2 OVERVIEW OF INVASIVE WOODY PLANTS AND THEIR COMPARATIVE ANALYSIS
2.1 INTRODUCTION
In this section an overview of woody plant species with special reference to the tropics is presented using the information contained in two databases containing information on, respectively, bibliographical references and information on 653 woody species. 
This is a preliminary overview, as data on a number of species and ecosystem attributes has yet to be entered in the database and many bibliographical references have yet to be traced. Wherever possible the data from the bibliographical database analyzed in 1994 (up to the year 1992) is compared with the 1998 analysis which includes data until the year 1996. During that period the data set of the bibliographical reference database has roughly doubled.
2.2 BACKGROUND
A rapid perusal of the literature indicates that there are hundreds, even thousands, of invasive plant species worldwide. In the plant kingdom a large proportion of invasive species is woody (including sub-shrubs with stems woody at the base). However, it is difficult to gain an overall picture of woody plant invasions because of the limited number of well studied cases and the difficulty in tracing published information on biological invasions. Therefore, only a long-term project can provide the basis to an overview of invasive woody plants. In order to do so, it is essential to gather information on any species exhibiting an invasive tendency as well as information on the biology and ecology of each plant and ecological and environmental information on the native and invaded ranges. This information gathering was initiated in the mid 1980s. To summarize and analyze some of the information gathered a relational database was set up. The Invasive Woody Species Database (using R:Base run on a PC-compatible 486 notebook) was set up between 1990 and 1993 and is based on information gathered over 10 years of literature search. A bibliographical reference database was initiated in 1990 and data entered thereafter. In 1993 a relational database on invasive woody plants was set up and includes a number of species (e.g. life-form, life cycle...) and invaded ecosystem attributes.
For practical reasons, mainly as a result of how data was initially entered into the database, sub-tropical areas (up to a latitude 30o) only include Florida, the Indian peninsula, Pacific islands, Northern Africa, South America, but fail to include southern Africa and parts of sub-tropical Australia.
2.3 INVASIVE WOODY SPECIES DATABASE
2.3.1 DATABASE STRUCTURE AND CONTENT
The Invasive Woody Species Database (due to lack of computer power) is in two main parts which can easily be connected: the bibliographical reference database (Reference Database) and the invasive plant database (Species Database). The Reference Database is composed of standard fields and contains over 18500 references of which about 6000 pertain to invasive organisms and the majority of the others contain ecological information relevant either to the autecology of these invasive organisms and or the ecology of the native and invaded ecosystems. In this study the term invasive is defined as the establishment of self-regenerating, usually expanding, populations of an introduced species in a free-living state in the wild (Binggeli 1994) and is somewhat different from that of Cronk & Fuller (1995) who do not include species invading man-made habitats. Pysek (1995) reviewed the terminology associated with invasive plants. Each reference record has a number code and a field containing a large number of key words (based on the text rather than the title or the abstract) including species names, geographic areas and life form. The Species Database is relational and consists of four tables, two of which contain information on respectively 42 species attributes and 10 ecosystem attributes. All information entered in the Species Database is linked to a reference in the Reference Database using a number code (where possible a copy of the original publication has been kept in files organised by genera or region). Since the quality and the quantity of the available information are often poor, the various attributes included in the Species Database were necessarily chosen according to the availability of information rather than their importance in generating hypotheses or making predictions. 
The great majority of biological information is not discrete. To simplify the database much of the information was categorized, often subjectively (for instance life-forms: tree vs shrub; degree of invasiveness).  Some of the results presented below may not represent distinct guilds but may prove be part of a continuum when more data has been added.
It is intended to continue refining and expanding the species database. This will result in the inclusion of new attributes and possibly the rejection of some existing ones. 
2.3.2 DATASET INCLUDED IN PRESENT ANALYSIS
2.3.2.1 Reference Database
Bibliographic references entered into the Reference Database have been obtained by scanning Forestry Abstracts, Current Advances in Botanical Sciences, journals' table of contents, floras and bibliographies of a large number of published papers. Any publication which reported the natural regeneration of a woody species outside its natural range was included in the database regardless of its degree of invasiveness or invaded habitats. In order to reduce some degree of bias all references dealing with three species investigated in detail by the author were not included. They are Acer pseudoplatanus L. (184 records), Hippophae rhamnoides L. (25 records) and Maesopsis eminii Engl. (33 records). The total number of references included in the present analysis is 3579, of which notes on or reprints of around 1000 publications are with P.B. Searching has been completed to the end of 1996 as many publications post 1996 publications have yet to be traced and entered. For the 1992 dataset searching had been carried out until late 1994 and includes 1924 records.
2.3.2.2 Species database
Published information pertaining to invasive woody plants is often subjective and difficult to interpret. For the purpose of this study, data for a number of attributes (e.g. degree of invasiveness, purpose of introduction and habitat types) have been amalgamated into a small number of broad categories. Wherever possible, invasive events are scored as possibly/potentially, moderately or highly invasive. Introduced species recorded as possibly/potentially invasive are locally regenerating but the extent of spread is not known or is limited. Moderately invasive species are spreading but still occur at low densities and are not considered an immediate problem. Highly invasive species have become dominant or co-dominant in the invaded region and are considered a threat to the native flora and ecosystem. These species are usually subjected to some form of control. Out of 1198 invasive events it has been possible to assess the degree of invasiveness in 1060 cases.
Throughout, the data are presented in relation to the degree of invasiveness. As each species can be invasive in more than one region with differing degrees of invasiveness, the highest degree of invasiveness is used for each species in the presentation of the data dealing with species attributes. Results are presented for attributes where data are currently (1/3/1998) available for at least 30% of the species. 

