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Alien Plant
Invasion: A Hot Ecological Issue
By: R. S.
Tripathi*
Migration of species from one geographical region to another across the natural
barriers such as high mountains, seas and oceans has been taking place since
time immemorial. The movement of species through natural dispersal agents has
been rather slow. However, with globalization there has been a phenomenal
increase in trade, tourism, travel and other human activities, and this has
caused both intentional and unintentional introduction of species from one
country to another at a pace that was never witnessed before. If a plant species
arrives in a territory where the habitat conditions are similar to its native
place, it germinates, survives, grows, reproduces and produces self-sustaining
populations in areas of natural and semi-natural vegetation in course of time.
Many exotic species may grow luxuriantly in the new environment and can expand
their range of distribution at a fast rate, and may even pose a serious threat
to the native species. Such introduced or exotic species are referred to as
invasive alien species. Invasive plant species are characterized by rapid
growth, high reproductive capacity, efficient dispersal mechanism, high
competitive ability and wide ecological amplitude. They have unique ability to
adapt physiologically to new environmental conditions. Most of the invasive
plant species possess high phenotypic plasticity coupled with hybridization
capacity and highly efficient reproductive strategies. These attributes of
invasive species contribute to their ecological success and capability to invade
new areas. It may be noted that all the alien/exotic species that enter a new
territory may not be invasive.
The problem of biological invasion has been recognized by SCOPE (Scientific
Committee on the Problems of Environment) as a central problem in the
conservation of biological communities. The invasive alien plants have serious
ecological implications for the conservation of native biodiversity, maintenance
of plant community structure, plant succession and ecosystem processes in the
areas invaded by them. Keeping in view the crucial role played by invasive alien
species in the conservation of biological diversity and its sustainable use, the
Convention on Biological Diversity (CBD) proclaimed invasive alien species as
the theme of this year's International Day for Biological Diversity. The United
Nations have declared 22 May the International Day for Biological Diversity and
the year 2010 the International Year of Biodiversity. The problem of plant
invasion, has engaged the attention of ecologists, conservationists and
environmentalists all over the world during the past 3-4 decades especially
after the launch of the Global Invasive Species Programme (GISP) by the SCOPE
during eighties. As of now, the problem of plant invasion has become a matter of
grave concern all over the world. However, in India, so far, the problem of
plant invasion has not been addressed as adequately as it
should have
been done, although several exotic plants have invaded the high-value
biodiversity areas and have adversely affected the natural and semi-natural
vegetation/ecosystems.
Biological
invasion and climate change are the two hottest topics of ecology these days.
The extent of distribution, rate of spread, and persistence of invasive alien
species directly influence the native biodiversity of the invaded region and
therefore, the trends in invasion by alien species has been identified as an
important indicator of the loss of biodiversity. The 'Convention on Biological
Diversity's framework for monitoring progress towards its “2010 Target” which
commits CBD to achieve by 2010, a significant reduction in the current rate of
biodiversity loss, regards biological invasion as one of the two major threats
to biodiversity. It has been reported by a number of researchers that the
invasion of plant species in the new environments is triggered by man-induced
habitat fragmentation, land degradation, forest degradation, land use and land
cover changes, fire regime and other kinds of anthropogenic stresses that impact
natural ecosystems. The facilitative effect of these drivers on plant invasion
may presumably be mediated through the reduction in biotic and physical
resistance of various kinds that would have been offered by the undisturbed host
plant community. The quantification of the extent of influence exercised by
different kinds of environmental resistances to the invading plant species in
the host community could be a very challenging area of ecological study. Apart
from causing depletion of native biodiversity, invasive alien species also alter
species composition, affect soil physical, chemical and biological properties,
and affect community development and ecosystem processes adversely, but no
reliable quantitative data is available on any of these aspects. The effects of
invasive alien species on the distribution, abundance and population dynamics of
native plant species in natural ecosystems, hydrology, soil biology, and
ecosystem processes need to be studied in detail. In India, a good number of
high-value biodiversity sites have been invaded by several invasive alien
plants, but unfortunately, the studies on the biology of plant invasion have
been only scanty. The biology and population dynamics of a number of exotic
weeds have been studied by the author and his collaborators at the department of
Botany, North-Eastern Hill University, Shillong during 1980- 2000. Besides,
researches on weed biology have also been conducted at some other universities
and research organizations in India, but the plant invasion perspective has been
missing in most of these studies.
Although the
problem related to the invasion of alien plant species has engaged the attention
of ecologists, foresters, agricultural scientists and government agencies over
the past few decades, nothing tangible has been done to eradicate and manage the
ever-increasing populations of invading plant species that have already
established in the invaded areas and are extending their range of distribution
due to man-induced habitat fragmentation and other kinds of anthropogenic
stresses. The economic and ecological costs associated with the invasion of
these alien plant species are indeed staggering. Some of the interesting
aspects, and the exciting points that emerge as a result of analysis and
synthesis of the scientific information gathered on relevant aspects of plant
invasion, are presented below.
-
The
genetic changes are likely to occur in alien species subsequent to invasion
in a new region, and these changes may hold key to their success in the
invaded land. An invading species that has colonized a novel environment has
to face a genetic challenge, because it has not experienced the selective
pressures presented by the new environment. Despite this, alien species
become successful invaders although they have to face challenges from the
already well adapted native species. The biologists need to find out the
under lying mechanisms and processes that make the invading species so
successful in their new environment.
-
Some
invasive alien species being intrinsically better competitors, offer strong
competition to native species in the invaded region. The native species show
a decline in resource use and invaders can increase their distribution and
abundance at the expense of the resident species of the area. This may cause
a drastic reduction in population size of several native species and some of
them may even be eliminated from their natural habitats.
