R Uma Shankar, Department of Crop Physiology, University of Agricultural Sciences, Bangalore, speaks to Down To Earth
"Serious study has not yet begun"
What are the modes of successful invasiveness?
There have been some studies on recruitment of pollinators by lantana. When lantana invades as an invasive species into a native ecosystemit proliferates and increases its density and biomass. Consequently, there is a chance that it usurps the native pollinators from the native vegetations. We are trying to study the function of the density of the lantana in a given patch of say a forest ecosystem, the sort of recruitment of pollinators that otherwise would be pollinating other native trees into the lantana.
Our initial studies show that there has been a significant recruitment of native pollinators - a finite population in a given ecosystem. So when we introduce competing weeds such as lantana and if the floral resource rewards are going to be high from lantana which in this case it isyou now have pollinators flocking to the lantana flowers instead of going to the native flowers. This is what invasive species can do in an ecosystem. Normally the concern has been that it displaces the native biological diversity and causes reduction in the recruitment of native biological diversity. But recent studies have tried to show that invasive species can have an indirect effect on the ecosystem by usurping the pollinators and their dispersal.
For example, the fruits of the lantana are favoured by birds. Obviously, the bird has little time and when native fruit species are displaced by the lantana, there is a possibility that the bird species are going to be recruited to the seeds offered by the lantana. So there is not only an effective dispersal of lantana, there is also an effective recruitment of native dispersal agents from native trees to the lantana. So on the one had we are in a way accelerating the spread of lantana and on the other regressing the multiplication of the native biological diversity. The sum of these two events probably causes decline in the native biological diversity in a given time.
There is a paper by L Chittka called 'The successful invasion of a floral market' published in Nature 2001. This paper examines a plant from the Himalayas called Impatience, which has somehow traveled into Europe. It is now effectively displacing at a fast rate the native plant species for just one reason: its high nectar content, which attracts pollinators. So the pollinators instead of flocking to the native floral species are recruiting into the Himalayan species. This is one way of actually invading and usurping the pollinators and effectively swamping the native biological diversity. And this is precisely what we say is happening with lantana here.
It is true of many other species of weeds which are pollinated by insect vectors. This might not be the case for species that are pollinated by wind. So we have a situation where a finite pollinator population is now going to be shared by an invasive species which wasn't there earlier.
I would also mention another small issue that has been considered but not studied in lanatana so far - cross contamination of pollen grains. Assuming pollinators are not really limitedbut then because of the pollen spread in the ecosystem and because of the pollinators visiting the flowers of different species at the same time, there is a clogging of the stigmatic surface of a native species by pollen grains from an exotic or an alien one. So effectively although these plants are pollinated, they are pollinated by the wrong type of pollen grains and so again there is a reduction in seeds.
Apart from this, there was a very old study published in Nature by Prof. Mohan Ram from the University of Delhi who tried to look at colour polymorphism in lantana - red and yellow flowers. Looking into the evolutionary significance of these colour typesit was found that a butterfly visiting a red flower may not visit a yellow flower but would only visit another red flower. So this is what is called as 'assortative pollination'. And because of this different lineages of lantana seem to have been evolving.
What are the parameters that would define a species' invisibility?
This is a tough question. You can only put broad parameters. But then the predictability of this is quite difficult as you may have a very small proportion of this species actually becoming invasive while a large proportion do not become invasive for various reasons.But on the same count there is theories that propose what species are likely to be invasive. One theory isa species that is going to be invasive is likely to be one that displays a large amount of genetic variability. Intrinsically, that species would have a high amount of genomic variability expressed because if only there is such variability, these species can go and find their own adoptive niche in the native ecosystem. Elsethey would be lost and really wouldn't be adaptive. That is one explanation. The other explanation is based on the classical explanation of pioneering species. Some species are called climax species and some are called pioneering species. Pioneering species are those that produce abundant amount of seeds and can literally swamp the entire ecosystem with their seed banks. Even if only a small proportion of the seeds germinate, they are successful. But I am afraid there has not been any great degree of success in predicting in advance who is going to be an invasive. But at this moment I would like to mention recent work that has been begun in Bangalore by a group of people led by Dr. Jagdeesh Krishnaswamy and his colleagues, who are now trying to see and ask if I know the present occurrence of an invasive species like lantana. Let us say I know that it occurs in 20 different spots in peninsular India. The question that would be interesting to ask is keeping everything else as constant where else can I expect this species to invade in future time to come. The data required for this kind of analaysis is quite straight forward. What you need to have is the latitude-longitude data that is the point coordinates of the occurrence of this species at 20-25 different points and from thatusing what is called an 'ecological niche' model. You can now use a long string of climatic data for the last say 30-40 years and project the future climate change over the Indian sub-continent and find out which are the places within the Indian sub-continent which would comply with the current climatic requirements of the species as given by the 20-25 points of real occurrence that we have got today. This can be projected in the future and you can have a very reasonable forecast of where else it could be spreading or where are the potential sites it could spread to.
