AN INITIAL ASSESSMENT OF EXOTIC AND INVASIVE PLANT SPECIES IN SRI LANKA's FLORA AND THEIR IMPACTS

Abstract

Globally, exotic naturalized plants that behave invasively and occupy wildlands areresponsible for greater losses of biodiversity than any other factor except habitat loss anddirect exploitation of plant species by people. About I - 2% of naturalized exotic speciesbecome invasive in behavior, infesting and sometimes destroying parks, preserves andrefuges. This occurs because the invasive plant species: have no natural enemies ordiseases present; spread rapidly due to high reproductive and dispersal capabilities; and outcompete native species.

Publications relating to the Sri Lankan flora, and Master's theses available in the library ofDept. of Forestry and Enviromental Science at the University of Sri Jayewardenepuraregarding various Sri Lankan biological communities, provide the opportunity for anexamination of Sri Lanka's exotic flora and an initial attempt to assess their impact on SriLanka's plant communities. Tallying herbs and woody species by life form, habitatpreference and origin (indigenous, endemic or exotic) revealed the following. Twenty-fivepercent 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 Ivines (19.5%) are the life forms with the highest percentage exotics. The largestproportion of the total number of exotic species is found in disturbed (22.6%) and wetforest (18.4%) habitats. Amongst herbacious species the proportion of exotics is also highin aquatic habitats (44.2%). Certain plant taxa contain species which readily naturalize inSri 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 bankscontain 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 ormature forest species can not germinate or survive, this would be cause for much concern.I! is certainly something that merits further research.

Initial observations in the field were made of % cover by indigenous + endemic and exoticspecies along 7 randomly located transects in several different habitat types. Fieldmeasurements indicate Sri Lankan wetzone forests are highly resistant to exotics andinhibit potential invasive behavior. Except in young forest gaps (where up to 20% of covercan 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 casewas quite different. Greater than 80% of the cover was exotic species in random transectssampled in these three habitat types

The total impact of the diverse assemblage of exotic species in Sri Lankan's should be ofsome concern. The exotics are competing with each other in some habitats which preventsanyone of them from taking over and forming a monospecific stand, but the indigenousspecies are forced out regardless. How widespread this is in other habitats in Sri Lankamerits further research so management recommendations can made.