Success and safety in the biological control of environmental weeds in New Zealand

Biological control, using specialist insect herbivores and plant pathogens, can be a self‐sustaining, cost‐effective and low‐risk tool for the management of environmental weeds. Agents have been recorded attacking non‐target plants in New Zealand and elsewhere, but the effects are usually minor and/or transitory. It seems probable that only two cases, worldwide, will result in significant damage to non‐target plants (representing 0.5% of the nearly 400 insect, mite, or fungal species used in classical weed biocontrol). Both of these cases were predictable from host range testing. Negative indirect, or ‘downstream’, ecological effects from specific weed biocontrol agents are difficult to predict and measure. They are probably insignificant compared to the impacts of the invasive plants that the agents are introduced to control. However, it is necessary to balance the risks associated with any introduction against the environmental benefits from controlling a weed to a predicted level. Recent analyses suggest that success rates are better than generally perceived. For New Zealand programmes, where enough time has lapsed to allow assessment, we calculate a full/partial success rate of 83%. Many of the costs associated with environmental weeds are difficult to quantify. Detailed risk assessment will make biological control programmes more expensive and time‐consuming, so that reliance on non‐biological management methods for environmental weeds may actually increase. The costs of biocontrol programmes against some New Zealand weeds can be kept down by using research already carried out in Australia and other countries, and the process is reciprocal. Developing international consortia of sponsors is also a potential way to fund programmes against weeds shared by several countries.     

The effect of targeted high‐threat weed control on wet forest understorey vegetation in the Central Highlands region,Victoria

The effective control of highly invasive weeds in Australia is an important conservation management action. In this study, we monitored the outcome of herbicide control on high‐threat weeds in the wet forests of the Central Highlands of Victoria. Twenty‐two control (no weed control) and 32 treatment (weed control) plots were surveyed annually over 24 months. Initial results show that weed cover and frequency decreased substantially in response to weed control; however, it is too early to determine the response of native species. We recommend that herbicide control and the associated monitoring programme be continued, and depending on the outcomes, data should be used to develop a more integrated management strategy.     

利用传统生物防治控制外来杂草的入侵

随着国际贸易的日益频繁,外来有害植物入侵,严重威胁我国的自然环境和生物多样性。利用从原产地引入食性较专一的天敌来控制外来杂草是杂草生物防治的主要方式之一,有保护环境一劳永逸的效果。简要介绍了国际生物防治概况,统计表明全世界至少有133种目标杂草进行生物防治,主要分布在菊科、仙人掌科和含羞草科,63科369种无脊椎动物和真菌作为杂草生物防治的天敌,利用最多的天敌是鞘翅目象甲科和叶甲科昆虫,其中大多数项目是治理外来杂草的。杂草生物防治最活跃的国家依次为美国、澳大利亚、南非、加拿大和新西兰。重点论述了利用传统生物防治方法防治外来杂草的经典项目、国内外研究概况,以及目前面临的问题和应用前景。我国杂草生物防治起步晚,传统杂草生防的目标杂草有4种,紫茎泽兰、空心莲子草、豚草和水葫芦,其中,空心莲子草的生物防治获得成功。共引进天敌14种,输出天敌23种,与世界上生物防治先进的国家比尚有距离。中国应充分借鉴国际成功经验,对外来杂草开展生物防治。中国的生物多样性在世界上占有十分独特的地位,将在生物多样性保护中发挥重要作用。     

Nipping aquatic plant invasions in the bud: weed risk assessment and the trade

The importation and sale of ornamental pond and aquarium plants is the most important pathway for the introduction of potential aquatic weeds into and subsequent spread of these within a country. Most current aquatic weeds were at one time deliberately imported for ornamental use. This article discusses a weed risk assessment approach to evaluating new potential weeds. It assesses the potential invasiveness of an aquatic plant based on its habitat versatility, competitive ability, reproductive output and dispersal mechanisms, range of potential impacts, potential distribution and resistance to management activities. The Aquatic Weed Risk Assessment Model (AWRAM) has been used to evaluate potential aquatic weeds in New Zealand, Australia and the USA. A similar approach could be used to guide the management of aquatic weeds in Europe. Banning the importation of highly ranked species effectively keeps biosecurity risks off-shore. Assessment of aquatic plant trade patterns, especially volumes of high-risk species, along with knowledge of current and potential distribution of those species and ease of management, are all factors to be considered when evaluating candidate plants for prevention of sale and distribution. This is a highly effective way of restricting both long-distance dispersal and density of propagules. A cooperative approach involving researchers, policy and trade representatives has been an effective way to achieve regulation of this risk pathway. European initiatives to prevent the distribution of potential aquatic weeds include the preparation of lists of known invasive aquatic species by the European and Mediterranean Plant Protection Organization (EPPO), with recommendations to member countries to consider measures to prevent their spread (e.g. banning importation of, banning sale and distribution of, and undertaking control programmes against those species). Belgian initiatives include an upcoming Royal Decree concerning the importation, exportation and possession of non-native invasive species, development of codes of conduct with the horticultural sector and prohibiting the sale, purchase and intentional release of these species in the wild. This article reviews these approaches and discusses other species of concern.     

