Officials in Brisbane, Australia, inspect containers.
A floating threat: sea containers spread pests and diseases
Oil spills garner much public attention and anguish, but “biological spills” represent a greater long-term threat and do not have the same high public profile.
It was an exotic fungus that wiped out billions of American chestnut trees in the early 20th century, dramatically altering the landscape and ecosystem, while today the emerald ash borer – another pest that hitch-hiked along global trade routes to new habitats – threatens to do the same with a valuable tree long used by humans to make tool handles, guitars and office furniture.
Perhaps the biggest “biological spill” of all was when a fungus-like eukaryotic microorganism called Phytophthora infestans – the name of the genus comes from Greek for “plant destroyer” – sailed from the Americas to Belgium. Within months it arrived in Ireland, triggering a potato blight that led to famine, death and mass migration.
The list goes on and on. A relative of the toxic cane toad that has run rampant in Australia recently disembarked from a container carrying freight to Madagascar, a biodiversity hotspot, and the ability of females to lay up to 40,000 eggs a year make it a catastrophic threat for local lemurs and birds, while also threatening the habitat of a host of animals and plants. In Rome, municipal authorities are ramping up their annual campaign against the tiger mosquito, an invasive species that arrived by ship in Albania in the 1970s. Aedes albopictus, famous for its aggressive biting, is now prolific across Italy and global warming will make swathes of northern Europe ripe for colonization.
This is why the nations of the world came together some six decades ago to establish the International Plant Protection Convention (IPPC) as a means to help stem the spread of plant pests and diseases across borders boundaries via international trade and to protect farmers, foresters, biodiversity, the environment, and consumers.
“The crop losses and control costs triggered by exotic pests amount to a hefty tax on food, fibre and forage production,” says Craig Fedchock, coordinator of the FAO-based IPPC Secretariat. “All told, fruit flies, beetles, fungi and their kin reduce global crop yields by between 20 and 40 percent,” he explains.
Trade as a vector, containers as a vehicle
Invasive species arrive in new habitats through various channels, but shipping, is the main one.
El Profesor Willem Van Cotthem de Bélgica, de quien ya he hablado en otras ocasones en este programa, ha trabajado en diferentes proyectos para la producción de alimentos con el patrocinio de la ONU en África, es inventor de TerraCottem, un sustrato para mejorar los suelos y autor del blog Desertification (https://desertification.wordpress.com/).
A continuación les ofrezco una traducción del artículo y al final haré mis comentarios:
“Muchos de quienes trabajan en el área de desarrollo están buscando plantas comestibles resistentes a la sequía, que puedan crecer con un mínimo de agua de riego. Desde luego, principalmente piensan en cultivos comestibles y hierbas, a menudo modificadas genéticamente.
Suena casi increíble, pero millones de personas en Centro y Sudamérica comen nopales sin espinas, sin que la gente en otros continentes sigan su ejemplo. Sólo una minoría sabe que muchas partes de la variedad sin espinas del nopal (Opuntia ficus-indica) se comen. No sólo proporciona comida fresca a la gente malnutrida, sino que puede usarse como una cerca viva alrededor de los barbechos. También puede ser importante para el ganado.
Photo WVC: Algeria 2007-04 Layoun P1000858.jpg
Llegará el día en que billones de personas en las zonas áridas cultivarán esta maravillosa cactácea en la casa o la escuela, los campos de refugiados, en los huertos familiares, en el suelo o en contenedores. Aunque esta especie suave y sin espinas se encuentra en todos los continentes y puede multiplicarse fácilmente plantando pencas individuales, casi nunca se usa para combatir la desertificación en forma de cercas vivas.
No puedo dejar de soñar en una GRAN MURALLA VERDE DE CACTOS a través de África ( o en áreas desérticas en otros continentes, donde sea que se necesite).
En lugar de gastar millones en plantar un cinturón de bosque durante varias décadas, desarrollando plantas en viveros, cavando hoyos, regando los arbolitos y perdiendo muchos de ellos a causa de la sequía, una gran barrera verde de cactos podría establecerse en un mínimo tiempo y a un ridículo bajo costo.
Costaría un cacahuate dispersar esta planta comestible en todas las regiones afectadas por la sequía.
