approaches to native and alien species

37 FUTURE FLORA

Trans. Suffolk Nat. Soc. 38 (2002)

APPROACHES TO NATIVE AND ALIEN SPECIES

C. D. PRESTON

Most botanists are aware of the apparently increasing number of alien species naturalised in Britain. Whereas it was commonplace twenty years ago to hear vice-county recorders expressing the view that their real job was to record the native flora, and that this left little time to spare for aliens, this is now very much a minority opinion. Stace’s New Flora of the British Isles (1991) did much to stimulate this interest in alien plants. Stace reviewed the plants which are currently naturalised or occur as frequent casuals, his criteria for inclusion in the New Flora, and as a result brought to the attention of British botanists many taxa which had not hitherto been included in Floras. Taxa which were added as a result of this work are discussed in Professor Stace’s contribution to this conference. (Stace, 2002) My intention in this paper is not to review the changing alien flora as such, but to examine the attitudes of botanists and conservationists to these alien species. How rapidly did the early botanists come to recognise the alien component in our flora? How have their successors classified this heterogeneous group of plants? And is the alien component of our biodiversity regarded as a threat to the native plants?

Definitions Some of the controversy which surrounds the subject of native species arises from uncertainty about definitions, or the differing definitions of different authors. The following definitions are based on those of Macpherson et al. (1996):

A native plant is one which arrived in the study area without intervention by man, whether intentional or unintentional, having come from an area in which it is native or one which has arisen de novo in the study area.

An alien plant is one which was brought to the study area by man, intentionally or unintentionally, even if native to the source area or one which has come into the area without man’s intervention, but from an area in which it is alien.

Two important points follow from these definitions. Native and alien species differ in their means of arrival, not their time of arrival. Although many native species arrived thousands of years ago, it is still possible for native species to colonise Britain by natural means of dispersal such as birds, sea currents or the wind. A species which has been introduced by man is defined by its means of introduction as an alien. One of the most frequent questions – how long does an alien species have to be established before it becomes a native? – is therefore based on a misapprehension. A species remains an alien however long it survives.

The second point to make is that the term native and alien are descriptions of the means of arrival of species, not value judgements. When Trevor Dines, David Pearman and I assessed the status of plants to be included in the New Atlas of the British and Irish Flora we were sometimes accused of ‘demoting’


Suffolk Natural History, Vol. 38 38

species to alien status, ‘devaluing species’ or told that we should regard species as ‘innocent until proved guilty’. However, the terms native and alien don’t carry any implication of rank, value or innocence. Organisations or individuals may of course apply their own value judgements – some prefer natives to aliens, others find aliens more interesting – but the terms themselves are not value judgements.

Recognition of the concepts of native and alien by early botanists Historians, with remarkable unanimity, regard William Turner (c. 1510–1568) as ‘the first scientific student of zoology and botany’ in Britain and as ‘the true pioneer of natural history in England’ (Raven, 1947: 49, 127)¹ Turner’s early books (Turner, 1538, 1548) show that he was well aware of the difference between plants which belonged to the wild flora of Britain and those which were known only in gardens. This distinction is implicit in such names as ‘Wylde carot’ (Daucus carota), ‘Wylde tasyll’ (Dipsacus fullonum) and ‘Wylde pappy’ (Papaver rhoeas or an allied species) in Turner’s Libellus (1538), and in the entry for Bellis, translated by Rydén et al. (1999):

Bellis …. is the herb that we call a Dasy, though among my Northumbrian countrymen the only one to be called a Dasy is the one with a red flower cultivated in gardens. They call the wild kind a Banwort.

However, the depth of Turner’s understanding of the difference between the wild and cultivated flora is more apparent in his second book, The names of herbes (1548). Despite its name, this book is much less exclusively concerned with names than the Libellus. Turner drew upon his travels in both Britain and mainland Europe in entries such as these:

Asarum [Asarum europaeum] …. in duche Haselwortz …. groweth in Germany wylde under Hasell trees, wherupon it hath the name. It groweth in Englande onely in gardines that I wotte of.

Glycyrrhiza [Glycyrrhiza glabra] ….. groweth in the Rockes of Germany wythout any settynge or sowyng.

These entries do not necessarily demonstrate a knowledge of the concept of alien species, as we understand it. For that, it is necessary to recognise the possibility of the transfer of plants from the garden to the wild by deliberate planting or accidental escape. Some entries in Turner (1548) suggest that he might be aware of the possibility of deliberate planting in the wild, such as that for ‘hoppes’:

Lupus salictarius [Humulus lupulus] …. do growe by hedges and busshes both set and unset.

