Gardens are a significant habitat for wildlife in the modern landscape and provide valuable ecosystem services. They are also the source of most invasive alien plants. The value of planting native species for garden wildlife is uncertain and still an area of active research. Conservation of genetic and physical resources can be effective only when the underlying ecological and historical processes are taken into account. Conservation of crop cultivars is an international activity. Conservation of ornamentals is less well developed internationally but a thriving activity in Britain.
Many proponents of ecological restoration and wildlife gardening emphasize the importance of native flora and fauna. The underlying rationale is that indigenous organisms give us a sense of place. However, alien plants can also become a local feature. Wormwood (Artemisia absinthium) originates from the Mediterranean region, but is now a characteristic plant of the Black Country in the English Midlands. Fritillary (Fritillaria meleagris), from central and southern Europe, is a valued feature of meadows in the Thames valley. Oxford ragwort (Senecio squalidus), from southern Europe, is a colourful weed of the British transport network. Examples could be multiplied. On a more local scale, many gardeners like to save their own seed, taking pride in their own race of this or that plant. Gardeners like to preserve their heritage.
Although such sentiments can be a powerful driver in selecting species for wildlife gardens, they are not in themselves the basis of a scientific approach to gardening. The Botanical Society of the British Isles (BSBI) is an arbiter of what is native or alien; the Society defines native and alien plants as follows.
On this basis, inland occurrences of salt-tolerant plants by motorways are marked on BSBI maps as alien, even though their dissemination from native coastal sources was quite unintentional.
Aliens have been arriving in Britain since 4000 BCE when Neolithic cultivators began to till the land. The archaeological record shows that few species arrived during the first 3000 years. It was not until the Iron Age, after 800 BCE, that many weeds such as corncockle (Agrostemma githago), cornflower (Centaurea cyanus), petty spurge (Euphorbia peplus) and common poppy (Papaver rhoeas) appeared. In the twentieth century, arable fields became progressively less weedy, and tended to lose rather than acquire weed species. Fiddleneck (Amsinckia micrantha), from western North America, is almost the only new arrival whose habitat is mainly arable.
European botanists divide alien plants into archaeophytes, which arrived before the discovery of America in about 1500, and neophytes, which arrived later. From 1800 onwards, the main source of new species in the wild has been gardens. Of the 20 vascular plants listed by the website DAISIE (Delivering Alien Invasive Species Inventories for Europe) as the worst in Europe, only three were not introduced for ornament. Two, including the highly allergenic ragweed (Ambrosia artemisiifolia), arrived as contaminants; one, common cord grass (Spartina anglica), arose by hybridization following the arrival of its American parent smooth cordgrass (Spartina alterniflora), probably also as a contaminant. This proportion is similar to that among the 15 seriously harmful plants in Britain (Table 21.1). Only three seriously harmful plants, grass weeds of cereal crops, were not introduced for ornament. Of the 350 less harmful but widely established British aliens, about 70% escaped from cultivation. About 30% were introduced by accident. The accidental arrivals have, with three exceptions, colonized habitats created by humans, such as gardens, arable fields, waste places and waysides. None is a threat to native vegetation.
Table 21.1 Numbers of vascular plant species in British gardens and in the wild, compared with totals for Europe
| Category | Number |
| World | 350,000 |
| Europe | 14,000 |
| Native | 11,000 |
| Alien not naturalized | 1200 |
| Alien naturalized | 1800 |
| British garden flora (including cultivars) | 75,000 |
| Species | 22,000 |
| Cultivars, subspecies, etc. | 53,000 |
| British wild flora | 4800 |
| Native | 1400 |
| Alien | 3400 |
| Introduced before 1500 (archaeophytes) | 160 |
| Introduced after 1500 (neophytes) | 3240 |
| Alien not naturalized | 2160 |
| Alien established in at least one county | 1240 |
| Alien established in 25% of counties | 362 |
| Accidentally arrived, including arable weeds | 112 |
| Escape from cultivation (food, medicine or wood) | 55 |
| Escape from cultivation (ornamental) | 195 |
| Harmful to agriculture, horticulture or environment | 75 |
| Seriously harmful | 15 |
Data on the size of the British wild flora are based on Stace, C.A. (2010) New Flora of the British Isles, 3rd edn, Cambridge University Press, Cambridge. Data on the garden flora are based on RHS Plant Finder 2013.
