An exotic species (also known as introduced species; alien species; foreign species; non-native species; non-indigenous species) refers to a plant species or an animal species that is non-native. It is introduced into an area where it does not occur naturally. The introduction of the species may be deliberate or accidental. It is not always considered a nuisance or invasive.
The term exotic in Forestry lakes a generally accepted definition simply ‘any species grown outside its natural range or distribution’ like Eucalyptus spp, Populus spp, Acacias, Prosopis juliflora, Broussonetia papyrifera, Ailanthus excela, Robinia pseudoacacia, Casuarina equisetifolia, etc are exotic in Pakistan.
The presence of an exotic species may have a significant effect on the local ecosystem. The ecological impact varies; it may generally bring adverse effects to the ecological balance in an ecosystem or it may be beneficial. At other times, the effect is not too significant to cause major changes in the ecosystem.
Difference Between Exotic and Invasive Species
An invasive species is defined as a non-native species that causes disruption in the local ecosystem. Therefore, it is a type of exotic species but it particularly became a nuisance because it “invaded” and outcompeted the native species in the local ecosystem. In other words, the species has become a “pest” to its new location. The nature of invasive species can be different from place to place. For instance, mesquite (Prosopis juliflora) is considered invasive in some areas of Pakistan; while at some locations it has its importance as fuelwood, medicinal value, industrial usage, etc.
The term exotic species has a wider scope and the invasive species is one of its subsets; some of the other subsets are naturalized species (i.e. exotic species that have become established and are able to sustain their population even without human help) and acclimatized species (i.e. exotic species that have physically and behaviorally adjusted to the new environment).
Introduction of Exotic Species
The nature of the introduction of an exotic species has to be anthropomorphic, i.e. human-mediated. As already mentioned, the introduction may be intentional (on purpose) or accidental. One of the common reasons for introducing a species to a non-native location is to serve as a biological control agent of certain species, particularly those that produce harm. The deliberate introduction may also be for economic gain. For instance, a plant that is not native to a new geographical location is cultivated so that it becomes locally available. As for accidental introductions, mostly human activities serve as the means for the species to be transported from one geographical location to another.
The Need of Exotic Species
Pakistan is spending a lot of foreign exchange on the import of timber and timber products annually, while the demand for the same is still increasing rapidly with the expansion of wood-based industries and rise in the standard of living. It is therefore imperative, that all its resources are fully exploited to achieve self-sufficiency in forest products. The production can be increased by putting more area under forest and by introducing fast-growing, more economic, environmental friendly exotic species.
Properties of Successful Exotic Species
Often, however, exotic plantations are formed on open grasslands sandy waste or another tract devoid of the forest, and here their creation is an advantage rather than otherwise, apart from utilitarian considerations.
The exotic trees grown in commercial forests for the production of wood have the following properties:
- Their produce is of high value
- Their rate of yield is high
In addition to these, exotic trees have the following properties in common:
- An adequate supply of seeds (or cuttings) is available
- They are easy to rise in nurseries.
- They are easy to establish and grow in plantations.
- Insects or diseases do not prevent their successful growth.
- Information is available on methods of growing them and on their rate of growth and yield, their diseases and control measures.
Trials for Introduction of Exotic Species
Following steps (trials) are involved for the successful introduction of exotics species:
The Species Elimination Trials
The trial is designed to eliminate from further consideration those species clearly unsuited to the environment of the new planting area.
Species elimination trials in Nigeria have given sufficiently reliable information to eliminate about half the species at a reasonable cost after two years.
The Species Growth Trial
The growth trial provides information on performance, growth rate, stem and crown form, agencies causing injury or death, and crop benefits to be derived from those species emerging successfully from elimination trials.
Growth trials are established on different soil types and different climatic zones of the region.
The Species Plantation Trials
Comparison of different spacing, fertilizers, cultivation and other Silvicultural treatments should be made on trial plots designed for that purpose. The species, which pass this test and meet the desired purpose, may then be recommended for field planting.
