Overstory #89 - Invasive Woody Plants
Invasive Woody Plants
Biological invasions are considered to be one of the major threats to the earth's biodiversity. Non-native woody species, introduced by humans, can spread into native forests, pastures, or cultivated areas. Such species are termed, "invasive." Many animals and plants are highly invasive and some species dramatically affect the structure and function of ecosystems.
Nearly all introductions of woody plants which have become invasive, have been introduced intentionally by horticulturalists, botanists, foresters or gardeners. The bibliographical data showed that as early as last century a number of authors realised the regeneration potential and sometimes the invasive potential of introduced woody plants. Since these problems have been known for quite some time, it is important to investigate how aware practitioners of introductions were of the potential problems associated with species introductions.
Botanists, conservationists, foresters, agroforesters and horticulturalists have, and often still are, to varying degrees, responsible for the introduction and planting of woody species. While there was awareness of invasive potential and related environmental impact, it appears that often it has been considered that introduction of potentially invasive species would do more good than harm. A number of examples are given below to illustrate the problem. In our view, many more cases could also be documented.
- A number of species introduced to tropical botanical gardens have become invasive but there is no evidence to show that scientists responsible for their introductions were aware of the potential problems. The appearance of articles warning that tropical botanical gardens could be the source of invasive species is a very recent development (Sheil 1994). However, Miller & Lonsdale (1987) have shown that the weedy nature and associated problem of introduced species at the Darwin Botanic Gardens was recognised early this century, but the botanists still failed to foresee the implications for the vegetation of the Darwin region.
- During the 1980s it was realised that a number of introduced trees were spreading in the logged and natural forests of the East Usambaras in Tanzania (Binggeli & Hamilton 1990). Because the biological importance of the East Usambaras and the threat posed by logging, deforestation and invasive species an IUCN project was initiated to help in the sustainable management the mountains. One aspect of the programme consisted in rehabilitating or demarcating Forest Reserves to prevent forest encroachment. In order to make the demarcation noticeable exotics including species known to be invading the natural forest, such as Cedrela odorata, were planted.
- The original vegetation, rich in endemic species, of the isolated Atlantic island of St Helena has almost entirely been destroyed. These changes have been induced by browsing, grazing, wood harvesting, forest clearance and plant introductions (Cronk 1989).
- Due to its prolific natural regeneration, Leucaena leucocephala, is generally considered to be a weed, yet it was planted, mainly as a fodder crop, in the Karnataka region of India in early 1980. Within ten years Patil & Kumar (1990) reported that the species had become a problematic weed in cultivated land, even today, new introductions of Leucaena germplasm are proceeding.
With respect to invasive species, little information from horticulture is readily available in the literature. Yet most invasive species have been introduced for ornamental purposes and this area necessitates much more attention. In many countries the introduction of species for ornamental purposes is subject to little regulations and is underreported.
Invasive Tendencies and Species Biology
Although most introduced species do not become invasive, it has been estimated that about 1% of introduced species do become invasive (e.g. Groves 1986). Until recently the problem of invasive woody plants has been publicized principally in relation to oceanic islands such as Hawai'i (e.g. Stone et al. 1992), Rodrigues (Strahm 1989), Réunion (Macdonald et al. 1991) and Madagascar (Sussman & Rakotozafy 1994). In recent years a number of invasive species have been reported from Africa (Binggeli & Hamilton 1993, Sheil 1994), Australia (Swarbrick & Skarrett, 1994) and India (Saxena 1991). It has now been realised that biological invasions may become a serious and ever-increasing problem in some continental regions.
Below a number of reasons are given for tendencies for invasive species to become increasingly problematical. These reasons are mainly related to increasing direct and indirect human induced disturbance and to human ever-increasing use of nature.
- Large gaps created by logging operations appear to be more readily invaded by exotics than natural tree fall gaps. The shrub Chromolaena odorata is readily found in selectively logged forests of the Western Ghats (India) but absent from natural forest (Chandrashekara & Ramakrishnan 1994). The reason for this is probably due to the fact C. odorata seeds are usually locally wind-dispersed but can be readily transported by vehicles (see species account). In heavily exploited and degraded forests of lowland of southwestern Sri Lanka Alstonia macrophylla (Apocynaceae) is commonly found in large gaps but appear to be absent from unlogged forest. In Tanzania Maesopsis eminii becomes dominant in logged forest but is capable of regenerating in natural forests in large treefall gaps. In the same forests the shrub Clidemia hirta is also becoming quite common in natural forest gaps.
- Why do exotics matter in the general context of conservation of biodiversity? Increase in turn-over rate has been observed (Phillips & Gentry 1994; Phillips et al., 1994), potential rapid climatic change should result in increased intensity of disturbance, increased human pressure for firewood and forest products will all lead to conditions more favourable to known invasive woody species. Most of these species require a substantial amount of disturbance to spread extensively.
- If climate changes many areas of natural vegetation are isolated and species will fail to move to other islands.
