3 INVASIVE WOODY PLANTS IN THE TROPICS
This section collates and summarizes relevant information not readily available elsewhere for an illustrative set of tropical species. Several major publications on invasive species have appeared in recent years (e.g. Drake et al. 1989, Ramakrishnan 1991, Cronk & Fuller 1995, Pysek et al. 1995, Carey et al. 1996, Sandlund et al. 1996, Williamson 1996). However there is need for a focus using, for each species, a set of headings covering life cycle attributes, consequences of invasive behaviour and integration into the invaded vegetation. Eighteen accounts are presented here on this basis. As a broader context four issues are discussed below. The first is the historical perspective. The second issue is how this relates to human aspirations, perceptions and actions. The biological basis of the contrast between extremely widely planted species apparently with no or minimal invasive tendencies and others which rapidly become problematic is the third issue. The final issue is possible future trends.
3.1 AN HISTORICAL PERSPECTIVE TO THE INTRODUCTION OF EXOTICS
Numerous woody species have been moved around the world, but the timing and the purpose of introductions, the species involved is poorly known. What remain of these introductions are agricultural, horticultural and forestry crops as well as invasive woody plants. Nevertheless, Spongberg (1990) has provided such an overview for North America. For most tropical regions such information has yet to be presented - but see Harris (1962) for the Leeward Islands, Duffey (1964) for Ascencion, Corlett (1992) for Hong Kong and Singapore, and Strahm (1993) - even though a foundation was laid long ago in the work of de Candolle (1860). Subsequently, Guppy (1917), Ridley (1930) and Sauer (1988) made significant advances in information collation and review where seed dispersal and plant migration are concerned. Crosby (1986) has related the pattern of introduction of plants, but particularly herbaceous agricultural weeds, to the European conquest of the world. A conspicuous omission from the studies carried out in the past has been the African situation. In this brief review we have therefore included a preliminary review for tropical Africa.
For the purpose of woody plant introductions into tropical Africa four main phases may be recognized:
•	early exploration and slave trade,
•	early colonial period, 
•	colonial exploitation, and 
•	post colonial development
Early exploration and slave trade
Europeans first settled in the Gulf of Guinea in the late 15th century. By the 18th century a number of French and English forts were scattered along the Golden Coast. These forts were involved in the slave trade and each of them had a gardens in which a number of crops, including many tropical American and Asian fruit trees, were grown (Juhé-Beaulaton 1994). These included Anacardium occidentale, Citrus spp. and Psidium guajava. A number of weeds were accidentally introduced when American crops were brought to Africa (Wild 1968). 
Early colonial period
In Tanzania, non-indigenous plants were introduced to the experimental gardens of the Bagamoyo Mission shortly after its inception in 1869. From 1893 273 species of mostly tropical plants, including many ornamental and forest trees, such as Acacia spp., Cinchona spp., Delonix regia, Eucalyptus spp. and Terminalia catappa, were tested in an experimental nursery at Dar es Salaam which functioned as a seed production and distribution centre. Large quantities of seeds of various timber species were distributed to private planters and government stations. By 1903 a total of 118 indigenous and exotic timber species were under cultivation in the Lushoto District (Schabel 1990). Following the closure of the Kwai Agricultural Station, founded in 1896, a station was established in the East Usambaras at Amani in 1902 as an experimental site for field testing of exotic species (Iversen 1991). It included a botanic garden of ca 750 acres, including ca 500 acres of plantations, spreading between 400 and 1100 m (Fernie 1948). Nearly all species were introduced from other continents because they were either potential cash crops or known to be fast growing and useful in reforestation of steep mountain sites. Although the Amani forests contain several quality hardwood species, the research station was built with timber imported from Germany - it was cheaper to do that than to utilise local woods (Joelson 1928). Ornamentals, including temperate species, were introduced and it was the Director's (Prof. A. Zimmerman) hobby to collect plants from all over the world and try them out at Amani (Joelson 1928). Amani became a major distribution centre for seeds and seedlings in Tanzania and the tropics (Anon 1930, Schabel 1990).