Photo Kenya: BOF Research 002_2.JPG
Algunos cuestionarán la posible naturaleza invasiva de esta planta. es recuerdo que esta variedad sin espinas de nopal es COMESTIBLE. ¿No debería uno soñar con una planta invasiva comestible por la gente y los animales en todas las zonas áridas?
Sólo tomen esto en cuenta: es Agosto 2014 y sugiero que comencemos lo más pronto posible con la construcción de la GRAN BARRERA VERDE DE CACTOS, tirando en el suelo pencas de nopal sin espinas. Dejemos a la naturaleza hacer su trabajo: las pencas crecerán fácilmente y las magníficas barreras ayudarán a combatir la desertificación.
¿Qué estamos esperando? “
COMENTARIOS DE FABIO RUIZ
Esto es lo que dice Willem Van Cotthem. Me parece excelente que en otros países promuevan una planta que conocemos de una manera muy cercana pero por otro lado, creo que es una desgracia o no sé como llamarlo para que no suene tan fuerte, que teniendo la planta y los conocimientos no la utilicemos en mayor medida en la región. Además de la variedad sin espinas que se menciona se pueden plantar otras variedades de nopal, por ejemplo las que aquí conocemos como chamacuero, joconostle, chabeño, etc. y también darle una oportunidad a los pitayos.
En el artículo original pueden ver fotos en las que se aprecia los diferentes modos en que se puede cultivar el nopal.
Sap-sucking insects may combat Kenyan cactus plague
“So when times are good it will continue to displace local plants and make more valuable pasture inaccessible.” – Arne Witt, CABI-Africa
Speed read
Prickly pear cactus is an invasive plant that threatens grazing areas in Kenya
A trial now shows that a species of bug can be used to control the cactus
But further safety testing and approvals are needed before rolling it out
An insect that sucks the sap out of cactus plants has been trialled in East Africa to contain the spread of an invasive cactus species that threatens local grazing areas.
The cochineal bug, known as dudu in Swahili, for biological control has been released on farmland in Kenya’s Laikipia region, which is used by Maasai for livestock herding. The trial showed that the bug feeds exclusively on the Opuntia stricta cactus, better known as prickly pear, which has invaded grasslands and drives out local plants used to feed cattle.
The Maasai community in Laikipia partnered with the Centre for Agriculture Biosciences International (CABI) to conduct the trial and halt the spread of the cactus. According to CABI, an non-profit science organisation from the United Kingdom, the trial, which concluded last month, has shown that the dudu bug will not be harmful to native and non-harmful imported plants in the region.
“The cochineal has not been found on other cactus species such as Austrocylindropuntia subulata and Cereus jamacaru that are growing in association with Opuntia stricta,” says Arne Witt, the coordinator of the invasive species programme at CABI-Africa. “In a nutshell, there is no risk.”
The prickly pear cactus was introduced in Kenya during colonial times as an ornamental plant capable of living in arid regions. Since then, the plant has colonised thousands of acres of fragile rangelands in northern Kenya, putting at risk the livelihood of animal herders.
According to CABI the cactus is also suspected to have caused the death of baby elephants after they consumed its fruit, meaning it poses a threat to local wildlife and related income from tourism.
The prickly pear (Opuntia ficus-indica) is an invasive species. Due to its hard spines it has almost no predators and known methods to destroy it are expensive.
On the contrary, the spineless variety (Opuntia ficus-indica var.inermis) is a widely cultivated plant in Central and South America, edible for men and animals.
Therefore, describing the prickly pear (Opuntia ficus-indica) as a noxious invasive species is a generalisation that is far too negative for its edible and ornamental spineless variety. Moreover, the prickly pear can also be used to produce an interesting biofuel.
Impacts of Plant Invasions Become Less Robust Over Time: Invasive Plants Are More Likely to Be Replaced by Other ‘Invasives’
Nov. 20, 2013 — Among the most impressive ecological findings of the past 25 years is the ability of invasive plants to radically change ecosystem function. Yet few if any studies have examined whether ecosystem impacts of invasions persist over time, and what that means for plant communities and ecosystem restoration.
(continued)
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SPINELESS CACTUS : A DROUGHT-TOLERANT JEWEL (Willem Van Cotthem)
The prickly pear (Opuntia ficus-indica) is seen by some as a dangerous invasive species. But what about the spineless variety (var. inermis), one of the best tools to combat desertification and hunger ? The pads and fruits of this variety can be eaten by humans and animals (see “nopales”). Wouldn’t we wish that this variety becomes an invasive plant in all desertfied areas ? For sure, I would not hesitate to plant it in semi-arid and arid areas (Photo WVC)
That was my first thought after reading the article mentioned below.
