Genetic Invasion In Marginal Areas

Genetic invasion can be said to have taken place when hybridization leads to the substitution of the genes in a

Fig. 4.15 The recently evolved Senecio hybrid species S. cambrensis (2n — 60) flanked by its parents, S. vulgaris (2n — 40) and S. squalidus (2n — 20). (Photo Professor R.J. Abbott.)

S. aethnensis X 5. chrysanthemifolius (2« = 20) (hi = 20)

Fig. 4.14 Recent hybrid speciation in Senecio. Origin of the (a) diploid hybrid species S. squalidus, (b) recombinant tetraploid hybrid species S. eboracensis, and (c) allohexaploid hybrid species S. cambrensis. (Reproduced with permission from Abbott et al., 2005.)

native species by those from an alien taxon. Given time, international scale as it is confined to the oceanic insidious hybridization by continuous backcrossing can regions of Europe from Spain to the Netherlands, with fundamentally alter the nature of an authochthonous over half the world population occurring in the British

(ancient indigenous) species and can therefore be Isles. A recent survey carried out by the wild plant regarded as a genetic invasion. In common with non- conservation charity Plantlife International (2004)

genetic invasions, it is marginal areas that are again most reported that one in six of the broad-leaved forest susceptible. Approximately 25 years ago smooth cord populations of H. non-scripta in the British Isles now grass (Spartina alterniflora) was introduced into Fran- show evidence of hybridization with the Spanish cisco Bay where it has since hybridized with the native bluebell (H. hispanica) and that in boundary areas such

Californian cord grass (S. foliosa). A molecular study of as hedgerows and road verges and urban areas the the parental chloroplast DNA (cpDNA) genotypes of proportion is even higher. The future viability of the the parental species has shown that hybridization pro- native bluebell is therefore at risk as a result of ceeds in both directions, assuming maternal inheritance hybridization, in addition to the dangers of habitat loss of cpDNA. The native species produces little pollen and illegal collection (Fig. 4.16).

with low viability whereas the hybrids produce abundant and virile pollen. Thus, it is anticipated that the spread

. o/-, i ill 4.6.1 Invasion and climatic warming of hybrids to other S. foliosa marshes could be an even greater threat to the native species than introductions of Invasion is much discussed in plant ecology in relation to alien S. alterniflora (Anttila et al., 2000). the possible consequences of climatic warming. The use

The common bluebell (Hyacinthoides non-scripta) of the term invasion is perhaps unscientific as it carries can be considered to occupy a marginal habitat on an an embedded negative-value judgement which should be

Fig. 4.16 Bluebells (Hyacinthoides non-scripta) in a wood near Oxford (England). This scene, which is typical of woodlands throughout the British Isles in spring, is likely to alter. Plantlife International (2004) has reported that one in six of the broad-leaved forest populations of H. non-scripta in the British Isles now shows evidence of hybridization with the Spanish bluebell (H. hispanica). The proportion of hybrids is even higher in hedgerows and road verges (see text).

Fig. 4.16 Bluebells (Hyacinthoides non-scripta) in a wood near Oxford (England). This scene, which is typical of woodlands throughout the British Isles in spring, is likely to alter. Plantlife International (2004) has reported that one in six of the broad-leaved forest populations of H. non-scripta in the British Isles now shows evidence of hybridization with the Spanish bluebell (H. hispanica). The proportion of hybrids is even higher in hedgerows and road verges (see text).

superfluous in the hopefully politically neutral world of plant migration. The question of where natural migration with changing climate ends and invasion begins is not easily answered. If invasion is used to describe the movement of exotic species then migration could describe species that may have been present in the area in question under a past climatic regime. It is probably more realistic to use a quantitative rather than a qualitative judgement in separating invasion from migration. The cases that are most usually described as invasions are those where large sections of the landscape are taken over by one or more species that were not there previously. Migration is possibly better reserved to describe more gradual changes. However, differentiation between these two scenarios will probably always remain elusive. Dramatic migrations which can be described as invasions are most often seen in areas where competition is minimal and where levels of resources fluctuate. In disturbed marginal sites or areas of open ground there can be rapid colonization by species that are new to the area, particularly if it is combined with some sudden and fortuitous supply of nutrients. It is fortunate given the extensive cultivation of exotic plants in many countries over many centuries that such situations are not globally overwhelming.

In many countries there is increasing concern about the ability of certain alien species to invade and repress or replace the indigenous flora. Exotic species that have the ability to reproduce and spread outside cultivation when an opportunity or space presents itself are usually described as invasive or alien components of the vegetation.

