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The ecology of biological invasions into nature reserves: An introduction
Biological Conservation 44 (1988) 1-8
The Ecology of Biological Invasions into Nature Reserves: An Introduction
M. B. Usher Department of Biology, University of York, York YO1 5DD, Great Britain
F. J. Kruger South African Forestry Research Institute, PO Box 727, Pretoria 0001, Republic of South Africa
I. A. W. Macdonald Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, Republic of South Africa
L. L. Loope Haleakala National Park, Box 369, Makawao, Hawaii 96768, USA
& R. E. Brockie Ecology Division, DSIR, Private Bag, Lower Hutt, New Zealand (Received 4 July 1987: accepted 31 July 1987)
A BS TRA C T The following five papers collectively form a report of the Working Group on Nature Reserves, associated with the SCOPE programme on the "Ecology of Biological Invasions'. This introduction ( i) defines the aims of the SCOPE programme, ( ii ) describes the approach of the Working Group, ( iii) provides definitions so that these do not have to be given repeatedly in the following papers, and (iv) 'signposts' the remainder of the report. l
Biol. Conserv. 0006-3207/88/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
2
M . B . Usher et al.
THE SCOPE P R O G R A M M E ON THE ECOLOGY OF BIOLOGICAL INVASIONS The programme
Since mid-1982 a SCOPE (Scientific Committee on Problems of the Environment, part of the non-governmental organisation, The International Council of Scientific Unions, based in Paris) programme has examined the ecology of invasive plants, animals and microbes, concentrating particularly on those species which have successfully invaded nonagricultural regions and which have disrupted natural ecosystem processes. The programme aims to answer three specific questions, namely: 1. 2. 3.
What are the factors which determine whether a species will be an invader or not? What are the site properties which determine whether an ecological system will be prone to, or resistant to, invasion? How should management systems be developed to use the knowledge gained from answering questions 1 and 2?
Within the international framework of the SCOPE programme there have been a number of national meetings, together with the establishment of specialist international working groups. Publications following national meetings include those from Australia (Groves & Burdon, 1986), the United States of America (Mooney & Drake, 1986), South Africa (Macdonald et al., 1986), the United Kingdom (Kornberg & Williamson, 1987), and the Netherlands (Joenje et al., 1987). The specialist working groups were planned to be concerned with tropical ecosystems, Mediterranean-type ecosystems, stressed ecosystems, modelling and nature reserves. The working group on invasions in nature reserves
The nature and degree of invasion by introduced species is thought to vary considerably between protected areas in different parts of the world. In general, reserves in the tropics and in subtropical arid lands have fewer invasive species, though there are important exceptions. Among temperate regions those in the southern hemisphere have often experienced severe invasions, whilst those in the northern hemisphere seem relatively free from invasions. At its worst, the problem of invasions seems insuperable on many oceanic island reserves. Variations in the degree of invasion may be due to inherent features of the ecosystems concerned, or may simply be due to different histories of settlement and land use.
Invasions into reserves: Introduction
3
Protected areas (a term used to include all areas managed primarily for the conservation of wildlife, and hence equating to such expressions as 'nature reserve', 'nature preserve', 'national park' and 'refuge') collectively constitute a useful sample of the world's ecosystems. They also provide ideal outdoor laboratories in that artificial disturbance is usually minimal. A study of the patterns of invasion in these areas therefore contributes substantially to the objectives of the SCOPE programme. Evaluation both of the consequences of these invasions for native biota and of the effects of control operations can provide useful practical guidelines for the future management of these areas. Reserve managers must decide how to allocate their limited resources to the control of invading species. However, both the consequences of the invasions and the prospects of successful control are often uncertain. An analysis on a global scale of the direct and indirect costs of biological invasions and of the benefits to conservation of their control, even if tentative, would be a useful step towards a rational policy in this respect. Formal management programmes for the control of introduced species have been underway for a decade or more in many protected areas. The experience gained can be usefully consolidated to give an overall prognosis for the problem and hopefully to contribute to cost-effective management.
