Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Theoretical ecology wikipedia , lookup
Occupancy–abundance relationship wikipedia , lookup
Latitudinal gradients in species diversity wikipedia , lookup
Habitat conservation wikipedia , lookup
Unified neutral theory of biodiversity wikipedia , lookup
Introduced species wikipedia , lookup
Island restoration wikipedia , lookup
Biodiversity wikipedia , lookup
Weed control wikipedia , lookup
Austral Ecology (2004) 29, 51–58 Weeds and the monitoring of biodiversity in Australian rangelands A. C. GRICE CSIRO Sustainable Ecosystems, Private Bag, PO Aitkenvale, Queensland 4814, Australia (Email: [email protected]) and Cooperative Research Centre for Australian Weed Management, Adelaide, South Australia, Australia Abstract Monitoring the biodiversity of Australian rangelands has been identified as a means of informing policy and supporting funding decisions in relation to the conservation of biodiversity. Australian rangelands are subject to invasion by alien plants that have the potential to have major impacts on ecosystem function and biodiversity, although there has been little quantitative documentation of these effects. Research is needed to improve our understanding of how and to what extent alien plants affect biodiversity in Australian rangelands so that this relationship can be considered when developing and implementing programmes to monitor biodiversity. It is also important to consolidate existing efforts to quantify the extent of alien plant invasions and monitor their progress, thus documenting a process that threatens biodiversity. Information on the presence and abundance of alien plant species should be considered for inclusion as a component of biodiversity monitoring programmes that are undertaken. Monitoring components of biodiversity can itself provide a basis for evaluating weed management strategies. Key words: biodiversity, invasive plants, weeds. INTRODUCTION An alien plant is one that is present because of intentional or accidental introduction by humans, and that reproduces consistently and sustains populations over many life cycles without human intervention. An invasive plant is a naturalized plant that produces reproductive offspring at considerable distances from parent plants and so has the potential to spread over considerable areas (Richardson et al. 2000). Invasion by alien plants is widely recognized as a major threatening process for a great variety of ecosystems worldwide. This is certainly true in Australia (State of the Environment Advisory Council 1996), where 1500–2000 alien species had naturalized by the mid 1990s (Humphries et al. 1991) and naturalization continues at an average rate of 10 species per year (Groves 1997). Numerous alien species are recognized as environmental weeds (plants that are in some way deleterious to the environment; Groves 1997) in that they invade natural ecosystems, as opposed to being restricted to, for example, areas used for agriculture. Many occur in Australia’s rangelands (Lonsdale & Milton 2002), which constitute approximately 70% of the Australian continent (Young et al. 1984). Alien plants, therefore, have the potential to influence the structure, function and composition of rangeland eco- Accepted for publication September 2003. systems and so constitute a threat to the biodiversity of those rangelands. Currently, there is considerable interest in monitoring the biodiversity of Australian rangelands (Smyth et al. 2004). Any programme for monitoring biodiversity must be built on an understanding of the processes that threaten biodiversity. The present paper discusses the significance of invasive species for monitoring the biodiversity of Australian rangelands. It provides a brief synopsis of the nature, scale and mechanisms of influence of invasive plant species on Australian rangelands and their biodiversity. It then discusses the implications of this information for the monitoring of Australian rangeland biodiversity. RANGELAND WEEDS The likely impact of alien plant species on the biodiversity of Australian rangelands can be evaluated in terms of the number of alien species present, the growth forms represented, the extent of their actual and potential geographical ranges and their relative abundances at the sites that they occupy. There is, however, no comprehensive, quantitative picture of the invasion of Australian rangelands by alien plant species. The number of alien plant species in Australian rangelands is lower than in non-rangeland parts of the continent (State of the Environment Advisory Council 1996), but floras of rangeland regions list significant 52 A. C. GRICE numbers of alien plant species. For example, Flora of Central Australia (Jessop 1981), which covers 30% of the Australian continent and 43% of its rangelands, documents 114 species of alien plants making up 6% of the total species listed for the region. Plants of Western New South Wales (Cunningham et al. 1981), which covers 5% of the continent and 7% of its rangelands, documents 400 species of alien plants making up 21% of the total species listed for the region. The alien flora of the Australian rangelands includes a broad