A global review of weeds that can germinate from horse dung

To assess the diversity of weed seeds dispersed via horse dung, we reviewed 15 studies on seed germination from horse dung – six from Europe, four from North America, three from Australia and one study each from Africa and Central America. Seed from 249 species from 43 families have been identified germinating from horse dung. Almost two‐thirds of the species were forbs and 33% graminoids, with over half being perennials and 32% annuals. Nearly every species (totalling 99% of those reviewed) is considered a weed somewhere, with 47% recorded as invasive and 19% international environmental weeds. Of the 2739 non‐native plants that are naturalized in Australia, 156 have been shown to germinate from horse dung. This includes 16 of the 429 listed noxious weeds in Australia and two weeds of national significance. Seed from 105 of the 1596 invasive/noxious plant species in North America have also been identified germinating from horse dung. Seed traits including seed size, length, width and mass affect dispersal via horse dung. Habitat disturbance from trampling facilitates germination of seedlings from dung in both natural and experimental studies. Some studies found that plants germinating from dung reach maturity and flower, while others found plants did not survive due to unfavourable growing conditions in the field. The diversity of species with seed that can germinate from horse dung highlights the potential of horses to disperse a range of seed over long distances. Whether such dispersal is beneficial or harmful depends on the plant and the context in which it germinates. To maintain the conservation value of protected areas, it is important to understand and manage the different potential weed dispersal vectors, including horses.     

Weeds and biodiversity conservation: A review of managing weeds under the New South Wales Threatened Species Conservation Act 1995

Summary   In New South Wales, alien plants pose the second greatest threat to biodiversity behind land clearing and habitat loss, yet current weed management does not always address the biodiversity at risk or put in place mechanisms to ensure their recovery. The problem arises in part from an assumption that control programmes which focus only on the weed will result in a biodiversity benefit, rather than acknowledging the need for an assessment of the biodiversity at risk and subsequent incorporation of such information into management strategies. The New South Wales Threatened Species Conservation Act 1995 (TSC Act) has been used as a tool to integrate weed control and biodiversity management through the listing of weeds as key threatening processes and the development and implementation of Threat Abatement Plans (TAPs). Through this process, weed management is forced to focus on actual biodiversity conservation outcomes by directing control to areas where the likelihood of a positive biodiversity response is maximized. Bitou Bush ( Chrysanthemoides monilifera ssp. rotundata ) was the first weed species listed under the TSC Act as a key threatening process and to have a TAP prepared. Implementation of the Bitou Bush TAP is now potentially assisting the recovery of over 150 native plant species and 24 ecological communities at more than 160 sites. The TAP process is now being used for Lantana ( Lantana camara ) nationally and for all widespread weed species that threaten biodiversity within each of the 13 Catchment Management Authorities across New South Wales. By focusing the objectives of weed control on biodiversity protection and recovery, and ensuring that sites throughout the distribution of the weed are prioritized, threat reduction and conservation outcomes are more likely to occur at a landscape scale.     

Distribution and spread of environmental weeds along New?Zealand roadsides

Most non-native weeds and other naturalised plants are in the early stages of invasion into New Zealand landscapes. For this invasion to be controlled, even partially, it is important to understand the dominant routes, mechanisms, and rates of weed spread across landscapes. With their linear corridors of disturbed habitats, roadsides are thought to play a large role in the spread of some weeds. We used both new surveys and existing data to assess which of the 328 environmental weeds listed by the Department of Conservation are most frequently found and where on roadsides, and whether distribution patterns are consistent with linear dispersal. We also analysed historical survey data for relationships between reserve weediness and proximity to roads. We surveyed 340 plots of 100-m-long stretches of roadside across four regions and found between 2 and 19 environmental weeds per plot; 128 species in total (Chao estimate 148). Especially abundant were agricultural (weeds and cultivated) species, species that have been naturalised for well over 50 years, and species that disperse externally attached to vertebrates. While we purposefully sampled within 10 km of town limits, we found no strong effect of distance from town on roadside weed richness, including richness of just ornamentally sourced weeds. Instead, number of houses within 250?m and presence of an adjacent house or other residential structure were both important, as was presence of woody vegetation on and adjacent to roadsides. Reserves adjacent to roads had significantly higher weed richness than reserves further from roads, although the causal mechanisms are unclear. Our results suggest that while roadsides include suitable habitats for most environmental weeds, distributions are patchy and roads show little sign of acting as linear dispersal corridors, instead they largely reflect neighbouring land uses. As such, roadside weeds should best be managed as part of the wider landscape.     