CONCEPTS A N D D E F I N I T I O N S The SCOPE programme is concerned with species, which have been accidentally or intentionally introduced by human agency to nature reserves, and which lie outside their natural distributional ranges, whatever the scale of the translocation. An introduced species is considered to be invasive if further recruitment to its local population has occurred without human intervention. Usually, this is accompanied by the expansion of its range in the new geographical location, also without overt human intervention. Introduced species are, therefore, contrasted with native species. The scope of the Working Group's study encompasses any conservation area which has statutory protection and which is managed as a reserve for the maintenance of native species and communities. Reserves included in the analysis have not necessarily met the I U C N criteria (IUCN, 1975). The approach adopted has been to select a number of reserves with welldocumented biological problems. Another aim has been to include a range of biogeographic regions of the world, these being defined according to Udvardy (1975). Each case study aims to include data on the invasions recorded, the physiography and history of the reserve, the nature of any disturbances to which the reserve has been subjected, and the characteristics
M . B . Usher et al.
4
o f the native biota. Analyses o f these d a t a serve as a basis for a c o n c l u d i n g synthesis. T h e b i o m e s c h o s e n for case studies at the meeting o f the W o r k i n g G r o u p in Paris in M a r c h 1985 were m o r e n u m e r o u s t h a n those in the following papers. D u e to the limitations caused by considering only p r o t e c t e d areas and b y requiring r e a s o n a b l e d o c u m e n t a t i o n , the case studies have been restricted to include the following f o u r h a b i t a t types: tropical a n d
TABLE 1 The World Coverage of Protected Terrestrial Areas, by Biomes, as Listed by Harrison et al. (1984). (The 24 case studies referred to in this issue of Biological Conservation are also listed (columns 3 and 4) so as to set them into an international context) Biome
Total area protected (ha, millions)
Case studies Number
Area (ha, millions)
47'5
0
--
11'3 27"8
0 0
--
Tropical humid forest Subtropical and temperate rainforest and woodland Temperate coniferous forest Tropical dry forest and woodland Temperate broad-leaf forest Evergreen sclerophyllous forest and scrubland Warm desert and semi-desert Cold-winter desert Tundra Tropical grassland and savanna Temperate grassland Mixed mountain ecosystems Mixed island ecosystems
76"4 14'4
5 0
6"88 --
8.3 60-3 7"6 101.2 8-8 3"3 28-4 1.6
7 3 3 0 0 0 0 6
0-40 1.78 1.01 ----0.74
Total
396.9
24
10'81
s u b t r o p i c a l d r y w o o d l a n d s a n d savannas, M e d i t e r r a n e a n - t y p e s h r u b l a n d s a n d w o o d l a n d s , arid 1ands, a n d oceanic islands. H a r r i s o n e t al. (1984) listed the a m o u n t o f each o f the earth's biomes t h a t is p r o t e c t e d for wildlife c o n s e r v a t i o n . So t h a t the case studies can be set in a w o r l d perspective, T a b l e 1 includes a pr6cis o f H a r r i s o n e t al.'s (1984) e n u m e r a t i o n t o g e t h e r with i n f o r m a t i o n o n the reserves included in the case studies. T h e g e o g r a p h i c a l l o c a t i o n o f each o f the case studies is s h o w n in Fig. 1.
Invasions into reserves: Introduction
5
b
--
oo
• eo*s 180°W
1200W
600W
O*
60*E
120*E
180 ° E
Fig. 1. A sketch map of the world indicating the geographical location of the 24 case studies. The symbols indicate nature reserves which are islands, Mediterranean-type shrublands and woodlands, tropical and subtropical dry woodlands and savannas, and arid lands.
METHODS
Compiling species lists In the analysis of the floras and faunas of reserves several conventions have been followed. For most groups of organisms, locally extinct native species have been excluded from the totals of native species known from the reserve. However, in the case of birds (and other highly mobile animals) the total number of native species, excluding in all cases species of marine habitats, ever recorded from the reserve is the total native fauna, regardless of how long ago the last record of a species was made. Locally extinct introduced species, which are known to have existed in the reserve since its proclamation, have been included. Where they had become locally extinct without active control measures having been implemented, they have been listed only in the 'total alien species' category and not in the 'invasive alien species' category. Where they have been actively eradicated from the reserve they have been listed as invasive. The total number of introduced species establishing in the reserve is considered the most useful statistic for comparing the susceptibility to invasion of different reserves by different taxonomic groups of introduced species.