Despite such tantalising entries, I’ve found nothing in The names of herbes which demonstrates conclusively that Turner knew of any species which owed

¹Allen (2001) suggests that Turner may appear to be the first scientific botanist as his

works are the first to appear in print. Evidence of his predecessors’ interests may only

survive in manuscript, or may not survive at all.

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its existence in the wild solely to populations we now describe as alien. However, such plants were identified in the botanical literature before the end of the 17th Century. Thomas Johnson (1633: 983), in revising Gerarde’s Herball of 1597, included his record of Paeonia mascula:

The male Peionie groweth wilde upon a cony berry in Betsome, being in the parish of Southfleet in Kent, two miles from Graves-end….

but added

I have been told that our Author himselfe planted that Peionie there, and afterwards seemed to finde it there by accident: and I do beleeve it was so, because none before or since have ever seen or heard of it growing wild since in any part of this Kingdome.

In fairness to Gerarde, I can’t help wondering whether the plant might have been a short-lived introduction. David Allen (in litt. 2002) tells me that peonies are regarded as outstandingly intolerant of being transplanted, and this might be a further flaw in Johnson’s accusation. A less controversial description of deliberate introductions is provided by Sir Thomas Browne’s observations on Acorus calamus, in a letter written to Christopher Merrett in 1668:

‘This elegant plant groweth very plentifully …. by the bankes of Norwich river [R. Yare] …. it hath been transplanted and set on the sides of marish [marshy] pondes in severall places of the countrey, where it thrives ….’ (Keynes, 1964: 344).

In the same letter, Browne says that he has found Urtica Romana [U. pilulifera] ‘to grow wild at Golston by Yarmouth, & transplanted it to other places’.

An early description of an accidental introduction again concerns Paeonia mascula, perhaps because this is such a conspicuous species and so obviously a garden plant. It is one of the records contributed to How’s Phytologia (1650: 95-96) by Walter Stonehouse, a botanist living in Darfield, Yorkshire:

Poeonia mas vulgaris. Male Peiony: Found growing in Mr. Feilds Well close in Darfeild: Which though farre from any house, I beleeve it came first out of a Garden with some dung. Mr. Stonehouse.

We can therefore conclude that the concept of the wild and the cultivated flora was well-known by the mid 16th Century, and probably pre-dated scientific botany. Cases of both deliberate and accidental introductions of alien species were documented by the mid 17th Century.

The distinction becomes controversial: an 18th Century dispute In the 18th Century, the distinction between native and alien became the subject of controversy. ‘The science of historical ecology began in 1769–72 with a debate in the Royal Society on whether or not the sweet-chestnut was an exotic tree’ (Rackham, 1980: vii). The controversy began with a paper, read in 1769 and published in 1770, by the Hon. Daines Barrington, better known

Plate 1: Peony (Paeonia mascula (L.) Mill.) an early introduction from cultivation. Naturalised on Steep Holm island in the Bristol channel for many years. (p. 39) (from a hand-coloured copy of Gerarde’s Herball reproduced by permission of the Syndics of Cambridge University Library).



as one of Gilbert White’s correspondents in The Natural History of Selborne. Barrington (1770) was investigating ‘the prevailing notion’ that Castanea sativa was an indigenous tree. He started by laying down ‘some general rules, from which it may be decided, whether a tree is indigenous or not, in any country.’

‘1. They must grow in large masses, and cover considerable tracts of ground; nor must such woods end abruptly, by a sudden change to other trees, except the situation and strata become totally different.

2. If the tree grows kindly in copses, and shoots from the stool, it must for ever continue in such a wood, unless grubbed up with the greatest care; nor is it then easily extirpated.

3. The seed of such tree must ripen kindly …. and in the greatest profusion.

Lastly; Many places in every country must receive their appellation from indigenous trees which grow there…’

Barrington then examined Castanea sativa in the light of these rules, concluding that it did not occur ‘in any considerable masses’ in Britain, and that ‘with us the nuts by no means ripen kindly’. ‘All these circumstances seem to afford a strong inference, that the Spanish chesnut cannot be a native of Great Britain’. Sweet chestnut timber reported in old buildings had turned out, when investigated by Barrington, to be oak. Some, indeed, had been reported as chestnut ‘not because any one hath found it to be so upon examination, but because there are no cobwebs on such roofs’.