variety of growth forms. Of the exotic species listed in Australian rangeland floras, 50–60% are annual forbs. Other growth forms contribute smaller proportions of exotic species: annual grasses (17%); perennial forbs (12–19%); perennial grasses (5–8%); shrubs (approximately 5%); and trees (1–2%) (Grice 2000). Some growth forms (e.g. smothering vines such as Cryptostegia grandiflora R. Br.; Tomley 1998) are likely to have greater impacts than others. Many of the alien species present in Australian rangelands already have extensive ranges there, as demonstrated by maps of current distributions of species nominated as Weeds of National Significance (WONS). However, bioclimatic analyses predict that for most species there remains much potential for further range expansion (Thorp & Lynch 2000). Although the number of species present, their growth forms and their geographical ranges indicate something of the threats posed by alien plant species, their impacts at any particular site will be driven more by their abundance, which can be expressed in terms of plant densities or biomass per unit area. Quantitative data on abundance of weeds across extensive areas are not available for Australian rangelands. Qualitative, categorical data are available for some species at State or national scales, as presented, for example, for species nominated as WONS (Thorp & Lynch 2000). Quantitative data on abundance are available for particular species at individual sites. These indicate that many alien species can reach very high densities and biomass, to the point where they dominate the stratum of vegetation within which they occur. For example, densities of Prosopis pallida (Willd.) Kunth. reach at least 1600–1800 plants per ha (Campbell & Setter 1999); dense stands of the shrub Mimosa pigra L. typically reach 10 000–30 000 plants per ha (Lonsdale et al. 1995); mature individuals of the annual forb Parthenium hysterophorus L. reach densities of 14 000 plants per ha (Navie et al. 1998). There are, therefore, numerous alien plant species of a wide variety of growth forms in Australian rangelands; many of them are currently widespread, although there remains considerable scope for further increases; and they often dominate the vegetation. Although these factors suggest that their combined effect on biodiversity is considerable, some species are likely to be more important than others. Some indi- cation of perceived relative importance of different alien species can be gained from legislative weed declarations. It must be acknowledged, however, that such declarations are rather subjective and relate strongly to particular land uses, especially for agricultural and pastoral industries. The information from central Australia (Jessop 1981) and western New South Wales (Cunningham et al. 1981) indicates that annual forbs, annual grasses and perennial grasses are underrepresented in lists of declared weeds relative to their contributions to exotic floras (Parsons & Cuthbertson 1992). In contrast, perennial forbs, shrubs and trees are overrepresented (Fig. 1). These ratios might reflect pastoral interests more strongly than they reflect environmental interests. For example, the perennial grass Cenchrus ciliaris L. (buffel grass) remains the most widely sown perennial pasture grass in northern Australia despite widespread concerns about its invasive characteristics and environmental impacts in central Australia, northern Western Australia and parts of Queensland (Griffin 1993). The nominations of WONS also indicate a predominance of woody species among exotic species that have been highlighted as most relevant to the rangelands, even though more than half of the exotic species are annual forbs (Table 1; Thorp & Lynch 2000). At least seven of Australia’s 20 WONS are relevant to rangelands. As well as being subject to invasion by alien plant species, Australian rangelands have experienced major shifts in the composition of the native flora. Some of these shifts involve increases in the abundance of native shrubs and small trees (Noble 1997) that are often referred to as invasions, although, strictly speaking, the species often do not conform to the definition of ‘invasives’ provided by Richardson et al. (2000), but rather are proliferations within their native ranges. Nevertheless, these shifts no doubt have significant consequences for biodiversity. Fig. 1. Percentage contribution of different growth forms to ( ) exotic plant and () declared weed lists species for Australian rangelands based on data from Cunningham et al. (1981), Jessop (1981) and Parsons and Cuthbertson (1992). WEEDS AND BIODIVERSITY MECHANISMS OF IMPACT ON ECOSYSTEMS Weeds affect the ecosystems that they invade in a variety of ways, and dramatic changes are often obvious. Weeds sequester resources (water, nutrients and light) that would otherwise be available to native plant species. The pathways and rates of water and nutrient cycling are then likely to be altered relative to the situation in the uninvaded state, as is the light regime (Standish et al. 2001). Freudenberger et al. (1997) link increases in the abundance of native shrubs in rangeland systems with landscape dysfunction; that