In New Zealand the rapid spread of introduced heather (Calluna vulgaris; Fig. 4.17) has been possible as it regenerates more rapidly after fire than the native upland scrub species (Wardle, 1991). In the British Isles and Ireland the number of alien species (or their hybrids with native species) that are able to survive outside

Fig. 4.17 Invasion of Scottish heather (Calluna vulgaris) in New Zealand. The heather has invaded a frost-flat at the northern edge of the Tongariro National Park. The trees in the distance are various species of Nothofagus, with those at the lower level being a mixture of red and silver beech (N. rubra, N. menziesii). There are some isolated stands of N. solandri at higher altitudes up to 1200 m. (Photo Dr Simon Fowler, Landcare Research, Lincoln, New Zealand.)

Fig. 4.17 Invasion of Scottish heather (Calluna vulgaris) in New Zealand. The heather has invaded a frost-flat at the northern edge of the Tongariro National Park. The trees in the distance are various species of Nothofagus, with those at the lower level being a mixture of red and silver beech (N. rubra, N. menziesii). There are some isolated stands of N. solandri at higher altitudes up to 1200 m. (Photo Dr Simon Fowler, Landcare Research, Lincoln, New Zealand.)

cultivation number 1326, which is a high figure when compared with a native flora which consists of only 1486 species (Preston et al., 2002). Most of these alien species inhabit disturbed or waste ground on the fringes of cultivation. Aquatic habitats due to their open nature are also highly susceptible to invasion.

Despite this presence of a large number of introduced species it is remarkable that in Europe in general there has not been a substantial alteration in the species composition of the major native plant communities. Nevertheless, in the Scottish Highlands changes in landuse over the past century have facilitated the spread of bracken (Pteridium aquilinum), a native species, and also rhododendron (Rhododendron ponticum), which is commonly considered as an introduced invasive plant species notwithstanding the fact that it was present in the Tertiary flora and eliminated by the Pleistocene glaciations.

For a long time the origins of the present 'invasive' population of R. ponticum was not known. A molecular study of the naturalized material, chloroplast DNA (cpDNA) and nuclear ribosomal DNA (rDNA) from 260 naturalized accessions of R. ponticum throughout the British Isles found that 89% of these accessions possessed a cpDNA haplotype that occurred in native material of R. ponticum derived almost entirely from Spain, while 10% of accessions had a haplotype unique to Portuguese material. These results therefore indicated an Iberian origin for a substantial sample of British material which could possibly permit the assertion that the present invasion is just a re-establishment of the ancient pre-Pleistocene European stock (Milne & Abbott, 2000).

The term invasion is often used in describing the changes that are taking place in a range of habitats from undergrazed prairies to upland and alpine pastures. In the case of alpine pastures, the advance of forests that is now taking place in much of Europe from the mountains of Scandinavia to the Alps is more in the nature of re-establishment rather than an invasion. Given time, however, migration can take place into any community. The above invasions or migrations are highly visible as they occur in open landscapes such as alpine meadows. Slow migrations are insidious and can take place so gradually that they may escape notice unless careful long-term records are kept and handed down from one generation to another.

There are already examples in Europe where species are moving from cultivated parks and gardens into the native plant communities due to longer growing seasons and a reduction in the risk of periodic exposure to intermittent very cold periods. One of the warmest areas in central Europe is southern Switzerland with a mean annual temperature of 12 °C and a generous mean annual precipitation of 2000 mm. Current climatic warming trends have enabled many exotic evergreen species introduced to this region in the eighteenth century, such as the southern magnolia of eastern America (Magnolia grandiflora), and the Nepal camphor tree (Cinnamomum glanduliferum), to become established in native plant communities (Walther, 2002).

In southern Switzerland, Quercus, Tilia and Fraxinus species dominate the present native forest, together with sweet chestnut (Castanea sativa) which was a Roman introduction. Over the past four decades the length of the growing season in the region has increased on average from 290 ±13 to 350 ±8 days and, what is probably more significant, the number of frost days has decreased from approximately 75 days per year to fewer than 10 in mild years. Comparisons of previous vegetation surveys between 1960 to 1975 with more recent surveys in 1994 and 1998 have also shown a striking increase in the presence of oceanic species, with holly (Hedera helix) climbing into the upper tree layer together with a more vigorous growth of ivy (Ilex aquifolium). Exotic broad-leaved evergreen tree species that have invaded from cultivation now include cherry laurel (Prunus laurocerasus) and the camphor tree (Cinnamomum glanduliferum). The overall change in the forest species composition has been described as laur-ophylization. This term has been suggested (Walther, 1999) as it portrays the spreading of evergreen broad-leaved (laurophyllous) species into deciduous forest, which represents a biome shift from deciduous to evergreen broad-leaved forest. Thus the margin between these two biomes is migrating and the ancient Tertiary forests of central Europe which were eliminated from the region by the Pleistocene glaciations may possibly be restored in coming decades.