- The conservation of biodiversity becomes increasingly significant in disturbed areas as areas of natural or semi-natural vegetation steadily decrease. In an overpopulated and resource hungry world secondary vegetation will become more and more important to the conservation of biodiversity. For instance in the British Isles motorway verges, railway embankments and abandoned quarries are now important assets to nature conservation.
- Susceptibility of natural areas to invasions is higher if a large seed source is available around it. The smaller the area of natural vegetation the more likely it is to be invaded.
- Data available from the tropics and sub-tropics on the incidence of invasions clearly show a higher reported number of cases in areas with more advanced economies and standards of living. This is probably not coincidental and rather reflects the large number of plants introduced and distributed in large numbers over wide geographical range for ornamental purposes. Assuming that the less developed world will increase its standards of living an ever increasing number of exotics species will be introduced to regions hitherto not widely affected by invasive species.
A combination of isolated natural areas surrounded by large tracts of potentially invasive species, higher human disturbance, higher natural disturbance, increased movement of exotic plants and potential impact of rapid climatic change (more important in areas without climatic gradients) will undoubtedly lead to an increased threat by introduced species.
The populations of introduced species often remain small and localized for long periods of time before they exhibit very rapid expansion. Until very recently little evidence was available to support a number hypothetical explanations for these observed time-lags or lag phases. The reasons for these time-lags are threefold (Hobbs & Humphries 1995):
- genotypic adaptations
- cyclical disturbance or a combination of environmental conditions
- species, with exponential growth, not observed until the population reaches a critical size.
The time between the introduction of a species and its first record of spread and pest status in tropical invasive woody species varies, respectively, from 3 years to around 50 years and from 4 years to around 90 years (see table below). Most woody plant species were introduced between 1838 and 1937 with a peak in the late 19th century. No obvious differences are observed when the degree of invasiveness is taken into account. This data further supports the view that the existence as well as the duration of time-lags is highly variable. A species may quickly become highly invasive even after it has already been present for a long time in a particular region.
In the tropics time-lags appear to be much shorter (see table below) than those observed in temperate species. Kowarik (1995) reported that in the German Brandenburg province the average duration of the time-lag between the introduction and the initiation of an invasion was 131 years and 170 years for, respectively, shrubs and trees.
Table. Duration (in years) of known time-lags between the introduction and first spread and pest status in tropical invasive woody plants.
- Acacia nilotica (N. Australia, ca 1900) ca 57 yrs
- Casuarina equisetifolia (Florida, ca 1900) ca 65 yrs
- Cecropia peltata (Ivory Coast, 1910) 69 yrs
- Chromolaena odorata (Ivory Coast, ca 1955) ca 20 yrs
- Cinchona succirubra (Galapagos, 1946) 40 yrs
- Lantana camara (Galapagos, 1938) 40 yrs
- Maesopsis eminii (East Africa, 1913) 65 yrs
- Miconia calvescens (Hawaii, ca 1975)
- 16 yrs Mimosa pigra (N. Australia, ca 1880) ca 90 yrs
- Pittosporum undulatum (Jamaica, 1883) 105 yrs
- Psidium guajava (Galapagos, 1858) 90 yrs
- Rubus sp. (Galapagos, 1983) 4 yrs
- Schinus terebinthifolius (Florida, 1898) 75 yrs
In many cases the original introduction of a woody species was limited to one or a few individuals, but a number of subsequent introductions, usually consisting of many individuals, were made for a different purpose. For instance, in Hawaii and Florida a few individuals of some tree species were planted as ornamentals but decades later the same species were either widely planted in forestry plantations or large quantities of seeds were aerially sown. Some of these species were observed to spread only following these secondary introductions.
The duration of the phase between the introduction of a species and its spread being so variable it is not possible to be certain that a species, although present for several decades, will not spread. For any particular region it is essential to understand the long-term disturbance regime as well as the specific ecology (inclusive of reproductive biology and regeneration requirements) of introduced species before any predictions can be made. Otherwise close monitoring of natural and semi/natural vegetation, particularly after exceptional disturbance events, is necessary for the early detection of new invasions.
Considering the difficulty in predicting the invasive potential of introduced species, it is therefore essential to look at the reasons and justifications for introducing plant species. Hughes (1994) has extensively discussed the pros and cons of species introductions and provided guidelines for the introduction or non-introduction of non native species. In case the introduction of a new species is the only means to satisfactorily fulfill a long-term need, Hughes (1994) suggested that it is then essential to make that its invasive potential will be as limited as possible. Procedures to reduce the risk of introducing a potentially invasive species should include extensive information searches prior to the proposed introduction and the establishment of limited trial plantings.
All introductions of non-indigenous plant species should be screened with great care in a process which considered the status in the native range and at other points of introduction. The following points require particular attention and should rapidly provide a clear indication to the introduced species invasive potential.
- Has the species or a related species has been reported as invasive elsewhere?