is, the ‘excessive flow of water and nutrients out of a landscape system’ (Tongway & Ludwig 1997). Invasive exotic shrubs might be expected to show similar associations. Resources that are acquired by a plant can be (i) retained by the individual; (ii) passed on to the succeeding generation by being used to produce seeds; or (iii) captured by consumers. The availability to consumer organisms of the resources that are acquired by plants varies depending on the plant species. For some alien weed species, there might be few, if any, herbivores capable of accessing the resources that have been sequestered by the weed. Specialist herbivores, 53 most notably insects, from the weed’s native range might not have been introduced. This forms part of the argument in favour of biological control as a weedmanagement technique. The ramifications for the food web could also extend to decomposers because litter derived from weed species can differ from that produced by native species (Pidgeon & Cairns 1981; Schulze & Walker 1997). Weeds also alter the structure of the vegetation that they invade. The growth form of the weed(s) is important in this regard and the effects are likely to be most extreme in cases where a weed is of a growth form that does not exist, or is only a minor component in the uninvaded community. Examples include the invasion of Top End floodplains, which are naturally dominated by herbaceous species, by the shrub Mimosa pigra L. (giant sensitive plant) (Lonsdale 1992); the invasion of Queensland Mitchell grass (Astrebla spp.) plains by the tree Acacia nilotica (L.) Del. (Mackey 1998); and the invasion of northern Australian riparian zones by the shrubby vine Cryptostegia grandiflora R. Br. (Tomley 1998). Weeds also affect ecosystems by altering fire regimes. These effects occur when weeds influence fuel loads, fuel distribution, fuel continuity or the timing of curing, with repercussions for the intensity, timing Table 1. Species that were nominated as Weeds of National Significance and that are relevant to rangelands (from Thorp & Lynch 2000) Scientific name Common name Growth form Xanthium spinosum L. Hyptis suaveolens (L) Poit. Argemone ochroleuca Sweet Xanthium occidentale Bertol. † Parthenium hysterophorus L. Themeda quadrivalvis (L.) Kuntze Bryophyllum tubiflorum Harvey Sida spp. Stachytarpheta spp. Eragrostis curvula (Schrader) Nees Sporobolus pyramidalis Beauv. S. natalensis Th. Dur & Schinz Pennisetum polystachion † Lantana camara L. Lycium ferocissimum Miers Jatropha gossypifolia L. Calotropis procera (Aiton) Aiton Bassia scoparia (L.) A. J. Scott † Mimosa pigra L. Gomphocarpus fruticosus (L.) Spreng. † Parkinsonia aculeata L. Senna obtusifolia (L.) H. S. Irwin & R. C. Barneby † Cryptostegia grandiflora R. Br. † Tamarix aphylla (L.) H. Karst Ziziphus mauritiana Lam. † Prosopis spp. † Acacia nilotica (L.) Del. Bathurst burr Hyptis Mexican poppy Noogoora burr Parthenium Grader grass Mother of millions Sida species Snakeweeds African lovegrass Giant rat’s tail grass Annual forb Annual forb Annual forb Annual forb Annual forb Annual grass Perennial forb Perennial forb Perennial forb Perennial grass Perennial grass Perennial grass Perennial grass Shrub Shrub Shrub Shrub Shrub Shrub Shrub Shrub Shrub Shrub/vine Tree Tree Tree Tree † Species declared Weeds of National Significance. Mission grass Lantana African boxthorn Bellyache bush Calotrope Kochia Giant sensitive plant Narrow-leaved cotton bush Parkinsonian Sicklepod Rubber vine Athel pine Chinee apple Mesquite Prickly acacia 54 A. C. GRICE and frequency of fires. Effects on fire regimes are cited among the impacts of the bulky perennial grasses Andropogon gayanus Kunth. (gamba grass) and Pennisetum polystachion (L) Schultes (mission grass) in the Top End, and Cenchrus ciliaris in central and Western Australia (Rossiter et al. 2003). EFFECTS OF WEEDS ON BIODIVERSITY Although some effort has been made to document mechanisms whereby weeds influence ecosystem structure and function, overall the impacts have not been widely quantified. There are few data specifically dealing with the effects of weeds on the biodiversity of Australian rangelands. A typical approach to examining the effects of weeds on plant communities is that used by Mullett and Simmons (1995). This example relates not to an invasive exotic species but to the indigenous Australian shrub Pittosporum undulatum Vent. (sweet pittosporum) that is spreading into communities or parts of the landscape that it did not occupy in pre-European times, as well as apparently expanding its range in Victoria. Mullett and Simmons (1995) quantified its environmental impacts by estimating the cover of all vascular plant species in 3 3-m quadrats located along transects running through small clumps of P. undulatum. Multivariate pattern analysis showed that fewer plant species occurred in quadrats where the abundance of P. undulatum was high and the abundance and cover of the indigenous species was inversely correlated with that of P. undulatum. A similar approach showed negative correlations between measures of abundance of native species and the shrub Chrysanthemoides monilifera (L.) T. Norl. (bitou