2 Addressing the underlying causes of invasions



2.1 Knowledge requirements



An understanding of the biology of the alien and the ecology of the invasion process should be fundamental to all attempts at managing alien plants, but can be difficult and costly to obtain. Therefore, an assessment has to be made of what level of information is desirable and is feasible to obtain. The primary objectives here are to gather necessary information on the following:



The taxonomic identity of the alien plant. In some cases specimens will need to be collected for identification by experts. Molecular biology techniques hold potential for distinguishing species or provenance from close relatives where this cannot readily be done by morphological criteria.



The ecological history of the invader. This entails an assessment of the potential time-lag between the species introduction and spread, the intensity, extent, rate of change and impact of the invasion as well as the environmental factors (e.g. disturbance) affecting the invasion.



The type of control that is likely to be most effective, or effective at least cost. The level of information needed depends primarily on the extent of the invasion, and the identification of underlying causes or predisposing factors that facilitate the invasion.



Details of the actual control method. The level of information needed depends mostly on the type of control. If the invasion is small, and containable with the use of manual control only, rather limited information is needed - how does the species respond to physical damage and herbicides? Much more detailed information is needed for biological control and, usually, for habitat manipulation.



2.2 Maintain the natural ecosystem processes



A key requirement is to maintain the ecosystem in as close to the 'original state', the most recent state before the impact of modern man, in which the native plant and animal communities evolved, as possible. The key processes are discussed below, though managers have little control over some of them.



Canopy disturbance regime. Many invasive species are dispersed more rapidly into disturbed areas and many are also competitively favoured in conditions of higher levels of available resources, especially light. Therefore, disturbance of the existing vegetation canopy, whether it is of trees, shrubs or herbs, is a key feature of many invasions. Disturbance caused by tree cutting or livestock grazing can clearly increase the rate of invasion by alien plants and can be limited by man.



Fire. Fire suppression policies carried out in many Mediterranean and warm temperate countries change the natural functioning of the ecosystem and so can lead to the invasion of fire sensitive species, as in the case of Pittosporum undulatum in Australia (Narayan 1993). The interaction between fire and invasive woody plants has been extensively reviewed by Bond & van Wilgen (1995).



Nutrient budgets. Many invasive species are competitively favoured in conditions of higher levels of available mineral nutrients than normally experienced by the native vegetation. In the Everglades, Florida, agricultural soil, derived from crushed limestone, was removed to exclude Schinus terebinthifolius (Doren & Whiteaker 1990).



Hydrological regime. Drainage, and consequent lowering of the water table below soil surface level, has been a major factor in the invasion of alien woody species in southern Florida. It could be predicted that where shortage of water is a limiting factor for plant growth, an increase in the availability of water is likely to favour invasive species more than native species.



Herbivores. Introduced pigs and goats create the disturbance essential for the establishment of many alien plant species on several islands (Stone & Loope 1987), or can preferentially eat native plants, giving the alien a competitive advantage. In such cases it will often make more sense to remove the animal (if this is feasible and politically acceptable), rather than try to directly control the alien plant species.


  Seed dispersers. Alien vertebrates, including birds and mammals, can be important dispersers of alien woody plants, (Cuddihy & Stone (1990) have reported a number of cases for Hawaii). Investigation of the main means of dispersal of the alien plant species should be carried out to determine if this provides the best opportunity to control the invasion.
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