2.4 RESULTS AND DISCUSSION
2.4.1 Reference database
Temple (1990) remarked that he was amazed that so few papers on exotic organisms were published in Conservation Biology, suggesting a lack of research in this important field of conservation. In fact, the number of references relating to woody invasive plants has steadily increased since the last century with the 1987-1996 period representing nearly half of the total (Table 2.1). The growth in published material on invasive woody plants, both worldwide and in the tropics, exhibit an increase similar to that of publications on African moist forests but compares favourably with the trend for sycamore (Acer pseudoplatanus) (Table 2.1). However, the rate of increase in the number of publications on invasive woody plants appears to have levelled off since 1990 and just over 200 publications have appeared yearly. The three data sets show the same levelling off or even decrease in publication rate associated with the World War II period. 
It is estimated that around 80-90% of all references published on sycamore, with the exception of those pertaining to suspension cell cultures, have been traced, but out of 2958 references only 30% and 17% are available from, respectively, Forestry Abstracts and Biological Abstracts searches. This clearly indicates that on-line databases only provide a starting point to a thorough literature search. It is estimated that the 3579 references represent no more than about half of all published material on invasive woody plants, although it does probably include the greater majority of key publications.
Table 2.1. Comparative increase in the number of references relating to woody invasive species worldwide and the tropics/sub-tropics, African moist forests and sycamore, Acer pseudoplatanus (data expressed as a %)
	Woody invasions	African	sycamore	
	world	tropics	forests		
N	3579	1312	1137	2958	
year					
>1896	0.7	0.6	0.3	5.4	
1896-01	0.1	0.1	0.7	1.2	
1902-06	0.2	0.2	0.6	1.1	
1907-11	0.2	0.2	0.7	1.6	
1912-16	0.3	0.4	0.2	1.2	
1917-21	0.3	0.3	0.1	0.7	
1922-26	0.6	0.5	0.6	1.0	
1927-31	0.8	1.1	0.7	1.7	
1932-36	0.9	1.0	1.7	1.5	
1937-41	0.8	1.0	1.4	1.2	
1942-46	0.8	1.1	0.5	1.0	
1947-51	1.5	1.7	3.0	2.4	
1952-56	2.5	1.9	3.2	5.3	
1957-61	2.2	2.0	4.4	4.6	
1962-66	3.7	3.6	4.0	4.2	
1967-71	4.7	4.4	5.4	8.0	
1972-76	6.9	5.9	5.5	10.0	
1977-81	8.9	7.2	7.8	11.0	
1982-86	15.0	13.5	10.1	11.0	
1987-91	22.7	24.6	24.6	14.9	
1992-96	26.2	28.7	24.5	11.6	

