Understanding Acceptable Level of Risk: Incorporating the Economic Cost of Under-Managing Invasive Species

Management of nonindigenous species includes prevention, early detection and rapid response and control. Early detection and rapid response depend on prioritizing and monitoring sites at risk for arrival or secondary spread of nonindigenous species. Such monitoring efforts require sufficient biosecurity budgets to be effective and meet management or policy directives for reduced risk of introduction. Such consideration of risk reduction is rarely considered, however. Here, we review the concepts of acceptable level of risk (ALOR) and associated costs with respect to nonindigenous species and present a framework for aligning risk reduction priorities with available biosecurity resources. We conclude that available biosecurity resources may be insufficient to attain stated and desired risk reduction. This outcome highlights the need to consider policy and management directives when beginning a biosecurity program to determine the feasibility of risk reduction goals, given available resources.     

A proposal to use plant demographic data to assess potential weed biological control agents impacts on non-target plant populations

Weed biocontrol programs aim to reduce the spread and population growth rate of the target plant while stabilizing or increasing populations of those native species considered under threat by invasive plants. This goal is not unique to weed biocontrol but applies to all other invasive plant management techniques, though such information is rarely collected. Without this information, success of management interventions can be ambiguous, and regulatory agencies, the public, policy makers, funders and land managers cannot be held accountable for chosen treatments. A fundamental reform, including use of demographic studies and long-term assessments, are essential to guide weed biocontrol programs. We propose to add use of plant demography (an assessment of how environmental factors and ecological interactions, for example competition, disease or herbivory, may affect plant populations by altering survival, growth, development and reproductive rates of plant individuals) during host specificity risk assessments of potential biological control agents. Demographic models can refine assessments of potential impacts for those plant species that experience some feeding or larval development during host specificity testing. Our proposed approach to focus on impact on plant demography instead of attack on plant individuals is useful in appropriately gauging threats potential weed biocontrol agents may pose to non-target species after field release.     

Modelling the policy and economic context for detection- based monitoring tools: the costs and benefits of detecting last survivors and first re-colonisers

Establishing and maintaining the success of pest or weed eradication programmes requires interpretation of failures to detect survivors and first re-colonisers. Recent developments provide statistical frameworks that allow sequences of such failures to be interpreted in a probabilistic context. For example, application of these methods allow managers of eradication programmes to decide a priori an acceptable risk of programme failure, and to use this decision to design monitoring regimes that deliver this level of certainty, given the detection characteristics of the search techniques at their disposal. Similar methods could be use to design monitoring regimes to detect an incursion by a previously eradicated species (i.e. an eradication breakdown), which have an acceptable risk of failure. However, the availability of these methods begs questions about how “acceptable” risks of eradication failure or breakdown should be specified, and the consequent effort that should be expended to locate last survivors and first re- colonisers. We use a risk-based bioeconomic framework to model and analyse these decisions. The analysis demonstrates critical trade-offs between the cost and efficacy of the detection techniques available, the “value” of the eradication programme, and the perceived risk that a breakdown can occur. While our focus is on island pest eradication, we suggest how the bioeconomic framework used could be usefully applied to the detection of rare, at risk species, and the management of sporadically frequent diseases.     

Biosecurity surveillance design: detecting non-indigenous vertebrate introductions using risk and power

