When a weed invasion is discovered a decision has to be made as to whether to attempt to eradicate it, contain it or do nothing. Ideally, these decisions should be based on a complete benefit-cost analysis, but this is often not possible. Partial analysis, combining knowledge of the demographics of the weed and economic techniques, can assist in making the best decision. This paper presents a general conceptual model to decide when eradication of a weed should be attempted. Decision rules are derived based on a few parameters that represent the rate of spread, the cost of controlling the invasion, and the cost of damage caused by the invasion. These decision rules are then used to identify the 'switching point' - the invasion size at which it is no longer optimal to attempt eradication. The decision rules are used to estimate the optimal duration of the eradication effort depending on the current size of the invasion. Sensitivity analysis is undertaken and the possibility of characterising an invasion based on five parameters is discussed.

Managers of deciduous perennial fruit crops must consider both biological and economic relationships in determining orchard design and life-time orchard management strategies. Orchardists require a good understanding of the many environmental, physiological and horticultural factors that influence tree growth, fruit production and fruit quality. Of particular importance in apple-tree management is knowledge of how the growing environment and horticultural manipulation of past years affect current and future growth habits and productivity of the tree. In addition to understanding biological factors that influence apple-tree productivity, a diverse range of orchard systems are currently available to orchardists. Each system, consisting of a particular combination of cultivar, rootstock, tree spacing and training method, has implications for fruit quality, quantity and ultimately profit. A dynamic simulation model of apple orchard production is developed in this research, and used to investigate a range of issues of relevance to the commercial apple orchardist. The model is developed in a bioeconomic framework and consists of biophysical and economic components. The biophysical component describes the vegetative and reproductive physiology of an apple tree, factors affecting the quantity and quality of apples produced and interrelationships between these factors. The economic component describes the costs and revenues associated with each orchard system from planting to maturity.

A stage matrix for 'Miconia calvescens' under Australian conditions is presented in this paper. The matrix contains eight stages - new seeds, seed bank, four juvenile stages, and two adult stages - reflecting the growth habit of 'M. calvescens' in tropical Australia, where it takes at least four years for a plant to reach maturity and where there are large differences in seed production between small and large adults. Life cycle analysis of the stage matrix is undertaken to give details of important events in the growth of this species, including age-specific survival and fecundity. Sensitivity and elasticity analyses are used to indicate the relative contributions of life stages to population growth. This information can be useful in planning control programs.

The detectability of invasive organisms influences the feasibility of eradicating an infestation. Search theory offers a framework for defining and measuring detectability, taking account of searcher ability, biological factors and the search environment. In this paper, search theory concepts are incorporated into a population model, and the costs of search and control are calculated as functions of the amount of search effort (the decision variable). Simulations are performed on a set of weed scenarios in a natural environment, involving different combinations of plant longevity, seed longevity and plant fecundity. Results provide preliminary estimates of the cost and duration of eradication programs to assist in prioritising weeds for control. The analysis shows that the success of an eradication program depends critically on the detectability of the target plant, the effectiveness of the control method, the labour requirements for search and control, and the germination rate of the plant.

The bitou bush ('Chrysanthemoides monilifera' subsp. 'rotundata' (DC.) T.Norl.) Threat Abatement Plan (TAP) aims to reduce the impacts of bitou bush on biodiversity in New South Wales. This is the first weed TAP in Australia and so its effectiveness in conserving threatened biodiversity, as well as its cost of implementation, must be examined to determine if this new approach should be adopted as a template for managing the biodiversity impacts of other major weed species. We therefore consider the question 'is the TAP a good investment in relation to protecting biodiversity'? We combine the costs of implementing the TAP with conservative, published estimates of the benefits of protecting biodiversity, to calculate the benefit-cost ratios of the investment. The ratios indicate that the benefits of the TAP exceed the costs under a wide range of economic conditions. While this result supports the approach, the cost of implementation should be analysed over the five years relative to the biodiversity outcomes in order to determine the ex post benefit of the TAP.

The emission of greenhouse gases, particularly carbon dioxide, and the consequent potential for climate change are the focus of increasing international concern. Temporary land-use change and forestry projects (LUCF) can be implemented to offset permanent emissions of carbon dioxide from the energy sector. Several approachesto accounting for carbon sequestration in LUCF projects have been proposed. In the present paper, the economic implications of adopting four of these approaches are evaluated in a normative context. The analysis is based on simulation of Australianfarm–forestry systems. Results are interpreted from the standpoint of both investors and landholders. The role of baselines and transaction costs are discussed.