Out of the 3579 references on invasive woody plants, very few (ca <5%) report case histories of biological invasions. The majority mention invasiveness only in passing or in a very descriptive manner. A large number of publications (Table 2.2) are review papers (12.5%) and there was a strong increase in their relative importance during the late 1980s, chiefly as a result of the SCOPE publications. In the 1990s the proportion of review papers still appears to be increasing. Papers on biological control and other forms of control represented respectively 11.5% and 16.6% of the total (note that some references classified under biological control also refer to other means of control). The proportion of publications relating to biological control, in both the world and the tropics, has gone through cycles with peaks around 1950, the late 1970s and 1990s whereas comparatively few publications on biological control appeared in the 1950s and around 1980. Prior to 1976 the proportion of biological publications was much higher in the tropics than worldwide, but since the late 1970s a similar proportion has been published (Table 2.2).
Table 2.2. Changes in the relative importance of review papers and publications on biological control relating to woody invasive species worldwide and in the tropics/sub-tropics.
	all references	general and review	biological control	
			references	references	
	world	tropics	world	tropics	world	tropics	
   N      	3579	1312	 446	 171	 410	 215	
   %      	100  	100	12.5	13.0	11.5	15.6	
							
year      	  N	  N	  % 	  %	  %	  %	
< 1896    	 26	  7	 7.7	  -	  -	  -	
1897-01   	  2	  1	  -	  -	  -	  -	
1902-06   	  6	  3	  -	  -	16.6	33.3	
1907-11   	  8	  3	  -	  -	  - 	  - 	
1912-16   	 11	  5	 9.1	  -	  -	  -	
1917-21   	 12	  4	16.7	  -	 8.3	  -	
1922-26   	 21	  6	 4.8	  -	 4.8	16.7	
1927-31   	 28	 14	 7.1	 7.1	14.3	 7.1	
1932-36   	 31	 13	 3.2	  - 	 9.7	23.1	
1937-41   	 29	 13	13.8	 7.7	10.3	23.1	
1942-46   	 28	 14	 7.1	 7.1	 7.1	14.3	
1947-51   	 57	 22	  - 	  - 	 5.3	13.6	
1952-56   	 91	 25	 6.6	 8.0	 6.6	24.0	
1957-61   	 79	 27	 7.6	 7.4	13.9	25.9	
1962-66   	132	 48	 9.1	12.5	12.1	22.9	
1967-71   	170	 58	 7.1	 6.9	12.3	24.1	
1972-76   	248	 77	 5.2	 3.9	 6.0	10.4	
1977-81   	319	 94	 8.8	11.7	 8.8	11.7	
1982-86   	535	177	 9.5	 7.3	10.5	13.0	
1987-91   	813	324	16.4	15.7	13.8	16.0	
1992-96   	936	376	18.2	20.2	13.6	17.8	

The biogeographical origin of the references is in decreasing order of importance from North America, Australia, South Africa, New Zealand, British Isles, Hawaii and tropical Africa (Table 2.3). It is followed by Germany, which in the English-speaking world, is not regarded as a region susceptible to woody species invasions and/or considered to be interested in the phenomenon (Germans scientists were not involved in the SCOPE programme). A comparison of the data for 1992 and 1996 indicate that there has been a relative decrease in the number of publications produced in South Africa and New Zealand.
It is worth noting that all the regions with the highest number of references are historically linked with Britain and it is not clear whether this is due to the historical factors such as interest in natural history (chiefly perception of nature) and/or horticultural interest and/or language barriers, or reflect the worldwide pattern and severity of woody invasions. The number of references relating to islands was 1276 (617 in the tropics) and to the tropics was 1312, including 137 for tropical Africa and 140 for Florida. The relative proportion of the figures for 1992 and 1996 are broadly similar.
Table 2.3. Geographical focus of bibliographical references (regions with more than 100 records in 1996 included).
Region	Year	
	1996	1992	
			
North America (except Florida)	547	272	
Australia	495	202	
South Africa	389	248	
Hawaii	318	148	
British Isles	316	169	
New Zealand	309	206	
Pacific (other than Hawaii)	224	87	
Florida	140	45	
Tropical Africa	137	78	
Germany	116	59	
			
Islands	-world	1276	659	
	-tropics	617	295	
World	3579	1924	
Tropics	1312	610	

Table 2.4 gives the number of publications in which a genus is given as invasive and includes genera with more than 70 and 20 records in respectively the world and the tropics/sub-tropics. Genera with more than 150 records include two genera each with only one invasive species (Ulex and Lantana) and four with several species (Acacia, Pinus, Psidium and Rubus). Some genera are reported in a large number of publications either in the tropics/sub-tropics (e.g. Psidium, Melaleuca and Chromolaena) or temperate zone (e.g. Pinus, Rubus and Hakea) while others are widely reported in both (Lantana, Acacia and Ulex). Between 1992 and 1996 the number of publications for some genera has increased much faster (e.g. Psidium and Miconia) or much slower (e.g. Ulex) than for the majority of genera. The rapid increase in the number of publications on Miconia and Cryptostegia reflects the rather recent realisation of their invasive potential and impact on native vegetation.
Table 2.4. Genera referred to in the largest number of publications.
WORLD			TROPICS AND SUB-TROPICS		
Genera	Year	Genera	Year	
	1996	1992		1996	1992	
						