Species Trials in Pakistan
The plant species have evolved through long and the continued selection and probably are the best suited to the local environment i.e. in the plain and arid area following species are grown Prosopis spicegera, Salvadora oleoides, Tamarix articulate, Acacia senegal, Zizyphus jujuba and Tecoma undulata.
Here is a research paper on the choice of exotic and indigenous tree species for planting on farmlands. The findings of the research show that ‘although the overall performance of an exotic species (E. camaldulensis) was better than other species the volume produced by an indigenous species (A. procera) equalled the former. The high water requirement of both the exotic species limits their potential for planting on farmlands since they compete extensively with the agricultural crop for water and nutrients. Planting of E. camaldulensis on waterlogged and saline soils is beneficial for biological amelioration of the land to make it suitable for cultivation of agricultural crops. Two of the indigenous species (A. procera and B. ceiba) are suitable for planting on farmlands particularly due to their importance for providing light quality timber and also fuelwood. Although the growth of D. sissoo and M. azedarach was slower than other species the importance of these two indigenous species for providing good quality timber increases their feasibility as long term investment’.
Exotic Species Recommended for Various Zones
Species trials and field practices have shown that these exotic species are recommended for various zones:
- Northern Hilly Tract: Populus deltoides, Robinia pseudoacacia.
- Sub-Mountain Range: Eucalyptus tereticornis
- Western Hilly Areas: Populus deltoides, Eucalyptus spp.
- Indus plain: Acacia cyanophylla, A. tortilis, Eucalyptus camaldulensis, Leucaena leucocephala, Poplus euramericana.
- Deserts: Acacia aneura, A. tortilis, A. victoria, Prosopis juliflora, P. cineraria.
- Saline Areas: A. nilotica, A. cyanophylla, E. camaldulensis, E. microtheca, Leucaena leucocephala, Prosopis juliflora.
- Waterlogged Areas: E. camaldulensis, E. robusta.
Please use the comments section at the end of the post for the addition of other successful exotic species in these zones bases on your field knowledge and personal experience.
Successful Exotic Species in Pakistan
Following are some of the successful exotic species introduced in Pakistan.
- Eucalyptus spp. Out of a number of species of Eucalyptus spp tried during the past half-century, Eucalyptus camaldulensis, E. citriodora, E. rudis, and E. terticornis have done best. These species can easily be grown with irrigation in the plain.
- Populus spp. Poplars are among the fast-growing trees in the world. The indigenous spp Populus ciliate, P. alba, P. euphratica, P. deltoids, P. euramericana occur in various areas.
- Acacias: Several Australian species have done well in the hills of India but they have not so far succeeded in Pakistan. But the African spp, notably Acacia tortilis and Acacia nilotica praise for the dry zone.
- Prosopis juliflora: It is the most successful introduction for really dry and saline/alkali sites and with the allied busy farm, P. glandulosa has become naturalized in some areas.
- Broussonetia papyrifera, Ailanthus excela and Robinia pseudoacacia: These are the most successful exotics in Pakistan. The species are extensively used in protective afforestation in the less arid uplands.
- Casuarina equisetifolia: It occurs on coastal sands and has been very successfully planted on similar sites outside its probable natural range. It has also been extensively planted far from the coast and on a wide variety of soil.
The Conocarpus Case
Introduction of Conocarpus
Mustafa Kamal as mayor (2005) under Karachi 2020 Strategic Plan earmarked 10 billion rupees for increasing the number of green areas in Karachi. Motivations included aesthetics, alleviating air pollution, and—anticipating future heatwaves—cooling and shade.
Monocultural Mass Planting of Conocarpus
Horticultural planners advised that a range of exotic and native trees, such as neem, gulmohar, and cypress, would be most suitable. But instead of a mixed ecology, in 2009 the government began a monocultural mass planting of Conocarpus erectus, a coastal wetland tree native to the Americas. Conocarpus is salt-, heat-, and sun-tolerant, making it suited to Karachi’s climate. It is also fast-growing—up to a foot a year—and cheap to cultivate on a large scale, at three rupees a cutting.
