Cabbages, cabbage trees and the dwindling web of life

By Mark Nicholson

“The Creator, if he exists, has a special preference for beetles.”

The quote above is often misquoted, as is the attribution. It may have been Charles Darwin, or the entomologist J.B.S. Haldane or even Stephen Jay Gould. The point of the quote was that when uttered, there were 300,000 described species of beetles. Today there are believed to be 1.5 to 2 million species of beetles and some think that is a gross underestimate. Compared to the modest number of 5400 mammal species and 11,000 bird species, the Creator must indeed have had an inordinate fondness for beetles and they may indeed reign when the rest of us have gone: one-fifth of all (recently) extant mammal species are either endangered, extirpated in the wild, or recently extinct.

At a conference I attended a few years ago in Amman, an American botanist told us a story of a cabbage. He had been surveying Alboran, a small, arid island off the south coast of Spain, for xerophytic plants. He came across a tiny member of the cabbage family, the Brassicaceae, and took a few seeds back to the US. He then invited colleagues back on the island to see the plants. But they had gone. The Armada (the Spanish Navy) had irrigated the whole island with sea water to keep the dust down for a helicopter landing site and the salinity had killed all the cabbages. Fortunately, his seeds germinated ex situ and were I think reintroduced in situ, saving it from extinction.

Cabbages are interesting for all sorts of reasons. Wild cabbage is native to the Mediterranean region of Europe and has been cultivated since the Roman Empire. There is also some evidence of cabbage use in the Shensi province in China dating back to 4000 b.p. The cabbage has also had more scientific names than any other plant. The accepted name today is Brassica oleracea (derived from the Celtic word bresic). There are also many cultivated varieties (known as cultivars or cvs.) such as Kohl rabi, cauliflower, kale, Brussels sprouts, Romanesco as well as related species including turnips, oilseed rape, mustard and Bilko Napa cabbage from China.

Another cabbage in the Brassicaceae and the sole member of its genus, Kerguelen cabbage, inhabits one of the remotest islands on earth, some 3000 km south east of Cape Town heading towards Antarctica at 50o south. The leaves of the plant contain a pale-yellow, highly pungent essential oil that is rich in vitamin C, for which reason sailors used it as a dietary supplement to prevent scurvy.

Both these unusual cabbages and cabbage relatives are locally endemic and both have qualities which could be selected for their adaptation to climate change. But both are threatened in the wild and could easily become extinct. A loss of a single species means a reduction in biological diversity, or biodiversity, which is a word still not understood by all. It essentially means the sum of all species and their interdependence. It is in fact ‘the web of life’ and the web is disentangling. The BBC last week reported that the UK was the most biodiversity-depleted country on the planet, with half of its biodiversity lost in the last fifty years.

Does it matter? Well, yes it does. All species depend on each other. The loss of one species can also affect another species’ survival. The most destructive species of all, Homo sapiens is destroying the very web on which we all depend. Loss of biodiversity happens for all sorts of reasons but the main ones are habitat loss, pollution, over-exploitation and climate change. In his book At Home Bill Bryson tells us that a hundred years ago oysters and 9kg lobsters were so common and cheap on the east coast of the USA that they were the daily fare of workers. A fishing boat off New England could catch ten tonnes of halibut a day: today the fish is no longer found off the northeast coast of America. Climate change can have a similar effect: the warming of shallow tropical seas causes coral bleaching. Coral reefs, along with the largely unexplored deep oceans have the highest biodiversity on the planet yet when the coral dies so do all the myriad species that cohabit with it.

Today a flock of fast-flying, bright yellow-billed Olive Pigeons flew overhead and headed for a distant patch of indigenous African forest. Why was that exciting? The area in Kenya where we live is dominated by thousands of acres of tea in which you will be lucky to see any bird species other than Stonechats and a few Rock Martins flying overhead. To dry the tea requires plantations of fast-growing, straight and thornless eucalyptus (which we call ‘gums’), Australian trees that coppice easily after repeated cutting. Four acres of tea require one acre of gums. The result is that indigenous African forest has been destroyed in our area because most African trees do not coppice, are not always straight and some have long thorns, which makes handling them difficult. Gums contain eucalyptus oil which may smell nice and help your arthritis but they are poisonous to us, and to most animals and insects, with the exception of Koala bears. Stand under a urinating koala and you will get a nasty-smelling shower of toxic liquid. If there were no indigenous forests near us, there would be no Olive Pigeons because they, and their cousins, the Green Pigeons only eat the fruit of African trees. No Green pigeons would mean that we would no longer hear their warbling, unpigeon like call described in my bird book as ‘mellow trills followed by lower, harsh creaking, barking, growling or whinnying, punctuated by sharp wick or tok notes’. Try imitating that.