Barrington went on to discuss Pinus sylvestris (‘there seems to be little doubt, therefore, that the fir was formerly an indigenous tree in northern parts of England’), Ulmus (‘I cannot think that the elm, which we see every where, is indigenous .… the Wych (or broad-leaved) elm, however, is certainly of natural growth in this country’), Tilia (‘I cannot allow the lime to be indigenous’), Acer pseudoplatanus (‘certainly a foreign tree’), Buxus (also considered alien) and some species about which he was less certain.

Barrington’s views were assailed two years later in papers by Ducarel (1772), Hasted (1772) and Thorpe (1772). Ducarel argued that large tracts of Castanea sativa grew in Kent, with the trees ‘never planted by any human hand’ but ‘by the bold irregular hand of nature’ and that the tree also grows in the Forest of Dean, whence it is mentioned in medieval documents. He also cited some place names derived from the tree’s name, and ancient trees such as the Tortworth Chestnut. ‘All these united evidences strongly co-operate to prove it a native of this island’. Thorpe (1772) described the numerous chestnut woods in Kent which contained ‘both …. stubs and stools of a large size’ and reported that ‘the nuts of the chesnut tree ripen kindly, and in great quantity’. Despite these ripostes, Barrington (1772) did ‘not see any reason for altering the opinions which I have happened to form on this subject’.

The status of Castanea sativa in Britain was not resolved until the 20th Century, when evidence from the pollen record demonstrated that it was an ancient introduction. Although introduced, it had been in England for long

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enough to assume those characteristics of a native species which were highlighted by Ducarel, Hasted and Thorpe. The controversy demonstrates how easy it is to fail to recognise alien species once they have become established members of the British flora.

Classification of alien species in the 19th and 20th Centuries The most influential classification of introduced species in the 19th Century was that of H. C. Watson, initially set out in Cybele Britannica (1847–59) and presented in its final form in the compendium to that work (1870). Watson (1870) recognised five categories:

native: an aboriginal British species;

denizen: a suspected introduction, but now growing in semi-natural habitats without need for further human intervention;

colonist: a suspected introduction, growing on cultivated or disturbed ground and therefore dependant on continued human intervention;

alien: a known, more or less established introduction;

casual: a chance straggler.

The Tortworth Chestnut, as depicted by J. G. Strutt in 1824.



Watson’s denizens included species such as Matthiola incana and Myrrhis odorata; he was uncertain whether or not to classify Smyrnium olusatrum as a native or denizen. The colonists were characteristically arable weeds such as Agrostemma githago, Papaver rhoeas and Ranunculus arvensis. Watson’s classification provided a useful way of coping with a complex situation in which some species (the denizens) were thought to be introduced but grew in semi-natural habitats whereas others (the colonists) were not known from historical evidence to be introduced but were confined to artificial habitats. It was widely used in county Floras in the second half of the 19th and the first half of the 20th centuries.

The subtlety of Watson’s approach was lost in Clapham, Tutin & Warburg’s (1952) Flora of the British Isles, which classified species as either native or introduced (although allowing varying degrees of doubt about the assignment to each category). This simple classification was also followed by Stace (1991).

Classification adopted for the New Atlas In preparing the New Atlas of the British and Irish flora (Preston, Pearman & Dines, 2002), the editorial team were contracted to produce maps which distinguished native and alien species. In reviewing the existing classification, we became unhappy with the current classification of species into the two categories native and alien. In particular, we agreed with Webb (1985) that a number of species which appear to be ancient introductions were currently classified as natives. We felt that some of these difficulties might be overcome if the distinction between archaeophytes and neophytes which is used in some European countries was adopted in Britain. Archaeophytes are ancient introductions, usually defined as plants which became established before AD 1500. They include arable weeds such as Agrostemma githago and Ranunculus arvensis, ruderals (e.g. Chenopodium urbicum, Malva neglecta), plants formerly grown for food (e.g. Smyrnium olusatrum) and useful trees (e.g. Castanea sativa). Neophytes are later introductions. Although they include some weeds from southern Europe (e.g. Anisantha diandra), many neophytes come from other continents. These neophytes include coniferous trees from western North America (e.g. Pseudotsuga menziesii), garden plants from eastern Asia (e.g. Fallopia japonica) or the New World (e.g. Michaelmas Daisies, Aster spp.) and plants of garden origin (e.g. Leucanthemum × superbum).

Further details of this classification will appear in the New Atlas, and David Pearman and I are currently preparing a detailed account of archaeophytes in Britain in collaboration with A. R. Hall.