Decades ago lignum was monocultured on the streets followed by eucalyptus and then conocarpus from 2005 to 2015. The recent study shows that 62 per cent of street trees are of conocarpus species, which is an over-representation of a single species,
Karachi exhibits low species diversity mainly due to repeated planting of one or few species on its streets over the years. Overreliance on a few species could be a potential threat of their complete elimination from the streets due to diseases, pests or environmental changes.
The Side effects of Conocarpus
According to reports, instead of alleviating the 2015 heatwave, the city’s several million Conocarpus made the crisis worse. Agricultural researchers had long argued that the tree’s pollen was a public health threat since it exacerbates asthma and causes respiratory problems. In a move typifying the “ordered disorder” characteristic of Karachi, Conocarpus was banned in 2016 in favour of a wider mix of species. Mass Conocarpus planting might have been part of MQM’s political strategy, but it turned out to be an ineffective gloss over deep-seated urban inequality and vulnerability.
Expert Opinion about Conocarpus Plantation
According to Dr Zafar Iqbal Shams (senior teacher in Karachi University with a doctorate in environmental sciences), “In the urban forestry nowadays, exotic species are not an issue but rather the invasive species — the ones which spread and replace other species without human assistance and are extremely dangerous for local flora.” he said, adding that all exotic species were not invasive.
Besides being an exotic species with harmful effects on the environment, the conocarpus (a North American species) grown extensively in the city is a shrub and not a tree – Dr Zafar Iqbal Shams What we need to address is the increasing levels of pollution, traffic noise and urban heat by planting a large number of native trees
On native species versus exotic species, he said that the study also showed that the corridor heavily comprised conocarpus species (84pc). Earlier in 1993, there was not only a better composition of native and exotic species, but there were more trees and less shrubs as well.
“In 1993, there were 2,198 shrubs and 2,212 trees whereas in 2013 there were 8,780 shrubs and 756 trees. The species diversity in 1993 was greater than 11 US cities,” he said, while showing slides with lists of exotic and native species found during the study.
The number of local species, he pointed out, should be increased which would naturally decrease the proportion of Conocarpus, which is not an invasive species,” According to Dr Zafar Iqbal Shams, the city cannot afford to remove all the exotic species as its tree cover is already very thin.
The urban forestry rule, states that not more than 10pc trees of any species should be planted, not more than 20pc trees of any genus should be planted and not more than 30pc trees of any family should be planted.
Should we Plant Exotic Species? A Debate
Statements in Opposed to Exotic Species
Prof. (Dr) SM Imamul Huq, former chairman of the Bangladesh Council of Scientific and Industrial Research (BCSIR), pointed out that exotic trees made the soil less productive.
It is suicidal to plant exotic trees in the country. Birds and animals don’t get any food from these trees. The authorities should conduct research on trees. Such trees are harmful to croplands and also destroy the soil’s fertility. Even birds don’t nest in these trees as they give no fruits.
Exotic trees such as pine, podocarpus and cypress trees have a huge impact on soil chemistry on the ground where they are planted, preventing native grass, plants and herbs from taking root underneath the canopy. It has a knock-on effect on wildlife, resulting in their vacating the areas.
The indiscriminate planting of foreign species of trees and plants was causing immense harm to the country’s bio-diversity and eco-system as well as native species, experts say. Many exotic plants, such as eucalyptus, acacia (akashmoni), ipil-ipil and pines, are being planted across the country, especially in the northern region, putting the environment to grave danger, they observe. The experts maintain that planting native species and throwing away the exotic plants would benefit the eco-system.
The plantation of eucalyptus is banned at the government level, but there is no legal bar at the private level. So, such exotic trees are being planted indiscriminately across the country. Such foreign species of trees are even being planted on the boundaries of crop lands, posing a dire threat to the environment. The government should replace such trees with native species for the sake of the environment
We should plant environment and wildlife-friendly trees that would induce birds to nest on their branches and insects to tap food from them.
