4 Assessing the effects of alien woody plants



All introduced species, but particularly invasive species, have some form of impact on the environment of the introduced region. Many species affect the landscape for instance as a result of their distinct morphology or striking floral or fruit displays. Many invasive species affect, to a varying degree, the structure and function of invaded ecosystems. However it is difficult to determine accurately their impact(s). Furthermore, even if demonstrated, these impacts cannot easily be classified as neutral, positive or negative because the cultural, political, social and employment background of the observer will to a large extent determine the conclusions reached. Natural regeneration can be given as a simple example. To foresters the regeneration of a desirable species (e.g. introduced timber trees) is judged positive but that of a forest weed (e.g. Chromolaena odorata, Clidemia hirta) as negative whereas to a manager of a 'natural' ecosystem the regeneration of any exotic species will be seen as negative. In this report we assume that an introduced species has a negative impact if it alters ecosystem structure and function.



Accurately determining the effects of an alien woody plant is difficult or impossible without long term research. This is due mainly to the length of the life cycle of woody plants and to the medium and long term disturbance cycles usually affecting the invaded ecosystems. Here we can only make a few general points about how someone could go about assessing the effects of an invasive plant.



4.1 Negative effects



The negative effects of an invasive species are closely correlated with the maximum density that the species can attain and its persistence at any one site, so the most heavily invaded areas should be investigated first. The principal questions are:



· How dependent is the alien on disturbance for its recruitment? How persistent is the species in the community under the prevailing long-term disturbance regime?



· What is the negative impact of the species? Does the species grow so densely or produce allelopathic compounds that native vegetation is very sparse beneath? In cases of early invasion, seed production or dispersal may be insufficient to lead to dense regeneration of the alien, but with more established populations the seed rain or soil seed bank can be so large that seed availability ceases to be a limiting factor.



· In areas with a high species richness and endemism, does the invader significantly affect the regeneration of rare and endangered plant species? Similarly what is the impact of the invader on animals with significant conservation value? Does the invader favour introduced animals to the detriment of native ones?



· Does the invader alter nutrient and hydrological cycles, light regime and fire susceptibility?


· Species may seriously affect human activities. A number of species create physical barriers to humans (e.g. Clidemia hirta) or may create a health problem (e.g. Lantana camara, Melaleuca quiquenervia). Some invasive species are poisonous to domestic animals (e.g. Lantana camara) while others become weeds of agricultural, forestry or pasture land (e.g. Mimosa pigra).



4.2 Positive effects



Beneficial effects can be great and have in the past sometimes been overlooked by protected areas managers, scientists and conservationists. The role of the weed in the local economy and the public perception of its value must considered. Possible beneficial effects of alien woody plants include:



· Timber; alien trees can produce good quality timber, (e.g. Pittosporum undulatum in Jamaica).



· Wood for use as firewood or charcoal; invasive woody plants can often be a highly productive source, often with a high coppicing ability.



· Coloniser of steep or bare slopes; the highly invasive Pueraria lobata and Pueraria thunbergiana stabilise steep slopes in, respectively, south eastern U.S.A. and Puerto Rico (Markin & Gardner 1993; Telford & Childers 1947).



· As a source of nectar and pollen for bees and insects. · As a source of fruit for animals and people; for example, in Fiji Psidium guajava (guava), a noxious weed of pasture, is very popular for making jellies and jams (Mune & Parham 1967).



· Aesthetic values; the public may prefer the pretty flowers or tallness of an alien tree to the more subtle appearance of a native tree, and do not share to such a degree concerns about the ancestry of the plant or how "natural" is its presence.



· Traditional medicine often uses invasive species although it is unclear whether these 'new drugs' have any practical purpose.



· Biodiversity value; on Philip Island (south Pacific Ocean) Olea europea L. subsp. africana, and associated leaf litter, is probably responsible for the survival of an endemic species cricket (Rentz 1993). On this rabbit infested island the introduced plant, which also protect the island's only naturally occurring clone of Hibiscus insularis, needs to be preserved until native shrubs flourish once again and provide the protection the cricket needs.



4.3 Selecting which alien to control



Generally, tropical plant communities susceptible to invasive woody plants tend to be invaded by more than one species at a time. Furthermore, in a given region, different species will spread in different plant communities. Resources rarely permit more than a few species to be controlled, so prioritising which species to control in which habitat is necessary. Not only must the cost and benefit of removing each species be considered, but the effect that each species may have on other alien weeds, and the consequences of its removal, should also be assessed. It is very difficult to predict the invasiveness of alien species and even more difficult to accurately predict the relative effects each may have, but some assessment is necessary. Some of the factors that should be considered are:


  · Invasiveness and potential impact of the species.
  · Current extent and density of the invading population.
  · Habitat types invaded and their recent disturbance history.
  · Effect that the species or its removal has on the invasion of other weeds.
  · Ease of control and the threat of re-invasion from outside the area.

· Species characteristics including dispersal agent and potential dispersal distance, presence or absence of seed bank (if present need to know size, duration of seed viability) and vegetative propagation (suckering, coppicing, layering) ability.



For important species for which there is much information, a Cost-Benefit type analysis can be done. With less information or more species, a simpler approach would be to:


  · Score for each species each factor that could affect the cost and benefit of control (such as potential impact on the ecosystem if the species were allowed to spread, relative extent, resistance to herbicides).
  · Weight each factor based on more or less subjective assessments of how important it is and whether it is 'good' (plus) or 'bad' (minus).

· Combine the scores to derive a ranking of control priority.



Since 1982 in the Hawaii Volcanoes National Park there has been a growing emphasis on controlling easily eradicable species (Tunison & Zimmer 1992). In 1988 forty-one alien plant species were being controlled, mostly species present in low numbers but judged to be potentially widespread and troublesome. This emphasis on the control of alien species with still only limited ranges is supported by uncertainties about which species will eventually become a problem. However, another 29 species were so widespread that manual control was no longer attempted against them, except in Special Ecological Areas (see below). Work was initiated on the biological control of three of these more widespread species (Tunison 1992). In Réunion in the Indian Ocean a ranking system based on five criteria has been established (Macdonald et al. 1991). Thirty three species were ranked for each of the following criteria:


  A. Current extent of invasions of primary habitats
  B. Difficulty with which a species can be controlled
  C. Potential extent of invasion
  D. Potential rate of spread

E. Ecological-impact ranking


  There is much subjectivity involved in ranking the species, particularly with criterion E, but the broad expertise and detailed knowledge of ecological characteristics of both species and habitats by managers should ensure that such a ranking system is fairly reliable.
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