Ecosystems can be severely damaged by the introduction, establishment and spread of non-indigenous species (NIS) including vertebrates, invertebrates and plants. Development and use of natural areas poses a biosecurity risk regarding the introduction of increase NIS invasion risks, so biosecurity systems including prevention and detection measures are required. Even with the most rigorous biosecurity efforts, there is potential for NIS to evade quarantine and go on to establish and spread. The cost of such an event can be great, both environmentally and financially (e.g. containment/ management or eradication). We have developed a surveillance design methodology, for an application where increased use of a natural area may result in NIS incursions, even with extensive biosecurity systems. The surveillance design methodology acknowledges heterogeneity of risk in the study area and stratifies the area to optimise surveillance deployment, achieving great efficiencies and improvement in statistical power of detection. Many of the risk decisions require lack data and so the system incorporates expert opinion with available data. The design covers the broad range of potential NIS that may be introduced by using exemplar species and a variety of surveillance system components (SSCs) (such as a combination of formal scientific surveys, trapping methods, and casual observation) distributed optimally over time and space. The mix of SSCs can be manipulated to take into account such factors as their relative financial costs and demands on expertise. The methodology has the flexibility to be applied to various groups of potential NIS (e.g. vertebrates, invertebrates and plants), and the design can evolve as data are collected (adaptive management). Overall, the surveillance design methodology allows for an efficient use of resources, providing sufficient power to detect incursions, resulting in reduced environmental and financial costs from NIS incursions.     

Weed Diversity and Uses: a Case Study from Tea Plantations in Northern Thailand

Based on concerns that useful weed diversity in agroecosystems as well as associated traditional knowledge may be declining due to agricultural intensification and indiscriminate eradication of weeds, we studied weed diversity and use by local ethnic groups in tea plantations in northern Thailand under different agricultural intensities and landscape complexities, namely agroforestry, organic, and Good Agricultural Practice (GAP) based conventional system. In each system, we sampled five tea plantations using belt-transects of 25 m². Use data were collected through interviews with specialist informants. We recorded 214 weed species (gamma-diversity) of which 66 were useful. Agroforestry system had the highest alpha- and beta-weed diversity, suggesting that the low agricultural intensity and high landscape complexity in this system is associated with higher weed diversity in tea plantations. The common weed species were clearly different in the three systems and only a small fraction (22 species) of widespread weeds was shared among them, resulting in low weed similarity between systems. The 66 useful species of weeds were mostly for food and medicine. Uses of some weeds were shared between systems. Many of the useful weeds are invasive species, suggesting that they are prevalent and inexpensive plant resources for rural people and could represent alternative resources in the future. Because the occurrence of invasive weeds may affect the natural habitats of the native flora, integrating the exploitation of weeds into weed management strategies may reduce the weed population while sustaining agrobiodiversity and conserving associated traditional knowledge in the long term.     

Evaluation of weed eradication programs: containment and extirpation

Weed eradication programs often require 10 years or more to achieve their objective. It is important that progress is evaluated on a regular basis so that programs that are 'on track' can be distinguished from those that are unlikely to succeed. Earlier research has addressed conformity of eradication programs to the delimitation criterion. In this paper evaluation in relation to the containment and extirpation criteria is considered. Because strong evidence of containment failure (i.e. spread from infestations targeted for eradication) is difficult to obtain, it generally will not be practicable to evaluate how effective eradication programs are at containing the target species. However, chronic failure of containment will be reflected in sustained increases in cumulative infested area and thus a failure to delimit a weed invasion. Evaluating the degree of conformity to the delimitation and extirpation criteria is therefore sufficient to give an appraisal of progress towards the eradication objective. A significant step towards eradication occurs when a weed is no longer readily detectable at an infested site, signalling entry to the monitoring phase. This transition will occur more quickly if reproduction is prevented consistently. Where an invasion consists of multiple infestations, the monitoring profile (frequency distribution of time since detection) provides a summary of the overall effectiveness of the eradication program in meeting the extirpation criterion. Eradication is generally claimed when the target species has not been detected for a period equal to or greater than its seed longevity, although there is often considerable uncertainty in estimates of the latter. Recently developed methods, which take into consideration the cost of continued monitoring vs. the potential cost of damage should a weed escape owing to premature cessation of an eradication program, can assist managers to decide when to terminate weed eradication programs.     