Evaluating the ecological impact of an introduced species The impact of an invasive alien species in a reserve has been rated as either severe, moderate or slight. A severe impact indicates that there is either a
6
M.B. Usher et al.
proven or potential local extinction of the reserve's population of any native species or a significant alteration of ecosystem functioning within the reserve. Examples of the former are the extinction of breeding populations of seabirds from island nature reserves brought about by the introduction of rats (Atkinson, 1985). An example of the latter is the effect of feral pigs on the accelerated loss of nutrients from catchments in the Great Smoky Mountains National Park in eastern North America (Singer et al., 1984). A moderate impact is one which leads to significant changes in the long-term mean densities of indigenous species within the reserve, but which is not thought to threaten any native species with extinction. All other lesser effects were rated as being slight, whether or not any effect on native species has been observed.
Endemism In order to quantify the endemicity of the native biota occurring in the case history reserves, the following four scales were used: endemic to the reserve itself; endemic to an area greater than a reserve of less than 1000 km 2, but with the species also being restricted to an area no more than 1000 km2; endemic to the biogeographic province (Udvardy, 1975) in which the reserve is located; and endemic to Udvardy's biogeographic realm. These endemicity criteria could only be assessed for taxonomic groups for which distributions at the appropriate scale are given in reference works.
Reference citation As far as is possible in the case history studies, the minimum number of references has been included for the sake of brevity. Original source references are not cited when recent synopses with full bibliographies are included. Whenever reserve species lists have been extracted from numerous sources and amended in the light of unpublished data, no reference is given for the list, as it is considered a new listing compiled either by the authors of the paper or by an acknowledged contributor.
THE F O L L O W I N G PAPERS The ordering of the four case study papers presented many problems, but the order chosen deals first with biomes that were thought to be the most affected by invasive species (oceanic islands) and last with those thought to be least affected (arid lands). The islands paper draws examples from many parts of the world. Many
Invasions into reserves: Introduction
7
tropical islands have protected status (Aldabra, Galapagos and Hawaii are used as examples), whereas for temperate islands it was more difficult to find appropriate case histories. Rhum is a well-documented reserve, but in Madeira, which could have provided good case history information, only the Salvage Islands are protected. In the sub-Antarctic, most islands, such as South Georgia and Marion Island, are not protected in the sense that there is any formal conservation declaration, and hence only Campbell Island is used as a case study. There is a similar international mixture of case studies in the paper on Mediterranean-type habitats (defined climatically, not geographically). Examples are drawn from USA in the northern hemisphere, and from Chile, Australia and South Africa in the southern hemisphere. Although the invasive species are frequently of European origin, well-documented case histories of invasive species in a Mediterranean (geographically defined) nature reserve are rare; the invasion by Oxalis pes-caprae, from South Africa, is one of the best known examples (Young, 1968). The paper dealing with invasions of savanna reserves draws on three case studies from Africa, one from Java and one from northern Australia. The absence of case studies in either the Neotropics or the Indian subcontinent is unfortunate; it results from the difficulty of finding suitable reserves in these two important savanna regions. The emphasis of the paper is therefore on African reserves, although the review is broadened by the two non-African case studies and by references to other Asian savanna reserves in the concluding sections. The arid lands paper is, to some extent, similar since the main focus of attention is on a series of national parks in USA, although an extensive reserve in Namibia is also considered. Again, although there are relatively few case studies, the aim is to relate these to the fragmentary data for arid land reserves elsewhere in the world. The final paper in this report aims to synthesise some of the data that have been collected, focusing on the questions which the SCOPE programme aimed to answer. The reason for the apparent difference between Europe and other areas of the world may relate to human perceptions of what is an alien species and when problems with invasive species become so critical that control measures need to be taken. In Europe, humans have moved species around for thousands of years, so that many introduced species appear natural and the status of some other species is indeterminate. In contrast, Europeans have colonised North America and the southern hemisphere continents only in the last 200-300 years, and the widespread movement of species is a more recent phenomenon. Indeed, after the passage of time, noninvasive introduced species may become worthy of conservation within their new geographical areas. Margules & Usher (1981) pointed out that Perring
8
M.B. Usher et al.