Lantana	286	137	Lantana	225	100	
Acacia	263	189	Psidium	189	83	
Ulex	207	155	Mimosa	132	45	
Psidium	194	83	Chromolaena	108	53	
Rubus	183	113	Melaleuca	106	64	
Pinus	172	124	Schinus	78	49	
Lonicera	138	64	Acacia	75	34	
Mimosa	134	45	Passiflora	72	48	
Chromolaena	127	80	Myrica	70	36	
Tamarix	124	61	Clidemia	65	32	
Melaleuca	106	64	Rubus	61	40	
Rhododendron	106	54	Cryptostegia	34	3	
Hakea	101	67	Ulex	34	20	
Schinus	96	59	Miconia	28	4	
Robinia	92	60				
Passiflora	79	51				

The difficulty in gaining information is illustrated in Table 2.5, where the journal, which published the largest number of papers on invasive woody plants, is noted to contain only 1.2% of the total number of references. Books, including floras and reports, account for 14.1% of the total while chapters in books and conference proceedings represent 19.3% of the total. For the tropics/sub-tropics alone the figures are markedly similar but journals publishing the largest proportion of papers is different. Since 1992 new publications have appeared and others have tended to publish fewer articles on invasive woody plants.
Table 2.5. Sources of bibliographical references (figures in brackets are %).
WORLD	TROPICS AND SUB-TROPICS	
							
Year 	1996	1992		1996	1992		
Total	3579	1924	 	1312	 610		
%	(100)	(100)		(100)	(100)		
							
Books (including floras  	 506	 264	Books (including floras 	 201	  90		
& reports)	(14.1)	(13.7)	& reports)	(15.3)	(14.8)		
							
Chapters in books &	 689	 423	Chapters in books &	 289	 141		
conference proceedings	(19.3)	(22.0)	conference proceedings	(22.0)	(23.1)		
							
Journals, bulletins &	2384	1237	Journals, bulletins &	 822 	 379		
magazines 	(66.6)	(64.3)	magazines	(62.7)	(62.1)		
							
Main Journals	 						
N.Z. J. Bot.	  42	  22	Pac. Sci.	  27	  10		
Biol. Conserv.	  40	  24	Biol. Conserv.	  16	  13		
BSBI News 	  31	  17	Proc. Hawn entomol. Soc.	  15	  14		
Pl. Prot. Quart.	  31	   6	Monogr. Syst. Bot. MO Bot. Gard.	  14	  11		
US Dept Agric., Agric. Handb.	  31	  28	Newsl. Hawn bot. Soc.	  14	   2		
Pac. Sci.	  27	  11	Aliens	  13	   0		
Ecology	  26	  11	Agnote	  11	  11		
J. Ecol. 	  26	  21	Atoll Res. Bull.	  11	   1		
J. appl. Ecol.	  24	  14	Pl. Prot. Quart.	  11	   3		
Aliens	  21	   0	Qld Agric. J.	  11	   4		
S. Afr. J. Bot.	  20	  17	Entomophaga	  10	   3		
Agric. Ecosyst. Environ.	  23	  16	Ind. Forester	  10	   8		
S. Afr. For. J.	  18	  16	Agric. J. (Fiji)	   9	   6		
S. Afr. J. Sci. 	  16	  15	J. Biogeogr.	   8	   7		