Other botanical and experimental gardens in Africa were established from about 1890 including in Cameroon (Limbe, formerly Victoria, 1891) by the Germans, in Guinea (Fouta-Djalon) by the French, in Zaire (Eala) by the Belgium and in Lagos (1887) and Calabar (1893), both in Nigeria and Uganda (Entebbe, 1898) by the British. 
Colonial exploitation
Soon after World War II a number of large-scale forestry plantations were established in order to enhance timber production. These  plantations consisted of introduced species including Eucalyptus spp., Gmelina arborea, Maesopsis eminii and various conifers, especially Cupressus and Pinus. A number of agricultural crops were widely planted.
Post colonial development
In the last 20 years an increasing diversity of non-indigenous germplasm has been introduced to Africa as it has been to other tropical regions (Hughes 1994). Hughes documents in some detail the increase in the trade including increases in the number of species and provenances distributed as well as the number of planting locations.
In other parts of the tropics similar patterns of species introductions, particularly in relation to crop plants, forestry and ornamental woody species, have occurred. Tropical botanical gardens were set up in more places around the tropics, including several small oceanic islands (Pamplemouses, Mauritius; St Vincent, Caribbean), Asia (Bogor, Indonesia; Calcutta, India; Peradeniya, Sri Lanka; Singapore), Australia (Darwin) and South America (Rio de Janeiro). Botanic gardens on islands were created as early as the 18th century whereas those on the continental land masses, particularly mainland Africa, much later (Cronk & Fuller 1995). The importance of Bogor (formerly Bruitenzorg) in Indonesia has been emphasised by Bouvier (1946). The main purpose of these gardens was to introduce exotic seeds and plants (e.g. Darwin, Miller & Lonsdale 1987) and usually to assess the commercial potential of a variety of crops. The name given by the Germans to what is now known as the Limbe Botanic Gardens, was the 'Research Institute for Land Improvement' (Timler & Zepernick 1987), and clearly indicates the original purpose of the gardens. A lack of knowledge of the local timber trees was often the reason for introducing exotic tree species. During the early years of these tropical gardens the botanical interest was secondary.
However, many differences in the patterns of introductions, resulting chiefly from different colonial histories and geographical locations, are likely. It is known that Polynesians introduced a large number of woody plants to the Hawaiian islands prior to European colonisation (Wagner et al. 1990, Bevacqua 1994). Botanical gardens were established in settled areas - the easily accessible or climatically ideal regions near coasts or in cooler upland areas. Large areas of South America were devoid of settlements and thus botanic gardens and forestry trial plots until comparatively recently. The colonial powers had different routes for the introduction of plant material. Often plants were first transported to Europe (e.g. to Kew, Bruxelles, Berlin-Dahlem for respectively Britain, Belgium and Germany) and then disseminated to tropical gardens in their respective colonial territories. 
During the past hundred years Africa has probably been subjected to less introduction of ornamentals than economically more advanced areas such as Australia and Hawaii which have developed major horticultural industries.
3.2 HUMAN PERCEPTION, IGNORANCE AND EXPERIENCE
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. These problems having been known for quite some time, it is then 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.
The French botanist Auguste Chevalier (1952) recommended the introduction of Chromolaena odorata to West Africa to control Imperata spp. and other coarse grasses despite the fact that he regarded the species as a weed (Chevalier 1949) and much earlier had published a paper on man's role in the dispersal of tropical plants (Chevalier 1931). 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). On many parts of this rugged island this has resulted in severe soil erosion. In the early 1990s a sustainable development strategy was initiated by Overseas Development Agency to address St Helena's environmental problems. In their final report Spooner et al. (1993) give a list of exotic species colonising eroded areas which they classify as useful, neutral and some which may lead to resource degradation. The latter group include a species which may have adverse effect on soils and another is an unpopular weed. The colonisation potential of other introduced species is not discussed and assumed to be positive. It is even stated that Acacia used in afforestation "are growing well, but there is limited seedling colonisation outside the planted zones" indicating that the lack of natural regeneration is considered to be a negative feature and concluded that "a high research priority should be given to identifying the regeneration requirements and constraints of the different colonists." Although new species have been recommended for introduction, Spooner et al. (1993) give no clear justification for new imports and trials of Leucaena for fodder and Casuarina equisetifolia currently under way, two species well known for their invasive potential. 