Prioritising islands in the United Kingdom and crown dependencies for the eradication of invasive alien vertebrates and rodent biosecurity

Invasive alien vertebrates (IAVs) pose a significant threat to island biodiversity worldwide, and their removal is an important nature conservation management goal. As methods advance, eradications from larger islands and of multiple species simultaneously are increasingly undertaken. Effective targeting to maximise conservation gain is important given limited resources. We build on existing prioritisation methods and use the islands of the UK and Crown Dependencies (UK) as an example of how vertebrate eradications might be prioritised and invasive-free status maintained through biosecurity. For each of the 9688 UK islands, we assessed ecological importance for native vertebrates and the anticipated impacts of the IAVs present to estimate the benefit of restoration based on the feasibility and sustainability of IAV eradications in relation to island size, human population and risk of unassisted reinvasion by swimming. As reinvasion poses a threat to the long-term benefits of eradication, we incorporated species-specific swimming distances and explored the effects of varying reinvasion probability from risk-averse to higher-risk strategies. The 25 islands that would benefit most from eradications were in Scotland, Northern Ireland and the Channel Islands. Our prioritisation method should be seen as an initial guide to identify islands that might benefit from intervention when faced with a large list of potential sites. Feasibility studies taking account of factors such as interspecific interactions, anthropogenic reinvasion, views of residents or ‘social feasibility’ and cost need to be undertaken before planning any eradication. We prioritised biosecurity for rat-free islands to highlight where comprehensive measures might be most beneficial.     

Ground beetle (Coleoptera: Carabidae) phenology, diversity, and response to weed cover in a turfgrass ecosystem

Despite being fragmented and highly disturbed habitats, urban turfgrass ecosystems harbor a surprising diversity of arthropods. The suitability of turf as arthropod habitat, however, likely depends on the extent and types of pesticides and fertilizers used. For example, moderate levels of weed cover in low-input lawns may provide alternative food resources. We conducted a 2-yr field study to: 1) characterize the ground beetle (Carabidae) species assemblage in turfgrass, and 2) assess the direct and indirect effects of lawn management on carabid communities. Weed cover and beetle activity were compared among four lawn management programs: 1) consumer/garden center, 2) integrated pest management (IPM), 3) natural organic, and 4) no-input control. Nearly 5,000 carabid beetles across 17 species were collected with the predator Cyclotrachelus sodalis LeConte numerically dominating the trap catch (87% and 45% of individuals in 2005 and 2006, respectively). Populations of C. sodalis underwent a distinct peak in activity during the third week of June, whereas omnivorous and granivorous species tended to occur at far lower levels and were less variable over the season. We found no evidence for direct effects of lawn management on carabid species diversity; however, we detected an indirect effect mediated by variation in weed cover. Seed-feeding species were positively correlated with turf weeds early in 2006, whereas strictly predaceous species were not. Thus, turf management programs that lead to changes in plant species composition (i.e., herbicide regimes) may indirectly shape epigeal arthropod communities more strongly than the direct effects of insecticide use.     

Predicting the benefits of banana bunchy top virus exclusion from commercial plantations in australia

Benefit cost analysis is a tried and tested analytical framework that can clearly communicate likely net changes in producer welfare from investment decisions to diverse stakeholder audiences. However, in a plant biosecurity context, it is often difficult to predict policy benefits over time due to complex biophysical interactions between invasive species, their hosts, and the environment. In this paper, we demonstrate how a break-even style benefit cost analysis remains highly relevant to biosecurity decision-makers using the example of banana bunchy top virus, a plant pathogen targeted for eradication from banana growing regions of Australia. We develop an analytical approach using a stratified diffusion spread model to simulate the likely benefits of exclusion of this virus from commercial banana plantations over time relative to a nil management scenario in which no surveillance or containment activities take place. Using Monte Carlo simulation to generate a range of possible future incursion scenarios, we predict the exclusion benefits of the disease will avoid Aus$15.9-27.0 million in annual losses for the banana industry. For these exclusion benefits to be reduced to zero would require a bunchy top re-establishment event in commercial banana plantations three years in every four. Sensitivity analysis indicates that exclusion benefits can be greatly enhanced through improvements in disease surveillance and incursion response.     