The other day, I was contacted by the Canadian Radio Station CBS for a live interview on my successful action SEEDS FOR FOOD (sending seeds of exotic fruits and vegetables to development projects in arid and semi-rid regions in order to contribute to the combat of hunger and poverty).
One of the questions was : “Sir, aren’t you afraid that one of the species could be invasive in those regions ?”. I replied that in fact I hoped that this would be the case. Indeed, suppose that one of the food plants, e.g. a tomato species or variety, would be invasive. It would spread gradually in that area and deliver free food to the hungry poor. What a splendid phenomenon would that be ! The Canadian journalist remained a couple of seconds silent and then said : “I got your point, Sir !”.
Can a Potentially Invasive Plant Bring a Positive Influence to a Region?
Oct. 25, 2013 — Can invasive species be beneficial for the region?
A recent study, published in the open access Journal of Hymenoptera Research, aimed to obtain empirical data on the activity and distribution of the bee species Braunsapis puangensis in the Suva area of Fiji and examine its association with the invasive creeping daisy Sphagneticola trilobata. The paper suggests that the invasive creeping daisy could in fact have a positive influence on a wild bee pollinator species, thus benefiting crops and biodiversity on the islands.
(continued)
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What if these tasty “balcony tomatoes”, grown in my new experimental pallet garden, would be invasive in desertified areas ? (Photo WVC)
Isn’t this a nice idea ? I keep hoping that some of our food plants could be invasive too. Who would mind ? (Willem)
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).
The 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.
Who would call this Opuntia ficus-indica var. inermis a pest ? (Photo WVC – Opuntia plantation in the N.E. Brazil 2000-06)
ScienceDaily (Nov. 30, 2012) — When people hear about exotic plants invading a new environment, there is usually a negative connotation, according to biology faculty member Matthew Heard in an article published in the journal Ecology Letters. They often think of plants like kudzu, Chinese privet, or Japanese honeysuckle, whose thuggish behavior can push out the native plants in their backyard or local parks.
Opuntia ficus-indica var. inermis : Prickly pear is generally seen ass an invasive species, but is this also the case for the spineless var. inermis ? (Photo WVC – Algiers 2006-12 – Opuntia as ornamental plant in a garden)
While this worse case scenario can happen, it isn’t always the case, said Heard, who wrote his Ph.D. dissertation at Brown University on how native and exotic plants coexist along the coasts of Rhode Island and Massachusetts.
“It turns out that in many places, native and exotic plants can actually live together,” Heard said. “And this means that exotic plants aren’t inherently bad like many people think, but it also means that it is important to figure out what is driving this balance between these two groups.”
In his paper, Heard notes that there has been little experimental fieldwork conducted to determine what factors allows native and exotic plants to live side by side. While there have been many potential explanations tossed out, it turns out that just being different is the main reason that they can actually coexist together.
“Basically, we found that exotics plants grow more and can essentially out-compete natives, which normally is a problem. But in these communities there are also insects, which prefer to eat exotic plants instead of natives and can keep their growth in check. As a result, native plants, which are less susceptible to these insects can thrive even when exotic plants that are better competitors are nearby,” said Heard.
How long this precarious balance will remain is unknown, but for now it isn’t just the case of exotic species being problematic. Instead it’s the story of how differences between two groups of plants allow them to survive along side each other.
ScienceDaily (June 8, 2011) — The recent field of invasion biology faces a new challenge as 19 eminent ecologists issue a call to “end the bias against non-native species” in the journal Nature.
Often called aliens, hitchhikers or invasives, non-native species could just as easily be coined “abductees” whose transport links to activities by humans, some scientists say.
The authors of the Nature comments section note that assumptions that “introduced species” offer only deleterious impacts are misguided and “that human-induced impacts, such as climate change, nitrogen eutrophication, urbanization and land use change are making the native-versus-alien species dichotomy in conservation increasingly meaningless.”
