels. Like many other hybrids Spartina anglica, despite its initial vigour as a colonizer, depends for its ecological success on having access to marginal areas.

4.5.4 Senecio squalidus - the Oxford ragwort

The cosmopolitan genus Senecio is rich in species that inhabit marginal areas. The potential for adaptation in this genus to peripheral areas is remarkably high as the genus as a whole crosses ecological boundaries with the same ease with which it has spread geographically. Habitat specific species of Senecio can be found from the New Zealand Alps (S. bellidioides), to the bogs of Chile (S. smithii), as well as on the arid volcanic ash of Mt Etna in Sicily where different species occupy distinct altitudinal zones. It is from this latter location that the Oxford ragwort (S. squalidus) has evolved over the past 300 years and provided one of the best-elucidated botanical evolutionary histories in this highly adaptable genus (Harris, 2002; James & Abbott, 2005).

It has long been thought that the British populations of S. squalidus came from the escape of introductions made directly to the Oxford Botanic Garden sometime after the garden opened in 1621 and before the end of the seventeenth century. However, a recent review of

Table 4.1. A selection of species commonly described as invasive and frequently found in marginal habitats

Species

Common name and notes

Acacia longifolia

Azolla filiculoides

Cabomba caroliniana

Crassula helmsii

Eichhornia crassipes

Elodea canadensis

Fallopia japonica

Rhododendron ponticum

The Sydney golden wattle: initially planted to stabilize sand dune systems in Portugal, now an extensive and escalating invasive species. Also a widespread invader of water systems in South Africa.

Water fern: an American species introduced as a plant for garden ponds from which it has escaped into watercourses. The plant has become very abundant in recent years possibly due to warmer winters.

Carolina fanwort: an aggressive invader of nutrient-rich freshwaters. The species is a native of eastern North America and South America but it is now widespread in North and South America. As a fully submerged aquatic plant that outcompetes native freshwater plants it can pose a risk of entanglement and drowning to swimmers.

Australian swamp stonecrop: C. helmsii originates from Australia and New Zealand and was introduced into Europe as an aquarium plant. Main problems are at present in the British Isles. Vigorous vegetative growth through most of the year, without any period of dieback in winter, blocks ponds and drainage ditches with dense mats that outcompete the native flora and impoverish the ecosystem.

Water hyacinth: this floating aquatic plant, a native of Brazil, has been for over 100 years a worldwide invasive species which jams rivers and lakes with uncounted thousands of tons of floating plant matter. An acre of water hyacinths can weigh up to 200 tons.

Canadian waterweed or common elodea: a submerged aquatic species in which only the small white flowers emerge at the water surface. Much used as an aerating species in aquaria and accidentally introduced into Ireland from North America. The first report of its occurrence anywhere in Europe was in Co. Down in 1836. It has subsequently spread across Ireland and Great Britain and is now common in lowland lakes, ponds, canals, and slow rivers, and became a troublesome aquatic weed in Britain despite the fact that only female plants occur in the British Isles. It is now considered a noxious weed in parts of Europe, Australia, Africa, Asia, and New Zealand.

Japanese knotweed: a native of Japan, Korea, Taiwan and China, but it appears that all plants grown in European gardens derive from a single Dutch import from Japan made in the 1820s. Almost all plants in the British Isles are female octoploids and almost all seed set is from hybrids with other species of Fallopia, e.g. F. baldschuanica. Originally a garden escape and first found in the wild in 1886. Its invasive spread has been from garden rubbish containing rhizome fragments.

Rhododendron: in parts of western Britain and Ireland it is an invasive species in oakwoods, spreading vegetatively by suckering from the tips of its procumbent branches. First introduced to the British Isles in 1761 from south-west Spain. Now extensively hybridized with subsequent introductions of the cold hardy species R. catawbiense and R. maximum, thus increasing its range particularly, in colder, montane regions (Milne & Abbott, 2000).