The biology of alien organisms is a vibrant branch of ecology with an enormous literature. In some cases, newly arrived plants and animals may spread very rapidly. Sometimes the expansion is followed by a rapid decrease. When Canadian waterweed (Elodea canadensis) first arrived in Britain in the 1840s, it choked waterways with its superabundant growth. After a few years it decreased in most sites, so that by the twentieth and twenty-first centuries it was a harmless component of the aquatic flora, valued as an aerator in aquaria. Floating pennywort (Hydrocotyle ranunculoides) was introduced from North America for ornament and escaped in Britain in about 1990. This may well show the same pattern of boom and bust; in the English Midlands, populations have been observed to crash in as little as 4 years.
The causes of such crashes are often not known, but are generally thought to be natural enemies. Crashes are quite alarming, even if sometimes beneficial. Useful garden plants can suffer the same fate. At the time of writing busy lizzie (Impatiens walleriana) has suffered a major setback from downy mildew. In the nineteenth century the Irish Potato Famine and the Phylloxera epidemic of vines were both caused by natural enemies that were themselves invasive aliens. The twentieth century saw widespread destruction of elms by an invasive alien fungus.
Sudden major crashes are likely to be the consequence of a single pest or disease. Natural enemies can also build up gradually over time, so that an alien species loses its competitive advantage. As natural enemies build up, the species should become a richer host for wildlife. In a highly influential paper, Richard Southwood proposed the hypothesis that ‘the number of insect species associated with a tree is a reflection of the cumulative abundance of that tree in the particular country throughout recent geological history [i.e. in the 12,000 years since the end of the last ice age]. This means that the dominant native trees will have most insect species, and recently introduced ones fewest.’ He tested his hypothesis against data for British trees. At the time when he wrote, the trees with the largest number of fossil records were willow, oak, birch and hazel. The first three also had the highest number of associated insects, so there was a significant linear regression between the number of insects and the number of fossil records. After taking out the regression, Southwood noted four trees with remarkably few associated insects – hazel, ash, holly and yew.
Like almost all attractively simple hypotheses in ecology, the Southwood hypothesis has borne up only moderately well to intensive testing. In a recent paper in the magazine British Wildlife, the hypothesis was tested against a much broader range of services to wildlife. These included mycorrhizal fungi, wood-decay fungi, diversity of foliage invertebrates, biomass of foliage invertebrates, leaf litter quality, blossom for pollen and nectar, fruits and seeds, and epiphyte communities. Sycamore (Acer pseudoplatanus) is a tree originally introduced from the mountains of west and central Europe. It has a rich epiphyte flora and a high biomass of foliar invertebrates. Although it does not support a high diversity of foliar invertebrates, it has many other beneficial features. Moreover, it now behaves like a native, seeding itself freely in established woodland. By chance it did not naturally cross the English Channel and is therefore alien in Britain. But viewed in the wider perspective of Western Europe it is a native.
Stories of alien organisms taking over the planet and making life intolerable for the human race are pure fiction. However, from the point of view of many other organisms, the colonization of the planet by humans has had equally dire effects. These effects are most extreme on oceanic islands such as Hawaii, New Zealand and St Helena. Many of us have seen pictures of mice eating albatross chicks alive on Gough Island. At the other extreme, most gardeners in the northern hemisphere are firmly located on the great land masses of Eurasia and North America, or, in the case of Britain, on a group of islands that were separated from the European continent about 6500 BCE. Continents and nearby islands are much less susceptible to invasion. Indeed, they are the source of almost all the really aggressive plants and animals that have invaded oceanic islands.