Twenty years ago I started an arboretum of indigenous East African trees on about 30ha at the Brackenhurst conference centre north of Nairobi. Today we have a closed-canopy forest of roughly 650 species of indigenous shrubs and trees that normally live around our altitude of 7000ft (2100m). Some are very rare and we are always searching for unusual ones. Trees from the coast generally will not survive here and plants from higher altitudes (3000-4000 m) will not be happy either. Our main threat is what are termed invasive species, yet another threat to biodiversity. Invasive plants are those which grow so vigorously that they shade out and outcompete local species. We have three nasty invading plants, including a Cestrum from Guatemala which was introduced as a garden plant a century ago and has spread rapidly into all the highland forests of Kenya. Birds love its fruit, but cattle will die in 24 hours if they eat the leaves. Another noxious species is wattle, an Australian Acacia which was introduced for tanning leather and as a fast-growing wood for steam engines. It fixes its own nitrogen but puts out poisons in its roots to depress the growth of other plants.

But a forest is more than trees: one tree may have hundreds of plants around it: understory herbaceous (non-woody) plants, epiphytes, hemi-parasites, orchids, grasses, vines, mosses, ferns, fungi, clubmosses, spikemosses and lichens. Those are just the visible ones. Every tree is a microbiome: every leaf, bark and root provides a home for tens of thousands of different species of microbiota such as bacteria and fungi about which we as yet know nothing. Every species of tree will have its own unique assemblage of bacteria, most of which are beneficial and may well provide the next therapy for cancer. The trees themselves are used both for medicine and for food. The oldest and arguably the best medicine in human history, aspirin, comes from both willow bark and the herb meadowsweet. A common tree here is African Rosewood, which is used to prevent benign prostatic hyperplasia (BPH) in men. And the roots of some of our Cabbage Trees with their curious leaves taste like mashed potato when cooked.

As well as the plants, we have the animals and birds. Look at the leaf above and many will tell me to spray it. The leaf is eaten by Chrysomelid beetles (>50,000 species), which are themselves eaten by a vole or a drongo or a Bushbaby. They in turn will fall prey to an owl, a civet cat or a Great Sparrowhawk. Meanwhile the flowers will provide nectar to bees, moths or to our eleven of the 38 species of Kenyan sunbirds.

When we planted the trees, many others found a home. We have recorded 188 bird species to date, some resident, some Eurasian migrants and some inter-African migrants. We have Black and White Colobus, Sykes monkeys, Bushbabies, Genet & Civet cats, Palm Civets, African hedgehog, Tree Hyrax, three types of mongoose, Bushpig, porcupine, African Clawless Otter, several bat species and almost as many butterfly species as are found in the UK. But the antelope are all gone, snared in the endless search for bush meat. Yet other lower altitude natural forests in Kenya have much greater biodiversity: Kakamega forest in western Kenya has over 400 species of butterflies and 27 species of snakes.

The trend both in Africa and worldwide of course is declining biodiversity. We fear and kill snakes and scorpions, yet the venom of both have now been found to be of use in treating diseases varying from Lupus to rheumatoid arthritis, from hypertension to Alzheimers. Sadly, we continue to kill the golden goose.

A well-known soldier and empire builder of 120 years ago did an animal count every time he approached by train from a few miles outside Nairobi. The average number of large wild animals he counted surpassed 6000 every time, including giraffe, rhino, zebra, lion, warthog, hyaena and seven species of antelope. Nairobi is fortunate enough to have a small National Park right on its doorstep but the number of animals today is nothing like those numbers. The rest of the area is now covered in concrete and tarmac. Is there room on the planet for 8 billion of us and the vast array of biodiversity or are we really in for the irreversible Anthropocene extinction?