Predicting potential impacts of environmental flows on weedy riparian vegetation of the Hawkesbury–Nepean River,south‐eastern Australia

Remnants of native riparian vegetation on the floodplain of the Hawkesbury–Nepean River near Sydney, have significant conservation value, but contain a large component of weeds (i.e. exotic species that have become naturalized). A proposal for the introduction of environmental flows required an assessment of potential impacts on 242 native and 128 exotic species recorded along 215 km of the river. The likely effects of frequency, season, depth and duration of inundation were considered in relation to habitat, dispersal season and tolerance to waterlogging. Overseas studies provided only limited information applicable to the study area; however, comparisons with similarly highly modified riparian habitats in New Zealand were instructive. Depth and season of inundation appear to be the variables with the greatest potential for differential effects on weeds and native plants. Because of likely spread of propagules and enhancement of growth under the present nutrient‐enriched conditions, environmental flows that would cause more frequent flooding to higher levels of the riparian zone were judged to be of more benefit to weed species than native species, unless supported by bushland management including weeding. Predictions were limited by incomplete data on Hawkesbury–Nepean species, but two types of environmental flow were judged to be potentially beneficial for native water‐edge plants, and worth testing and monitoring: first, flows that maintain continuous low‐level flow in the river, and second, higher level environmental flows restricted to the river‐edge habitat in autumn (the season in which a greater proportion of native species than weed species are known to disperse propagules). In summary, the presence of environmental weeds in riparian vegetation constrain the potential for environmental flows to improve river health. However, with ongoing monitoring, careful choice of water level and season of flow may lead to environmental flows that add to our knowledge, and benefit riparian vegetation along with other river system components.     

Weed invasions in wetlands of Australia's Top End: reasons and solutions

Weed invasions are an increasing threat to the extensive wetlands of the Northern Territory's wet-dry tropics. Although the conservation value of these wetlands is in some ways undisputed, it is evident from the Government's multiple land use policy that it is also misunderstood. This policy aims to maximise economic returns from wetlands while protecting their ecological integrity at a time when ecological and economic costs associated with weeds are, at least in the short term, set to worsen. The underlying reasons behind wetland loss and degradation in Australia parallel those identified in Mediterranean Europe where there was antipathy from bureaucracies toward science and ecology. Several case studies from the Northern Territory explore how ecological, anthropogenic, political and economic factors contribute to weed problems. Caution is necessary when translating experience from agricultural weeds to environmental weeds. Managers have not always heeded the advice of specialists and practitioners, whose understanding of the ecological basis to weed invasions is not in as parlous a state as sometimes thought. Even when faced with sound information from which to manage, it was non-ecological reasons that slowed down or prevented effective weed control. If the floristic identity and diversity of Australia's natural wetlands is to be retained, then weeds need serious and immediate attention. Weed impacts progress beyond loss of wetland habitat and biodiversity to regional changes in landscape processes. We advocate that governments and industry recognise and address the underlying non-ecological reasons that exacerbate weed problems and set priorities to fund relevant practical studies and control programs that enable inventive weed management. Cooperation between land users, custodians and the wider community can help to overcome bureaucratic obstacles and enable judicious weed control that contributes effectively to wetland protection.     

Establishment and impact of insect agents deployed for the biological control of invasive Asteraceae: prospects for the control of <Emphasis Type="Italic">Senecio madagascariensis</Emphasis>

Several invasive Asteraceae have been targeted for biological control worldwide, with variable success. Senecio madagascariensis Poiret, which invades agricultural lands in Australia and Hawaii, is a recent target. Since several potential insect agents were recorded in the plant’s native range in South Africa, we assessed biocontrol efforts against asteraceous weeds to determine those most likely to deliver success. Some 108 insect species, from five orders and 23 families, were deployed against 38 weed taxa, mostly in the mainland USA, Canada, Australia and New Zealand. Coleoptera (mainly Curculionidae and Chrysomelidae), Diptera (Tephritidae) and Lepidoptera (Tortricidae) featured the most. Despite high establishment success (73% of releases across countries), only 37% of successful releases achieved meaningful impact. Although root-feeding and stem-feeding insects appeared to be the best candidates, neither insect family nor feeding guild significantly influenced the probability of success. This synthesis of the global contribution of different guilds of specialist herbivores to the management of invasive Asteraceae is guiding the selection of candidate agents for the biocontrol of S. madagascariensis in Australia.     