The majority of publications relating to invasive woody plants are either not abstracted or make no reference to invasive woody plants in the title, abstract or key words, again highlighting at the difficulty in gathering information. The high ranking of, for instance, BSBI News (not abstracted) in Table 2.5 points out that much of the available information is anecdotal and often collected by amateur botanists. As a result the quantity of information available for a particular species can be very limited. Some genera are widely used in review papers as typical examples of invasive woody plants and some of the species may have be cited in a high proportion of review papers, and this is particularly the case of Myrica and Melaleuca with respectively and third and a quarter of their publications.
Some of the changes observed between the 1992 and 1996 datasets are undoubtedly due to changes in publication rates in various countries, genera and journal/magazine/bulletin, however some of the variation is also due to literature hitherto unavailable or not scanned. 
2.4.2 Species database
2.4.2.1 Taxonomy
Worldwide 653 woody plant species, belonging to 315 genera and 110 families, have been recorded as being invasive. The Rosaceae and legume families, and to a lesser extent the Pinaceae and Myrtaceae, contain a large number of invasive woody plants (Table 2.6). However most species of the Rosaceae are possibly/potentially or moderately invasive and relatively fewer are highly invasive. On the other hand families such as the Asteraceae, Myrtaceae and Melastomaceae have a number of highly invasive species but few possibly/potentially invasive species and are chiefly tropical/sub-tropical species. Regardless of the degree of invasiveness the Mimosaceae contains the largest number of invasive species in the tropics.
Table 2.6. Families with highest number of invasive woody species (Degree of invasiveness (DI): 1 = possibly/potentially invasive, 2 = moderately invasive, 3 = highly invasive, N = no. of species).
WORLD								
Total          	  	DI 1           	  	DI 2            	  	DI 3		
Family         	 N	Family         	 N	Family          	 N	Family        	 N	
								
Rosaceae       	98	Rosaceae       	34	Rosaceae       	50	Mimosaceae    	17	
Mimosaceae     	49	Papilionaceae  	12	Mimosaceae     	21	Rosaceae     	14	
Papilionaceae  	27	Mimosaceae     	11	Myrtaceae      	13	Asteraceae    	 7	
Pinaceae       	27	Pinaceae       	 8	Pinaceae       	12	Pinaceae      	 7	
Caesalpiniaceae	21	Salicaceae     	 7	Caesalpiniaceae	11	Myrtaceae     	 6	
Myrtaceae      	20	Caesalpiniaceae	 7	Ericaceae      	11	Papilionaceae 	 5	
Asteraceae     	17	Caprifoliaceae 	 6	Oleaceae       	11	Caprifoliaceae	 4	
Ericaceae      	17	Berberidacea   	 5	Solanaceae     	11	Melastomaceae 	 4	
Oleaceae       	17	Cornaceae      	 4	Papilionaceae  	10	Solanaceae    	 4	
								
 TROPICS AND SUB-TROPICS		
Total          	  	DI 1           	  	DI 2           	  	DI 3		
Family         	 N	Family         	 N	Family         	 N	Family       	 N	
	
Mimosaceae     	31	Mimosaceae     	 4	Mimosaceae     	15	Mimosaceae   	12	
Myrtaceae      	14	Caesalpiniaceae	 3	Rutaceae       	 9	Myrtaceae    	 5	
Caesalpiniaceae	13	Papilionaceae  	 3	Caesalpiniaceae	 8	Asteraceae   	 4	
Rosaceae       	11	Acanthaceae    	 2	Myrtaceae      	 8	Melastomaceae	 4	
Rutaceae       	10	Myrsinaceae    	 2	Euphorbiaceae  	 6	Rosaceae     	 4	

2.4.2.2 Biogeography
In the temperate zone a large number of invasions have been reported from most regions with the exception of Southern America and Asia (Table 2.7). In the tropical zone most invasions are reported from Pacific and Indian Ocean islands, with no records from the Amazon region! Worldwide the same number of invasions of continents and oceanic islands has been reported (Table 2.8). When one takes the degree of invasiveness into account the ratio between possibly/potentially, moderately and highly invasive species is similar on continents and islands (Table 2.8). However, there are relatively few highly invasive woody species on islands situated on continental shelves (e.g. British Isles) than isolated ones (e.g. Hawaii). In the tropics there are relatively fewer invasive species on continents and on continental shelf islands. The relative importance of possibly/potentially invasive species is much lower in tropical regions than for the world but this is probably due to under-recording. 
Table 2.7. Biogeographical distributions of invasive events (DI as in Table 2.6, DI available for 1060 invasive events, N = no. of species).
	Total	DI1	DI2	DI3	
Region	N	N	N	N	
					
Europe	250	107	122	21	
Pacific Islands	155	31	83	41	
North America	143	37	82	24	
New Zealand	134	43	78	13	
Australia	88	35	33	15	
Indian Ocean Islands	74	9	50	15	
Southern Africa	62	13	29	20	
Tropical Africa	57	13	32	12	
Asia	42	7	22	13	
Southern America	27	4	18	5	
West Indies	12	1	10	1	
Atlantic Islands	11	3	8	-	
Madagascar	8	-	4	4	
Others	2	-	2	-	