bush) that invades coastal vegetation in south-eastern Australia (Weiss & Noble 1984). Working in central Queensland woodlands, Fairfax and Fensham (2000) compared floristic diversity across boundaries between uncleared land and cleared land with either native (exotic species < 10% of total ground cover) or exotic (exotic species > 10% of total ground cover) pastures. The total number of native species was greater in uncleared land than in either native or exotic pastures on cleared land. In cleared land, native species richness was greater in native pastures than in exotic pastures. One of the difficulties with such studies is in separating the effects of invasion by exotic pasture species from the effects of livestock grazing, because cleared land is generally subject to higher livestock densities. McIvor (1998) examined the effects of various pasture-management treatments, including over-sowing with legume/grass mixtures, on species diversity of experimental pastures in northeastern Queensland. The presence of sown species, including the grasses Cenchrus ciliaris and Urochloa mosambicensis (Hackel) Dandy and the legumes Stylosanthes hamata (L) Taub. and S. scabra Vogel, was associated with reduced numbers of native species at plot and quadrat scales. This is also a case in which the exotic species were deliberately sown and the effects of their presence cannot be separated from the effects of the sowing techniques. Even less research has examined the effects of weeds on animal groups, where the focus has been more on the ecological responses of individual vertebrate species. Again, quantification of effects has not been a strong point of much of the work. Perhaps in contrast to the situation with vascular plants, weeds can affect individual vertebrate species either positively or negatively. This is evident from a list of exotic plant species that are eaten by a variety of Australian bird species (Loyn & French 1991). It is not apparent from this kind of evidence how important the weeds are in the diets of the species listed, nor does the information indicate the net effects on bird species or communities. Through their role as weed-dispersal agents, birds facilitate changes in plant communities that could continue for many decades or even hundreds of years, with repercussions for other components of biodiversity and ecosystem processes (Williams & Karl 1996). In central Australia, Griffin et al. (1989) examined bird, reptile and plant communities in riparian zones dominated by the native tree Eucalyptus camaldulensis Dehnh. and the invasive Tamarix aphylla (L.) H. Karst. Areas infested by T. aphylla supported fewer birds and reptiles, had different avian community structures and significantly different and less-diverse herbaceous plant communities compared with sites dominated by E. camaldulensis. Published reports contain several examples of native Australian mammal species possibly benefiting from the presence of exotic plant species. These include the use of Ulex europaeus L. (gorse) by the eastern barred bandicoot (Perameles gunnii Gray), of Rubus fruticosus L. (blackberry) by the broad-toothed rat (Mastacomys fuscus Thomas) (Brown et al. 1991) and of Mimosa pigra by the red-cheeked dunnart (Sminthopsis virginiae Tarragon) (Braithwaite & Lonsdale 1987). In the first two of these examples, the apparent associations between weed and mammal are not quantified or evaluated experimentally. These examples are sufficient to indicate that native plant species richness is generally lower in places where one or more weed species are abundant and that individual native animal species can respond positively or negatively to invasion by a weed species. Causal relationships are generally implied but not demonstrated. These conclusions fall well short of providing a comprehensive picture of how biodiversity responds to exotic plant invasions, although it is generally agreed that the net effect of weed invasions is negative. In WEEDS AND BIODIVERSITY particular, a more specific understanding of the relationships between weeds and biodiversity in rangelands is not available. Most work on the environmental impacts of weeds, including effects on biodiversity, has examined single weed species. However, it is common for natural ecosystems to be invaded by more than one species. In effect, new plant communities are emerging within Australian ecosystems as weed complexes combine with native species that can coexist with them. The presence of one weed species can facilitate invasion by others. For example, a weed species that has characteristics that attract birds (e.g. fleshy fruit) could facilitate invasion by other bird-dispersed weed species. Under these circumstances, there will be synergistic effects of weeds on biodiversity. Obviously, alien plant invasions are only one of the threats to biodiversity, and both the relative importance of various threatening processes and the interactions between them must be considered. Leigh and Briggs (1992) (cited in Groves & Willis 1999) evaluated numbers of extinct and endangered Australian plant species threatened by various factors. They considered that 57 species (13% of total extinct and