An invasive plant and climate change threat index for weed risk management: Integrating habitat distribution pattern and dispersal process

Invasive plants pose a significant threat to the integrity and biodiversity of native systems. Weed risk assessment and management provides a framework for assessing this threat. However, relatively little attention has been paid to the threat posed to biodiversity by invasive plants in a rapidly changing climate. This paper aims to estimate the impacts of climate change on exotic plant habitats, and incorporates elements of dispersal to develop a management index for identifying invasive plant threat under climate change. The spatial distribution of habitat suitability is modelled at the landscape scale for multiple exotic plant species under current climate and a climate change scenario for the year 2030. Expert opinion of the dominant dispersal mechanisms and weed status is used to model relative dispersal threat of each exotic plant species. These pattern and process outputs are integrated to create a multi-species management priority layer in an effort to synthesise the inherently complex outputs from multiple models of multiple species. The overall multi-species management index thus combines pattern and process to identify geographic locations at greatest threat from invasion under climate change.     

Evaluating the Feasibility of Eradicating an Invasion

The detectability of invasive organisms influences the costs and benefits of alternative control strategies, and the feasibility of eradicating an infestation. Search theory offers a mathematically rigorous framework for defining and measuring detectability taking account of searcher ability, biological factors and the search environment. In this paper, invasive species detectability is incorporated into a population simulation model. The model is applied to a base set of parameter values that represent reasonable values for a hypothetical weed. The analysis shows the effects of detectability and search effort on the duration of an eradication program. For a given level of detectability and search time, the analysis shows that the variables with the greatest influence on the duration of the eradication effort are search speed, kill efficiency, germination rate and seed longevity. Monte Carlo simulations are performed on a set of four weed scenarios, involving different combinations of plant longevity, seed longevity and plant fecundity. Results of these simulations are presented as probability distributions and allow us to calculate how the probability of eradication will be affected by search strategy.     

Many eyes on the ground: citizen science is an effective early detection tool for biosecurity

Early detection of target non-indigenous species is one of the most important determinants of a successful eradication campaign. For early detection to be successful, and provide the highest probability of achieving eradication, intense surveillance is often required that can involve significant resources. Volunteer based monitoring or “citizen science” is one potential tool to address this problem. This study differs from standard citizen science projects because the participants are personnel or contractors of a company working on Barrow Island, Western Australia. We show that personnel can contribute successfully to a surveillance program aimed at detecting a broad taxonomic range of non-indigenous vertebrate and invertebrate species. Using data collected over a five year surveillance period on Barrow Island, we show that eighteen of the nineteen (95%) non-indigenous invertebrate species new to the island were detected by personnel working on the island, and that the number of detections made by these workers was significantly related to the number of personnel on the island at any one time. Most personnel detections (91%) were made inside buildings where the majority of active surveillance tools could not be implemented. For vertebrates, 4 NIS species detections (100% of detections) were made in the built environment by personnel. Although reporting of suspect non-indigenous species is voluntary, personnel are required to attend inductions and toolboxes where reporting of suspect biosecurity risk material is encouraged. These results demonstrate the value of industry led ‘citizen science’ programs, resulting in sustained stewardship and conservation of areas with high environmental value.     