There is evidence from the temperate zone that some woody species (e.g. Ulex europaeus, Lupinus arboreus) facilitate the establishment of native forest. However this evidence is entirely lacking for the tropics and it is generally accepted that invasive plants displace rather than facilitate succession on tropical oceanic islands. Thus nitrogen-fixing plants such as Acacia and Casuarina spp. main be successfully used to prevent soil erosion but there is, at present, no evidence that they will eventually encourage the regeneration of native plants and that of endemic plants in particular.
Until recently foresters have expressed little concern about the invasive potential of introduced trees. In 1980, a book (Anon. 1980, 1983) on the choices of species to be used for fuelwood production globally and their silviculture was produced by BOSTID (Board on Sciences and Technology for International Development). For most species the ease of natural regeneration is discussed without indicating the geographical areas where prolific regeneration could lead to a weed problem. In fact about half of the species included are known to be invasive and some being serious pests in parts of the world. The authors were aware of the invasive problem but simply dealt with it by inserting a "warning" on the page facing the table of contents consisting of a single paragraph. In recent years all foresters have been aware of the invasive potential of some of their introductions and some have now started to confront the problem (e.g. Hughes & Styles 1986, Hughes 1994).
In India, 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 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 under-reported. A perusal of the B&T World Seeds Master List (Anon. 1996) indicate that seeds of around 50% of the species included in the Species Database can be readily ordered. As B&T World Seeds can supply reproductive material for around 35,000 species or cultivars the potential for the introduction of known and potential invasives is enormous.
A major source of ambiguity in many references to invasive species is taxonomic instability. This is compounded by poor historical records, uncertainty of the sources of introductions and frequent use of local rather than scientific names. 
Species have sometimes been wrongly identified. For instance, In Hawai'i Melaleuca quinquenervia was formerly referred to as M. leucadendron which is not synonymous (Little & Skolmen 1989). Species identification can be difficult as a number of species hybridize readily. These taxonomic difficulties have been dealt with in some detail by a number of authors including Stirton (1979) and Swarbrick (1986). 
In many cases taxonomically difficult groups are involved. Species of the genus Cecropia have been introduced to many parts of the tropics and are spreading spontaneously. Cases reported include C. peltata in Malaysia, Cameroon and Ivory Coast (Putz & Holbrook 1988, McKey 1988, Ake Assi 1979), C. leucocoma in Zaire (Léonard 1951) and C. schreberiana in Madagascar (C.C. Berg pers. comm. 1995). However, there is confusion about the identity of the various invading populations. Species in the genus Cecropia have strong morphological similarities but have somewhat distinct geographical and/or ecological ranges. The species C. peltata s.s. (central America), C. pachystachya (southern America) and C. concolor (Amazon basin) can be regarded as C. peltata s.l. The names given in the literature are sometimes inaccurate. Only when the origin of the material is known, is it possible to identify the specimen with confidence. Thus, in Madagascar C. schreberiana (native to the Caribbean) has often misidentified as C. peltata, although clear morphological differences exist between these two species. The Cecropia species reported from Zaire as C. leucocoma and from Malaysia as C. peltata have both recently proved to be C. pachystachya. The identity of the further African material remains unclear (C.C. Berg pers. comm. 1995). In Hawaii C. obtusifolia has also been erroneously identified as C. peltata (Wagner et al. 1990).
