To be or not to be a noxious invader (Willem VAN COTTHEM)


It remains a remarkable fact that exotic plants are generally seen as potential invaders of the native flora, while adventive exotic invaders of harbours, airports and railway stations do not get that stigma.

What do we mean by “adventive plants” ? It are plants “not native to and not fully established in a new habitat or environment; locally or temporarily naturalized.”

In and around harbours, airports and railway stations all over the world one will find numerous “exotics”, mostly introduced as foreign (not native) seeds with the ships, airplanes and trains.  Are these potential invaders ?  Sure !  Do we ring the alarm bell ?  Never !  Over a certain period of time these “invaders” may even adapt to their “new” habitat and become part of the local “weeds”.

This sounds like the well-known tunes of the intentional historical introduction of many ornamental species and food species.  Do we call them “invaders” too when their seeds start spreading and they adapt to their new habitat ?

Suppose some drought-tolerant tomato variety would easily adapt to the drylands and starts “invading” those dry environments.  Would we consider it as an “invasive species” or be happy with that “new food plant” ?

This music sounds completely different when one considers a number of  “noxious invaders“.

Take for instance Prosopis juliflora and Opuntia ficus-indica.

Prosopis juliflora is a small tree or a shrub native to Mexico, S. America and the Caribbean, where it is used for forage and wood.  Its roots are able to grow very deep in search of water.

In my view, this rooting characteristic makes it an excellent species to be used for reforestation projects in desertification affected drylands.  Therefore, I did not hesitate to plant hundreds of Prosopis saplings in several reforestation projects in the Sahel countries, particularly on barren soils.

Since long, Prosopis juliflora has become established as a weed in Australia, Africa and Asia.  However, in some of these regions it is considered as “the Devil Tree”, invading arable land so strongly that is denies the native plants to develop in a normal way, deteriorating agricultural production.

One can wonder why Prosopis is not invading arable land in Mexico, S.America and the Caribbean and why local people are still using it for forage, wood and environmental management.  Is this a Dr. Jekyll and Mr. Hyde story ?

Anyway, the Prosopis trees that I planted on barren fields in many Sahel countries between 1985 and 2005 developed swiftly into small trees and shrubs, in the shadow of which numerous local weeds invaded the barren soil, forming gradually a dense vegetation cover in which step-by-step local shrubs and trees appeared, thus transforming the former barren area into a lusciously green wood.  A “Devil Tree” ?  No Sir, an “Angel Tree” for those areas.

Let me now turn to Opuntia ficus-indica, the Indian fig opuntia, barbary fig, cactus pear or prickly pear cactus.

Since long, this cactus with the sharp spines has become an important domesticated fruit crop in many arid and semi-arid parts of the world (sweet fruits, tunas, barbary figs).

There are many other uses of this species :

  • Young green paddles or pads are eaten as “nopales”
  • Jams and jellies
  • Alcoholic drinks
  • Cattle feed
  • Boundary fence
  • Dispersant for oil spills
  • Limiting erosion
  • Etc.

And yet again, in some countries this extremely interesting species is called a “noxious invader”, even a “pest”, because of its ability to spread rapidly beyond the areas in which it is cultivated.

Let us accept that it is quite unpleasant to see this prickly cactus with its “terrible” spines developing all over the arable land.  It can indeed be very difficult to halt the spreading of such a “pest”.

But what if this same cactus would not have these long, sharp spines which make the plant almost untouchable ?  Well, such a variety without the long spines exist !  Remarkably enough it exists already on all the continents.  It is called Opuntia fius-indica var. inermis (inermis is Latin for unarmed).

P1000353 copy 1The spineless Opuntia ficus-indica in a garden at Staoueli (Algiers, 2007) – (Photo WVC)

I believe that this throws a completely different light on the qualification of this “noxious invader”, because such a spineless cactus has all the same qualities (and uses) as the prickly one, but it can easily be handled, even if it would be spreading rapidly (although one could not use it for fencing).

For me the var. inermis of Opuntia ficus-indica is Dr. Henry Jekyll, while the spiny prickly pear cactus is Mr. Edward Hyde.  That’s why I am still looking for a clear answer : to be or not to be a noxious invader.

2000-06-BRASIL-OPUNTIA01Who would call this Opuntia ficus-indica var. inermis a pest ? (Photo WVC – Opuntia plantation in the N.E. Brazil 2000-06)

Author: Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.

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