In contrast to the above insidious invasions or reestablishment of past vegetation communities, there are regions of the world where the native plant species have been almost extinguished by invading species from another hemisphere. In New Zealand, introduced

European species have largely transformed the vegetation of the coastal plains. Even the sand dune plant cover has been drastically altered (Fig. 4.18), with the native dominant graminoid, a sedge, pingao (Desmoschoenus spiralis), being replaced almost totally by the alien marram grass (Ammophila arenaria). It is probable that the geographical isolation of New Zealand has left its flora vulnerable to artificially introduced plants. It is also somewhat surprising that it is possible to raise the altitudinal level of the treeline through the introduction of exotic northern hemisphere conifers (Wardle & Coleman, 1992). Fortunately for conservation, this ability of invaders to outperform the native species in their own habitat and persist without human assistance is not universal, and individual colonies of so-called aliens in disturbed ground remain at present the commonest situation due to their inability to reproduce and persist in a foreign environment.

4.6.2 Climatic warming, disturbance and invasion

Detailed examination of the distribution of invasive species, as described in an analysis of the changes in the British flora in the interval between two extensive mapping surveys (1987 and 2004), has revealed disturbance and habitat change as major factors influencing the distribution of invasive species (Braithwaite et al., 2006). One of the most outstanding changes has been the spread of halophytic vegetation along road verges due to the large quantities of salt that are used for road de-icing. Notable examples include the grass-leaved orache (Atriplex littoralis) in eastern England and the lesser sea spurrey (Spergularia marina) in Scotland. These cases stand out as in the past these species have been restricted to the more southern parts of the British Isles and here climatic warming is probably an important factor in their advance northwards. The spread of wall barley (Hor-deum murinum) which has increased by 20% (Braithwaite et al., 2006) mainly in built-up areas in the south may also be due to climatic warming.

A coastal species that has been extinct in the Channel Islands (last recorded in the Scilly Isles in 1936) and is now very rare in Ireland, occurring only in two sites in County Wexford (Preston et al., 2002), is the cottonweed (Otanthus maritimus). Given that coastal species migrate readily and this species is still found along the west coast of France and the Iberian Peninsula it might be hoped that climatic warming could aid the restoration of this species to the British flora (Fig. 4.19).

Mapping does not always record the true extent of the reproductive capacity of a species. Bracken (Pteridium aquilinum; Fig. 4.20) occurs in many plant communities (Marrs & Watt, 2006). It is limited by frost and waterlogging, and although the latter is increasing it is likely that the elevation of the frost line will allow bracken to colonize higher altitudes on mountain slopes. Changing land management in upland areas is failing to keep the spread of this species under control. Thus merely mapping in 10 km squares for presence or absence does not give true indication of the ability of this species to invade pastures and impinge on plant communities with a high conservation interest.

4.6.3 Theories on habitat liability to invasion

A general theory of invasibility in relation to fluctuating resources in plant communities has suggested particular characteristics that make certain environments susceptible to invasion (Davis et al., 2000). The theory proposes the following predictions in relation to the probability of invasions:

(1) strong fluctuations in resource supply

(2) coincidence of increased abundance of invader propagules with a greater availability of resources

(3) increased availability of resources due to disturbance and other effects which reduce resource capture by the resident vegetation, e.g. disease, and pest outbreaks

(4) a lengthy interval between an increase in the supply of resources and the eventual capture or recapture of the resources by the resident population.

The above-mentioned study (Davis et al., 2000) also points out that there are no observed relationships between either diversity or productivity and invasi-bility. This is a resource-based theory rather than an assessment of Darwinian fitness as measured by reproductive success. There is nevertheless a connection in that greater growth will usually lead to greater reproductive success. Fluctuating resources are a feature of marginal habitats and therefore likely to have a significant effect particularly in pioneer terrestrial communities or in aquatic habitats where variable environments are a regular phenomenon.

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