threatened species) were threatened by ‘weed competition’, making this factor the third most important after ‘grazing and agriculture’ and ‘low population numbers’. WEEDS AND BIODIVERSITY MONITORING Monitoring the biodiversity of Australian rangelands has been identified as a means of informing policy and supporting funding decisions in relation to the conservation of biodiversity (Smyth et al. 2004). There is already sufficient information available to conclude that invasion of Australian ecosystems by alien plants is one of the important processes that threaten biodiversity. What, therefore, should the links be between the monitoring of biodiversity and work on alien plant invasions in Australian rangelands? First, it is useful to quantify the progress of alien plant invasions, thus documenting a threatening process as far as biodiversity is concerned. Although Australia does not have a national system for mapping the occurrence of alien plants, many States do. There would be value in at least improving the compatibility of these systems even if a single national approach proves elusive. Mapping at the relatively coarse scales currently used would be sufficient to inform judgements about where biodiversity is at risk from weeds and which weeds are involved. It is important that weed-mapping systems include the capacity to document changes over time and incorporate at least qualitative estimates of the abundance of different weed species. It will be necessary to consider the trade-offs 55 between the quality and quantity (scale, frequency) of information recorded. Information on the distribution and abundance of alien plant species in relation to biodiversity would be used to inform decisions about where to direct weed-management resources. Second, there is a need for research to improve our understanding of how and to what extent alien plants affect biodiversity, expressly in Australian rangelands. Many of the examples provided in the present paper are from outside the rangelands. In particular, attention should be given to understanding (i) how the spatial and temporal characteristics of rangelands impinge on the interactions between weeds and biodiversity; (ii) the interactive effects of livestock grazing and alien species; (iii) how alien species alter fire regimes and so impact on biodiversity; (iv) how different taxonomic groups respond to different types of weed invasion; and (v) how biodiversity is affected by invasions of multiple weed species. It could be useful to conduct further research to define functional groups that consist of alien plants that affect biodiversity in similar ways (Díaz et al. 2002). A growth-form or functional-group approach might help predict or evaluate the effects of weeds on biodiversity. Weed growth forms that are novel to the invaded system are likely to be more significant for biodiversity than growth forms for which there is a native ecological analogue. However, there is also the possibility that there are subtle but important differences between species that could otherwise be regarded as being in the same functional group or having the same growth form as some native, non-weedy species. For example, Cenchrus ciliaris is, in many ways, structurally and functionally similar to the native, perennial, tussock grasses of central Australia, but it might be less useful to native granivores than the native grasses because of the characteristics of its seeds. Invasion by Cenchrus ciliaris could therefore lead to a decline in the diversity of granivores. Third, information on the presence and abundance of alien plant species should be considered for inclusion as a component of biodiversity monitoring programmes that are undertaken. There are two reasons for doing so: (i) the occurrence, distribution and abundance of weeds could be useful as an indicator of biodiversity; and (ii) invasion by alien plants is a major threatening process for Australian natural ecosystems. Of course, the effort applied to incorporating alien species as a component of biodiversity monitoring must be determined in relation to the purpose of the programme and the resources available to carry it out. However, including information on alien plant species might be a relatively small and efficient addition to a monitoring programme that was already gathering information on native vegetation. Research is needed to establish the indicator value of alien plant species, although one can expect a general 56 A. C. GRICE negative correlation between presence and abundance of native and exotic plant species. Current evidence indicates that the shapes of the relationships between weed abundance and biodiversity measurements are variable (Panetta & James 1999). It seems unlikely that biodiversity decline is a linear function of weed abundance; rather, the impacts of weeds are likely to be greater when weeds are in an advanced stage of invasion. There may be thresholds such that the rate of decline or loss of species or communities will be much greater at high levels of weed abundance. Such relationships and thresholds should be quantified and effort should be made to understand the ecological processes that underlie them. The