Mark Davis, lead author and professor with Macalester College in Saint Paul, Minnesota, says that a nativism perspective — native species equals good, non-native species equals bad — has dominated conservation efforts over the past few decades. He points to a number of ecologists, “including those who rightly could be called legendary for their contributions to the field over the past decades, who believe there has been way too much ideology and not enough good science associated with the anti-non-native species perspective.”
The authors believe that a shift in the field is needed to consider outcomes and impacts of an organism on an environment rather than focus on native origins.
Tamarisk Biocontrol Efforts Get Evolutionary Boost
ScienceDaily (July 12, 2012) — UC Santa Barbara scientists trying to control the invasive tamarisk plant have been getting a boost from evolution, in the form of a rapidly evolving beetle that has been changing its life cycle to more efficiently consume the noxious weed.
Their findings are published in the journal Evolutionary Applications.
“This is one of the clearest cases of rapid evolution,” said co-author Tom Dudley, who is the principal investigator at UC Santa Barbara’s Marine Science Institute Riparian Invasive Research Laboratory. The tamarisk leaf beetle, he explained, has managed to delay its entry into hibernation to adapt to the shorter days of the southern region of the United States. That adaptation in turn allows the beetle to survive until spring, while prolonging the time it has to reproduce, and increasing its effectiveness at controlling the invasive weed.
Range rehabilitation using reseeded enclosures in Lake Baringo Basin, Kenya
Text and photos by Stephen MUREITHI (Univ. of Nairobi, Kenya)
Editing by Willem VAN COTTHEM (Univ. of Ghent, Belgium)
Background
Land in the Njemps Flats in the Lake Baringo Basin in Kenya is legally community owned. Uncontrolled utilization, overgrazing and mismanagement have precipitated in decades a ‘tragedy of the commons’-scenario, manifested in severe land degradation.
To address the socio-economic problems caused by severe soil degradation, the Rehabilitation of Arid Environments (RAE) Trust, initiated in 1982, established large-scale “communal” enclosures, rehabilitating the land following a participatory approach.
Communal enclosures, varying in size from 6 to 400 ha, are perimeter-fenced using a solar-powered electric fence. After de-branching the noxious woody vegetation, mainly Acacia reficiens and A. mellifera, the land was prepared for planting through chiseling to break the surface crust and construction of micro-catchments, e.g. embankments.
Drought-resistant indigenous fodder trees were planted and a mixture of grass seeds broadcasted by hand. The main grass species used included Cenchrus ciliaris, Enteropogon macrostachyus and Eragrostis superba.
Direct benefits derived from the enclosures include :
grazing resources (dry season or livestock fattening),
sales of cut grass for fodder or thatching,
sales of grass seed, poles and fuel-wood,
beekeeping.
The rehabilitation success attained in these communal enclosures soon fostered many local inhabitants to establish “private” enclosures. This gave rise to a mosaic of enclosures, differing with respect to the years since establishment, and based on two distinct management types.
A good vegetation cover within these enclosures has been reported (see http://tinyurl.com/6eje59u), resulting in a very sharp contrast between the enclosure and the surrounding open rangeland. The private enclosures, reseeded and managed by individual farmers with limited financial and technical resources, range in size from less than 1 to 20 ha. Fencing of these private enclosures is mainly done using cut-thorn bushes (Acacia reficiens and Prosopis sp.), and/or planted Pricky Pear cactus Opuntia ficus-indica and O. eliator. The latter cactus species is unfortunately invasive, and the RAE Trust has been teaching local people on choosing the right species for fencing, and how to manage and control the invasive species. Not all the farmers however, consult the RAE Trust when establishing private enclosures, but the organisation is reaching out to try to educate them on sustainable enclosure management. Read more about the enclosure story at http://www.springerlink.com/content/p5058m7107034l5m.
Errata
Our previously (2011-06) posted article, ‘Range rehabilitation by fencing it with the Prickly Pear cactus in Lake Baringo Basin’ was aimed at showing the trends of installing rangeland enclosures by private farmers in the Lake Baringo Basin. We compared photographs taken in 2005 and 2011. A lot of farmers are adopting the rangeland enclosure technique and some have used Opuntia species for fencing. Unfortunately, the readily available species Opuntia eliator, is an invasive one. At the same time, as the photos below will show, Prosopis juliflora has also aggressively invaded the area. The referred article was not aimed at implying that the RAE Trust is planting Prosopis (in fact, it has never planted any Prosopis species) and or planting / promoting Opuntia as a live fence and a rehabilitation tool. For any such impression that arose from the article, we hereby convey our apologies to Dr. Elizabeth Meyerhoff and the RAE Trust.