A test of the Australian Weed Risk Assessment system in China

The Australian Weed Risk Assessment system (AWRA) is an effective pre-border weed-screening tool that has played an active role in preventing the introduction of alien weeds into Australia and has been utilized in several other countries worldwide. Here, we selected 131 species of naturalized exotic plants (including 76 species of given non-weeds and 55 species of given weeds) to evaluate the AWRA in China for the first time. The AWRA performed better for discriminating major weeds than non-weeds and minor weeds, as it correctly rejected 84% of major weeds and did not wrongly accept a major weed. Among non-weeds, 76% were correctly classified with the final outcome of “accept” and 7.9% were wrongly rejected by the AWRA. This system correctly rejected 56% of minor weeds but accepted only 2.8% of minor weeds. The remaining 23% of all alien plants tested were classified as “evaluate further” by the AWRA. The area under the ROC curve (AUC) was 0.944, suggesting that the AWRA would be highly efficient at discriminating alien plants in China. In addition, we compared the scores of seven attributes of the AWRA between prior plant categories and analyzed their correlation with weed status. The average score for each attribute differed significantly between the two prior categories (weed and non-weed), but the average scores of the attribute “undesirable traits” did not significantly differ between any two of the three categories (non-weeds, minor weeds, and major weeds). There was a significant positive correlation between the scores of each attribute of the AWRA and weed status. The correlation coefficient for “dispersal mechanisms” and weed status was the highest and that for “undesirable traits” was the lowest. We believe that the AWRA can serve as an important weed-screening tool for plant introduction management in China.     

What’s a Weed? Knowledge,Attitude and Behaviour of Park Visitors about Weeds

Weeds are a major threat to biodiversity globally degrading natural areas of high conservation value. But what are our attitudes about weeds and their management including weeds in national parks? Do we know what a weed is? Do we consider weeds a problem? Do we support their management? Are we unintentionally spreading weeds in parks? To answer these questions, we surveyed visitors entering a large popular national park near the city of Brisbane, Australia. Park visitors were knowledgeable about weeds; with >75% correctly defining weeds as ‘plants that grow where they are not wanted’. About 10% of the visitors, however, provided their own sophisticated definitions. This capacity to define weeds did not vary with people’s age, sex or level of education. We constructed a scale measuring visitors’ overall concern about weeds in parks using the responses to ten Likert scale statements. Over 85% of visitors were concerned about weeds with older visitors, hikers, and those who could correctly define weeds more concerned than their counterparts. The majority think visitors unintentionally introduce seeds into parks, with many (63%) having found seeds on their own clothing. However, over a third disposed of these seeds in ways that could facilitate weed spread. Therefore, although most visitors were knowledgeable and concerned about weeds, and support their control, there is a clear need for more effective communication regarding the risk of visitors unintentionally dispersing weed seeds in parks.     

Can differential responses to nutrients explain the success of environmental weeds?

Abstract. Question: The use of plant traits to predict weed impact is a long‐standing goal in weed ecology. In particular, trait plasticity, i.e. the variability of a trait response to environmental change, is widely considered to contribute to weed success. However, the generality of the role of trait plasticity in determining weed impacts has never been systematically tested. Methods and location: We tested the hypothesis that high‐impact environmental weeds have greater plasticity in growth responses to nutrient availability than low‐impact species. In a glasshouse experiment, we supplied a complete nutrient solution at five different concentrations to seedlings of 24 species of high‐ and low‐impact environmental weeds from south east Queensland, Australia. Results: Almost all species showed plasticity in biomass accumulation in response to the nutrient treatments, but plasticity in biomass accumulation did not differ between related high‐ and low‐impact species. There was no evidence of nutrient‐related plasticity in root: shoot allocation. Seedling survival was greater at higher nutrient concentrations, and also differed greatly between families. Survival among low‐impact species was marginally (p= 0.0610) lower than among high‐impact species. Conclusion: We conclude that the impact of environmental weeds in south east Queensland cannot be predicted from nutrient‐related plasticity in seedling growth. The effects of nutrients on seedling survival warrant further research.     