Gardeners hardly need to be reminded of the detrimental effect of alien animals. In Britain, grey squirrels, rabbits and brown rats are often troublesome. These mammals are well established, and although they may be controlled and sometimes eliminated on offshore islands, there is no prospect of eradicating them from the mainland. The aquatic mammals muskrat and coypu have been successfully eradicated from England. Attempts, possibly successful, have been made to eradicate American bullfrog (Lithobates catesbeianus), which is a vector of the deadly chytrid fungus Batrachochytrium dendrobatidis. Invertebrate animals are much harder to eradicate. Colorado beetle (Leptinotarsa decemlineata) has been eliminated in Britain, but this may be due as much to the British winter as to human intervention. Recent outbreaks of gypsy moth (Lymantria dispar) and oak processionary moth (Thaumetopoea processionea), which defoliate trees, will probably result in long-term establishment despite strenuous efforts to eradicate them. The first is thought to have been introduced by accident as a stowaway and the second by import of young oaks from mainland Europe to British nurseries. Following these and other pest outbreaks, government and the horticultural industry have become more aware of the dangers from introduced pests.
Alien plants may also become invasive. The most costly invasive plant in Britain is Japanese knotweed (Fallopia japonica), which can have a large effect on the price of urban property. Costs of eradication from urban gardens often exceed 5000 GBP, excluding contingent building costs, which may be greater. The campaigning charity Plantlife has been urging the governments of Scotland, England and Wales to ban Japanese knotweed and other invasive plants from sale. The procedure for doing this consists of first adding the name of an invasive plant to an approved schedule, and then persuading the government to apply the ban. The first such ban came into effect in April 2014. The five banned species are floating pennywort (Hydrocotyle ranunculoides), floating water primrose (Ludwigia hexapetala), New Zealand pigmyweed (Crassula helmsii), parrot's-feather (Myriophyllum aquaticum) and water fern (Azolla filiculoides). With the exception of water primrose, these plants are currently well established in Britain. The ban on sale of the other four will have little or no direct environmental effect. Water fern has been established in the wild since about 1900, having escaped from botanic gardens. Although it sometimes covers the whole surface of a water body, it has the habit of suddenly disappearing after attack by the weevil Stenopelmus rufinasus.
It is no coincidence that the first five species banned from sale are non-European water plants. Water plants are readily transported by anglers, children and boats and are very difficult to eliminate. If they had been European plants they would have been present from ancient times. Plantlife gives a list of 21 further species that it would like to see banned from sale. This list, surprisingly, includes water lettuce (Pistia stratiotes), which is far too frost-tender to survive out of doors in northern Europe. The Plantlife list does not include trees, but does include the hybrid shrub Rhododendron superponticum, generally known in Britain simply as rhododendron. Recent studies have shown this to be a hybrid swarm between R. ponticum and two or more American species including R. catawbiense and R. maximum. R. superponticum has all the character of a true invasive, being an ecosystem engineer that transforms moorland and open woodland into an impenetrable evergreen thicket.
The Plantlife list consists entirely of plants with some horticultural or aquacultural merit. Plantlife has also teamed up with the RHS to produce a leaflet ‘Gardening without harmful invasive plants’, which contains an extensive list of plants that are either not likely to escape into the wild or are native British species. Of course, there are plenty of other harmless garden plants, as well as many native plants that are invasive in a garden context.
A notable environmental effect of horticulture is through imported pests and diseases. New Zealand flatworm (Arthurdendyus triangulatus) and several other species of flatworm have become established in Britain, almost certainly imported with ornamental plants from the southern hemisphere. Citrus longhorn beetle (Anoplophora chinensis) and Asian longhorn beetle (Anoplophora glabripennis) are not established in Britain, but are notifiable pests that have been transported with ornamental trees from Asia to other parts of the world. The scale of UK trade with continental Europe is so large that almost any pest or disease (see Chapter 17) that is common in northern Europe will become established in Britain. The outlook for the future is not reassuring. We can expect a high rate of arrival of new pests and diseases in the coming years, for various reasons, and some of these (but by no means all) are likely to be the result of the international trade in plants and plant products.
Almost all gardeners take pleasure in some aspects of wildlife, but gardens are not nature reserves. Curiously enough, they are often richer and more diverse than nature reserves. A peat bog, for example, is a species-poor natural habitat. Its flora and fauna are specialized and fascinating but not diverse. Many bog plants such as sphagnum moss are hard to cultivate.