RAE recognizes that Prosopis juliflora is an invasive species and has worked in partnership with many organisations for years to eradicate that species from the Lake Baringo lowlands. Opuntia elatior is also recognized by RAE as an invasive species, especially as more and more people use it for fencing. RAE does not plant or promote the use of this Opuntia for fencing, but has worked with the people to manage and control it, in particular using the species as fodder during severe droughts – burning off the thorns using the wood of Prosopis and then feeding the Opuntia to cattle, thereby managing and ridding the area of both invasive species.
2005-08: Land without any fence in Lamalok, Lake Baringo Basin (Photo Stephen Mureithi)2011-01: The same land now a private enclosure fenced by the farmer with Opuntia elatior in Lamalok, Lake Baringo Basin. This species of Opuntia is invasive. (Photo Stephen Mureithi).2011-01: Private enclosure fenced by the owner with Opuntia elatior in Lamalok, Lake Baringo Basin. This species of Opuntia is invasive. Note that the Prosopis juliflora is invading too. (Photo Stephen Mureithi)2011-01: Prosopis juliflora is invading this newly established private enclosure. The use of an Opuntia elatior fence by the owner also poses a problem as the species is very invasive (Photo Stephen Mureithi).2011-01: An access road in Lamalok, Lake Baringo Basin is susceptible to gulley erosion during rain storms (Photo Stephen Mureithi).2011-01: Private enclosure fenced with Opuntia elatior in Lamalok, Lake Baringo Basin. This species of Opuntia is invasive (Photo Stephen Mureithi).2011-01: An Opuntia elatior fence has been established to the left of this open communal grazing land, which is invaded by Prosopis juliflora. Note, Opuntia elatior is also a potential weed/invader in degraded areas (Photo Stephen Mureithi)2011-01: A homestead enclosed with Opuntia elatior cactus. Note the cut-thorn bush placed on top of the juvenile live fence by the owner to protect it from being uprooted by livestock. The gulley in the foreground is naturally recovering, following the trapping of sediments and slowing down of flood water by the Opuntia fence (Photo Stephen Mureithi).2011-01: A few private enclosures in the Lake Baringo Basin are as large as 50 acres or more (Photo Stephen Mureithi).2011-01: University of Nairobi researchers sampling soils inside a communal enclosure at Lamalok (Photo Stephen Mureithi).2011-01: Rain water harvesting embankment inside a reseeded communal enclosure in Lamalok (Photo Stephen Mureithi)2011-01: Most households keep many goats, while the number of cows has reduced due to lack of pastures and loss during droughts. Here, 11.00 a.m., a farmer’s flock has just been let out heading to pasture (Photo Stephen Mureithi)2011-01: The Opuntia elatior cactus fence offers the right conditions for germination of Prosopis juliflora seeds below it, due to moisture trickling from accumulating dew at night, and shading from scorching sun during the day. However, the ‘unthankful Prosopis shrubs’ aggressively invade the Opuntia cactus fence, denying it sunlight, and eventually choking it to death (Photo Stephen Mureithi).2011-01: Fluvisols are the dominant soils in the Njemps Flats. Some pockets with Vertisols, like the one featured here, exist. Vertisols are characterised by serious cracking during the dry season (Photo Stephen Mureithi).2011-01: Good grasses cover inside a private enclosure at Lamalok. University of Nairobi researchers in the background carrying out sampling for biomass and cover using a line transect and quadrat method (Photo Stephen Mureithi).2005-09: A private enclosure protected by a live Opuntia elatior cactus fence, with the farmer’s cows grazing in the background. Notice the invading Prosopis juliflora around the fence (Photo Stephen Mureithi).2005-09: Farmers starting a live Opuntia fence, lying the paddles in a line (some farmers slightly bury the cut-tip for faster establishment of the roots). The rains are ‘good news’ for every plant in the arid areas (Photo Stephen Mureithi).2005-09: Lying of the paddles to establish an Opuntia fence. Soil surface sealing causes instantaneous flooding (Photo Stephen Mureithi).2005-2006 : Once the paddles are rooted to the ground, it marks the start of an Opuntia fence. Most farmers place cut-thorn bush on top of juvenile Opuntia to protect it from easily being uprooted by livestock as they bite the ‘flesh-juicy paddle’ of a young cactus plant (Photo Stephen Mureithi).2005-09: Communal enclosure in the background, protected by both Opuntia cactus and solar electric (not visible) fences. A duo fence: Prickly Pear cactus and a solar electric fence for a Communal enclosure is uncommon. In this photo, the driver of the small pick-up was about to ‘sleep in the bush’, his vehicle not coping with the wet Fluvisols prone to surface sealing (Photo Stephen Mureithi).