Identifying hotspots of alien plant naturalisation in Australia: approaches and predictions

The early detection of newly naturalised alien species is vital to ensuring the greatest chance of their successful eradication. Understanding where species naturalise most frequently is the first stage in allocating surveillance effort. Using Australia’s Virtual Herbarium, we compiled the collection records for all plant species in Australia. We controlled for potential spatial biases in collection effort to identify areas that have an elevated rate of first records of alien species’ occurrence in Australia. Collection effort was highly variable across Australia, but the most intense collection effort occurred either close to herbaria (located in cities) or in remote natural environments. Significant clusters of first records of occurrence were identified around each state’s capital city, coinciding with higher collection effort. Using Poisson point process modelling, we were able to determine the relative influence of environmental and anthropogenic factors on the spatial variation in the risk of species naturalisation. Effort-corrected naturalisation risk appeared to be strongly related to land use, road and human population densities, as well as environmental factors such as average temperature and rainfall. Our paper illustrates how the risk of naturalisation at a location can be estimated quantitatively. Improved understanding of factors that contribute to naturalisation risk enhances allocation of surveillance effort, thereby detecting novel species sooner, and increasing the likelihood of their eventual eradication.     

After biocontrol: Assessing indirect effects of insect releases

Development of biological control agents for weeds has been motivated by the need to reduce the abundance and distribution of a pest plant where chemical and mechanical control were not cost effective. Primary objectives have been direct reduction in abundance of the target and, secondarily, the increase of desirable species. Recently, wildland weeds have become a focus of biological control projects. Here, desired outcomes include both reduction of the target and indirect effects of increased diversity and abundance of native species and restoration of ecosystem services. However, goals and benefits of biocontrol programs are not always well-articulated and direct and indirect impacts are not easily predicted. We evaluated the extent to which several successful biological control projects for weeds of rangelands and waterways measured indirect impacts on invaded ecosystems. We also examined biocontrol of a wildland pest tree for which the principal objective is restoration of ecosystem services. We found few quantitative assessments of the impacts of pest plant reduction on community composition or ecosystem processes. All examples documented variation in the impacts of agent(s) across the invasive range of the target plant as well as variation in impacts on the invaded ecosystem. However, without appropriate quantitative information, we cannot evaluate site characteristics that may influence vegetation responses. Most successful weed management programs integrated the use of biocontrol agents with other weed management strategies, especially modifications of disturbance and competing vegetation. Discussion and evaluation of responses of nontarget species would improve our understanding of the context-specificity of outcomes.     

Increasing habitat connectivity in agricultural landscapes as a weed management strategy reconciling ecology and agronomy

While studies have explored how habitat amount drives weed assemblages in agroecosystems, knowledge remains limited of the effects of habitat connectivity. The response-effect trait framework provides insights into the mechanisms underpinning the relationship between landscape structure and the taxonomic diversity and abundance of weed assemblages. This study evaluated how habitat connectivity and habitat amount affect weed diversity and abundance in winter cereal fields, and whether these effects are driven by the functional composition of weed assemblages. We sampled weeds in 27 winter cereal fields. We measured habitat connectivity and habitat amount provided by wooded, grassland and cropland elements. We selected five traits related to the dispersal, establishment, and competitive abilities of weed species likely to respond to landscape structure: seed number per plant, type of reproduction, seed dry mass, plant vegetative height and seed germination rate. The functional composition of weed assemblages was assessed using community weighted mean trait values. Weed diversity and abundance were used as proxies of weed management. The taxonomic approach did not reveal any effect of landscape structure on weed diversity and abundance. Only the grassland elements that contributed to habitat connectivity, and to a lesser extent to habitat amount, drove the functional composition of weed assemblages. High habitat amount favoured species with many seeds, while high habitat connectivity favoured species with fewer seeds, a higher ability to reproduce vegetatively and higher seed germination rates. In turn, higher seed germination rates increased weed evenness and reduced weed abundance. Some of these relationships were influenced by the presence of rare species. Overall, high connectivity provided by grassland elements increases weed evenness and reduces weed abundance by shaping weed functional composition. Our study suggests that land-use planning policies that enhance the connectivity provided by grassland elements could be considered as a weed management strategy reconciling ecology and agronomy.     