The most obvious wild animals in many gardens are those that fly: birds, bees, moths, hoverflies and butterflies. Most of these come in from the surrounding countryside or from nearby urban greenspace. The individual garden, especially if it is a small one, is only a part of their habitat. Jennifer Owen studied her urban garden in Leicester from 1972 to 2001, identifying 2673 species of plants and animals from an area of 741 m2. Parasitic wasps (553 species) and beetles (442 species) were the most numerous groups. There were 23 species of butterflies, of which four were observed to breed in the garden. Likewise, she caught 282 species of macro-moths, of which 50 were found breeding. Some breeding moths ate a wide range of plants, the most extreme being angle shades (Phlogophora meticulosa), which was found on 54 plant species. Poplar hawk-moth (Laothoe populi) ate just poplar and willow, which are in the same family. The snout (Hypena proboscidalis) had just one food plant, nettle (Urtica dioica).
Owen addressed the question of whether native plants were more likely to be used as food plants than aliens. Her results suggest that this is not the case. Indeed, buddleia (Buddleja davidii), a native of China, was the plant fed on by the largest number of moth species. The next most widely used plant was shrubby cinquefoil (Potentilla fruticosa). This is a very rare British native, and prior to her study its lepidopteran fauna had not been reported at all. Gardens provide a range of food resources for invertebrates that may not be available in the wild.
Because mobile garden animals come from far and wide, their diversity and abundance are determined by the wider environment. Owen observed a general decline in numbers of butterflies, macro-moths, hoverflies, social bees and wasps, and ladybirds over the 30 years of observation. She tentatively attributed this to loss of habitat in and near Leicester, as the urban area increased and agriculture intensified in the wider countryside. Solitary wasps and beetles other than ladybirds, on the other hand, became more abundant over the 30 years. She did not offer an explanation for the difference, but it is possible that solitary wasps and beetles are on average less mobile, so that for them the immediate neighbourhood of her garden was more important than the wider environment.
Owen's study is unparalleled in its length and depth but lacks breadth. Her garden was to some extent managed for wildlife and cannot be taken as typical. Indeed, there is no such thing as the typical garden. In 2000, a more wide-ranging study of urban gardens was initiated. This was BUGS (Biodiversity in Urban Gardens in Sheffield). Ken Thompson's semi-popular account No Nettles Required is the best introduction for the general reader. The BUGS gardens ranged in size from 32 m2 to 940 m2. In 61 gardens, invertebrate animals (mainly insects) were caught in three pitfall traps, and in 23 gardens flying insects were caught in one Malaise trap (a tent-like structure with a wide bottom, narrowing upwards to a collecting vessel at the top). For animals likely to be taken by these methods, it emerged that trees have a major positive effect, that big gardens are no richer than small gardens, and that richness did not increase away from the city centre.
These results should not be taken to mean that big gardens do not overall contain more species. Big gardens generally have a wider range of trees, garden plants and habitats. What the results do emphasize is that animals do not recognize garden boundaries. A small garden may be part of a street mosaic that overall is just as diverse as the range of habitats in a larger garden.
Pitfall traps and Malaise traps are not good for catching butterflies and moths, so the BUGS results tell us little about the Lepidoptera. The butterflies found in gardens are wide-ranging habitat-generalists making use of abundant nectar resources. The moth Blair's shoulder-knot (Lithophane leautieri) is a specialist feeder on cypress, so suburbs are its main habitat. Gardens are also a significant habitat for juniper pug (Eupithecia pusillata). No doubt most garden moths are also habitat-generalists, in the sense that their food plants are widely distributed across the urban and rural landscape.
Ecosystems provide us with many benefits. A report by the Millennium Ecosystem Assessment (2005) emphasized the importance of such ecosystem services and drew attention to the contribution of relatively unmodified wild nature in producing them. Gardens are by definition highly modified and not at all wild. They do, however, provide a number of ecosystem services: food, recreation, ornament and other social benefits (see Chapter 22). They are also good for pollinators and the percolation of water. Trees and shrubs provide cooling and shelter from the wind.