Table 2.8. Relative incidence of invasions on continents and oceanic islands (DI as in Table 2.6, DI available for 1060 invasive events, N = no. of invasive events).
	WORLD	TROPICS AND SUB-TROPICS	
	 	DI 1	DI 2	DI 3	   	DI 1	DI 2	DI 3	
	 N  	  % 	  % 	  % 	 N 	  % 	  % 	  %  	
									
Continents	529 	 30 	 52 	 18 	133	 23 	 56 	 21 	
Oceanic islands 	531 	 27 	 56 	 17 	260	 16 	 60 	 24 	
									
Oceanic islands									
   isolated  	396 	 22 	 58 	 20 	247	 16 	 60 	 24 	
   continental shelf	135 	 41 	 50 	  9 	 13	 15 	 70 	 15 	

2.4.2.3 Ecology
Species attributes
Shrubs (height < 5 m) are the commonest invasive life-form followed by small trees (height 5-15 m) and trees (height > 15 m). The relative importance of shrub species is much greater among potentially invasive species than in the other two categories (Table 2.9). There are few invasive woody climbers. In tropical regions the relative importance of possibly/potentially and moderately invasive shrubs is much lower.
Table 2.9. Life-forms of invasive woody species (DI as in Table 2.6, N = no. of species).
WORLD 	
	Height	Total		DI 1	DI 2	DI 3	
	      	 N 	 %	 N	 %	 N 	 %	 N 	 % 	
										
Trees      	 >15m 	166	25	34	20	102	28	 30	25	
Small trees	5-15m 	181	28	32	18	115	32	 34	29	
Shrubs     	 < 5m 	264	41	97	56	125	35	 42	35	
Climbers   		 42	 6	11	 6	 18	 5	 13	11	
										
TROPICS AND SUB-TROPICS	
   	Height 	Total	DI 1 	DI 2 	DI 3	
		  N	 %	 N	 %	  N	 %	  N	 %	
										
Trees      	 >15m  	 73	30	 7	23	 51	34	 15	24	
Small trees	 5-15m	 90	36	11	35	 62	41	 17	27	
Shrubs     	 <5m   	 63	26	10	32	 29	19	 24	38	
Climbers   	       	 19	 8	 3	10	  9	 6	  7	11	

The relative proportions of evergreen and thorny species increase with the degree of invasiveness, although overall the total number of thorny species is low (Table 2.10). The majority of tropical species are evergreen. Wind-pollination is commoner in possibly/potentially invasive species and insect-pollination more frequent in moderately and highly invasive species and in the tropics the relative importance of wind-pollination increases with the degree of invasiveness (Table 2.10). Some moderately and highly invasive species are amphophilous, bird-pollinated or self-pollinated. A variety of fruit types characterize invasive woody plants including, in decreasing degree of importance, berries, pods, capsules, drupes and cones. Birds are the main dispersal agent followed by wind. The relative importance of wind-dispersed species decreases with the degree of invasiveness (Table 2.10). The relative proportions of fruit types and dispersal agents of tropical species are similar to that of the world, although some fruit types are not represented among the tropical examples.
In the majority of species, fruit size (to the nearest cm) was 1 cm and 3cm with greater variation among pods (1-60 cm) than among other fruit types (1-15 cm). Most fruits contained one or two seeds, but a few had a larger number (up to a thousand). The size of most seeds was between 1 mm and 6 mm with a few reaching 50 mm.

Table 2.10. Various species attributes characterising invasive woody plants (DI as in Table 2.6, in brackets sample size, N = no. of species).
		WORLD	TROPICS AND SUB-TROPICS	
		DI1		DI2		DI 3		DI 1		DI 2		DI 3		
	 	N	%	N	%	N	%	N	%	N	%	N	%	
														
