Here is interesting research paper from one of our members. I am
forwarding it as Sunday reading.

regards
Pankaj Oudhia
========================================

Impact of Invasive Plants on the Structure and Composition of Natural
Vegetation of Northwestern Indian Himalayas1
RAVINDER K. KOHLI, KULDIP S. DOGRA, DAIZY R. BATISH, and HARMINDER
PAL SINGH2
http://wssa.allenpress.com/wssaonline/?request=get-document&issn=0890-
037X&volume=018&issue=05&page=1296

Abstract: Himachal Pradesh situated in the lap of northwestern
Himalayas is one of the richest repositories of plant diversity in
India. However, during the past three decades, because of the
increased pace of development and interference of humans through
introduction of invasive exotics, the ecology of the state has
changed tremendously. Ragweed parthenium, billy goat weed, and
lantana—the three exotics from South America—have caused much harm in
the state because of their invasive potential. A study conducted to
assess the changes in the structural composition and dynamics of
vegetation shows that density and diversity of native flora were
adversely affected because of invasion by these three exotics.
Because all the three exotic weeds are known to exhibit allelopathy,
it might be one of the major nonresource–based hypothesis for the
successful invasion by these exotic species.

Nomenclature: Billy goat weed, Ageratum conyzoides L.; lantana,
Lantana camara L.; ragweed parthenium, Parthenium hysterophorus L.

Additional index words: Diversity, dominance, ecological status,
Importance Value Index, invasive weeds, species richness, vegetation
analysis.

Abbreviation: IVI, Importance Value Index.








INTRODUCTION Return to TOC

Invasive plants are known to exert significant impact on the native
communities, resulting in their displacement and hence an imbalance
in natural and agricultural ecosystems (Pimentel et al. 2001 ; Sakai
et al. 2001 ). This imbalance results in the formation of large
monocultures of invasive plants in the alien environment. In fact,
invasive species are an important component of global change
(Vitousek et al. 1996 ). Invasion may be accidental or deliberate, as
a result of introduction of some species on purpose. The success of
these species in the alien and new environment may be attributed to
several reasons. Many theories have been proposed in this direction.
Possession of a set of traits by the invasive species as proposed by
Baker (1974) in case of ideal weeds may not be always true because
some species possessing only a subset of traits may be very invasive
in nature (Williamson and Brown 1986 ). Among the theories proposed
for the success of invasive species, absence of natural enemies
(pathogens, predators, or pests) known as Natural Enemies Hypothesis
or alternatively allelopathy (through the release of chemicals into
the environment) has been proposed (Heirro and Callaway 2003 ).
Besides, invasive species also have an ability to undergo genetic
changes due to selection pressure imposed by the alien environment
and exhibit quick response to anthropogenic disturbances (Sakai et
al. 2001 ). It is thus essential to study the mechanism of
invasiveness in alien environment especially regarding their
colonization, expansion, establishment, and ecological impact so as
to take timely action for their management.

India, one of the 12 megacenters of origin of cultivated plants, is
also one of the richest centers of biodiversity. Its rich diversity
is attributed to varied climatic conditions and its geography.
Himalayas, the well-known mountain ranges in India, passing through
many sovereign states of the country, are the storehouses of plant
diversity. Himachal Pradesh is one such state in the northwestern
India. Unfortunately, increasing anthropogenic disturbances, tourism,
pollution levels, rapid industrialization, and urbanization, besides
introduction of exotics, have put a great stress on the native
vegetation. Invasive plant species are a cause of concern, causing a
serious threat to the structure and dynamics of the natural plant
communities. A number of exotic species introduced either
deliberately or accidentally have been encountered in this state. Of
these, three weedy species, viz., billy goat weed, lantana, and
ragweed parthenium, all native to tropical America, are the most
abundant (Anonymous 2003 ). These are rapidly invading the forest
areas, grassland, agricultural lands, and pastures at the cost of the
native species, thereby causing a serious impact on the structure and
dynamics of the native vegetation.

A study was therefore conducted to evaluate the incidence of these
three exotic invasive weeds at different altitudinal ranges of the
state to study their impact on the ecological structure and
composition of the native vegetation.



MATERIALS AND METHODS Return to TOC

Study Site. This study was conducted in the hilly state of Himachal
Pradesh (30°22′40 to 33°12′40N and 75°45′55 to 79°4′20E)
located in
northwestern India (Figure 1 ). The state has an area of 5.97 Mha,
with altitude varying from 244 to 6,750 m and the climate ranging
from subtropical to alpine/semiarctic. Nearly 66.5% of the state has
been recorded as under forest cover (Forest Survey of India 2001 ).
The altitude increases from west to east and from south to north.