The benefits of urban trees are especially great in countries with hotter summers than Great Britain. A cost-benefit analysis in the United States found that the monetary benefits of urban trees outweighed costs, mainly because trees enhanced property values. Benefits from shade, shelter and improved percolation of water are large, and apply equally to trees in gardens. Shade trees planted close to buildings can reduce power demand for air conditioning in hot cities by 20–40%. Net carbon sequestration by urban trees in the United States is reckoned at 19 million tonnes of carbon per year, but will fall as the trees mature and cease to grow or are cut down. The effect of urban trees in absorbing pollutants, on the other hand, is small because, at realistic densities, trees remove only about 1% of air pollutants.
Pollination is a well-known ecosystem service. Pollinating insects have declined on farmland because they have lost nesting sites and food resources and because they are sometimes killed by insecticides. Gardens, however, support much higher densities of pollinators than the countryside, so that an orchard next to a flower garden is sure to benefit. Whether urban gardens can benefit the wider countryside depends on the foraging range of the pollinators. Mobile butterflies range widely, but without returning to a nest. Bumblebees can forage over 1.5 km. The honeybees of Sheffield regularly fly 6 km in August to reach a really good resource, heather (Calluna vulgaris) on moorland. Earlier in the year (which is when pollinators are needed for orchards and clover), the city itself has more to offer. In May the bees only fly about 1 km. It seems likely, therefore, that countryside within about 2 km of gardens may benefit from enhanced pollination, but countryside further away will probably receive little of economic value.
Until the end of the twentieth century, plant collecting for gardens was a significant threat to plants in the wild. James Edward Smith, who founded the Linnean Society of London, wrote in 1790 that the lady's-slipper orchid (Cypripedium calceolus) ‘has not only been admired and cherished by the scientific botanist, but it has among gardeners always been sold at the highest price of any British vegetable’. It was ruthlessly persecuted by plant hunters, so that by the 1970s only one colony remained, watched over by volunteers day and night. By the 1980s only one plant remained, so seeds were sent to the Royal Botanic Gardens, Kew, and propagated. Several thousand of the resulting seedlings were planted out in the wild as part of an official Species Recovery Programme. One of the reintroduced plants first flowered in 2000, and by 2012 there were 80 flowering plants at one locality, Gait Barrows in Lancashire.
In the nineteenth century, ferns were especially persecuted. Killarney fern (Trichomanes speciosum) was exterminated from many localities in Ireland and Britain. Curiously enough it did not need a recovery programme as its prothalli (sporelings) can persist indefinitely in many sites that are currently unsuitable for the full-grown fern.
From these and other sorry tales, private gardeners and many in the horticultural trade have learnt their lesson. In the United Kingdom it is illegal to dig up a plant without the landowner's permission. Stronger statutory protection is given to many wild plants, so that even the landowner may not dig them up. The UK Joint Nature Conservation Committee (JNCC; a statutory body) supplies a Conservation Designations Spreadsheet of formidable size (70 Mb). In this we learn that the lady's-slipper orchid is protected under the Wildlife and Countryside Act 1981, the Bern Convention, the EU Habitats Directive, the EC CITES Annex B, the UK Biodiversity Action Plan, and the Conservation of Habitats and Species Regulations 2010. This bewildering list is not aimed at gardeners, who would surely not attempt to source the plant from the wild. Only one supplier is listed in the 2013 RHS Plant Finder, and none for Killarney fern.
Most importers of plants now have a responsible attitude. A good starting point for advice on this complex matter is the RHS advisory note Importing and Exporting Plants.
On a more positive note, the role of botanic gardens and, to a lesser extent, private gardens in ex situ conservation is substantial. The most famous example of a species that has survived mainly in gardens is the ginkgo (Ginkgo biloba). Small wild populations persist in the Dalou mountains of southwest China, but the species is secure for the future in gardens. Botanic gardens hold not only living plant collections, but worldwide maintain 235 seed banks. One of the largest is the Kew Millennium Seed Bank, in Sussex. This stores seeds from all native British plant species, and from more than 24,000 species found elsewhere in the world.