& Farrell (1977) had included at least 21 species of flowering plants, which are either known introductions or are not certain to be native, in the British Red Data Book. These species include Isatis tinctoria, which is thought to have been introduced to Britain for use as a dye more than a thousand years ago and persists in a naturalised state in two localities, and Juncus dudleyi, much more recently introduced, which also occurs in only two localities. It is important to define aims of conservation management and to consider thresholds which affect h u m a n perception. Not all introduced species of plants, animals and microbes are adverse factors in conservation management: it is the group of invasive introduced species that is the focus of attention in the following papers.
REFERENCES Atkinson, I. A. E. (1985), The spread of commensal species of Rattus to oceanic islands and their effects on island avifaunas. In Conservation of island birds, ed. by P. J. Moors, 35-81. Cambridge, International Council for Bird Preservation. Groves, R. H. & Burdon, J. J. (eds) (1986). Ecology of biological invasions: An Australian perspective. Canberra, Australian Academy of Science. Harrison, J., Miller, K. R. & McNeely, J. (1984). The world coverage of protected areas: Development goals and environmental needs. In National parks, conservation and development, ed. by J. McNeely and K. R. Miller, 24 31. Washington, DC, Smithsonian Institution Press. IUCN (1975). Worlddirecto O, ofnationalparks and protected areas. Morges, I UCN. Joenje, W., Bakker, K. & Vlijm, L. (eds) (1987). The ecology of biological invasions. Proceedings o.1"the Koninklijke Nederlandse Akademie van Wetenschappen, series C, 90, 1-80. Kornberg, H. & Williamson, M. H. (eds) (! 987). Quantitative aspects q/the ecology o/ biological invasions. London, The Royal Society. Macdonald, I. A. W., Kruger, F. J. & Ferrar, A. A. (eds) (1986). Ecology and management of biological invasions in southern Africa. Cape Town, Oxford University Press. Margules, C. & Usher, M. B. (1981). Criteria used in assessing wildlife conservation potential: A review. Biol. Conserv., 21, 79 109. M ooney, H. A. & Drake, J. A. (eds) (1986). Ecology of biological invasions of North America and Hawaii. New York, Springer-Verlag. Perring, F. H. & Farrell, L. (1977). British Red Data Books: 1. Vascular plants. Lincoln, Society for the Promotion of Nature Conservation. Singer, F. J., Swank, W. T. & Clebsch, E. E. C. (1984). Effects of wild pig rooting in a deciduous forest. J. Wildl. Manage., 48, 464-73. Udvardy, M. D. F. (1975). A classification of the biogeographical provinces of the world. IUCN Occasional Paper, 18. Young, D. P. (1968). Geraniales. LXXXII. Oxalidaceae, I. Oxalis L. In Flora Europaea, 2, Rosaeeae to Umbelliferae, ed. by T. G. Tutin, V. H. Heywood, N. A. Burges, D. M. Moore, D. H. Valentine, S. M. Waiters and D. A. Webb, 192-3. Cambridge, Cambridge University Press.
The Ecology of Biological Invasions into Nature Reserves: An Introduction
M. B. Usher Department of Biology, University of York, York YO1 5DD, Great Britain
F. J. Kruger South African Forestry Research Institute, PO Box 727, Pretoria 0001, Republic of South Africa
I. A. W. Macdonald Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, Republic of South Africa
L. L. Loope Haleakala National Park, Box 369, Makawao, Hawaii 96768, USA
& R. E. Brockie Ecology Division, DSIR, Private Bag, Lower Hutt, New Zealand (Received 4 July 1987: accepted 31 July 1987)
A BS TRA C T The following five papers collectively form a report of the Working Group on Nature Reserves, associated with the SCOPE programme on the "Ecology of Biological Invasions'. This introduction ( i) defines the aims of the SCOPE programme, ( ii ) describes the approach of the Working Group, ( iii) provides definitions so that these do not have to be given repeatedly in the following papers, and (iv) 'signposts' the remainder of the report. l
Biol. Conserv. 0006-3207/88/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
2
M . B . Usher et al.