Vegetation Analysis. Geographically, the state can be generally
divided into three distinct regions, viz., lower or Shivaliks (350 to
1,500 m), middle (1,500 to 4,500 m), and the upper or the alpine
Himalayas (>4,500 m). For this study, the state was divided into 12
grids (equivalent to district)—four in each region. Each district was
divided into five zones and each zone into five locations. At each
location, five quadrats (2 m2) were laid randomly. In other words,
the study comprised nearly 300 sites and 1,500 quadrats (500 in each
region), and these were laid in such a manner so as to cover the
whole area evenly. In each quadrat, vegetation analysis was done, and
all the plants appearing were sampled, identified, and their
Importance Value Index (IVI) was calculated (Misra 1968 ). The
analysis was done over a span of 2-yr period, i.e., 2002 and 2003.
IVI was calculated using the following formula:

where



Furthermore, the vegetation was analyzed for species richness,
evenness, diversity, and dominance through ecological indices to
reduce complexity of data (Whittaker 1953 ). Though a great variety
of indices are available, to avoid conceptual and technical problems
and to get precision, only a few such as Margalef's richness, Hill's
evenness, Shannon's diversity, and Simpson's index of dominance are
applicable (Ludwig and Reynolds 1988 ). These indices were computed
as per the following formulas, using the statistical package of
Ludwig and Reynolds (1988) .

where S = total number of species, N = total number of individuals
of all the species, and ni = number of individuals of the ith species.

Furthermore, the IVI value of the species in the different study
areas was analyzed using the lognormal distribution to describe
abundance patterns. For this, species were arranged in order of their
abundance (starting from most common to most rare species) and
plotted against log10 of IVI values, thereby indicating abundance
pattern of vegetation.

Statistics. To cover whole of the districts, sites and locations were
so chosen. Quadrats were laid randomly at selected locations,
maintaining five replications. Data were analyzed by two-sample t
test wherever necessary.


RESULTS AND DISCUSSION Return to TOC

As per survey study, in 2002 to 2003, 9.4% of total species in the
state of Himachal Pradesh were found to be exotic, whereas the rest
(90.6%) were native ones. It was in sharp contrast to just 5% exotic
species reported in the survey conducted during 1987 to 1988 (H. P.
Forest Department 1988). In other words, the number of exotic species
has almost doubled over a period of 15 yr or so. Because most of the
exotics have a tendency to be invasive, the IVI and abundance data of
three most invasive weeds were calculated from all the three altitude
ranges of the state.

As regards billy goat weed, the reason for its greater IVI than
ragweed parthenium is attributed to climatic conditions because the
weed grows luxuriantly in the agricultural fields during winters.
Surprisingly, ragweed parthenium, which has played havoc in the
plains of India, is still in the process of colonization in the hilly
state of Himachal Pradesh (Figure 2 ). The data indicate that the
process of invasion by these three weeds is continuous and expanding
in the lower and middle Himalayas, but they are not present in the
upper Himalayas.

Further studies on the selected weeds were undertaken in the highly
infested areas of lower Himalayas so as to determine their impact on
the structural composition of the vegetation. In the presence of
billy goat weed, a total of 12 plant species were encountered,
whereas in the lantana- and ragweed parthenium–infested areas, 10 and
14 species, respectively, were counted in comparison with 25 in
uninfested areas (Table 1 ). Likewise, density and biomass in the
respective areas infested with three selected weeds were
significantly less compared with native vegetation (Table 1 ). The
density was reduced by nearly 64.4, 82.5, and 67.6% in areas infested
with billy goat weed, lantana, and ragweed parthenium, respectively.
Likewise, biomass was reduced by nearly 52.7, 72.4, and 59.6%,
respectively.

Richness index, a measure of numerical strength of vegetation, was
significantly less in the weed-infested than in the uninfested areas.
It was nearly 2.2 and 2.6 times more in the uninfested areas compared
with areas infested by billy goat weed and lantana, respectively
(Table 1 ). Likewise, the evenness index was comparatively more in
the uninfested areas, indicating that species are evenly distributed,
whereas the lesser values in the weed-infested areas indicate
patchiness in distribution. The Shannon's index also indicated
greater plant diversity in the uninfested areas, whereas this index
was reduced by 36 to 51% in the weed-infested areas (Table 1 ). The
higher values of index of diversity indicate the variability in the
type of species and heterogeneity in the communities, whereas the
lesser values point to the homogeneity in the community. On the
contrary, the index of dominance was minimum in uninfested controls
compared with weed-infested areas. It was maximum in billy goat weed–
infested areas, followed by lantana and ragweed parthenium (Table
1 ). The higher the value of index of dominance, the greater is the
homogenous nature of the community and vice versa. In other words,
such communities are dominated by single species. Furthermore, more
the homogenous nature of the community, lesser is the diversity and
hence stability, i.e., such habitats are subjected to renewed
disturbances and are occupied by more opportunistic and
environmentally flexible species with wider ecological amplitude. In
this study, where higher values of index of dominance vis-à-vis the
lesser diversity was observed in the weed-infested areas, indicating
the greater instability in such areas. Such a pattern is also clear
when the IVI values in all the weed-infested areas were plotted with
species rank using a lognormal distribution pattern (Figure 3 ). In
all three weed-infested areas, an abrupt decrease in vegetation
compared with uninfested areas was noticed. The lognormal
distribution for the uninfested areas indicates a normal curve of
vegetation (Figure 3 ). The changed structure of vegetation in the
infested areas may be due to the allelopathic influence of these
weeds because this has been reported to be an alternative hypothesis
among nonresource interactions besides several other factors (Bais et
al. 2003 ). Reasons for the depletion of vegetation were not explored
in this investigation; however, it is certain that these three weeds
are adversely affecting the structure and composition in the lower
and middle ranges of Himalayas. Thus, it calls for timely steps to
check their further spread.