Mapping Grey Willow (Salix cinerea) stand architecture using airborne laser scanning: implications for large‐scale tree weed control

We investigated the potential of airborne laser scanning (ALS) for mapping the stand architecture of Grey Willow (Salix cinerea), an invasive wetland weed in New Zealand. In particular, we focused on two metrics, tree height and canopy density, both of which influence the efficacy and nontarget impacts of herbicides aerially broadcast by helicopter to control tree weeds. We compared ground‐based measures of Grey Willow height and canopy density with ALS‐derived data, and the relationship between canopy density as estimated by each method and aerial herbicide deposition at three wetland sites in New Zealand. Analysis revealed strong linear relationships between ground‐based and ALS metrics, indicating that ALS data could be used to generate accurate, high‐resolution digital maps of Grey Willow height and canopy density. These maps coupled with computer‐guided variable flow rate technologies, which enable optimal placement of herbicide, could maximise Grey Willow mortality while reducing the mortality of nontarget indigenous plants. We recommend the application of ALS‐derived maps and computer‐guided variable flow rate technology is investigated for more targeted large‐scale tree weed control.     

Frugivore gape size and the evolution of fruit size and shape in southern hemisphere floras

Abstract The present study uses differences among frugivore faunas of the southern hemisphere landmasses to test whether frugivore characteristics have influenced the evolution of fruit traits. Strong floristic similarities exist among southern landmasses; for example, 75% of New Zealand vascular plant genera also have species in Australia. However, plants in Australia and South America have evolved in the presence of a range of mammalian frugivores, whereas those in New Zealand, New Caledonia and the Pacific Islands have not. In addition, the avian frugivores in New Zealand and New Caledonia are generally smaller than those of Australia. If frugivore characteristics have influenced the evolution of fruit traits, predictable differences should exist between southern hemisphere fruits, particularly fruit size and shape. Fruit dimensions were measured for 77 New Zealand species and 31 Australian species in trans‐Tasman genera. New Zealand fruits became significantly more ellipsoid in shape with increasing size. This is consistent with frugivore gape size imposing a selective pressure on fruit ingestability. This result is not a product of phylogenetic correlates, as fruit length and width scaled isometrically for Australian species in genera shared with New Zealand. Within‐genus contrasts between New Zealand and Australian species in 20 trans‐Tasman genera showed that New Zealand species have significantly smaller fruits than their Australian counterparts. Within‐genus contrasts between New Zealand and South American species in nine genera gave the same result; New Zealand species had significantly smaller fruits than their South American counterparts. No difference was found in fruit size or shape between New Zealand and New Caledonia congeneric species from 12 genera. These results are consistent with the broad characteristics of the frugivore assemblage influencing the evolution of fruit size and shape in related species. The smaller‐sized New Zealand frugivore assemblage has apparently influenced the evolution of fruit size of colonizing taxa sometimes within a relatively short evolutionary timeframe.     

Responses of invertebrates to herbicide in Salix cinerea invaded wetlands: Restoration implications

The use of herbicides to control weeds, particularly large invasions, has now become an essential management tool in many ecological restoration projects. The herbicide glyphosate is routinely used to control the invasive weed, Grey Willow (Salix cinerea), within New Zealand wetlands. However, little is known about the effects of glyphosate on invertebrates. We determine the short‐term effects of glyphosate on the abundance and composition of the nontarget canopy invertebrate community in wetlands invaded by Grey Willow in New Zealand. Initially, the application of glyphosate and a surfactant showed no detectable effect on the canopy invertebrates examined in this study. However, 27 days after herbicide application, significant Grey Willow canopy loss caused dramatic decreases in the abundance of invertebrates in the glyphosate‐treated plots compared with the unsprayed plots. Invertebrates appeared to be sensitive to changes in vegetation structure, such as canopy loss. These results agree with previous studies that have shown that the negative impacts of glyphosate on invertebrate communities are related to indirect effects via habitat modification as the herbicide‐treated vegetation dies. From a terrestrial invertebrate perspective, this study suggests that the use of glyphosate herbicide is suitable for the control of invasive weeds within wetland restoration projects as it appears to have negligible impact on the canopy invertebrate assemblage.     

Impacts on invertebrate fungivores: a predictable consequence of ground-cover weed invasion?