Details of early introductions are often vague in the extreme. Some species are initially described as native and only much later is their true origin discovered. For instance Ocotea angustitepala (Lauraceae), collected in 1912 in undisturbed forest (Robyns & Wylczek 1950), was described from Cameroon as a new species. It was later discovered to be a neotropical plant (Ocotea patens) introduced to the Victoria (Limbe) Botanic Gardens (Rowher 1989). Evidently, the species started to spread into natural forests soon after its introduction. Its current status in native vegetation is unknown. For a number of species which have been clearly shown to be spreading it is difficult, if not impossible, to determine whether they result from natural or human dispersal. These species reached new areas prior to the 19th century. Uncertainty concerning their status arises from the dearth of historical records. For instance, in the tropics it has been suggested that Hibiscus schizopetalus (Malvaceae) in Tanzania (Hawthorne 1993) and Ceiba pentandra (Bombacaceae) in Africa (Irvine 1961) are invaders, whereas Mora excelsa (Caesalpiniaceae) in Trinidad migrated naturally (Beard 1945).
The status of Mimosa pigra in the tropics is a good illustration of the difficulties in establishing the introduction and spread of a woody invasive plant. M. pigra originated from tropical America and was apparently widespread in tropical Africa by the mid 19th century. It has been suggested that its seeds floated across the Atlantic ocean, however as the seed-pod sections float by surface tension rather than buoyancy they are unlikely to travel such a long distance. Therefore the introduction to and the widespread distribution of M. pigra in Africa remains a mystery (Lonsdale et al. 1989). It was originally thought that M. pigra had been introduced to Northern Australia in the late 1930s but subsequent detailed investigations of photographs and grey literature by Miller & Lonsdale (1987) suggest that the shrub was introduced prior to 1891. Early accounts referred to M. rubicaulis, a misnomer for M. pigra or simply as mimosa and already referred to the species weediness. The case of M. pigra clearly shows that painstaking investigations are necessary to unravel some of the historical aspects of an invader.
Woody plant species introduced for crop purposes usually have a set of characteristics which include very fast growth, ease of propagation (including suitability for aerial seeding), were often nitrogen fixers and resistant to a variety of biotic and abiotic agents such as pests and diseases, aperiodic drought, inundation and fire (e.g. Maydell von 1989, Hughes 1994). The view is bluntly expressed by Vietmeyer (1986) writing about the Casuarina invasion of Florida where, in his view, the tree's "aggressiveness is a threat, not a benefit" he adds: "But isn't an unstoppable tree just what we need? in our deforested world the whole idea of a 'weed tree' seems like a paradox. If anything, weed trees are vital to the planet's future. Adaptable, resilient, irrepressible, resistant to disease - what could be more vital for Ethiopia, Chad, Nepal, Haiti, and the other bedrock spots of environmental degradation where the people are too busy trying to survive to worry about nurturing trees?" In parts of semi‑arid western Africa the local population's main reasons for preferring Azadirachta indica for single tree planting is a combination of good availability of seedlings and the ease of vegetative propagation, fast growth and low risk as well as shade and insect repellence (von Maydell 1989) .
3.3 INVASIVE TENDENCIES AND SPECIES BIOLOGY
In the Invasive Woody Plant Database the purpose and date of the original introduction are included. However, the database does not contain any information on secondary introductions or plantings or on the use of a species for other purposes than the original introduction. The original introduction is usually limited to a very small number of plants often in a botanic garden or an experimental plot or a private garden. A close look at many cases of invasive species show that secondary introductions or plantings were usually carried out on a very large scale much later than the original introduction. In Hawaii seeds of many tree species introduced for ornamental purposes prior to or around 1900 were aerially sown in the 1920s and 1930s for forestry purposes (Little & Skolmen 1989). Similarly in Tanzania Maesopsis eminii was widely planted 50 years after its original introductions (Binggeli 1989). This has several implications including: artificial dispersal to new sites and rapid build up of a very large population of seed producing trees. This might explain the time-lag (see below) observed between the introduction and the start of the invasion of some woody species.
