In defence of aliens

Why I prefer to grow exotic plants rather than just native species.

In an article in this month’s Gardener’s World magazine, Monty Don argues that gardeners should grow more native plants, rather than non-native ‘alien species’. This is not the first time he has suggested that we should stick to native British planting. However much I like and respect Monty, however, I won’t be following his advice, and here’s why.

I studied geology at University. The UK has some of the most varied and interesting rocks in the world: you only have to look at the multitude of swirling colours on a geological map to see that rocks from virtually every geological period outcrop somewhere in the British Isles, from the ancient rocks of the Lewisian Gneiss complex in northwest Scotland (an incredible 3 billion years old), to interglacial deposits laid down in Norfolk half a million years ago containing bones of sabre toothed cats, mammoths, and the tools of the early humans who lived alongside them. The modern science of geology was invented here, as canal construction in the 18^th century started to cut through and reveal this multitude of strata. If you are interested in rocks, the UK is one of the best places to find them. We can even do dinosaurs.

Unfortunately, the same cannot be said for plants, and our native flora is disappointingly depleted. Of course our wild flowers, ferns, mosses and lichens are worthy of study and protection and are often beautiful in their own way. I even grow a few in my garden. But our native flora simply cannot get my pulse racing in the same way as plants from, say, South Africa, Australia or the rainforests of Southeast Asia in terms of excitement, diversity and sheer awe-inspiring beauty.

Sabah is a Malaysian state, slightly smaller in size than Scotland, on the island Borneo. There are more species of orchids, just one of over 400 families of flowering plants, native to Sabah than the total number of all native flowering plant species in the UK.

The British native tree flora is particularly sparse at a little over 30 species. The number of tree species in Borneo is still unknown, but thought to be around one hundred times greater and the total number of native flowering plant species growing in Borneo is around 12,000. Admittedly not many plants from Borneo will grow outdoors in the UK, but the comparison holds true in other biodiversity hotspots such as the mountains of Southwest China, with a climate more similar to our own, and from where many familiar garden plants originate.

The reason for this difference is that the forests of Borneo (or China) have existed continuously for millions of years without any major climatic changes, providing the ideal conditions for plant growth and speciation: constant and abundant warmth, light and moisture and a complexity of habitats. 50 million years ago, southeast England enjoyed very similar conditions to tropical Asia, and relatives of the plants now still growing there also grew near London. The London Clay, the geological layer underlying much of London (and with which London gardeners will be very familiar) contains the fossils of these plants, including mangroves dominated by Nipa palms, and tropical forest trees and climbers and even the remains of dipterocarp trees, the great signature trees of the southeast Asian rainforest.

So why do we have so few native plant species today? Our previously abundant flora was wiped out following the onset of the Ice Age over 2 million years ago which covered the country with thick ice sheets in the north and barren tundra in the south. When the ice retreated around 12,000 years ago, sea levels were still lower than they are today and Great Britain was a peninsular of Europe, enabling colonisation of the country by plants from continental Europe, until the land bridge was cut off around 4,000 years later by rising sea levels.

Our entire native flora therefore is the result of colonisation of the country by European plants during a post-glacial window of only a few thousand years (and at the same time as it was re-colonised by people). In a sense, all of our native plants are “invasive species”.

It is difficult to imagine the excitement among botanists and gardeners when new and exotic plants unlike anything seen in Britain or Europe started arriving from overseas, first as a trickle in the 17^th century and then a flood by the mid 18^th century, thanks to global trade and exploration. Perhaps the paucity of our own native flora created a latent thirst for more exciting plants, and heightened the excitement of obtaining new, exotic seeds, and discovering how to germinate them and keep them alive in our climate. By the end of the 18^th century, no other country had so many plants in cultivation and today the number of non-native plant species grown in British parks and gardens is estimated at a staggering 14,000 species (and over 50,000 varieties and cultivars).

Of course, this golden age of new plant introductions had a dark side: it was European colonisation of the world that enabled this 300 year period of plant-hunting, and the botanical resources of these countries were looted and plundered by the British and other colonising  nations.

Sir Joseph Banks, the great botanist and explorer who was the first to bring many iconic Australian and New Zealand plants back to Europe during his 1768-1771 Endeavour voyage, went on to turn the Royal pleasure gardens at Kew into a scientifically-run botanic garden, which he described as a “great botanical exchange house for the empire”. Plants with economic benefit were identified, brought back to Kew for their cultivation requirements to be determined, and then transported to be grown on an industrial scale in colonies where the climatic conditions suited their cultivation, and then traded across the globe resulting in huge profits. The botanical riches that ultimately found their way into our gardens were the by-product of this industry, highly unethical by today’s standards.