There is a group of people called the Kurubas in Pudupatti village off Natham in Dindigul district in Tamil Nadu, who have been using lantana as a substitute for bamboo in weaving basket and assorted articles for the last 60-80 years. They get a revenue of Rs. 30per month exclusively from this freezero investment resource. They don't pay anything to acquire this resource they go to the forests go to the wasteland and harvest these stems and weave it. When asked why they adopted this practice, they said that bamboo was difficult to obtain and they did not have the money to buy it. What is more interesting, however, is that the Kurubas use five other species of plants also regarded as weedy, such as alingiumphyllanthus reticulataand azima which are all available near their lakes and streams. And they procure these and use these also to weave the baskets. Here is an example where one could probably look at substitution as an important phenomenon in the process of either maintaining livelihood or also conserving the natural resources.
What are the impacts of an invasive species on an eco-system?
First of allI would like to mention that to the best of my knowledge in India we don't have a comprehensive study on invasive species with regard to their impacts, both direct and indirect.
In an ecosystemthe effects could be long lasting and it is very difficult to have a handle on it not only because of the time-scale but also our own difficulty in putting it together and anticipating all the processes that it might have an effect on. So most studies by default generally tend to look at immediate effects and do not look even three or four years ahead.
In BR Hills and MM Hills in south India, we looked at the effects of lantana on the vegetation and we also looked at the communities of birds and butterflies to see the scale and assemblages of these different taxonomic units as a function of different levels of lantana in those areas. We get varied responses in terms of whether vegetation and recruitment is affected or not. When we talk about vegetation and recruitment being affectedwe again have to travel down from community scale to individual species. So it is a whole conundrum of exercise which, unless we have a sharply defined focus of study becomes a little difficult to pinpoint.
There are not many studies done in India and I would only quote our own work where we have looked at some detail. We are now looking at the landscape approach of lantana whose densities are different and therefore we can impose a pseudo-replica and ask.' If lantana were to be at a certain density, what is the effect it poses on the rest of the vegetation or in this case the animal community, the butterflies and birds.' We are also proposing manipulative experiments. One of the propositions that has not got complete consensus is that we can remove lantana from huge parts of the forest and keep it in different parts to measure impacts. Because, all said and done, the impacts could be confounded by many other intertwined processes.
What are the worst invasive weeds in India?
In India, in both the ecological and management context. I would say lantana and chromolena are the worst culprits. Ecologically, these have made great strides in our forests be it the Western Ghats or the eastern Himalayas, which are the two mega diversity hotspots of our country. Management wise the very fact that they are in such numbers speaks volumes of our quality of management. There have been attempts to do biological control for both but they have met with failures. There have been attempts to control these weeds by manual processes, but this has not been possible. It is both because of sheer numbers and because they are reasonably tolerant to fire. Both also have the propensity to produce enormous numbers of seeds, so they are highly fecund. And the other problem is also the problem of managing such weeds in a forest where you cant take up herbicide spray, so to that extent they are out of bounds of any management control.
One has to understand micro climatic variables to put in place some kind of management. Such as asking where and why they don't occur. There is a very clear finding that lantana occurs in certain habitats and not others. We are now trying to study what the features of those habitats where lantana doesn't occur are. Is there a projection available from such a study, which can make a meaningful management? I think sometimes we have been so paranoid of invasives that we run out of common sense. Can we now think of using this, such as what we are proposing for lantana, maybe turning it into a livelihood issue. There is also a case of an acquatic weed called alternanthea phyllexeroides. Research is being carried out to see if this weed can be used for anything. It appears that this weed also called the alligator weed, is one species that has the highest leaf protein content - of about 24 - 25 per cent. People in the Hebbal area use it as cattle feed. They have converted a weed, which would otherwise clog the water surface into something useful.
What are the losses in economic terms of an alien species invasion?