Alien species and plant conservation: the current orthodoxy Naturalised aliens are widely regarded as a threat to native plant communities and species. The current orthodoxy can be summed up in the following propositions:

• conservation efforts should be devoted to native species (and archaeophytes?);

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• native plants, in addition to their intrinsic worth, are significantly more

valuable to other wildlife than alien species;

• alien species have caused great damage to ecosytems in many parts of the

world;

• native plants should be encouraged in Britain in landscape plantings,

whereas alien plants require control;

• vigilance is needed to prevent the further spread of established or new

alien species.

When these propositions were put to the conference the overwhelming majority of the audience of 400 indicated their agreement with them; only about five of those present indicated dissent.

Alien species and plant conservation: opponents of the current orthodoxy Although the orthodox views outlined in the previous section are held by many botanists and conservationists, they are opposed by a minority of ‘heretics’. The ‘heretics’ approach the subject from a variety of directions, including a philosophical interest in the issues (e.g. Peretti, 1998; Kendle & Rose, 2000), an interest in urban ecology (e.g. Gilbert, 1994, 2001; Dickson, 1998), forestry (e.g. Brown, 1997) or the current relationships between people and plants (e.g. Mabey, 1996). Although many of the heretics would accept that there are cases where alien species threaten native plant communities, they would all challenge the widely held view summarised by Kendle & Rose (2000) as ‘native plant good, alien plant bad’. Some of their arguments are summarised below.

Problems of definition The definition of a native plant may vary from author to author – some definitions would classify plants which were introduced by man before the Neolithic as native, for example, and others as introduced. Peretti (1998) considers that the distinction is ‘arbitrary and unscientific’ and Kendle & Rose (2000) suggest that it relies on ‘a selective categorisation of which types of humans can legitimately act as modes of dispersal, rather than reflecting biological attributes’. Even if definitions are agreed, the evidence needed to apply them is often incomplete and there is therefore doubt about the status of particular taxa. We ‘are usually forced to rely on partial natural history records’ and this is one of the reasons that Peretti (1998) gives for considering that ‘the study of biological invasion does not rest on a rigorous scientific foundation’.

The alien problem is exaggerated Several authors have suggested that the problems caused by invasive aliens are exaggerated. Williamson (1998) concluded that in Britain the impact of aliens was not significantly different from that of natives. The plants which are controlled, or which Wildlife Trusts consider ought to be controlled, at nature reserves in northern England, for example, comprise 26 native species and 7 aliens. The native Pteridium aquilinum requires control in 12 reserves, more than any other species; the alien which is most often troublesome is



Rhododendron ponticum (7 reserves). Kendle & Rose (2000) cite other examples of invasive native species. However, as Williamson (1998) points out, the alien species, although no more undesirable than many natives, do represent an additional problem and therefore an additional cost.

Alien plants provide valuable habitats…. Ever since Southwood’s (1961) classic paper, it has been agreed that native species support more associated species than exotics. Kendle & Rose (2000) accept this conclusion for invertebrates associated with vascular plants, but point out that it need not necessarily hold for other interactions, and that it would be wrong to consider that the conservation value of alien species is negligible. A study of epiphytic bryophytes in Britain, for example, concluded that the alien tree Acer pseudoplatanus was ‘a moderately important habitat’, similar in the average number of species it supported per tetrad to the native A. campestre (Bates et al., 1997). There seems to be a need for a critical review of the published information on this topic, and for further carefully planned recording.

…. especially in urban habitats. Alien plants may be particularly valuable in urban habitats. Gilbert (1994, 2001) and Hart et al. (1997) have even commented on the beneficial influence of the reviled Fallopia japonica in certain urban habitats. On riversides in the Glasgow area its stands support a diverse array of vernal species normally associated with woodland. Buddleja davidii is a more popular urban alien.

Aliens may have valued aesthetic or historical associations Alien species may have been associated with an area for sufficiently long to contribute to a ‘sense of place’. Shelter belts of Pinus sylvestris are a characteristic feature of the East Anglian Breckland. Prunus cerasifera is one of the traditionally planted shrubs of Cambridge and Platanus × hispanica, London Plane, is a characteristic species of the streets of London (Kendle & Rose, 2000; Rackham, 1986). Brown (1997) draws attention to the contribution to the cultural landscape provided by ‘the grove of wind-torn sycamores sheltering a Pennine farmhouse’. Crocus vernus is protected in a meadow at Inkpen, Berkshire, and Paeonia mascula is cherished on Steep Holm, where it was first recorded in 1803.