Modelling biosecurity change: Ecology, economics, technology and social values

The threats and responses to biosecurity are constantly changing, creating decision problems for policy makers setting priorities for future biosecurity systems. In the United Kingdom during 2003–04, the Department for Environment, Food and Rural Affairs (DEFRA) commissioned a Horizon Scanning project to predict the future (20–30 years) of biosecurity needs in the United Kingdom. This project created an integrated model of key ecological, economic and technological processes involved in the development and control of invasive species, across a range of taxa, and also sought views on social values that could limit response options and affect the economic and political importance of introduced species. The model demonstrates the ability to make useful probability- based estimates of economic impact given practical assumptions on ecological, economic and technological inputs. Sensitivity analyses show where improved data could reduce uncertainty. The model establishes a framework that has been used to identify major drivers of biosecurity change affecting the next generation: increased and more diverse trade and travel increasing the entry of new species; climate change affecting establishment and spread of pests introduced from new zones that could approximate Britain’s climate; social values affecting attitudes to control measures such as animal culling and greater concern for environmental and amenity resources rather than agriculture; and technological improvements in pest detection. An important economic issue affecting the value of the impact caused by invasions is the time scale over which the impact is felt, ranging from immediate in the case of many livestock diseases through to the long-delayed recognition of loss of environmental quality from competition or diseases affecting native plants. New pest detection technology offers substantial opportunity to improve eradication of introduced species and could affect the prevention versus cure paradigm for many species for which general exclusion systems are presently adopted. An integrated modelling framework allows some quantification of these drivers and offers a tool to guide biosecurity planning.     

Agricultural biosecurity

The prevention and control of new pest and disease introductions is an agricultural challenge which is attracting growing public interest. This interest is in part driven by an impression that the threat is increasing, but there has been little analysis of the changing rates of biosecurity threat, and existing evidence is equivocal. Traditional biosecurity systems for animals and plants differ substantially but are beginning to converge. Bio-economic modelling of risk will be a valuable tool in guiding the allocation of limited resources for biosecurity. The future of prevention and management systems will be strongly influenced by new technology and the growing role of the private sector. Overall, today''s biosecurity systems are challenged by changing national priorities regarding trade, by new concerns about environmental effects of biological invasions and by the question ‘who pays?’. Tomorrow''s systems may need to be quite different to be effective. We suggest three changes: an integration of plant and animal biosecurity around a common, proactive, risk-based approach; a greater focus on international cooperation to deal with threats at source; and a commitment to refocus biosecurity on building resilience to invasion into agroecosystems rather than building walls around them.     

Climate change increases risk of plant invasion in the Eastern United States

Invasive plant species threaten native ecosystems, natural resources, and managed lands worldwide. Climate change may increase risk from invasive plant species as favorable climate conditions allow invaders to expand into new ranges. Here, we use bioclimatic envelope modeling to assess current climatic habitat, or lands climatically suitable for invasion, for three of the most dominant and aggressive invasive plants in the southeast United States: kudzu (Pueraria lobata), privet (Ligustrum sinense; L. vulgare), and cogongrass (Imperata cylindrica). We define climatic habitat using both the Maxent and Mahalanobis distance methodologies, and we define the best climatic predictors based on variables that best ‘constrain’ species distributions and variables that ‘release’ the most land area if excluded. We then use an ensemble of 12 atmosphere-ocean general circulation models to project changes in climatic habitat for the three invasive species by 2100. The combined methodologies, predictors, and models produce a robust assessment of invasion risk inclusive of many of the approaches typically used individually to assess climate change impacts. Current invasion risk is widespread in southeastern states for all three species, although cogongrass invasion risk is more restricted to the Gulf Coast. Climate change is likely to enable all three species to greatly expand their ranges. Risk from privet and kudzu expands north into Ohio, Pennsylvania, New York, and New England states by 2100. Risk from cogongrass expands as far north as Kentucky and Virginia. Heightened surveillance and prompt eradication of small pockets of invasion in northern states should be a management priority.