- How similar is the site of the proposed introduction to that of the species in its native and invaded regions? This includes comparisons of soil, climate, disturbance (both in terms of intensity and periodicity including that of fire, wind, flood) and human disturbance. Conditions favouring or limiting the species spread should be identified.
- Knowledge of the species reproductive biology is important and in particular that of seed production, seed longevity, and dispersal ability.
- Susceptibility of young individuals to grazing (e.g. whether the plant is thorny or not),
- Assessment of practical and effective methods of control in the case of weediness problems. The economics of control must be carefully considered.
If the above points indicate that the proposed species has no invasive potential an introduction may be considered. On the other hand if the proposed species exhibits invasive tendencies a strong justifications will have to be made for its introduction.
The following points are essential in detecting the early stages in an invasion:
1. awareness that the species is a problem in another region where climatic and environmental factors are similar,
2. first-hand knowledge in identifying and recognizing the species in the wild is essential, and
3. active governmental or voluntary organisation in the field of plant invasions is necessary to provide background response.
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Binggeli, P. & Hamilton, A.C. 1993. Biological invasion by Maesopsis eminii in the East Usambara forests, Tanzania. Opera Bot. 121, 229-235.
Chandrashekara, U.M. & Ramakrishnan, P.S. 1994. Successional patterns and gap phase dynamics of a humid tropical forest of the Western Ghats of Kerala, India: ground vegetation, biomass, productivity and nutrient cycling. Forest Ecol. Mgmt 70, 23-40.
Cronk, Q.C.B. 1989. The past and present vegetation of St Helena. In: J. Biogeogr. 16, 47-64.
Groves, R.H. 1986. Plant invasions of Australia: an overview. In Groves, R.H. & Burdon, J.J. (Eds) Ecology of biological invasions: an Australian perspective, pp. 137-149. Australian Academy of Science, Canberra.
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Hughes, C.E. 1994. Risks of species introductions in tropical forestry. In: Commonw. For. Rev. 73, 243-252.
Kowarik, I. 1995. Time lags in biological invasions with regard to the success and failure of alien species. In Pysek, P., Prach, K., Rejmánek, M. & Wade, P.M. (Eds) Plant invasions - general apsects and special problems, pp. 15-38. SPB Academic Publishing, Amsterdam.
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Miller, I.L. and Lonsdale, W.M. 1987. Early records of Mimosa pigra in the Northern Territory. Pl. Prot. Quart. 2, 140-142.
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Phillips, O.L. and Gentry, A.H. 1994. Increasing turnover through time in tropical forests. Science 263, 954-957.
Phillips, O.L., Hall, P., Gentry, A.H., Sawyer, S.A. and Vasquez, R. 1994. Dynamics and species richness of tropical rain forests. Proc. Natl. Acad. Sci. 91, 2805-2809.
Saxena, K.G. 1991. Biological invasions in the Indian subcontinent: review of invasion by plants. Ramakrishnan, P.S. (Ed.) Ecology of biological invasion in the tropics, pp. 53-73. International Scientific Publications, New Delhi.
Sheil, D. 1994. Invasive plants in tropical forests: warnings from the Amani Botanic Gardens, Tanzania. Bot. Gardens Conserv. News 2(3), 23-24.
Sussman, R.W. and Rakotozafy, A. 1994. Plant diversity and structural analysis of a tropical dry forest in southwestern Madagascar. Biotropica 26, 241-254.
Stone, C.P., Smith, C.W. and Tunison, J.T. (Eds) 1992. Alien plant invasions in native ecosystems of Hawai'i: management and research. University of Hawaii Press, Honolulu.
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Swarbrick, J.T. and Skarrett, D.B. 1994. The Bushweed 2 database of environmental weeds in Australia. The University of Queensland Gatton College.
This article is excerpted with the kind permission of the authors from:
Binggeli, P., J.B. Hall, and J.R. Healey. 1998. An Overview of Invasive Woody Plants in the Tropics. School of Agricultural and Forest Sciences Publ. No. 13, University of Wales, Bangor. "This publication is an output from a research project funded by the United Kingdom Department for International Development (DFID) for the benefit of developing countries. R4742 Forestry Research Programme."
About the Authors
Dr. Pierre Binggeli is an expert in woody plant ecology and conservation, with wide experience in tropical and European ecosystems. His current position is Research Officer, at the University of Wales Bangor, working on the project "Biodiversity conservation in ancient church and monestry yards in Ethiopia".
Dr. John B. Hall is a Senior Lecturer in Forestry, with widespread experience in tropical woody plant ecology, especially in West, East and Southern Africa.
Dr. John R. Healey is a Lecturer in Tropical Forestry, who has worked on invasive plant ecology and management in Jamaica, as well as other aspects of forest ecology and management in Jamaica, West and East Africa, and South East Asia.
The authors can be contacted at: School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd, LL57 2UW, UK; Tel: 44 1248 383703 or 382446; Fax: 44 1248 354997; Web site: http://www.safs.bangor.ac.uk/IWPT