INITIAL ASSESSMENT OF EXOTIC AND INVASIVE PLANT SPECIES IN SRI LANKA’s
FLORA AND THEIR IMPACTS
Carla C Bossard
Fulbright fellow, University of Sri Jayewardenepura, Sri Lanka
Associate Professor, St. Mary’s University of California, USA
Globally, exotic naturalized plants that behave invasively
and occupy wildlands are responsible for greater losses of biodiversity
than any other factor except habitat loss and direct exploitation of plant
species by people. About 1 - 2% of naturalized exotic species become
invasive in behavior, infesting and sometimes destroying parks, preserves
and refuges. This occurs because the invasive plant species: have
no natural enemies or diseases present; spread rapidly due to high reproductive
and dispersal capabilities; and out compete native species.
Publications relating to the Sri Lankan flora, and
Master’s theses available in the library of Dept. of Forestry and Enviromental
Science at the University of Sri Jayewardenepura regarding various Sri
Lankan biological communities, provide the opportunity for an examination
of Sri Lanka’s exotic flora and an initial attempt to assess their impact
on Sri Lanka’s plant communities. Tallying herbs and woody species
by life form, habitat preference and origin (indigenous, endemic or exotic)
revealed the following. Twenty-five percent of Sri Lanka’s flora
is exotic species (15.6% herbacious and 9.4% woody species) not including
cultivated species not known to escape. Herbacious (34.6%) and liana’s
/ vines (19.5%) are the life forms with the highest percentage exotics.
The largest proportion of the total number of exotic species is found in
disturbed (22.6%) and wet forest (18.4%) habitats. Amongst herbacious
species the proportion of exotics is also high in aquatic habitats (44.2%).
Certain plant taxa contain species which readily naturalize in Sri Lanka.
Four plant families, Fabaceae, Verbeneaceae, Myrtaceae, and Rutaceae contain
>55% of all woody exotics. The Poaceae, alone account for 28.6% of
all exotic herbs.
Species lists from Masters theses done on two wet
zone forest reserves indicate seed banks contain about 30% exotic species,
while mature wet zone forests contain only 5 - 11% exotic species.
If seed banks contain the seeds of exotic species underwhich the native
or mature forest species can not germinate or survive, this would be cause
for much concern. It is certainly something that merits further research.
Initial observations in the field were made of %
cover by indigenous + endemic and exotic species along 7 randomly located
transects in several different habitat types. Field measurements
indicate Sri Lankan wetzone forests are highly resistant to exotics and
inhibit potential invasive behavior. Except in young forest gaps
(where up to 20% of cover can be exotic species) almost all cover is indigenous+endemic
species. In disturbed lands, wet and low zone grasslands and aquatic
habitat (fresh water pond) in Sri Lanka the case was quite different.
Greater than 80% of the cover was exotic species in random transects sampled
in these three habitat types.
The total impact of the diverse assemblage of exotic
species in Sri Lankan’s should be of some concern. The exotics are
competing with each other in some habitats which prevents any one of them
from taking over and forming a monospecific stand, but the indigenous species
are forced out regardless. How widespread this is in other habitats
in Sri Lanka merits further research so management recommendations can