situation is likely to be further complicated by the fact that some species increase in the face of weed invasions. What measures of weed presence and abundance could be considered as part of a monitoring programme? Weed species richness (weed species per unit area) on its own is unlikely to be a good measure of how severely weeds are affecting biodiversity. Some weed species have a far greater impact than others, although the presence of a large number of alien species might be correlated with other forms of disruption to the invaded ecosystem and so with the overall effects on biodiversity. Relative abundance of weeds at monitoring sites will be more informative than presence/absence. Typically, the severity of infestations of weedy trees and shrubs are described in terms of plant density (Campbell & Setter 1999). This is a practical means of making intersite or temporal comparisons of abundance of weed species whose individuals are of approximately equal size. However, where there is a need to make comparisons between species whose individuals differ in size, measurement of biomass will be far more informative. In a biodiversity monitoring programme, site-specific data on the biomass of weeds and how that Table 2. Alien plant species of Australian rangelands that warrant priority consideration in monitoring programmes Mode of impact/Species Trees and shrubs that invade and dominate open savannas and grasslands Acacia nilotica Parkinsonia aculeata Prosopis spp. Ziziphus mauritiana Mimosa pigra Jatropha gossypifolia Perennial grasses that alter fire regimes Cenchrus ciliaris Pennisetum polystachion Andropogon gayanus Themeda quadrivalvis Smothering vines Cryptostegia grandiflora biomass is distributed between species and growth forms would generally be preferable to simple lists of species present. Effort to gather data on relative weed abundance should be most heavily concentrated on those species and growth forms that make the greatest contribution to the biomass of the weed community or that have the potential to do so. Such species are the ones most likely to have significant impacts on ecosystem structure and function. This effort should be tempered by the possibility that peculiarities of an invasive species could give it an impact out of proportion to its abundance. Moreover, an alien species might not dominate the vegetation overall, but might still have a major impact on a particular stratum of the vegetation and so on biodiversity and ecosystem function. The current state of knowledge suggests that priority species or groups should be trees and shrubs that invade open savannas and grasslands, perennial grasses that are likely to significantly contribute to altered fire regimes or that are not ecological surrogates of native tussock grasses, and smothering vines that are relevant to more densely vegetated woodlands and forests (Table 2). This is not to say that other species and growth forms could not be significant transformers of rangeland ecosystems. Fourth, monitoring components of biodiversity can provide a basis for evaluating weed-management strategies. Monitoring would help assess management strategies (as opposed to experimental treatments) in terms of both their effectiveness against target weeds and the side-effects of strategies on non-target components of an ecosystem. Some management techniques and strategies will produce more favourable results than others. Weed management practices themselves can have effects on ecosystem function and biodiversity. For example, burning to control riparian infestations of Cryptostegia grandiflora affects indigenous species as well (Grice 1997). Monitoring will always be important in cases where eradication of alien plant species is being attempted (Groves & Panetta 2002). In Australia, relatively little effort has been directed at evaluating weed-management strategies. Finally, if alien plant species are to be incorporated into a monitoring programme, careful consideration should be given to the timing of monitoring and the placement of monitoring sites. Decisions should be based on the stage and rate of weed invasion, the phenological and demographic characteristics of the weed(s), the distribution of the weed(s) across a region and landscape, the status of weed management and the purpose of the monitoring programme. Where the emphasis is on alien plant species as a threatening process, more frequent monitoring would be required for species whose populations increase and whose ranges expand rapidly than for species that increase WEEDS AND BIODIVERSITY and spread more slowly. Monitoring of weeds in relation to biodiversity should be timed to take into account the phenological cycles of the weeds involved. This is especially important where annual weeds are concerned. In rangelands, riparian zones and other nutrient- and moisture-rich parts of the landscape often support a greater variety and abundance of weed species than upland areas. These areas could warrant more intense monitoring or a greater emphasis on alien species. Riparian zones also support species and communities that do not occur elsewhere in rangeland landscapes. Their importance for biodiversity