The International Center for Living Aquatic Resources Management (ICLARM) has demonstrated that coastal village communities in Solomon Islands can successfully farm giant clams. The production technology is simple and does not require a large capital investment. The main inputs are clam seed, labour and time. Labour is used for activities such as planting, cleaning, thinning and harvesting. In this paper, a bioeconomic model is used to explore optimal farm management for two species of giant clam fanned for the aquarium and seafood markets. The theoretical basis for this analysis is found in the economic theory of optimal forestry exploitation. Optimal management involves finding the combination of the decision variables and the cycle-length that maximises a stream of discounted profits. The decision variables considered here are husbandry which relates to cleaning, and the frequency with which thinning is undertaken. The optimal cycle-length is determined for both a single clam harvest and multiple harvests for various management scenarios. The labour requirements for these management scenarios are identified for the multiple-harvest case and input substitution between optimal combinations of labour and cycle-length is investigated. Results indicate that profits are maximised for both species when husbandry is excellent and labour usage is most intensive. Thinning is only necessary for seafood clams for which the optimal cycle-length is longer. Village farmers may not be profit maximisers however, and labour spent on giant-clam farming takes them away from other activities. Rather than investing more labour and harvesting the clams earlier, a village farmer with other objectives may devote less labour and harvest the clams later, and spend more time on other activities. In general, these results are consistent with extension advice provided to village farmers by ICLARM. Optimal solutions were found to be very stable when incorporated into global optimisation routines and sensitivity analysis of a wide range of parameter values.

While a number of models have been developed to assist managers of deciduous fruit tree crops with specific aspects of decision making, most are non-optimising predictive models and few employ detailed mechanistic models of fruit-tree growth that would enable the simulation of any orchard system from planting to maturity. This paper details the complex biological and economic relationships present in an apple orchard system and describes a dynamic simulation model based on these interactions. The model is bioeconomic in nature, and may be used to investigate a range of issues of relevance to the commercial apple orchardist. These issues include understanding how biological factors influence apple-tree productivity, and how to choose among a diverse range of apple orchard systems. Each system, consisting of a particular combination of cultivar, rootstock, tree spacing and training method, has implications for fruit quality, quantity and ultimately profit. The choice of system is made at planting, while an important annual decision is the optimal rate of thinning, both of which determine potential yield over the lifetime of the orchard. These decisions also influence costs and revenues per hectare and, by necessity, are made in the context of unknown future prices of inputs and outputs. The bioeconomic model is used to maximise net present value of one orchard system by selecting optimal thinning strategies over a 15-year period.

Mitigation of, or adaptation to, dryland salinity will require large-scale changes in land and water use. Analysis of the problem requires the simultaneous observation of hydrological processes, which drive salinisation, and economic conditions, which influence the decisions of land managers. Given the long-term nature of the salinity problem, the time lags involved and the very large number of possible land-use combinations for a particular catchment, it is necessary to resort to modelling as a decision tool and for policy analysis . This paper presents suggestions for combining hydrology and economic models . A simple model of a catchment is developed and used to identify minimum data requirements. Questions regarding model design and implementation are raised and the advantages and disadvantages of different approaches are discussed.

1. - Weeds have a wide variety of impacts on society, theenvironment and the economy. Some of the economicimpacts are benefits but most are costs.2. - The costs of particular weeds in given areas have beenestimated by many writers in a rich literature on theassessment of the impacts in agriculture. Only Combellack(1987) has attempted to estimate the nationwide impactof weeds in general.3. - In his innovative study, Combellack valued the economiccosts of weeds in 1981–82 to be $2,096m. New methodsof weed control and techniques of farm managementhave since been developed, and new weed species nowoccur. Therefore the current costs of impacts cannot bereadily compared with those of 1981–82.4. - The nationwide impact of weeds needs to bere-estimated to provide a more recent benchmark thatreflects current costs, prices and technologies, and thecurrent distribution of impacts within the community.A current estimate provides useful information fordecisions on the allocation of resources, cost sharing,and management of specific weed problems.5. - In this report, we attempt to estimate the economiccosts of weeds across Australia. In addition, we offer aneconomic framework to help consider the problems thatweeds create, and the generation and use of informationto resolve those problems.