The plantation of the alien conocarpus tree would destroy the ecosystem of the federal capital, similar to the planting of paper mulberry trees.
You may know that experiment of plantation of conocarpus has badly failed in Karachi, as trees are harming the health of our citizens and our local ecosystems. Islamabad is presently suffering from a similar situation as paper mulberry, an exotic tree, was planted decades ago. This does not suit our environment and has caused pollen allergy to thousands of residents of our capital city,
Alien tree species would not only create problems for citizens in the form of allergies but would also destroy underground species, and grass and trees that grow naturally in the city.
Dr Anisur Rehman (Islamabad Wildlife Management Board (IWMB) Chairman) said, “Our local trees are very beautiful and they are environment-friendly. Currently, an American species – Lantana Camara – has been spreading in the Margalla Hills National Park area. It has captured almost 10pc of the national park. It is creating problems for the local species, and even grass cannot grow under the tree”.
Susceptible to diseases and can easily be ravaged by insects since they are not used to local environmental conditions, exotic trees increase the acidity of the soil, which prevents other plants to grow and multiply.
Exotic trees, on the other hand, have lower wood density and shorter lifespan, thus, they are not as effective as the premium native tree species when it comes to carbon sequestration.
Native trees provide habitat for smaller animals and insect predators, which are important in the control of crop and pasture damaging pests, thereby reducing the use of pesticides.
Likewise, these species of trees that originated from abroad are not protective of wildlife as animals, including many endangered species, prefer to nest and inhabit areas with native trees and avoid those they are not familiar with, she stressed.
Most insect predators such as bats, birds, predatory insects and parasites build their homes on native trees that create different layers of diverse, healthy and thriving vegetation underneath the canopy, which reduce the tunnel effect of strong winds and double the ability of trees in erosion control.
By complementing the local landscape, native trees also give an area a unique feel and provide a beautiful landscape while providing valuable resources for the survival of fauna species, given that some local animals are dependent on certain native trees in order to survive and thrive.
These trees whether naturalized or localized in our country are still invasive species. Because of this, our own native endemic and indigenous trees and plants are being displaced,
Likewise, according to the book, if a fast-growing species such as gmelina/melina (Gmelina arborea) or the known mahogany (Swietenia mahogany) are planted in a certain area, the slower native trees such as the kamagong (Diospyros discolor) will not be able to compete. Eventually, the much slower-growing tree will die, which may later lead to species extinction,
One of the justifications offered for the introduction of exotic trees is that they lead to better soil stability of the slopes on which they are planted. However, conservationists contradict this assertion. “The argument that exotics lead to soil stability is also fallacious, as the roots of these trees are very shallow, and the trees can get uprooted by high-velocity winds and heavy rain, which characterise the monsoons.
Statements Supporting Exotic Species
Moringa, native to parts of Africa and Asia (locally called sohanjana ki phalli), is a miracle tree with immense health benefits and should be planted in the city on a wider scale. The species was is in calcium and protein. Its leaves could be used for making herbal tea while the seed oil could be used to have a glowing look, a participant informed the attendees.
Forest Department officials said that the claim by green activists regarding exotic trees was baseless and no tree was harmful to the environment.
“The growth rate of eucalyptus and akashmoni is good. Poor people could get benefits within 10 years after planting such trees. This is not possible when it comes to native species,” Mohammad Shamsul Azam, deputy chief conservator of the forests department, told The Independent. “Moreover, these exotic trees have adapted to Bangladesh’s climate and soil,” he said.
The forest official also said that 90 per cent birds consume insects, while 5 per cent consume fruits and 5 per cent are herbivores. Besides, the wood of eucalyptus and akashmoni are used in brick kilns, he added.