Deciduousness 	(383)						(112)						
	deciduous 	58	57	100	47	27	39	2	18	22	30	3	11	
	evergreen 	36	35	100	47	35	51	7	64	46	63	20	71	
	semi-evergreen 	8	8	12	6	7	10	2	18	5	7	5	18	
Thorniness 	(220)						(110)						
	thornless 	36	97	114	90	41	73	8	88	62	86	26	70	
	thorny	1	3	13	10	15	27	1	12	10	14	11	30	
Pollinating agent	(161)						(61)						
	amphophilous		-		-		3	7	-		-		1	5	
	bird	-		1	1	1	2	-		-		-		
	insect	11	38	57	63	22	54	4	100	32	86	12	60	
	wind 	18	62	32	35	14	35	-		4	11	6	30	
	self-pollinated	-		1	1	1	2	-		1	3	1	5	
Fruit type	(520)						(188)						
	achene	2	1	11	4	7	7	-		4	4	3	6	
	berry	32	23	59	21	27	27	4	16	21	19	12	23	
	capsule	20	15	55	19	19	19	5	20	24	21	11	21	
	catkin	8	6	8	3	1	1	-		-		-		
	cone	10	7	18	6	9	9	-		3	3	3	6	
	drupe	17	13	63	22	11	11	7	28	30	27	8	15	
	follicle	10	7	13	5	1	1	-		3	3	-		
	nut 	2	1	6	2	-		-		-		-		
	pod 	26	19	37	13	23	23	9	36	23	20	15	29	
	pome	7	5	7	2	1	1	-		1	1	-		
	samara	4	3	9	3	1	1	-		2	2	-		
Dispersal agent	(287)						(117)						
	mammal	2	3	9	6	4	5	-		3	5	4	10	
	bird	29	49	71	46	38	49	5	62	27	46	17	43	
	explosive	-		1	1	3	4	1	13	1	2	2	5	
	insect	-		-	2	3				-		2	5	
	mammal	-		3	3	5	6	-		2	3	4	10	
	water	1	2	2	1	3	4	-		2	3	3	7	
	wind	27	46	65	43	22	29	2	25	24	41	8	20	

The purpose of introduction of species which have become invasive is not known for a large number of invasive events. Introductions for amenity purposes, and to a lesser extent for forestry and agricultural purposes, have been responsible for most reported cases of invasions (Table 2.11). Agricultural introductions result in relatively fewer cases of highly invasive species whereas the reverse is true for forestry introductions. In the tropics the relative proportion of introductions for agricultural, botanical and forestry purposes is greater, however this trend is not observable in highly invasive species rather than amenity purposes.
Table 2.11. Main purpose of introductions (DI as in Table 2.6, N = no. of invasive events).
WORLD								
	DI1		DI2		DI3		Total		
	N	%	N	%	N	%	N	%	
Agriculture	37	3.5	67	6.3	16	1.5	120	11.3	
Botanic gardens	3	0.3	11	1.0	6	0.6	20	1.9	
Forestry	27	2.5	70	6.6	34	3.2	131	12.3	
Amenity	114	10.8	164	15.5	49	4.6	327	30.9	
Landscape	7	0.7	11	1.0	6	0.6	24	2.3	
Accidental	1	0.1	-		1	0.1	2	0.2	
Unknown	114	10.7	250	23.6	72	6.8	436	41.1	
Total	303	28.6	573	54.0	184	16.4	1060	100.0	
									
TROPICS AND SUB-TROPICS				
	DI1		DI2		DI3		Total		
	N	%	N	%	N	%	N	%	
Agriculture	22	5.6	43	10.9	9	2.3	74	18.8	
Botanic gardens	3	0.8	9	2.3	4	1.0	16	4.1	
Forestry	4	1.0	38	9.7	13	3.3	55	14.0	
Amenity	16	4.1	42	10.7	23	5.8	81	20.6	
Landscape	-		3	0.8	4	1.0	7	1.8	
Accidental	1	0.2	-		1	0.2	2	0.4	
Unknown	27	6.9	96	24.4	35	9.0	158	40.3	
Total	73	18.6	231	58.8	89	22.6	393	100.0	

Ecosystem attributes
In only half of the reported invasive events is it possible to determine the invaded habitat (Table 2.12). Most invasions occur in forests - both natural and disturbed. The majority of invaded habitats are highly disturbed and in most instances this has resulted from human disturbance. Forests appear to be the natural habitats most susceptible to invasions by woody plants. Results for tropical regions do not differ markedly from world trends.
Table 2.12. Habitat types invaded by invasive woody plants (DI as in Table 2.6, N = no. of invasive events).
WORLD
	DI 1	DI 2	DI 3	Total	
	N	%	N	%	N	%	N	%	
Disturbed grounds	35	3.3	71	6.7	36	3.4	142	13.4	
Forests	-natural	10	0.9	30	2.8	19	1.8	59	5.5	
	-disturbed	26	2.4	82	7.7	40	3.8	148	13.9	
	-plantation	4	0.4	7	0.7	3	0.3	14	1.4	
Hedges	2	0.2	5	0.5	1	0.1	8	0.8	
Grasslands	12	1.1	23	2.1	18	1.7	53	4.9	
Riverbanks	9	0.9	23	2.1	10	0.9	42	3.9	
Roadsides	11	1.0	23	2.1	3	0.3	37	3.4	
Swamps	4	0.4	4	0.4	2	0.2	10	1.0	
Dunes	5	0.5	8	0.8	3	0.3	16	1.6	
Bogs/heathland/fynbos	1	0.1	9	0.9	7	0.7	17	1.7	
Cliffs	3	0.3	5	0.5	-	0	8	0.8	
Unknown	181	17.1	283	26.7	42	3.9	506	47.7	
Total	303	28.6	573	54.0	184	17.4	1060	100.0	