Need for Opuntia fences control
011-01: Note the pyramid-like growth of the Opuntia elatior fence. Without any deliberate effort to trim the outgrowing paddles (they fall and become new plants themselves), the fence can encroach meters of land on both sides. It has also been reported to harbour dangerous animals like snakes when it grows uncontrolled (Photo Stephen Mureithi).
Prosopis upsurge in Kenya
African drylands were severely degraded by the prolonged Sahelian droughts of the 1970’s and rehabilitation through tree planting was prioritized. In Kenya, some species of the Prosopis genus were enlisted for screening because they had shown potential in rehabilitation of quarry mines (Maghembe et al 1983). The introduced species included Prosopis chilensis (Mol.) St., Prosopis juliflora, Prosopis cineraria (L.) Druce., Prosopis pallida (Humb. & Bonpl. ex Willd.) Kunth and Prosopis tamarugo Phil. (Barrow 1980, Herlocker et al., 1980, Maghembe et al., 1983). From these and other efforts, P. chilensis and P. juliflora emerged as the most promising (Barrow 1980, Herlocker et al. 1980) and their seeds were sporadically exchanged across the country (Paetkau 1980), without a trace of provenance origins.
Introduction and distribution of Prosopis germplasm did not take cognisance of the invasive traits of the species as manifested in prolific seeding, seed dispersal through livestock and prolonged viability of seeds in the soil seed bank (Otsamo et al.,1993, Shiferaw et al., 2004), hybridization (Pasiecznick et al., 2001, Vega and Hernandez, 2005, Landeras et. al., 2006), allelopathy (Goel et al., 1989, Al-Humaid and Warrag, 1998, Rizvi et. al., 1999, Nakano et al., 2003) and root plasticity (Elfadi and Luukkanen, 2006).
Prosopis invasions became a reality when the weedy potential of P. julilora was first observed in the Tana Riparian Ecosystem in 1985, but its control was considered too expensive and almost impossible because of its prolific seeding and seed dispersal by livestock (Otsamo et al.,1993). This led to planting restriction in the Tana region, but planting in other parts of Kenya continued unchecked. For example, in the Turkana District extensive planting of P. chilensis and P. juliflora continued up to early 1990s, when rehabilitation projects were active. This and other related efforts in the country contributed to widespread planting of the species that are now considered problematic outside the rehabilitation areas (Choge et al., 2002), with the wetlands being single most critical ecosystems that are under threat (Source: Muturi GM, 2011).
2005-08: Photo taken at Lamalok, Lake Baringo Basin. There was no sight of any Prosopis juliflora invasion, though the place was severely degraded without any herbaceous cover (Photo Stephen Mureithi).2011-01: Severe invasion of the same degraded communal grazing land in Lamalok (Photo Stephen Mureithi).2011-01: Open degraded communal rangelands, invaded by Prosopis juliflora (Photo Stephen Mureithi).2011-01: Open degraded communal rangelands invaded by Prosopis juliflora (Photo Stephen Mureithi).2011-01: Private enclosure being invaded by Prosopis juliflora (Photo Stephen Mureithi).
As a plant biologist I still wonder why we accept that cargo vessels full of bulk food (wheat, corn, rice, bananas, … ) import tons of weeds, insects etc. in every harbour in every country, while one is shivering when we send an envelope with carefully washed and dried seeds of a clean papaya, melon or watermelon to hungry people in a developing country.
You never know, they could be invasive and disrupt the local biodiversity equilibrium !
I hope to see once in my lifetime invasive tomatoes, carrots, onions or beetroots taking over the waste dumps or barren soils. Imagine the Sahara and the Gobi desert full of flowering food crops. Imagine the Horn of Africa transformed in a Garden of Eden with all those invasive delicious plants, offering food or fodder.
DESERTIFICATION 
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