Contradictions in conservation biology Peretti (1998) suggests that there is a contradiction between the emphasis given by conservation biologists to the importance of species migration and the consequent need for wildlife corridors, and the restriction of conservation action to native species. It is becoming difficult for plants to migrate naturally in the modern landscape, but those populations which happen to have been dispersed by man are regarded as alien and therefore do not merit conservation.

Arguments from ecological and evolutionary theory If ecosystems are stable communities in which the species are dependant on numerous, complex inter-relationships with other species, it is easy to understand how they might be disrupted by an invasive alien. However,

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ecologists have tended in recent years to view nature ‘as a chaotic, random, and structurally open system’ with species migration as a natural and normal component (Peretti, 1998). Peretti suggests that in some mixed assemblages, native and alien species ‘coexist with some degree of harmony’.

On a longer timescale, hybridisation of species which are brought into contact in periods of plant migration is believed to be one precursor to plant speciation. Such speciation in the future might result in the evolution of new species, which might then fill niches in a changing environment. Prevention of the spread of alien species, or elimination of alien species before establishment, might inhibit these processes.

Does ‘nativism’ represent a viable conservation strategy ? Both Peretti (1998) and Kendle & Rose (2000) appear to be arguing that the traditional attitude of conservationists does not represent a sustainable strategy in the long term. ‘People have been moving biota for thousands of years on five continents .... it becomes extremely difficult to identify the natural, native, or original conditions of an ecosystem’ (Peretti). ‘Classifications such as native and natural provide full scope for humans .… to act as forces of degradation but make no allowance for positive influence when we are acting in harmony with nature …. What is the future of native conservation if every restorative intervention is instantly degraded by the fact that people did it?’ (Kendle & Rose).

Political considerations ‘The purism of biological nativism has historically been associated with fascist and apartheid cultures and governments’ (Peretti, 1998). In particular, the use of native shrubs and trees in gardens became an ideological doctrine in Nazi Germany. Those unfamiliar with the Lingua Tertii Imperii, Victor Klemperer’s term for the debased language of Nazi Germany (see Klemperer, 2000), will read with astonishment of the ‘Blood-and-Soil-Rooted-Garden’ and of the garden designer Willy Lange’s view that in the formal garden, the Nordic race spiritually perished in the race-morass of the South (Wolschke-Bulmahn & Groening, 1992). Peretti (1998) considers that such associations represent ‘compelling reasons to challenge biological nativism’ and warns that ‘environmentalists must be careful not to reinforce a politically conservative nativist agenda’. (This advice would appear to place the politically conservative environmentalist in something of a dilemma.)

Brown (1997), despite reservations elsewhere in his paper about the definition of the term native, argues that there may ‘be considerable value in being able to be able to [sic] identify whether a particular species is native to a given site’ but regards the use of the term at the country scale as ‘a bizarrely nationalistic view of biogeography’ which ‘has neither ecological nor practical value’. In my view, this overlooks the practical value to the Flora writer of using a single word, ‘introduced’, to describe the status of a species at all sites in a country.

Kendle & Rose (2000) argue ‘that the “native plant good, alien plant bad” terminology, although not directed at people, nevertheless can have undesirable overtones for immigrant communities. A more positive social message …. is to celebrate the rich origins of those species that co-exist in cities … rather than condemning them because they “did not originate here” ’.



Do our views matter anyway? There is an element of fatalism amongst the heretical literature, best summed up by Barker (2000): ‘Nativism is meaningless in the context of geological timescales, and in view of the magnitude and inevitability of change and our powerlessness in the face of change. Changes will occur: ecologists study them.’

Acknowledgements I am grateful to Dr T. D. Dines and, in particular, to D. A. Pearman for allowing me to draw on some of our joint work for the New Atlas of the British and Irish Flora. P. H. Oswald drew my attention to Thomas Johnson’s treatment of Gerarde’s ‘Peionie’ record and Dr M. O. Hill introduced me to the paper by Peretti (1998), the stimulus for the second part of the lecture. D. E. Allen kindly commented on a draft of this paper. The illustrations of the Tortworth Chestnut from Strutt’s Sylva Britannica and of Paeonia mascula from a hand-coloured copy of Gerarde’s Herball are reproduced by permission of the Syndics of Cambridge University Library.

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C. D. Preston CEH Monks Wood, Abbots Ripton, Huntingdon, Cambs. PE28 2LS

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