Species

Common name and notes

Salvinia molesta

Spartina anglica

Tamarix spp.

Giant salvinia: an aggressive and competitive species and possibly one of the world's worst aquatic weeds. Excessive growth results in complete coverage of water surface which degrades natural habitats. Mats of floating plants prevent atmospheric oxygen from entering the water and decaying remains consume dissolved oxygen necessary for fish and other aquatic life.

Common cord grass: a recent hybrid salt marsh grass first recorded in 1870 (see text) has been used worldwide for coastal protection. It has a high capacity for invasion of mudflats and can displace native estuarine species of Zostera and Salicornia. It can also lead to the loss of feeding habitat for wildfowl and waders. However, with rising sea levels its role as a coastline protector needs to be reassessed and set against the negatively perceived invasive properties of the species.

Tamarisk or salt cedars: introduced from Asia. Invaders of riparian areas throughout the American West where they accumulate salt in their tissues, which is later released into the soil, making it unsuitable for many native species. Because of their tolerance to alkaline and saline conditions, tamarisks are valuable as shade and ornamental plants. However, in many regions they have become a serious problem because they have formed extensive stands and cause great water losses as they take in much more water than the native plants, causing severe desiccation in desert wetland oases.

herbarium and literature records has concluded that material morphologically similar to British S. squalidus (2n = 20), a short-lived, herbaceous perennial plant, was collected from Mt Etna, Sicily, and first grown in the British Isles, not as commonly thought in the Botanic Garden at Oxford, but at the Duchess of Beaufort's garden at Badminton and then later transferred to the Oxford Botanic Garden sometime before 1719 (Harris, 2002).

Subsequent taxonomic studies have been unable to recognize in Sicily the taxon S. squalidus as found in Britain, and it has been suggested that British S. squalidus is a product of hybridization between the Sicilian diploid species S. aethnensis and S. chrysanthemifolius (Crisp, 1972 in James & Abbott, 2005). Both Sicilian species occur on Mt Etna, with S. chrysanthemifolius present up to approximately 1000 m altitude and S. aethnensis from approximately 1600 to 2600 m altitude with a series of hybrid swarms occurring between these two species at 1300 m ± 300 m, where certain plants can be found that bear a close morphological resemblance to British S. squalidus. It has therefore been suggested that the material introduced to the British Isles in the seventeenth century was collected most likely from these hybrid populations.

Following a century of cultivation in the Oxford Botanic Garden, stabilized derivatives of this hybrid material escaped at the end of the eighteenth century and subsequently spread to many parts of the British Isles.

Further molecular studies of S. squalidus in Britain have shown that all the individuals examined were of mixed ancestry and that the taxon is indeed a hybrid derivative of S. aethnensis and S. chrysanthemifolius (James & Abbott, 2005) and that this species must have originated within a period of 90 years, i.e. between the time hybrid material was first grown in the British Isles in the early eighteenth century and when the species was first reported growing on walls in Oxford away from the Botanic Garden in 1794 (Sibthorp, 1794).

Subsequent to the escape of S. squalidus from Oxford, and in its continuing spread northwards through England reaching central Scotland in the mid 1950s, the process of hybridization has continued. Crossing with the native tetraploid species, Senecio vulgaris L. (2n = 40), led to the recent origin of three recognized, sexually reproducing, hybrid taxa: an introgressant form of S. vulgaris, S. vulgaris var. hibernicus, a recombinant tetraploid hybrid species, S. eboracensis and the allohexaploid S. cambrensis (Figs. 4.14-4.15) (James & Abbott, 2005). All these species are inhabitants of marginal areas and are at risk from extinction. Despite extensive searches, the species Senecio cambrensis has not been found growing at any ofits previously recorded sites in Edinburgh or at other potential sites in the area since 1993. The lineage was present in Edinburgh from at least 1974 and therefore survived in the wild for a minimum of 19 years. The species remains well established in parts of North Wales (Abbott & Forbes, 2002).

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