-
Many
invasive plant species release chemical compounds into the environment ,
which are not generally harmful to them, but those chemicals suppress the
growth of other plant species growing in close proximity of such invasive
species. This negative effect (often referred to as allelopathic effect) of
invaders on the native species confers a tremendous competitive advantage on
the former. The 'chemical release hypothesis' offers a plausible explanation
for the spectacular success of invasive plant species in the new areas that
they invade.
-
The
herbivores and parasites or pathogens, the natural enemies of the invasive
species, that were regulating their population growth in their native place
are absent in the invaded region. Invading species generally arrive in their
new environments without their co-evolved natural enemies from their natural
habitats that they had occupied in their native place. This may provide
invaders opportunities for luxuriant growth and more prolific reproduction,
which allows them to out-compete native species, and expand their range of
distribution. This is the basis of the so-called 'enemy release' or 'escape'
hypothesis which is employed to
explain
the spectacular success of invasive alien plants in their new environments.
The above
hypotheses or approaches explain why and how alien species become more
successful in the invaded land compared to their native place. It may be
mentioned that the majority of studies on invasive alien species have been
conducted in the invaded territory, and surprisingly, we do not have any
quantification regarding their abundance, competitive success, aggressiveness
and response to the natural enemies in their native land. The soundness of these
“invasion hypo-theses” can be tested only when we have comparative
bio-geographical approach towards the problem of biological invasion and have
sufficient relevant data from the native as well as invaded regions.
For any
species to become successful, it is essential that it genetically adapts itself
to its new environments. It may also be mentioned here that preserving genetic
diversity is absolutely necessary for species to continually adapt genetically
in a changing environment. Therefore, investigating the genetic adaptability of
invasive alien plant species in the new environments should also be an issue of
focus among population ecologists and conservation biologists.
Besides
impacting native biodiversity through direct competitive suppression, the
invasive alien species having strong allelopathic potential may al so influence
plant diversity by converting a complex plant community into a much simpler one,
which is characterized by the dominance of only a few species. In extreme
situations where the impact is severe, the single species-dominance may also
result. This kind of effect of invasive species has serious implications for
food chain length and complexity of food web, which are pre-requisites for
ecosystem stability and smooth functioning of ecosystems. Unfortunately, the
researches related to impact of invasive plant species on ecosystem processes
are scarce. Ecological impact of plant invasion is a serious cause of concern to
all of us. The ecological consequences of plant invasion are many-fold and we
need to have zero-tolerance towards invasive alien species. The issue of alien
plant invasion needs to be addressed very seriously like the problem of climate
change.
Indeed, the
problem has engaged the attention of ecologists during the past few decades, but
the coordinated rigorous effort involving multinational research teams has been
missing. In India where the problem of alien plant invasion is quite severe, the
Union Government has recently taken some initiative to address the issue in a
befitting manner. The state governments should also follow suit. Some of the
aspects/points related to alien plant invasion in India on which we need to
focus our attention without any further delay, are as follows.
-
The status of alien plant invasion in India.
-
Identification of the worst alien species that have invaded the natural
ecosystems in different bio-geo-graphic regions of India, their
distribution, rates of invasion, and their population dynamics.
-
Invasive alien species and their impact on native biodiversity, plant
community composition and eco-system processes.
-
Attributes and ecological strategies of the worst invasive alien species and
their pathways of invasion.
-
Identification of the habitats and ecosystems that are most vulnerable to
plant invasion.
-
Exploring the causes of spectacular success of the invasive plants in the
natural ecosystems of India.
-
Plant invasion as related to various kinds of anthropogenic disturbances,
fire regimes, species richness and species composition and habitat
characteristics of the host plant communities.
-
Plant invasion in relation to elevated concentration of carbon dioxide,
climate change and other global changes.
-
Genetic adaptability of exotic species in contrasting ecological habitats.
-
Impact of invasive alien plants on physical, chemical and biological
properties of soil of the areas which they invade.
-
Effects of invasive alien plants on ecosystem processes.
-
Strategies and action plans for effective management of some of the worst
invasive plant species at the local, eco-region and national levels.
There is a
need to launch a coordinated/network research program at the national level in
India encompassing the points outlined above. We need to develop a policy
framework for tackling the problem of plant invasion in India. Special emphasis
has to be laid on some of the worst invasive alien weeds that have been
spreading very fast and have become established and naturalized in several parts
of India. Notable among such alien species are
Ageratum
conyzoides L., Eichhornia crassipes
Solms.,
Eupatorium odoratum
L. (syn.
Chromolaena odorata
(L.) King &
Robinson),
E.
adenophorum
Spreng. (syn.
Ageritina adenophora
(Spreng.)
King & Robinson),
E. riparium
Regel.
(Ageritina
riparia
(Regel.) King
& Robinson,
Galinsoga
ciliata
Blake,
Lantana
camara
L.,
Mikania micrantha H.B.&.K., Parthenium hysterophorus L. and
Prosopis
juliflora
DC.
There is a
need to launch a national website and create a National Authority that should
cover the entire gamut of the problems associated with biological invasion. The
proposed Authority could monitor the invasion and spread of alien species,
educate people about the adverse impact of invasive alien species on native
flora and integrity of natural ecosystems, and on human and animal health. The
Authority could also formulate strategy and action plan for preventing alien
invasion, and for the control and eradication of invasive exotic species, and
suggest suitable mitigation measures where the preventive and control measures
fail to yield the desired results.
*INSA
Senior Scientist, National Botanical Research Institute, Lucknow-226 001
E-mail:
[email protected]
____________________________________________________________________________________________________________________
Financial
support from the Indian National Science Academy, New Delhi under the INSA
Senior Scientist Scheme is gratefully acknowledged. |