Mr Rao Sadiq (M.Sc Forestry and experience of more than a decade with indigenous and exotic species), “In my opinion, we should move with the world because we need to adopt new species as per our market requirements. If we research, the world is moving towards new (exotic and genetically modified) species”. “While talking about exotics we forget that Shisham (most planted species in Punjab) and Acacia (in Sindh) are also exotics while these have adopted themselves with the local environment and considered as local,” he added. “The introduction of Moringa, Teak, Paulownia, etc will revolutionize the planting industry in near future and these are all exotics”.
“Yes, introduce exotics after detailed study about their silvicultural characteristics, suitability to conditions, economic value, and above all its use as habitat for wildlife,” Dr Muhammad Rafique (M.Sc Forestry) Ph.D. Forestry USA, Retired Chief Conservator of Forests (CCF) Punjab Forest Department.
Syed Nasir Shah (Conservator of Forests Balochistan Forest & Wildlife Department) stated, “We have tried so many exotic species on a trial basis in Loralai and Dhadar areas (of Balochistan) and few species were found very well eg Acacia victoriae. From the last 15 years, growth is good.” Mr Zubair Ahmed (Range Forest Officer) added that since A. victoriae is shallow-rooted (one of the drawbacks of exotics) and some of the said trees fell down due to heavy wind storms. He added that it is a fast-growing species. Leaves have high fodder value and it is drought resistant.
“Among other exotics, Mesquite is also a well-growing species. On the other hand, Quetta Pine (Pinus halepensis) (which is considered as local) is, in fact, an exotic species. Its old name was Persian Pine”, added by Syed Nasir Shah CF BFWD. “Throughout Quetta, the result of Quetta Pine is the same (it is drying). Same is a sign of exotic. Lahori saroo is also drying in Quetta (because of being exotic). The replacement for Quetta Pine can be Aasmani,” he further said.
“Being a Forester I would like to discourage this opinion (of not planting exotics species), as trees are of significant importance, one or other way native or non-native add colours to nature, the introduction of exotic species other than own habitat gives the idea or impression of innovation, indeed I would add it, a mode of innovation in growing trees which adds to depleting cover of forests. We can gauge trees of all kinds in terms of, shade, timber, wildlife and other related minor produce. hence any move which is directed to thwart exotic species may result in disaster to Forestry awareness”, stated by Mr Hosh Muhammad – Divisional Forest Officer (DFO), Sindh Forest Department.
What needs to be done?
Dr Muhammad Rafique concluded the question ‘should we stop planting exotic species’ with these words, “In my opinion, mere drawing some conclusion on the basis of experience as obtained with Mesquite and Eucalyptus is not fair. We need to study the species being introduced as exotic in its true veracity with all ramifications especially focusing on the purpose, and economics involved. The species management regime will go a long way in achieving the objectives of that introduction. I strongly urge to the introduction of forestry species aiming at reducing the gestation/harvesting period, and thus making that species a viable entity for quick financial returns, a drawback in our indigenous species and people’s reservation to involve in forestry enterprise than of agriculture. There is a lot of scope of agroforestry in the country, the species with cuppressi form growth and giving minimum shading effect on crops may also be considered in this regard. The drought conditions and aridity are alarmingly hampering all our all forestry activities throughout the country, some drought-tolerant but luxuriantly performing species side-by-side of Acacia, Prosopis and especially Tamarix.”
There needs to be a better understanding of the best ecological restoration practices that can be implemented when working in a landscape, experts argue, stating that the Forest department, too, needs to rework its forest regeneration strategies. Large-scale planting of exotics in ecologically-sensitive areas must be discouraged. The government should come up with a policy that stipulates that only native flora should be planted in public spaces because of its ecological significance or prior introducing any exotic proper research should be done and the introduction should be done after successful trials. “If a species has several benefits, a single drawback is acceptable,” concluded by Syed Nasir Shah CF BFWD
So what do you think should we stop planting exotic species? Provide your valuable feedback (based on your experience and expertise) in the comments section below.
- Biology Online
- Environment and Society Portal
- The Hindu
- The Independent
- Special thanks to all Forestry Experts who provided their valuable feedback on the topic.