It has been estimated that only about 1% of introduced species become invasive (e.g. Groves 1986). The reasons why most introduced species do not become invasive are not known. Suggestions for a certain number of species can be made which apply to the tropics but may be less relevant to the temperate zone. In tropical regions most widely introduced species have some sort of commercial value whereas potentially valuable species, ornamentals or botanically interesting plants have been introduced rather locally in a few botanic gardens. Cecropia peltata is an escape from the Limbe Botanic Gardens (Cameroon) and is now competing with Musanga cecropioides in secondary vegetation (McKey 1988). This species was also introduced to the Amani Botanic Gardens (Tanzania). However, Fernie (1948) reported that only one tree survived at Amani, and as the species is dioecious, it cannot spread even with suitable environmental conditions.
A number of fast growing introduced tropical trees have exhibited invasive tendencies. Janzen (1986) has argued that many of these species, including a large number of legumes, are the sort of species which can become invasive and have a strong impact on the rate of succession and the species composition of the vegetation. However, species of extensively planted genera such as Eucalyptus and Tectona have caused few problems. In the Sierra of southern Peru Eucalyptus globulus is the dominant tree at most altitudes and essential to the local economy (Dickinson 1969). Yet, it does not regenerate. Although a few hundred species of Eucalyptus have been introduced and often extensively planted throughout the world (Penfold & Willis 1961), few instances of invasions have been reported and, those which have, tend to be relatively minor (Kirkpatrick 1977). Abandoned logged Eucalyptus tereticornis in the Western Ghats (India) show that although the species fails to regenerate, its coppicing ability allows it to remain dominant in the secondary regrowth (George et al. 1993) but the understorey species composition suggest that the typical complement of native species will eventually dominate the canopy. Kirkpatrick (1977) identified the poor dispersal ability of Eucalyptus combined with scattered distribution of suitable habitats for seedling survival as the reason for the very slow spread of Eucalyptus species in California. Seedling establishment is only successful in areas more or less free from plant competition. The observed limited spread has occurred despite various species being widely used for timber, windbreak and ornamental purposes. 
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 such 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. Some evidence supporting some of these views have recently been reported.
•	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. 
3.4 TIME-LAGS
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 (Table 3.4.1). 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 (DI = 3) 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 (Table 3.4.1) 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 3.4.1. Duration (in years) of known time-lags between the introduction and first spread and pest status in tropical invasive woody plants. 
		Year of	noticed	Perceived as	
		initial	after	problem/pest	
	Region	introduction	(yrs)	after (yrs) 	
					
Acacia nilotica	N. Australia	ca 1900	ca 50	ca 57	
Casuarina equisetifolia	Florida	ca 1900	ca 56	ca 65	
Cecropia peltata	Ivory Coast	1910	<48	69	
	Cameroon	ca 1910	ca 23	ca 36	
	Zaire	1911	19	40	
	Malaysia	1953	19	35	
Chromolaena odorata	Ivory Coast	ca 1955	ca 7	ca 20	
Cinchona succirubra	Galapagos	1946	26	40	
Lantana camara	Galapagos	1938	32	40	
Maesopsis eminii	East Africa	1913	14	65	
Miconia calvescens	Hawaii	ca 1975	ca 16	ca 16	
Mimosa pigra	N. Australia	ca 1880	ca 36	ca 90	
Pittosporum undulatum	Jamaica	1883	ca 60	105	
Psidium guajava	Galapagos	1858	?	< 90	
Rubus sp.	Galapagos	1983	3	4	
Schinus terebinthifolius	Florida	1898	50	75	

FACTORS DETERMINING TIME-LAGS
For a number of the more comprehensively documented species reasons for the observed time-lags have been inferred and sometimes demonstrated (Table 3.4.2). All reported instances show that major changes in biotic and abiotic factors determine the duration of time-lags.
Abiotic factors include wind, flooding and deforestation. In the case of Acacia nilotica in Australia the time-lag resulted from a series of above average wet years in the early 1950s. In Florida Casuarina equisetifolia started spreading following the disturbance caused by two hurricanes. Cecropia peltata was introduced to south western Ivory Coast as a shade tree in coffee plantations in 1910. By the late 1950s the species had spread by 20 km. However, following the destruction of most of the remaining forest the rate of spread increased markedly in the 1960s: the species had reached sites 100 km away from where it was introduced.
