3 The detection and extent of the species

3.1 Detection



The early detection of alien plants is crucial. This may be achieved by paying attention to the following:



ˇ Establish a list of plant species known to be invasive in regions with similar climatic and ecological characteristics.



ˇ Determine which of these species are present in the protected area and surrounding region. Set up and regularly update a list of newly introduced ornamental plants. A database of recorded locations of introduced species should be initiated and distribution maps for fieldworkers produced.



ˇ The vegetative and reproductive potential of the alien species should be assessed. The mode of dispersal of propagules (i.e. fruit, seed, vegetative material) needs to be identified and in the case of animal dispersal the identification of the animal(s) is required.



ˇ Monitor potential natural regeneration. The frequency of monitoring should be determined by the biology of the weed(s) or potential weed(s), particularly the time taken to reach reproductive maturity, and should be more frequent in habitats in which the weed is more likely to establish or grow more rapidly. For wind-dispersed species only the 'seed shadow' needs to be investigated whereas for vertebrate-dispersed species the area to be monitored will depend on the behaviour and range of the disperser. Species dispersed by water may disperse over very long distances during heavy flooding and this should be taken into consideration during monitoring operations.



ˇ Roads and tracks often act as corridors for the spread of invasive plants into natural areas and need to be looked at in detail. Propagules may be transported by animals, humans (e.g. shoes), vehicles (e.g. tyres), soil and dumping of horticultural refuse and the extent of these activities needs to be monitored.



ˇ Very large disturbance events occur at irregular intervals (often centuries) and may affect large areas. Conditions caused by such events are ideal for the spread of most introduced species and special care should be shown to monitor the recovery of vegetation and possible spread of invasive plants.



ˇ Staff of protected areas should be made aware of the threat from alien plants and should be instructed to look out for weeds that may be likely to invade, both within and outside protected areas, based on evidence from similar habitats elsewhere. Staff should be issued with invasive woody plant distribution maps and identification guides.

3.2 Determining the extent and density

The initial step in developing an effective management programme should be the mapping of the current and potential range of the species, its density and size class distribution. However, care must be taken not to use so much time or resources in this step that the population is allowed to expand significantly before control measures are initiated. The level of detail required is determined by the purpose of the assessment. A survey to assess the extent of alien plant invasions in a large number of areas (perhaps as part of a programme to compare the conservation status of these different areas) would need less detailed information than one to plan an actual manual control operation.

The survey method depends on the extent of the invasion, the terrain in which it occurs and the resources available. Regular surveys should be undertaken to provide information on the rate of weed expansion and the effectiveness of any control measures.

Survey methods include:

Remote sensing. At the largest scale satellite images could be useful, especially where an alien woody plant has heavily invaded large areas of a treeless vegetation. However, the use of LANDSAT data to map the distribution of Melaleuca quinquenervia in south Florida was unsuccessful (Bodle et al. 1994). On rangelands remote sensing techniques, including computer based image analysis, are effective in assessing infestations of noxious plants as long as the right phenological stage of the targeted species is investigated (Everitt et al. 1995).

Aerial photographs have been used and are generally more useful, allowing much greater resolution and a longer historical record. In the case of species which can regenerate under forest canopy, such as Miconia calvescens in Tahiti, aerial photographs will only detect canopy individuals and fail to reveal the true extent of the invasion (Meyer 1996). To monitor the spread of Melaleuca quinquenervia into non-forested ecosystems of Florida, the use of infrared photography at a scale of 1:12,000 was suggested to detect individual trees. A survey at a scale of 1:40,000 was carried out but only allowed the detection of mature, monotypic stands larger than one hectare (Bodel et al. 1994). The rate of spread can be estimated from aerial photographs for a number of different years (e.g. Mimosa pigra in northern Australia, Lonsdale 1993). Aerial photographs must be verified with ground surveys. In some specific cases an invader may be detected in natural vegetation using aerial photographs or in mountainous areas from vantage points. Although the phenological status of plant is critical, some species with contrasting foliage, such the pale-leafed Aleurites moluccana, can be readily recognized in forest stands at any time of the year either from aerial photographs or from vantage points in mountainous areas (Kepler 1990, p. 71).

In North America Tamarix chinensis has been readily detected and it distribution mapped in late November, when its foliage turned a yellow-orange to orange-brown colour prior to leaf drop, using airborne video data in conjunction with global positioning system and geographic information system technologies (Everitt et al. 1996).

Ground survey. The various techniques of ground survey are not dealt with fully here, but should include areal stratification if the area is composed of relatively distinct sub-areas, defined by degree of invasion or forest type, for example. Plots (permanent or temporary) are usually the best means to record the information, transects being useful if the intention is to sample along a gradient of some vegetational or environmental factor. Nested sub-plots are useful for sampling successively smaller individuals in smaller areas. The level of data collection should be dictated by the objectives and available resources, but ranges from:

ˇ Assessments of the frequency of alien plants as seen from roads throughout a region (as with Henderson & Musil 1984). Regular monitoring near points of introduction of alien species (e.g. forestry plantations, amenity planting, botanic gardens) is desirable.

ˇ Simple estimations of coverage of alien and native species in each sampling unit or plot.

ˇ The permanent marking, measuring and identification of all individuals of all species, native and alien, within each sampling unit or plot (refer to Alder & Synott (1992) for techniques of permanent sample plots).

Historical survey. Much useful information can be gained from research of local records and unpublished material. For management the greatest value of historical records is where they provide evidence that the spread of a species is dependent on occasional events, such as major hurricanes or changes in livestock grazing pressure. In this case the present rate of invasion (in the absence of such occasional events) may provide little indication of the potential rate. A number of highly invasive species are known to have spread only after major environmental disturbances (see case histories for details).

Local knowledge and public awareness. People who live in the area may have much information, some of it from their predecessors, on the spread of alien plants and other information to do with their use and effects. Locals may also be able to monitor future changes.

It can be very difficult to interest the public in the problem of weeds. In Hawaii for example, where the publicity about invasive plants is probably higher than anywhere else in the world, new tourist developments in or near natural vegetation still landscape their gardens almost exclusively with exotic plants (Yee & Gagné 1992).

In order to prevent the spread of the highly invasive tree Miconia calvescens to the remaining islands Society Islands a colour leaflet outlining the problem and providing instructions to local inhabitants on what to do if they came across the species (Meyer undated). It is not known how effective the dissemination of such material is.


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