THE SCOPE P R O G R A M M E ON THE ECOLOGY OF BIOLOGICAL INVASIONS The programme
Since mid-1982 a SCOPE (Scientific Committee on Problems of the Environment, part of the non-governmental organisation, The International Council of Scientific Unions, based in Paris) programme has examined the ecology of invasive plants, animals and microbes, concentrating particularly on those species which have successfully invaded nonagricultural regions and which have disrupted natural ecosystem processes. The programme aims to answer three specific questions, namely: 1. 2. 3.
What are the factors which determine whether a species will be an invader or not? What are the site properties which determine whether an ecological system will be prone to, or resistant to, invasion? How should management systems be developed to use the knowledge gained from answering questions 1 and 2?
Within the international framework of the SCOPE programme there have been a number of national meetings, together with the establishment of specialist international working groups. Publications following national meetings include those from Australia (Groves & Burdon, 1986), the United States of America (Mooney & Drake, 1986), South Africa (Macdonald et al., 1986), the United Kingdom (Kornberg & Williamson, 1987), and the Netherlands (Joenje et al., 1987). The specialist working groups were planned to be concerned with tropical ecosystems, Mediterranean-type ecosystems, stressed ecosystems, modelling and nature reserves. The working group on invasions in nature reserves
The nature and degree of invasion by introduced species is thought to vary considerably between protected areas in different parts of the world. In general, reserves in the tropics and in subtropical arid lands have fewer invasive species, though there are important exceptions. Among temperate regions those in the southern hemisphere have often experienced severe invasions, whilst those in the northern hemisphere seem relatively free from invasions. At its worst, the problem of invasions seems insuperable on many oceanic island reserves. Variations in the degree of invasion may be due to inherent features of the ecosystems concerned, or may simply be due to different histories of settlement and land use.
Invasions into reserves: Introduction
3
Protected areas (a term used to include all areas managed primarily for the conservation of wildlife, and hence equating to such expressions as 'nature reserve', 'nature preserve', 'national park' and 'refuge') collectively constitute a useful sample of the world's ecosystems. They also provide ideal outdoor laboratories in that artificial disturbance is usually minimal. A study of the patterns of invasion in these areas therefore contributes substantially to the objectives of the SCOPE programme. Evaluation both of the consequences of these invasions for native biota and of the effects of control operations can provide useful practical guidelines for the future management of these areas. Reserve managers must decide how to allocate their limited resources to the control of invading species. However, both the consequences of the invasions and the prospects of successful control are often uncertain. An analysis on a global scale of the direct and indirect costs of biological invasions and of the benefits to conservation of their control, even if tentative, would be a useful step towards a rational policy in this respect. Formal management programmes for the control of introduced species have been underway for a decade or more in many protected areas. The experience gained can be usefully consolidated to give an overall prognosis for the problem and hopefully to contribute to cost-effective management.
CONCEPTS A N D D E F I N I T I O N S The SCOPE programme is concerned with species, which have been accidentally or intentionally introduced by human agency to nature reserves, and which lie outside their natural distributional ranges, whatever the scale of the translocation. An introduced species is considered to be invasive if further recruitment to its local population has occurred without human intervention. Usually, this is accompanied by the expansion of its range in the new geographical location, also without overt human intervention. Introduced species are, therefore, contrasted with native species. The scope of the Working Group's study encompasses any conservation area which has statutory protection and which is managed as a reserve for the maintenance of native species and communities. Reserves included in the analysis have not necessarily met the I U C N criteria (IUCN, 1975). The approach adopted has been to select a number of reserves with welldocumented biological problems. Another aim has been to include a range of biogeographic regions of the world, these being defined according to Udvardy (1975). Each case study aims to include data on the invasions recorded, the physiography and history of the reserve, the nature of any disturbances to which the reserve has been subjected, and the characteristics
M . B . Usher et al.