Empirical impact evidence exists for few of New Zealand’s environmental weeds. Financial constraints prevent managers examining all impacts of all weeds. Therefore it is useful to seek generalisable rules which allow managers to predict impacts of new invasions. Invasive weeds may indirectly affect fungivorous invertebrates through mechanisms such as altered litter-fall, decomposition rates, and microclimate, all of which may alter fungal activity. I tested the hypothesis that fungivorous invertebrates would be consistently affected by three invasive weeds, and that this effect would be more pronounced than for other invertebrate functional groups. Using pitfall traps, I compared invertebrates beneath climbing asparagus, tradescantia and ginger with invertebrates of uninvaded lowland forest. Five out of six Coleoptera taxa which differed in abundance between invaded and uninvaded sites were fungivores; the remaining taxon was a saprophage. At least two taxa of fungivorous Coleoptera responded to each of the three weed species. Acari (mainly Oribatids, some of which are fungivores), Isopoda and Amphipoda (decomposers) were the only taxa to respond to the presence of all three weed species. These results provide some support for the hypothesis that fungivores are particularly responsive to ground-cover weed invasion, but suggest this could be extended to include decomposers as well. However, the direction of effect differed among weed species. Therefore, while changes in fungivore and decomposer abundance may be predicted as one of the more likely consequences of ground-cover weed invasion in New Zealand low-land forest, site- and taxa-specific effects make it difficult to predict the direction of effect.     

Ecology and management of Melaleuca quinquenervia,an invader of wetlands in Florida,U.S.A.

Invasive weeds are potent agents of environmental change. Wetlands are valuable environments that frequently are impinged by a variety of threats including invasive weeds. Melaleuca quinquenervia (Cav.) S.T. Blake (broad-leaved paperbark), though experiencing major diminishment of native populations in Australia, is naturalized and highly invasive in most wetland habitats of south Florida, U.S.A., where it infests more than 202,000 ha. Wetlands in south Florida, including such renowned areas as the Everglades, are being transformed into M. quinquenervia swamps, with major environmental and economic impacts. Current management methods include herbicides, mechanical or hand removal of plants, flooding, and prescribed burning. Insufficient information, high costs, non-target impacts, and the resilience of M. quinquenervia (trunk and root sprouts and massive canopy seed banks) greatly constrain the effectiveness of these control methods. Biological control offers long-term management potential, most likely by reducing the rate of spread and the vitality and growth rate of plants, thus rendering them more vulnerable to other environmental stresses and control methods. The leaf weevil Oxyops vitiosa Pascoe, a natural enemy of M. quinquenervia in Australia, will likely be the first biocontrol agent released against the weed in Florida. More information is needed, especially ecological data, to better understand the invasiveness of M. quinquenervia in Florida and to facilitate its management there.     

The Thrips orientalis group from South-east Asia and Australia: some species identities and relationships (Thysanoptera, Thripidae)

Abstract  A thrips associated in Thailand with the flowers of Paederia foetida , a rubiaceous weed in Florida that is a threat to Australia, is here recognised as Thrips morindae Priesner, a species previously considered a synonym of T. javanicus Priesner. Systematic relationships of these species with others in South-east Asia and the Australasian region are discussed. Two new species-groups are designated: the T. obscuratus group of six species from New Zealand and New Caledonia, and the T. orientalis group of 12 South-east Asian species that are associated with scented white flowers such as Gardenia . Four species of this second group are recorded from tropical Australia.     

长期稻鸭共作对稻田杂草群落组成及物种多样性的影响

2000~2003年连续4年研究了稻鸭共作条件下田间杂草群落的特征及其动态变化规律。结果表明,在长期稻鸭共作条件下,田间杂草密度逐年降低,下降趋势符合阻滞模型y=k+a·e^bx,模型参数b反映了杂草种群的下降速率。在稻田6种主要杂草中,水虱草(Fimbristylis miliaceae)、陌上菜(Lindernia procumbens)、丁香蓼(Ludwigia prostrata)种群数量降低较快,鸭舌草(Monochoria vaginalis)、异型莎草(Cyperus difformis)次之,稗(Echinochloa crusgalli)最慢。稻鸭共作使稻田杂草群落的物种多样性持续降低,群落均匀度提高,群落相似性与稻鸭共作前相比逐年降低。说明稻鸭共作改变了田间杂草的群落结构,有利于限制杂草的发生危害。随着稻鸭共作的连年进行,对田间杂草的控制效果逐渐上升,4年后达99%以上。稻鸭共作是稻田替代化学除草的一种非常有效的生物、生态控草措施,具有显著的经济和生态效益。