TROPICS AND SUB-TROPICS
	DI1		DI2		DI3		Total		
	N	%	N	%	N	%	N	%	
Disturbedgrounds	12	3.0	25	6.4	21	5.3	58	14.7	
Forests	-natural	7	1.8	17	4.3	13	3.3	37	9.4	
	-disturbed	9	2.3	45	11.4	15	3.8	69	17.5	
	-plantation	1	0.3	4	1.0	3	0.7	8	2.0	
Hedges	-		-		-		-		
Grasslands	2	0.5	8	2.1	10	2.5	20	5.1	
Riverbanks	-		-		2	0.5	2	0.5	
Roadsides	1	0.3	5	1.2	-		6	1.5	
Swamps	1	0.3	1	0.3	2	0.5	4	1.1	
Dunes	1	0.3	1	0.3	1	0.3	3	0.9	
Bogs/heathland/fynbos	-		-		-		-		
Cliffs	-		-		-		-		
Unknown	39	9.8	125	31.8	22	5.7	186	47.3	
Total	73	18.6	231	58.8	89	22.6	393	100.0	

2.5 FUTURE PROSPECTS
2.5.1 CURRENT AND FUTURE PROBLEMS
When this invasive woody database was set up it was clear that many problems would arise. Some were predicted. Others have arisen since. Many of the difficulties are linked with our perceptions of nature and difficulties in communicating information. The main problems are:
1.	The literature is very scattered.
2.	Much more information is available than is currently held in the database.
3.	Taxonomic difficulties, including hybridization, are major hurdles for many genera and since this is well-known the reader is referred to other literature for details (e.g. Stirton 1979, Swarbrick 1986).
4.	Invasive hot spots have been clearly identified but have yet to be explained. In the literature on biological invasions historical, cultural, social and economic factors associated with humans have received little attention. Do invasive hot spots result from:
a.	cultural bias in recording,
b.	high levels of species introductions or,
c.	ecosystem susceptibility to invasions?
or a mixture of these and other ecological factors. Above it has been pointed out that most cases of invasions have been reported by, and focus on, countries historically linked with Britain. Therefore, a cultural bias should not be underestimated.
5.	Few species and ecosystem attributes have been looked at in any detail. Basic information such as life-history traits (e.g. pollinating agent, seed dispersal) are rarely reported even in case studies. Such information may often be obtained from other sources. A check list of attributes to be answered by workers in the field should be considered, which would provide us with a standardized set of data.
6.	The status of some species is unclear, are they biological invasions or the result of natural dispersal? (see section 2 for details)
7.	The families with the highest numbers of invasive woody species are given in Table 2.6. However, the invasive potential of a family can only be determined when the ratio of invasive species to the total number of woody species in a particular genera is known.
8.	As in all literature based investigations, at least 95% of the data is secondary, dealing with species and habitats unfamiliar to the compiler.
2.5.2 DATABASE USE
In addition to the overview of invasive woody plants, the database has two main objectives: 
To have an easily retrievable source of information on a particular species, including publications relating to it, and information on a number of species and invaded ecosystem attributes, and 
to use the information to predict the invasive potential of a species in a particular habitat and/or region.
For predictive purposes invasive species must be compared with non-invasive species and with a sub-set of local floras for which the same type of information will need to be gathered. Ideally these non-invasive species should be chosen at random, but since information on them will be hard to obtain or unavailable, for present purposes well-documented species will be selected.
Invasive woody plants are a minor component of biological invasions. The time, financial resources and effort involved in producing this database, as well as what is still required to make it reasonably comprehensive, indicate that much needs to be done before an overall picture of biological invasions emerges.