However, Kew’s mission has long since evolved, and it is now part of a global network of botanic gardens committed to identifying, documenting and conserving the world’s plants. In the same way, we gardeners can also change our attitude to growing exotic plants, ensuring that they are sourced and grown ethically and responsibly, and by growing them we can even contribute to plant conservation efforts. The principles that botanical resources belong to the country of origin, that plants and seeds may only be collected with the permission of that country, and that any benefits arising from them be shared with the country of origin, are now enshrined in international law. By embracing these principles, modern-day plant hunters, nurseries and gardeners can make growing exotics a force for good. A good example is the Wollemi pine, an extremely beautiful but rare tree discovered in Australia in 1994. As part of its conservation programme, research into commercial propagation allowed the plant to be made available to the public through selected nurseries, both to raise funds for conservation work but also to ensure it exists in cultivation around the world, so that gardeners can contribute directly to its conservation.

Perhaps the biggest charge levelled against non-native plants is their perceived negative impact on the environment; specifically that exotic species grown in gardens support fewer invertebrates (and therefore, indirectly, fewer vertebrates), and that they escape into the wild where they become invasive, driving native species to extinction. The Royal Horticultural Society has for a number of years been running an experiment to investigate the relative value of native and non-native plants to invertebrates.  The findings so far are that the non-native plants (including plants such as  Alstroemeria, Callistemon, Fuchsia and Osteospermum) supported a good number of plant-dwelling invertebrates, but around 20% fewer than UK natives. However, they also found that native plants are not always the first choice for pollinating insects visiting gardens, and that non-native plants prolong the flowering season. The important thing for attracting pollinators was offering flowering plants for as long a season as possible, not the origin of the plants, and so exotics may have the upper hand here. This is of particular importance, since there is strong evidence that nationally the number of pollinating insects is in decline.

A small number of non-native plants do become naturalised and have a negative impact on natural habitats (and, in the case of Japanese knotweed and buddleia, for example, the economy). However, in the UK only 10% of plant introductions are self-sustaining outside cultivation, and just 0.2% lead to widespread problems. It is worth remembering also that many plants we now accept as part of our biodiversity such as elm, horse chestnut and sycamore trees, snowdrops and many corn-field ‘wildflowers’ are, in fact, introduced non-natives. Fossil evidence shows that Rhododendron ponticum, introduced into the UK from Spain and Portugal in the 18^th century and now an invasive weed in the west of the country,  was in fact native to Britain before the ice age. A growing number of studies on the impact of invasive species have found no evidence that non-native plant species increase in abundance in the countryside at the expense of non-native species, and concluded that the negative effects of non-native plants on British biodiversity (and elsewhere) have been exaggerated, and that other factors such as changes in land use and management and climate change are much more important drivers to changes in the abundance and distribution of native plants. Exotic species may, in fact, help natives in many environments, in a number of ways.

These are complex but fascinating ideas. However, what concerns me as much as the effect of exotic plants on the environment is some of emotive language used, especially in the media, in framing this debate. A call for ‘native only’ planting and ‘eradicating alien species’ has uncomfortable nationalistic or even xenophobic overtones, especially in a multicultural city like London. It’s no wonder that the Daily Mail loves reporting on the ‘threat’ to the country from ‘foreign’ plants and animals, using language designed to conjure up fear. As the ecologist Mark Davis points out, perhaps it’s time to abandon the native vs alien concept, and judge species on whether they are producing benefits or harm to biodiversity, human health, ecological services and economies, rather than from where they originated.

References

Davis, M. et al. (2011). Don’t judge species on their origins. Nature 474, 153-154.

Hamilton, G. Aliens to the rescue. New Scientist 15 January 2011.

Head, S. and Thompson, K. What is the relative value for wildlife of native and non-native plants in our gardens? Wildlife gardening forum website.

Johnston, M., Nail, S. and James, S. (2012). ‘Native versus aliens’: the relevance of the debate to urban forest management in the UK. In Trees, people and the built environment, Forestry Commission, pp. 181-191.

Kendle, A.D. and Rose, J.E. (2000). The aliens have landed! What are the justifications for ‘native only’ policies in landscape plantings? Landscape and Urban Planning 47, 19-31.

Loram, A. et al. (2008). Urban domestic gardens (XII): The richness and composition of the flora in five UK cities. Journal of Vegetation Science. Journal of Vegetation Science 19 (3), 321-330.

Manchester, S.J. and Bullock, J.M. (2000). The impacts of non-native species on UK biodiversity and the effectiveness of control. Journal of Applied Ecology 37 (5), 845-864.

Salisbury et al. (2015). Enhancing gardens as habitats for flower-visiting aerial insects (pollinators): should we plant native or exotic species? Journal of Applied Ecology 52, 1156-1164.

Smith, R.M. et al (2006). Urban domenstic gardens (IX): Composition and richness of the vascular plant flora, and implications for native biodiversity. Biological Conservation 129, 312-322.

Thomas, C.D. and Palmer, G. (2014). Non-native plants add to the British flora without negative consequences for native diversity. Proceedings of the National Academy of Sciences of the USA 112, 4387-4392.