ACKNOWLEDGMENTS

This work was carried under the B. P. Pal National Environment
Fellowship Award support to Ravinder Kumar Kohli by the Ministry of
Environment and Forests, Government of India. We are thankful to the
research staff of the laboratory for their help in carrying out this
work.




LITERATURE CITED Return to TOC

Anonymous. 2003. Report of Two Days Workshop on Management of Weeds
(Congress grass, Lantana, Neela Fulnu) Through Community
Participation. March 25–26, 2003. Shimla, India: State Council for
Science, Technology and Environment.

Bais, H. P., R. Vepachedu, S. Gilroy, R. M. Callaway, and J. M.
Vivanco. 2003. Allelopathy and exotic plant invasion: from molecules
and genes to species interactions. Science. 301:1377–1380.

Baker, H. G. 1974. The evolution of weeds. Ann. Rev. Ecol. Syst. 5:1–
24.

Forest Survey of India. 2001. State of Forest Rep. 2001. Dehradun,
India: Forest Survey of India. 58 p.

Heirro, J. L., and R. M. Callaway. 2003. Allelopathy and exotic plant
invasion. Plant Soil. 256:29–39.

Hill, M. O. 1973. Diversity and its evenness, a unifying notation and
its consequences. Ecology. 54:427–432.

H. P. Forest Department. 1988. Himachal Forests. Shimla, India: H. P.
Forest Department. Pp. 1–2.

Ludwig, J. A., and J. F. Reynolds. 1988. Statistical Ecology, A
Primer on Methods and Computing. New York: J. Wiley. Pp. 85–103.

Margalef, R. 1958. Temporal succession and spatial heterogeneity in
phytoplankton. In A. A. Buzzati-Traverso, ed. Perspective in Marine
Biology. Berkeley: University of California Press. Pp. 323–347.

Misra, B. 1968. Ecology Work Book. New Delhi, India: Oxford and IBH.
Pp. 31–51.

Pimentel, D., S. McNair, and J. Janecka. et al. 2001. Economic and
environmental threats of alien plant, animal, and microbe invasions.
Agric. Ecosyst. Environ. 84:1–20.

Sakai, A. K., F. W. Allendorf, and J. S. Holt. et al. 2001. The
population biology of invasive species. Annu. Rev. Ecol. Syst. 32:305–
332.

Shannon, C. E., and W. Weaver. 1963. The Mathematical Theory of
Communication. Urbana, IL: University of Illinois Press. Pp. 31–35.

Simpson, E. H. 1949. Measurement of diversity. Nature. 163:688

Vitousek, P. M., C. M. D'Antonio, L. L. Loope, and R. Westbrooks.
1996. Biological invasions as global environmental change. Am. Sci.
84:218–228.

Whittaker, R. H. 1953. A consideration of climax theory, the climax
as a population and pattern. Ecol. Monogr. 23:41–78.

Williamson, M., and K. Brown. 1986. The analysis and modeling of
British invasion. Philos. Trans. R. Soc. Lond. Ser. B. 314:505–522.



----------------------------------------------------------------------
----------

TABLES Return to TOC



Table 1.Ecological parameters of the three selected weedy species






----------------------------------------------------------------------
----------

FIGURES Return to TOC


Click on thumbnail for full-sized image.



Figure 1. Map of India showing study site (Himachal Pradesh)




Click on thumbnail for full-sized image.



Figure 2. Importance Value Index of three selected weeds in the lower
and middle Himalayas, Himachal Pradesh, India




Click on thumbnail for full-sized image.



Figure 3. Lognormal distribution of vegetation in the areas infested
by the selected weed species or in uninfested areas. Species are
ranked in order of their decreasing Importance Value Index values



1Received for publication January 14, 2004, and in revised form May
20, 2004.

2Professor and Coordinator and Research Fellow, Centre for
Environment, and Associate Professor and Research Scientist,
Department of Botany, Panjab University, Chandigarh 160014, India.
Corresponding author's E-mail: rkkohli45@...




----------------------------------------------------------------------
----------