is disproportionate to the area of land that they occupy, so monitoring weeds as part of a biodiversity monitoring programme is more important for riparian zones. It is important to evaluate and respond to the threats posed by invasive species in the context of the overall threats to biodiversity and to consider how the various threatening processes interact. Invasive species are one threat among many. Ground-based monitoring techniques are time-consuming and expensive. Although it is possible, for particular species and situations, to quantify the distribution and abundance of individual weed species by remote sensing (Abbott et al. 1999), it is unlikely that weeds in general can be monitored by remote techniques, particularly in the early stages of invasion. Monitoring of weeds in rangelands can be included in a programme for monitoring rangeland vegetation in general, but interpreting the data appropriately is critical. Monitoring can indicate correlations between measures of weed abundance (e.g. weed species richness, total weed biomass) and measures of biodiversity, but such correlations do not indicate the importance of weeds as causes of change to biodiversity, or the mechanisms that might be involved (Watson 2004). Manipulative experiments would be a more reliable means of testing the effects of weeds on biodiversity, although for many taxonomic groups, the experiments would have to be long-term. Any such work should be directed toward identifying generalizations in relation to how different types of weeds (species, growth forms or functional groups) affect biodiversity in different habitats or environments, which components of biodiversity are affected, and in what ways. ACKNOWLEDGEMENTS The material in the present paper was originally presented at a workshop ‘Biodiversity Monitoring in Rangelands’, held in Alice Springs, Australia, in October–November 2002. I thank Alan Andersen, Angas Hopkins, David Keith, Hugh Pringle, Jeff Richardson and Jeremy Wallace for helpful comments on an earlier draft of the manuscript. 57 REFERENCES Abbott B. N., Grice A. C. & Bastin G. N. (1999) Low-level aerial photography can detect infestations of the invasive woody vine Cryptostegia grandiflora in northern Australia. In: People of Rangelands, Building the Future. Proceedings of the VI International Rangeland Congress;19–23 July 1999, Townsville, Queensland, Australia (eds D. Eldridge & D. Freudenberger) pp. 602–3. VI International Rangeland Congress Inc, Townsville. Braithwaite R. W. & Lonsdale W. M. (1987) The rarity of Sminthopsis virginiae in relation to natural and unnatural habitats. Conserv. Biol. 1, 341–3. Brown P. R., Wallis R. L., Simmons D. & Adams R. (1991) Weeds and wildlife. Plant Prot. Quart. 6, 150–53. Campbell S. D. & Setter C. L. (1999) Mortality of Prosopis pallida following burning. In: People of Rangelands, Building the Future. Proceedings of the VI International Rangeland Congress; 19–23 July 1999, Townsville, Queensland, Australia (eds D. Eldridge & D. Freudenberger) pp. 596–7. VI International Rangeland Congress Inc, Townsville. Cunningham G. M., Mulham W. E., Milthorpe P. L. & Leigh J. H. (1981) Plants of Western New South Wales. NSW Government Printer, Sydney. Díaz S., Briske D. D. & McIntyre S. (2002) Range management and plant functional types. In: Global Rangelands, Progress and Prospects (eds A. C. Grice & K. C. Hodgkinson) pp. 81–100. CABI Publishing, Oxford, UK. Fairfax R. J. & Fensham R. J. (2000) The effect of exotic pasture development on floristic diversity in central Queensland, Australia. Biol. Conserv. 94, 11–21. Freudenberger D., Hodgkinson K. & Noble J. (1997) Causes and consequences of landscape dysfunction in rangelands. In: Landscape Ecology, Function and Management: Principles from Australia’s Rangelands (eds J. Ludwig, D. Tongway, D. Freudenberger, J. Noble & K. Hodgkinson) pp. 63–77. CSIRO, Melbourne. Grice A. C. (1997) Fire for the strategic management of rubbervine Cryptostegia grandiflora, an invasive shrub. In: Proceedings of the Australian Bushfire Conference; 8–10 July 1997, Darwin, Northern Territory, Australia (eds B. J. McKaige, R. J. Williams & W. M. Waggitt) pp. 239–44. CSIRO Tropical Ecosystems Research Centre, Darwin. Grice A. C. (2000) Weed management in Australian rangelands. In: Australian Weed Management Systems (ed. B. M. Sindel) pp. 431–58. R.G. and F.J. Richardson, Melbourne. Griffin G. F. (1993) The spread of buffel grass in inland Australia: land use conflicts. In: Proceedings of the 10th Australian Weeds Conference and the 14th Asian Pacific Weed Science Society Conference; 6–10 September 1993, Brisbane, Australia (ed. J. T. Swarbrick) pp. 501–4. Weed Society of Queensland, Brisbane. Griffin G. F., Stafford Smith D. M., Morton S. R., Allan G. E., Masters K. A. & Preece N. (1989) Status and implications of the invasion of Tamarisk (Tamarix aphylla) on the Finke River, Northern Territory. Aust. J. Environ. Manage. 