The introduction of pollinators, marked changes in grazing and trampling intensities are examples of biotic factors determining the duration of time-lags. The sudden introduction of a pollinating agent is one of the main biotic factor determining some time-lags. For instance, Ficus microcarpa widely planted as an ornamental in Florida only became a pest about 45 years after introduction, when its natural pollinator - the fig wasp (Parapristina verticillata) was introduced. Although no evidence for the existence of genetic adaptations in determining time-lags is available for the tropics, limited data from the temperate zone indicate that this factor is probably of some importance in tropical regions too. It has been shown for the Chinese ornamental tree, Ailanthus altissima, introduced to and widely planted in North America, that native and introduced populations are genetically different although their genetic variability is the same. However, it has been argued that the differences are the result of human selection rather than adaptation to local environmental conditions. 
A number of other biotic and abiotic factors may also affect time-lags and are well-known to produce marked increases in woody plant population sizes including drought, frost, human-induced disturbance, hybridization and diseases.
It is often difficult and even impossible to judge if there is a time-lag or simply marked increases in population size resulting from various biotic and abiotic factors - examples are given in Table 3.4.2. In the case of Pittosporum undulatum spread into the natural forest only started after hurricane disturbance, although it had already invaded disturbed forest.
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.
In areas where monitoring is not regular some time-lags will be an artifact of erratic recording interest.
Table 3.4.2. Biotic and abiotic factors known to influence or determine the time-lag and marked population increase of invasive woody plants in the tropics.
	time-lag	marked population increase	
 ABIOTIC FACTORS			
Logging/deforestation	Cecropia peltata	Maesopsis eminii	
Wind	- hurricane 	Casuarina equisetifolia	Pittosporum undulatum	
	- treefall		Passiflora mollissima	
Flooding	Acacia nilotica 		
 	Mimosa pigra 		
Fire 		Chromolaena odorata	
	Pinus pinaster		
Nutrient enrichment 	 	Schinus terebinthifolius	
BIOTIC FACTORS			
Pollinator	Ficus microcarpa		
Grazing	Ulex europaeus 		
Disperser	 	Psidium guajava	
Digging by mammals	 	Clidemia hirta	
	Passiflora mollissima		
Trampling by mammals	Ulex europaeus 		

Table 3.4.3. Age (years) of first reproduction of tropical invasive woody species.
Age   	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	no data	
Frequency	8	6	3	1	4	-	1	-	-	3	-	-	-	-	1	218 	

SUCCESSIONAL STATUS AND DISTURBANCE
Most short time-lags (i.e. <10-15 years) are caused by a lack of seed production resulting from the juvenile state of the plants. Although limited, the available data indicate that under favourable conditions most species start sexually reproducing by the time they are 10 years old (Table 3.4.3). Longer time-lags, observed in a number of species, are essentially consequences of major, and often dramatic, disturbance events (e.g. hurricane, flood, fire...) which create the conditions favouring prolific regeneration (invasion windows). When an introduced species requires a particular form and/or intensity of disturbance occurring at irregular intervals the time-lag is variable.
Early successional species, which exhibit early and yearly fruiting and efficient dispersal, and/or massive seed banks, are able to exploit invasion windows created by exceptional disturbance events. Species with irregular fruiting and no seed bank will often fail to regenerate every time suitable conditions arise and one or more invasion windows may be necessary before the species spread.
Considering that the majority of tropical woody species have been introduced within the last hundred years and included relatively few late-successional tree species (usually not used in amenity planting and forestry) it is not surprising that few of these species have been reported as invading tropical regions. In the temperate zone several late successional species are now spreading into semi-natural vegetation (e.g. Quercus rubra, Prunus serotina and several conifers). 
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 autecology (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.