4
o f the native biota. Analyses o f these d a t a serve as a basis for a c o n c l u d i n g synthesis. T h e b i o m e s c h o s e n for case studies at the meeting o f the W o r k i n g G r o u p in Paris in M a r c h 1985 were m o r e n u m e r o u s t h a n those in the following papers. D u e to the limitations caused by considering only p r o t e c t e d areas and b y requiring r e a s o n a b l e d o c u m e n t a t i o n , the case studies have been restricted to include the following f o u r h a b i t a t types: tropical a n d
TABLE 1 The World Coverage of Protected Terrestrial Areas, by Biomes, as Listed by Harrison et al. (1984). (The 24 case studies referred to in this issue of Biological Conservation are also listed (columns 3 and 4) so as to set them into an international context) Biome
Total area protected (ha, millions)
Case studies Number
Area (ha, millions)
47'5
0
--
11'3 27"8
0 0
--
Tropical humid forest Subtropical and temperate rainforest and woodland Temperate coniferous forest Tropical dry forest and woodland Temperate broad-leaf forest Evergreen sclerophyllous forest and scrubland Warm desert and semi-desert Cold-winter desert Tundra Tropical grassland and savanna Temperate grassland Mixed mountain ecosystems Mixed island ecosystems
76"4 14'4
5 0
6"88 --
8.3 60-3 7"6 101.2 8-8 3"3 28-4 1.6
7 3 3 0 0 0 0 6
0-40 1.78 1.01 ----0.74
Total
396.9
24
10'81
s u b t r o p i c a l d r y w o o d l a n d s a n d savannas, M e d i t e r r a n e a n - t y p e s h r u b l a n d s a n d w o o d l a n d s , arid 1ands, a n d oceanic islands. H a r r i s o n e t al. (1984) listed the a m o u n t o f each o f the earth's biomes t h a t is p r o t e c t e d for wildlife c o n s e r v a t i o n . So t h a t the case studies can be set in a w o r l d perspective, T a b l e 1 includes a pr6cis o f H a r r i s o n e t al.'s (1984) e n u m e r a t i o n t o g e t h e r with i n f o r m a t i o n o n the reserves included in the case studies. T h e g e o g r a p h i c a l l o c a t i o n o f each o f the case studies is s h o w n in Fig. 1.
Invasions into reserves: Introduction
5
b
--
oo
• eo*s 180°W
1200W
600W
O*
60*E
120*E
180 ° E
Fig. 1. A sketch map of the world indicating the geographical location of the 24 case studies. The symbols indicate nature reserves which are islands, Mediterranean-type shrublands and woodlands, tropical and subtropical dry woodlands and savannas, and arid lands.
METHODS
Compiling species lists In the analysis of the floras and faunas of reserves several conventions have been followed. For most groups of organisms, locally extinct native species have been excluded from the totals of native species known from the reserve. However, in the case of birds (and other highly mobile animals) the total number of native species, excluding in all cases species of marine habitats, ever recorded from the reserve is the total native fauna, regardless of how long ago the last record of a species was made. Locally extinct introduced species, which are known to have existed in the reserve since its proclamation, have been included. Where they had become locally extinct without active control measures having been implemented, they have been listed only in the 'total alien species' category and not in the 'invasive alien species' category. Where they have been actively eradicated from the reserve they have been listed as invasive. The total number of introduced species establishing in the reserve is considered the most useful statistic for comparing the susceptibility to invasion of different reserves by different taxonomic groups of introduced species.
Evaluating the ecological impact of an introduced species The impact of an invasive alien species in a reserve has been rated as either severe, moderate or slight. A severe impact indicates that there is either a
6
M.B. Usher et al.
proven or potential local extinction of the reserve's population of any native species or a significant alteration of ecosystem functioning within the reserve. Examples of the former are the extinction of breeding populations of seabirds from island nature reserves brought about by the introduction of rats (Atkinson, 1985). An example of the latter is the effect of feral pigs on the accelerated loss of nutrients from catchments in the Great Smoky Mountains National Park in eastern North America (Singer et al., 1984). A moderate impact is one which leads to significant changes in the long-term mean densities of indigenous species within the reserve, but which is not thought to threaten any native species with extinction. All other lesser effects were rated as being slight, whether or not any effect on native species has been observed.
