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The ecology and management of invasive plant species on tropical islands

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  • Tropical island ecosystems have proven to be inordinately vulnerable to invasions by exotic plants and animals. Today these islands only contain small remnant populations of the original flora and fauna, and these populations are facing increasing pressure from invasive plants. This paper attempts to answer four important questions whose solutions will help explain the phenomena of greater invadeability on islands. These questions are 1) Why is the vegetation on tropical islands so vulnerable to invasions? 2) What are the attributes of the woody plants which invade islands? 3) What are the impacts of the invasions on the natural plant communities? 4) What are the methods of control and their costs and benefits? Section two contains a management plan for the control of the invasive species Psidium cattleianum in Ranomafana National Park, Madagascar. The vulnerability to invasion of vegetation on tropical islands was attributed to evolution with different selective forces, the history of human disturbance on islands, disharmony, and an inherent vulnerability of island flora. The attributes of invasive woody plants which promoted invasions was explained in terms of safe site occupancy, demography, physiology, dispersal characteristics, and seed and fruit characteristics. The impacts of invasive plants on tropical islands can cause long-term changes in ecosystems. These changes include; a change in community structure, a change in fire distribution patterns, a change in nutrient status, a change in the soil-water regimes and hydrological processes, and changes in diversity. Ecological control methods in natural systems were examined as were their costs and benefits. The objectives of the control operation must be clearly understood before an integrated weed control program can be implemented. Mechanical, chemical, biological and legislative control options are testable, and such tests are recommended. The target plant should be understood on both an individual and population level. The economic, sociological and ecological implications of the control operations must be understood before an operation can begin. Often two or more control strategies may be used simultaneously to optimize the effectiveness of control. Once the desired level of control is achieved, native species for the site must be established to insure a maximum degree of ecological restoration. The objectives of the management plan in Ranomafana National Park, Madagascar are to control and greatly reduce the extent and dominance of strawberry guava within the boundaries of the park, and to occupy the current guava infestation sites with native species. The management plan will be implemented in two phases. The first phase will be for research and development, during which the baseline information regarding extent of infestation, control methods, and nursery and outplanting technology will be developed. The second phase will be the integrated management program, in which the control measures and the restoration efforts will be accomplished. An estimated minimum budget for an eight-year management plan without biological control is $214,740.
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