3.5 MANAGEMENT AND POLICY IMPLICATIONS
Although much information has been gathered on biological invasions (e.g. Drake et al. 1989), our ability to identify potential invaders has been doubted (e.g. Crawley 1987). Willis (1991) in his review of Drake et al. (1989) suggested that some general pointers to the probability of a successful invasion: e.g. high habitat disturbance and broad ecological amplitude, short generation time, high reproductive output, high diffusivity and high numbers of invading individuals. However, these pointers do not explain why a species is a successful invader, although they may suggest factors which regulate the speed and extent of an invasion. Indeed, experimental work by Bergelson (1994) shows that fecundity may not be a useful predictor of invasiveness. A number of diverging views concerning the invasive potential of various plant species exist. For instance, Swarbrick (pers. comm. 1994) suggested that any species given the right location, climate and environment is potentially invasive, whereas it is often considered that pioneer and not 'climax' spp can become invasive (e.g. Whitmore 1991).
In Southern Africa, several biological invasions have proved problematical, Richardson & Cowling (1992) have offered a tentative flow diagram to distinguish between species with low and high probabilities of becoming invaders in the fynbos biome. Other work recently carried out on the subject of predictability has been carried out on plants introduced to North American and Australia and on the genus Pinus. Reichard (1994) compared 235 invasive plant species with 114 introduced species which have failed to spread and found that the best single predictor for invasiveness was the behaviour of the species in other regions where it has been introduced: once an invasive, often an invasive.
Using multiple logistic regressions Scott & Panetta (1993) found that for agricultural weeds introduced from southern Africa to Australia the highest level of variation was explained by weed status in native region. Using their regressions Scott & Panetta (1993) predict the future weed status of a number of recently arrived or not yet introduced species. However no suitable predictor of invasive plants (environmental weeds) were identified.
Rejmánek & Richardson (1996) by using a discriminant function, conclude that invasiveness in the genus Pinus can be predicted from three characteristics: mean seed mass, minimum length of the juvenile period and mean interval between large seed crops. They found that short juvenile phase, small seed mass and short intervals between large seed crops are predictive traits of invasive pine species. They then suggest that their discriminant function derived from demographic parameters of pines can be used as a general screening tool for the detection of invasive woody seed plants. Using their discriminant function they correctly classified 38 out of 40 invasive species as invasive. As the two exceptions were due to efficient vertebrate dispersal and concluded that this factor should be taken into account. They then tested cultivated but non-invasive plants which were all correctly classified as non-invasive. However out of 13 species or genera thought to be non-invasive by Rejmánek & Richardson 6 are recorded as potentially or moderately invasive in the Invasive Woody Plant Database! Clearly further work needs to be carried out to extend Rejmánek & Richardson's pine predictions to other woody plants. One major difficulty is that one of their predictors, interval between seed crops, is generally poorly known or unknown. This characteristic has not been included in the Invasive Woody Plant Database due to the scarcity of reliable information. Although included in the database, information on the age of first reproduction (phase change) is available for fewer than 50 of the invasive woody plants. At present the database cannot be used to test the applicability of Rejmánek & Richardson's predictions to other woody plants.
Vegetative propagation is the main, if not the only, mean of spread of a number of invasive woody plants and this is ignored by Rejmánek & Richardson. It is likely that Rejmánek & Richardson criteria will eventually proved not to be applicable to most woody plants and that families and even genera will need to be investigated independently. At present, species known to be invasive elsewhere in the world under similar climatic conditions is still the best predictor of invasiveness.
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. Carter (1994) has suggested that "all plant introductions should be presumed weeds until proven otherwise" and that a plant introduction protocol should be set up, based on the methodology used in the introduction of biological control agents. In case the introduction of a new species is the only mean to satisfactorily fulfil 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.
a.	Has the species or a related species has been reported as invasive elsewhere?
b.	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.
c.	Knowledge of the species reproductive biology is important and in particular that of seed production, seed longevity, and dispersal ability.
d.	Susceptibility of young individuals to grazing (e.g. whether the plant is thorny or not),
e.	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.