29, 297–315. Groves R. H. (1997) Recent Incursions of Weeds to Australia 1971– 1995, Technical Series no. 3. Cooperative Research Centre for Weed Management Systems, Adelaide. Groves R. H. & Panetta F. D. (2002) Some general principles for weed eradication programs. In: Proceedings of the 13th Australian Weeds Conference; 8–13 September 2002, Perth, Australia (eds H. Spafford Jacob, J. J. Dodd & J. H. Moore) 58 A. C. GRICE pp. 307–10. Plant Protection Society of Western Australia, Perth. Groves R. H. & Willis A. J. (1999) Environmental weeds and loss of native plant biodiversity: some Australian examples. Aust. J. Environ. Manage. 6, 164–71. Humphries S. E., Groves R. H. & Mitchell D. S. (1991) Plants invasions of Australian ecosystems: a status review and management directions. Kowari 2, 1–116. Jessop J. (1981) Flora of Central Australia. Reed Books, Sydney. Leigh J. H. and Briggs J. D. (1992) Threatened Australian Plants. Overview and Case Studies. Australian National Parks and Wildlife Service, Canberra. Lonsdale W. M. (1992) The biology of Mimosa pigra. In: A Guide to the Management of Mimosa pigra (ed. K. L. S. Harley) pp. 8–32. CSIRO, Canberra. Lonsdale W. M., Miller I. L. & Forno I. W. (1995) Mimosa pigra L. In: The Biology of Australian Weeds, vol. 1 (eds R. H. Groves, R. C. H. Shepherd & R. G. Richardson) pp. 169–88. R.G. and F.J. Richardson, Melbourne. Lonsdale M. & Milton S. (2002) People and plant invasions of rangelands. In: Global Rangelands, Progress and Prospects (eds A. C. Grice & K. C. Hodgkinson) pp. 101–15. CABI Publishing, Oxford, UK. Loyn R. H. & French K. (1991) Birds and environmental weeds in south-eastern Australia. Plant Prot. Quart. 6, 137–49. Mackey A. P. (1998) Acacia nilotica ssp. indica (Benth.) Brenan. In: The Biology of Australian Weeds (eds F. D. Panetta, R. H. Groves & R. C. H. Shepherd) pp. 1–18. R. G. and F. J. Richardson, Melbourne. McIvor J. G. (1998) Pasture management in semi-arid tropical woodlands: effects on species diversity. Aust. J. Ecol. 23, 349–64. Mullett T. & Simmons D. (1995) The ecological impacts of sweet pittosporum (Pittosporum undulatum Vent.) in dry sclerophyll forest communities, Victoria. Plant Prot. Quart. 10, 131–8. Navie S. C., McFadyen R. E., Panetta F. D. & Adkins S. W. (1998) Parthenium hysterophorus L. In: The Biology of Australian Weeds (eds F. D. Panetta, R. H. Groves & R. C. H. Shepherd) pp. 157–76. R. G. and F. J. Richardson, Melbourne. Noble J. C. (1997) The Delicate and Noxious Scrub. CSIRO, Melbourne. Panetta F. D. & James R. F. (1999) Weed control thresholds: a useful concept in natural ecosystems? Plant Prot. Quart. 14, 68–76. Parsons W. T. & Cuthbertson E. G. (1992) Noxious Weeds of Australia. Inkata Press, Melbourne. Pidgeon R. W. J. & Cairns S. C. (1981) Decomposition and colonisation by invertebrates of native and exotic leaf material in a small stream in New England (Australia). Hydrobiologia 77, 113–27. Richardson D. M., Pyek P., Rejmanek M., Barbour M. G., Panetta F. D. & West C. J. (2000) Naturalization and invasion of alien plants: concepts and definitions. Divers. Distrib. 9, 169–76. Rossiter N. A., Setterfield S. A., Douglas M. M. & Hutley L. B. (2003) Testing the grass-fire cycle: alien grass invasion in the tropical savannas of northern Australia. Divers. Distrib. in press. Schulze D. J. & Walker K. F. (1997) Riparian eucalypts and willows and their significance for aquatic invertebrates in the River Murray, South Australia. Regul. Rivers: Res. Manage. 13, 557–77. Smyth A. K., Foulkes J. & Watt A. (2004) Biodiversity monitoring in Australia’s rangelands: Introduction. Austral Ecol. 29, 1–2. Standish R. J., Robertson A. W. & Williams P. A. (2001) The impact of an invasive weed Tradescantia fluminensis on native forest regeneration. J. Appl. Ecol. 38, 1253–63. State of the Environment Advisory Council (1996) Australia: State of the Environment. CSIRO, Melbourne. Thorp J. R. & Lynch R. (2000) The Determination of Weeds of National Significance. National Weeds Strategy Executive Committee, Launceston. Tomley A. J. (1998) Cryptostegia grandiflora Roxb. ex R.Br. In: The Biology of Australian Weeds (eds F. D. Panetta, R. H. Groves & R. C. H. Shepherd) pp. 63–76. R. G. and F. J. Richardson, Melbourne. Tongway D. & Ludwig J. (1997) The nature of landscape dysfunction in rangelands. In: Landscape Ecology, Function and Management: Principles from Australia’s Rangelands (eds J. Ludwig, D. Tongway, D. Freudenberger, J. Noble & K. Hodgkinson) pp. 49–61. CSIRO, Melbourne. Watson I. (2003) Regional scale monitoring systems – making a case for causality. In: Biodiversity Monitoring in the Rangelands: a Way Forward,Vol 2 [CD] (eds A. Smyth, C. James and G. Whiteman) Final report to Environment Australia on an expert technical workshop held from 29 October to 1 November 2002 in Alice Springs by the Centre for Arid Zone Research, CSIRO Sustainable Ecosystems, Alice Springs, NT. Weiss P. W. & Noble. I. R. (1984) Status of coastal dune communities invaded by Chrysanthemoides monilifera. Aust. J. Ecol. 9, 93–8. Williams P. A. & Karl B. J. (1996) Fleshy fruits of indigenous and adventive plants in the diet of birds in forest remnants, Nelson, New Zealand. NZ J. Ecol. 20, 127–45. Young M. D., Walker P. A. & Cocks K. D. (1984) Distribution of influences on rangeland management. In: Management of Australia’s Rangelands (eds G. N. Harrington, A. D. Wilson & M. D. Young) pp. 333–46. CSIRO, Melbourne.