A few studies have been carried out to ascertain the economic costs. There was a study by Pi Menthal called 'Environment and economic cause of non-indigenous species' published in Bioscience in 2000. It estimates that the cost to the US exchequer annually is millions of dollars from alien species. When we talk about costing, it is subjective to a large extent, but we fail to now fix the value of such invasive species. With lantana, for instance, we never count the cost to the ecosystem consequent to the reduction of seeds which is of course consequent to the pollinators being driven away which is due to the invasion of lantana. This kind of biological auditing is rarely done.
What ultimately are the effects of an invasive?
This is quite a straight-forward issue. When we are talking of lantana's effects in complex ecosystems such as in forests or in eucalyptus rubber and mango plantations, we are talking of two issues. One is of course the ecological consequence and the other is the management consequence. From the point of view of ecological consequence, it certainly is more complex in a complex ecosystem because we have not really attended to a number of species there that could be directly or indirectly affected by lantana. In a plantation, the interaction is just two-way. The effects could be quite less on the pollinators.
What causes an invasion to be successful?
I have a feeling this is a valid question for any alien species. The parameters that define the growth adaptability and fecundity of an alien species into another habitat will depend upon what factors predispose. There have been several hypotheses put forward for the possible success of an invasive. One of them is the 'enemy release' hypothesis. For example, lantana is supposed to have come from South America when the British brought it here as an ornamental plant. No, win South America, it is not a weed as we see it here, because it has got various checks and balances. A given species in a given ecosystem does not act on its own volition, it interacts with assemblage of species which then decides which is dominant and which is subordinate. So there is some sort of predominance and this is through some sort of interactive assemblage through the process of the evolution of that ecosystem has come into place. Now, assume that when you pull out of the system one of the species which is not predominant and implant it in an alien habitat, it is likely that this species can become the dominant species simply because it is not any more subjugated by the rest of the flock it has lived with or evolved with. Now that is what happens to an alien when it gets out of its habitat. But it is not necessarily true that it should happen all the time and to all the aliens. There is a possibility that it can get relieved of the suppression it is put in. The 'enemy release' hypothesis believes that in native habitats, because it has evolved in that area, it has also evolved with a whole load of herbivores who keep a check on it. But when you put the species in an alien habitat, you are not sending the baggage of herbivores along.
Can India evolve a system of research and management of invasives?
Yes perhaps it can. As you may know, India has a fairly well-established plant quarantine system in place coordinated by the Indian ministry and the national genetic resource centres in in the Indian Agriculture Research Institute campus, Delhi. It is vested with complete control of what material comes into the country and what goes out. We cannot transfer any biological material without its permission. But there can never be a foolproof method because even a small consignment can play havoc. I feel we probably should have a good monitoring device which would play the role of a watch-keeper who in advance comes out with an information package which tells where the signals are if something is really getting out of hand.
Do you think our focus should be within the country or outside?
We have been talking about invasives as occurring through trans-national and trans-political boundaries. But this is a misplaced definition of invasives. Plants do not recognize political boundaries, they recognize habitat boundaries. So if a plant were to come across the Pakistan border, say from Karachi to Amritsar, we call it an alien. But when it comes from Amritsar to Bangalore, we don't call it an alienal, though the distance separating Amritsar from Bangalore is much more. On top of that, the subcontinent's landscape is very heterogenous. We believe our focus should be all-inclusive, whether it comes from Orissa to Karnataka or Canada to Orissa, it should be treated on one currency - of how much habitat jump has been made by the species rather than how many countries it has jumped. If this is done, we will have a greater degree of sensitivity to deal with aliens and non-indigenous species.
For instance, the acacia consinna, which is used to make soap powder is normally found in moist deciduous forests. But our studies have shown that in certain evergreen forests, such as in the sacred groves in the Western Ghats, it has been found. Nowhere is a deciduous species which has in my opinion invaded an evergreen habitat. So, to me this is also an alien to that habitat although it is not an alien in terms of the Indian landmass. It can create havoc in the evergreen forest by usurping the evergreen species.
What we propose is a re-look at the definition of invasive - is it an exotic invasive or is it an indigenous invasive? We should merge these together and have a common definition of species that move from one habitat to another.
Is this new definition gaining acceptance?
No, it is not widely accepted. The problem seems to be most people talk about trans-boundary plants as invasives.
Has biological control been successful in stemming invasives?
There have been some attempts at biological control. But I believe none of them have really taken off in any great degree. There is always an apprehension that these biological control programmes may fail and have an effect that was not intended.
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