If a species introduction is initiated following the assessment of its invasive potential, then tests trials should be carried out. These trials should be established at some distance from natural vegetation and have a clear buffer zone. This buffer zone . The field trials would assess the following parts of the plant's reproductive cycle over a few years after the species has started producing viable seeds:
-	seed production.
-	germination requirements under varied environmental and climatic conditions.
-	seedling establishment in a number of habitats and disturbance regime.
It is also important to establish why an introduced species fails to produce any seeds. This might be due to the absence of pollinator (see the case of Ficus above) or a period of unusual unfavourable climatic conditions.
Carter (1994) identified a number of difficulties with screening of plants in this way. These include:
-	unpredictable behaviour of the introduced species after removal from native pathogens, insects and browse animals,
-	unpredictable behaviour under new climatic, management and fire regimes, and
-	genetic drift or hybridization which may change the character of the plant introduced. Thus it is essential that, once species are introduced, there should be:
-	Long-term and regular monitoring (problems may not show for 100 years, see time-lags above), and 
-	rapid action if weediness appears a problem.
Case studies reported by Waterhouse (1994) on Chromolaena odorata in Northern Australia, Loope et al. (1992) on Miconia calvescens in Hawaii and Wester & Wood (1977) on Clidemia hirta in Fiji and Hawaii clearly show that in the case of highly invasive species, eradication is possible if the invader is controlled at a very early stage of the invasion. Once a plant has become a weed it is usually impossible to eradicate it (Carter 1994).
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.
The above three points require resources, a good and extensive higher level educational system, good environmental awareness (particularly of biological invasions) as well as an efficient information system. At present the information system is a problem worldwide and the setting up of database(s) on invasive species is being considered or set up by various national and international bodies . Awareness exist in regions where many highly invasive species have already altered the ecosystem structure and function. Resources and to an extensive education system are only available in richer countries.
The problem of invasive plants is more likely to become more severe rather than recede in the coming decades. At the same time resources available to deal with the problem of invasive plants are unlikely to increase significantly. Only by using resources, not just financial but also intellectual, more efficiently will be there be more progress made to control invaders. Muniappan & Viraktamath (1993) have pointed out that the effort to control invasive plants in India has been sporadic and uncoordinated. They stress that to deal with the problem of invasive plants cooperative projects must be undertaken by researchers with a variety of scientific backgrounds. In fact Muniappan & Viraktamath's remarks do not just apply to India but is highly relevant to scientists worldwide. As plant introductions and invasions are a global problem coordination and communication must be tackle with on a global scale.
3.6 RESEARCH NEEDS
One of the main problems relating to invasive woody plants and plant introductions is a lack of fundamental knowledge of the basic characteristics of woody plant species as well as the lack of an efficient and easily accessible international information system. Furthermore the type and amount of information required by various professionals varies immensely.
INFORMATION SYSTEM 
There is a need for an efficient information system which should provide the following information: 
a.	current distribution, habitat type and degree of invasiveness of woody plant species.
b.	bibliographical resources including the supply of relevant literature.
c.	control methods including by mechanical, chemical and biological means.
d.	directory of people and organisations involved in the study and control of particular woody species.
e.	provide species accounts on all highly and a number of moderately invasive woody species
FUNDAMENTAL RESEARCH
The assessment of the invasive potential of a species and the control of invaders can only be successful if detailed knowledge of the autecology of the invader and the invaded ecosystem is available. This information is either not available or difficult to obtain. The following areas of research require special attention:
a.	species autecology with special reference to the reproductive cycle including dispersal ability. Studies must be carried out in both the native and invaded regions.
b.	ecosystem dynamics but particularly the variation in space, time and intensity of disturbance and its impact on species composition and population densities.
c.	impact of potential climate change on invading species and on introduced, but non invasive, species. Further work on prediction of invasibility is required.
d.	The characteristics of invasive species need to be investigated in more detail and should be compared to control groups such as a sub-set of local floras and to introduced but non-invasive plants. Stratification of the data based on lifeform and uses may proved advantageous.
e.	more efficient control procedures and field methods need to be designed.