Reintroducing plants to the wild from gardens is not easy. If a population died out because the species was persecuted by plant collectors, then conditions for successful re-establishment may well still exist. Far more often, however, wild populations die out because the habitat has deteriorated. If the plant is reintroduced, the same process will repeat itself and the plant will die out again. A review in the journal Biological Conservation found that two-thirds of reintroductions failed rather rapidly and that very few of them led to successful establishment of a subsequent generation from seed. If only a few transplants are introduced, the population may lack sufficient genetic variation to be viable. Moreover, conservationists making a reintroduction often lack sufficient understanding either of the biology of their plant or of the environment at their site. Short-term success may not be a good guide to longer-term prospects. Plant extinction, as opposed to plant spread, is usually a slow process. A few individuals may survive, either in the seed bank or as established plants, long after the site is no longer suitable for population growth.
Conservation of cultivars for gardens forms a part of the wider activity of conserving genetic resources for the future. Genetic diversity is vital for combating disease and adapting to new climates and soils. Where a genetically uniform cultivar such as the Cavendish banana, for example, has been planted on a wide scale, the risk is enormous. By 2013, Cavendish banana plantations were being devastated in many parts of the globe by virulent new strains of the fungal pathogen Fusarium oxysporum. In the face of such threats, great efforts have been made to conserve local races of existing crop plants and their wild relatives. International bodies such as the Global Crop Diversity Trust and Biodiversity International (founded 1974 as the International Board for Plant Genetic Resources) promote gene banks and on-farm conservation. In gene banks, seeds and tissue are frozen and stored.
Crops grown in gardeners' vegetable plots are therefore relatively well served by international conservation activities. Fruit varieties are less well served internationally, but in Britain the National Fruit Collection at Brogdale, Kent, is managed by Reading University and includes over 3500 named cultivars of apple, pear, plum, cherry, bush fruit, vine and cobnut. Many are commercially available. The RHS Plant Finder 2013 lists 1070 varieties of apple (Malus domestica). When it comes to roses, the same source has 3700 records, including species and cultivars.
High numbers of cultivars present a real problem for conservation, particularly as plant breeders are constantly bringing new varieties to market. Old cultivars are not necessarily better than modern ones. Indeed, they are often worse, and they may become worse still over time as their genomes are supplemented by viruses and other diseases. Viruses can be eliminated, but at a cost, by tissue culture (see Chapter 11). Another cause of deterioration is the presence of new pests and diseases in the environment outside the plant. For example, hollyhock rust (Puccinia malvacearum) first appeared in Europe in 1869 and hollyhocks have never been the same since.
Given that diversity per se may be a threat as well as a blessing, we need criteria by which to select for conservation worthy cultivars that are otherwise likely to disappear from commerce or become extinct. Four such criteria are the following.
In Britain the process of selecting cultivars for conservation is highly developed through the charity Plant Heritage. Plant Heritage was established in 1978 as the National Council for the Conservation of Plants and Gardens (NCCPG) and in 2009 was rebranded as Plant Heritage. Its Threatened Plants Project selects cultivars for conservation on the basis of rarity in cultivation, horticultural merit, heritage value and usefulness to people. A major plank of Plant Heritage's activity is to promote living collections of properly sourced and well-named plants, called National Plant Collections (NPCs). NPCs include common as well are rare cultivars and species, and have been likened to living libraries. In 2013 there were about 630 of them, maintained by botanic gardens, RHS gardens, horticultural trusts, commercial nurseries and individual volunteers.
Whereas the conservation of crop plants is a major international activity, the conservation of ornamental plants is much less so. In this matter, British gardeners, who have a strong sense of heritage, have led the way.
Native plants are not necessarily better for wildlife. Most invasive alien plants were originally introduced for ornament. Any garden with trees or large shrubs offers a large resource of herbage in which animal life is little affected by human activity on the ground. Garden flowers are a substantial resource for pollinating insects. Conservation of wild plants and cultivars in gardens is most effective when underpinned by sound scientific principles.
Written for the third edition by Mark Hill, independent consultant (to replace parts of Chapters 18 and 19 in the second edition).