Endemism In order to quantify the endemicity of the native biota occurring in the case history reserves, the following four scales were used: endemic to the reserve itself; endemic to an area greater than a reserve of less than 1000 km 2, but with the species also being restricted to an area no more than 1000 km2; endemic to the biogeographic province (Udvardy, 1975) in which the reserve is located; and endemic to Udvardy's biogeographic realm. These endemicity criteria could only be assessed for taxonomic groups for which distributions at the appropriate scale are given in reference works.
Reference citation As far as is possible in the case history studies, the minimum number of references has been included for the sake of brevity. Original source references are not cited when recent synopses with full bibliographies are included. Whenever reserve species lists have been extracted from numerous sources and amended in the light of unpublished data, no reference is given for the list, as it is considered a new listing compiled either by the authors of the paper or by an acknowledged contributor.
THE F O L L O W I N G PAPERS The ordering of the four case study papers presented many problems, but the order chosen deals first with biomes that were thought to be the most affected by invasive species (oceanic islands) and last with those thought to be least affected (arid lands). The islands paper draws examples from many parts of the world. Many
Invasions into reserves: Introduction
7
tropical islands have protected status (Aldabra, Galapagos and Hawaii are used as examples), whereas for temperate islands it was more difficult to find appropriate case histories. Rhum is a well-documented reserve, but in Madeira, which could have provided good case history information, only the Salvage Islands are protected. In the sub-Antarctic, most islands, such as South Georgia and Marion Island, are not protected in the sense that there is any formal conservation declaration, and hence only Campbell Island is used as a case study. There is a similar international mixture of case studies in the paper on Mediterranean-type habitats (defined climatically, not geographically). Examples are drawn from USA in the northern hemisphere, and from Chile, Australia and South Africa in the southern hemisphere. Although the invasive species are frequently of European origin, well-documented case histories of invasive species in a Mediterranean (geographically defined) nature reserve are rare; the invasion by Oxalis pes-caprae, from South Africa, is one of the best known examples (Young, 1968). The paper dealing with invasions of savanna reserves draws on three case studies from Africa, one from Java and one from northern Australia. The absence of case studies in either the Neotropics or the Indian subcontinent is unfortunate; it results from the difficulty of finding suitable reserves in these two important savanna regions. The emphasis of the paper is therefore on African reserves, although the review is broadened by the two non-African case studies and by references to other Asian savanna reserves in the concluding sections. The arid lands paper is, to some extent, similar since the main focus of attention is on a series of national parks in USA, although an extensive reserve in Namibia is also considered. Again, although there are relatively few case studies, the aim is to relate these to the fragmentary data for arid land reserves elsewhere in the world. The final paper in this report aims to synthesise some of the data that have been collected, focusing on the questions which the SCOPE programme aimed to answer. The reason for the apparent difference between Europe and other areas of the world may relate to human perceptions of what is an alien species and when problems with invasive species become so critical that control measures need to be taken. In Europe, humans have moved species around for thousands of years, so that many introduced species appear natural and the status of some other species is indeterminate. In contrast, Europeans have colonised North America and the southern hemisphere continents only in the last 200-300 years, and the widespread movement of species is a more recent phenomenon. Indeed, after the passage of time, noninvasive introduced species may become worthy of conservation within their new geographical areas. Margules & Usher (1981) pointed out that Perring
8
M.B. Usher et al.
& Farrell (1977) had included at least 21 species of flowering plants, which are either known introductions or are not certain to be native, in the British Red Data Book. These species include Isatis tinctoria, which is thought to have been introduced to Britain for use as a dye more than a thousand years ago and persists in a naturalised state in two localities, and Juncus dudleyi, much more recently introduced, which also occurs in only two localities. It is important to define aims of conservation management and to consider thresholds which affect h u m a n perception. Not all introduced species of plants, animals and microbes are adverse factors in conservation management: it is the group of invasive introduced species that is the focus of attention in the following papers.
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