Ken Thompson - Where Do Camels Belong?: The Story and Science of Invasive Species

WHERE DO
CAMELS
BELONG?

THE STORY AND SCIENCE
OF INVASIVE SPECIES

DR KEN THOMPSON was for many years a lecturer in the Department of Animal and Plant Sciences at the University of Sheffield. He now writes and lectures on gardening and ecology. He has written five previous books, the most recent of which was Do We Need Pandas? The Uncomfortable Truth about Biodiversity.

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The Book of Weeds

WHERE DO
CAMELS
BELONG?

THE STORY AND SCIENCE
OF INVASIVE SPECIES

KEN THOMPSON

First published in Great Britain in 2014 by

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Copyright Ken Thompson, 2014

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A CIP catalogue record for this book is available from the British Library.

eISBN 978 184765 9958

INTRODUCTION

WHERE DO
CAMELS
BELONG?

W here do camels belong? Ask the question and you may instinctively think of the Middle East, picuring a one-humped dromedary, some sand and perhaps a pyramid or two in the background. Or if you know your camels and imagined a two-humped Bactrian, you might plump for India and central Asia. But things arent quite so simple if were talking about the entire camel family.

Camelids (the camel family) evolved in North America about 40 million years ago. Titanotylopus, the largest camel that has ever lived, stood 3.5 m high at the shoulder and ranged through Texas, Kansas, Nebraska and Arizona for around 10 million years. Other species evolved very long necks and probably browsed on trees and tall shrubs, rather as giraffes do today. Much, much later camels spread to South America, and to Asia via the Bering Strait, which has been dry land at various times during the recent Pleistocene glaciations. Camels continued to inhabit North America until very recently, the last ones going extinct only about 8,000 years ago. Their modern Asian descendants are the dromedary of north Africa and south-west Asia and the Bactrian camel of central Asia. Their South American descendants are the closely related llamas, alpacas, guanacos and vicuas (llamas are only camels without humps; all you need to do is look one in the eye for this to be pretty obvious).

Now you know all that, let me ask you again: where do camels belong? Is it:

1. in the first place you think of when you hear the word camel, i.e. the Middle East.
2. in North America, where they first evolved, lived for tens of millions of years, achieved their greatest diversity, and where they became extinct only recently.
3. in South America, where they retain their greatest diversity.

   Or, just to muddy the waters a bit more, is it:

4. in Australia, where the worlds only truly wild (as opposed to domesticated) dromedaries now occur.

Finally, if you felt able to give a confident answer, can you explain why?

If you think camels belong where they evolved, the question has only one answer: North America. If it means where they have been present for the longest time, the answer is the same. If it means where camels have been present during recent millennia, then the answer is Asia and South America. If camels belong wherever they can thrive without human assistance, then it must also include Australia. These are all perfectly reasonable interpretations of belonging.

And there is nothing particularly special about camels. Dispersal over huge distances is not at all unusual among land animals, and it is almost routine among birds. Horses are much the same as camels, and frogs, toads, shrews, deer, cats, weasels, otters, hares, skinks, chameleons and geckos are among the many other groups that now occur almost everywhere, and do so as a result of relatively recent dispersal without human assistance often starting out in Africa or south-east Asia. None of these species have an an obvious answer to the question about where they belong whether they are natives or aliens any more than camels. Indeed, once you adopt a view of the world that doesnt assume that theres something very special about where things happen to be right now (or in relatively recent history), asking where anything belongs tends not to have an obvious answer.

The Earth is home to just short of two million species of living organisms. At least, those are the ones we have recognised, described and named. There are certainly many more, maybe up to 10 million, possibly even more. Each of those species has a characteristic distribution on the Earths land surface, or in its oceans, lakes and rivers. Some are common, some are rare, some have very wide ranges, others are confined to tiny areas such as single islands. But in every case, that distribution is in practice a single frame from a very long movie. Run the clock back only 10,000 years, less than a blink of an eye in geological time, and nearly all of those distributions would be different, in many cases very different. Go back only 10 million years, still a tiny fraction of the history of life on Earth, and any comparison with present-day distributions becomes impossible, since most of the species themselves would no longer be the same. Go back further still, and the Earth itself starts to become unfamiliar, with some continents drifting further apart, others colliding.

Only rarely do we get a really good view of what a dynamic, unstable place the world and its inhabitants really is, but when we do it can be quite startling. Recently, Dutch researchers drilled down over half a kilometre to obtain sediment cores from the Bogot basin in the tropical High Andes of Colombia. The pollen grains preserved in this sediment column tell us what the vegetation was like at every moment during the last two million years and the researchers found something remarkable. This is what they concluded:

Present-day montane forest and pramo vegetation reflect a frozen moment in a long and dynamic process of almost continuous reorganization of floristic elements. It indicates that on a Pleistocene timescale present-day plant associations are ephemeral. Most of the record reflects no-analogue vegetation associations.

In other words the plants (and the vegetation they formed) that would have been familiar to a human observer at any moment during the last two million years would have seemed quite unfamiliar to anyone from any other point in time. Not only that, but (that final no-analogue comment) none of the various kinds of vegetation that grew during that immense span of time has any close modern equivalent, and all would be unfamiliar to a present-day observer.

What all this tells us is that there is nothing special about the plants or camels or anything else we have now, nor about exactly where they happen to be, i.e. where they are currently native. The only unusual thing about