Daniel Spring

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.

Applying Search Theory to Determine the Feasibility of Eradicating an Invasive Population in Natural Environments

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.

Economics of a nest-box program for the conservation of an endangered species: a reappraisal

An optimization model is developed to identify timing and placement strategies for the installation of nest boxes and the harvesting of timber to meet joint timber–wildlife objectives. Optimal management regimes are determined on the basis of their impacts on the local abundance of a threatened species and net present value (NPV) and are identified for a range of NPV levels to identify production possibility frontiers for abundance and NPV. We apply the model to a case study focusing on an area of commercially productive mountain ash (Eucalyptus regnans F. Muell.) forest in the Central Highlands region of Victoria, Australia. The species to be conserved is Leadbeater’s possum (Gymnobelideus leadbeateri McCoy), which is locally limited by a scarcity of nesting hollows. The modeling is exploratory but indicates that nest boxes may offer a promising population recovery tool if consideration is taken of their placement and areal extent through time. Résumé : Les auteurs ont développé un modèle d’optimisation pour identifier les stratégies concernant le moment et l’emplacement pour l’installation de nichoirs et la récolte de bois afin de rencontrer les objectifs conjoints pour la matière ligneuse et la faune. Les régimes optimaux d’aménagement sont déterminés sur la base de leurs impacts sur l’abondance locale d’une espèce menacée et la valeur nette actualisée. Ils sont identifiés pour une gamme de niveaux de valeur nette actualisée afin d’identifier les limites possibles de production en fonction de l’abondance et de la valeur nette actualisée. Nous appliquons le modèle à une étude de cas concentrée dans une zone de forêt commerciale productive d’eucalyptus géant (Eucalyptus regnans F. Muell.) sur le plateau central dans la région de Victoria en Australie. L’espèce à conserver est le bucorve du Sud (Gymnobelideus leadbeateri McCoy) qui est localement en nombre limité à cause de la rareté des cavités pouvant servir de nichoir. La modélisation est exploratoire mais indique que les nichoirs peuvent constituer un outil intéressant pour rétablir la population si leur emplacement et leur répartition dans le temps sont pris en considération. [Traduit par la Rédaction] Spring et al. 2003

Agent-based Bayesian approach to monitoring the progress of invasive species eradication programs

Eradication of an invasive species can provide significant environmental, economic, and social benefits, but eradication programs often fail. Constant and careful monitoring improves the chance of success, but an invasion may seem to be in decline even when it is expanding in abundance or spatial extent. Determining whether an invasion is in decline is a challenging inference problem for two reasons. First, it is typically infeasible to regularly survey the entire infested region owing to high cost. Second, surveillance methods are imperfect and fail to detect some individuals. These two factors also make it difficult to determine why an eradication program is failing. Agent-based methods enable inferences to be made about the locations of undiscovered individuals over time to identify trends in invader abundance and spatial extent. We develop an agent-based Bayesian method and apply it to Australia’s largest eradication program: the campaign to eradicate the red imported fire ant (Solenopsis invicta) from Brisbane. The invasion was deemed to be almost eradicated in 2004 but our analyses indicate that its geographic range continued to expand despite a sharp decline in number of nests. We also show that eradication would probably have been achieved with a relatively small increase in the area searched and treated. Our results demonstrate the importance of inferring temporal and spatial trends in ongoing invasions. The method can handle incomplete observations and takes into account the effects of human intervention. It has the potential to transform eradication practices.

Building a Regionally Connected Reserve Network in a Changing and Uncertain World

Habitat connectivity is required at large spatial scales to facilitate movement of biota in response to climatic changes and to maintain viable populations of wide-ranging species. Nevertheless, it may require decades to acquire habitat linkages at such scales, and areas that could provide linkages are often developed before they can be reserved. Reserve scheduling methods usually consider only current threats, but threats change over time as development spreads and reaches presently secure areas. We investigated the importance of considering future threats when implementing projects to maintain habitat connectivity at a regional scale. To do so, we compared forward-looking scheduling strategies with strategies that consider only current threats. The strategies were applied to a Costa Rican case study, where many reserves face imminent isolation and other reserves will probably become isolated in the more distant future. We evaluated strategies in terms of two landscape-scale connectivity metrics, a pure connectivity metric and a metric of connected habitat diversity. Those strategies that considered only current threats were unreliable because they often failed to complete planned habitat linkage projects. The most reliable and effective strategies considered the future spread of development and its impact on the likelihood of completing planned habitat linkage projects. Our analyses highlight the critical need to consider future threats when building connected reserve networks over time.

Finding needles (or ants) in haystacks: predicting locations of invasive organisms to inform eradication and containment

To eradicate or effectively contain a biological invasion, all or most reproductive individuals of the invasion must be found and destroyed. To help find individual invading organisms, predictions of probable locations can be made with statistical models. We estimated spread dynamics based on time-series data and then used model-derived predictions of probable locations of individuals. We considered one of the largest data sets available for an eradication program: the campaign to eradicate the red imported fire ant (Solenopsis invicta) from around Brisbane, Australia. After estimating within-site growth (local growth) and intersite dispersal (saltatory spread) of fire ant nests, we modeled probabilities of fire ant presence for >600000 1-ha sites, including uncertainties about fire ant population and spatial dynamics. Such a high level of spatial detail is required to assist surveillance efforts but is difficult to incorporate into common modeling methods because of high computational costs. More than twice as many fire ant nests would have been found in 2008 using predictions made with our method rather than those made with the method currently used in the study region. Our method is suited to considering invasions in which a large area is occupied by the invader at low density. Improved predictions of such invasions can dramatically reduce the area that needs to be searched to find the majority of individuals, assisting containment efforts and potentially making eradication a realistic goal for many invasions previously thought to be ineradicable.

Estimating eradication probabilities and trade-offs for decision analysis in invasive species eradication programs

Eradication of invasive species can have substantial benefits but programs often fail and have high costs. Costs can be reduced by substituting lower cost management actions for higher cost actions that have a similar impact on the probability of eradication. A first step towards minimizing costs is to determine all combinations of management actions that achieve eradication with the same probability, which is a form of trade-off analysis. Trade-off analysis can have a high computational cost when large numbers of management alternatives are compared with a complex spatial simulation model. Here, we apply a practical method for conducting trade-off analysis in which statistical methods are applied to data generated by a complex spatial model to derive a simpler model (the meta-model) that is used to determine trade-offs. We demonstrate this approach with a case study focusing on Australia’s largest eradication program, the campaign to eradicate red imported fire ants (Solenopsis invicta). Trade-offs were estimated for two surveillance methods, remote surveillance and ground surveillance, that are currently used in the eradication program. The methods are applied adaptively, with remote surveillance applied over all potentially infested locations (“area-wide surveillance”) and ground surveillance applied near remotely sensed detection points to find and remove remaining undetected individuals (“local surveillance”). The meta-modelling approach provided useful insights for management. When area-wide and local surveillance methods are applied adaptively, increases in the sensitivity of area-wide surveillance can allow for large reductions in the area of local surveillance and treatment. This can potentially result in substantial cost savings in circumstances where local search methods have a high cost.

Look before you treat: increasing the cost effectiveness of eradication programs with aerial surveillance

Most successful invasive species eradication programs were applied to invasions confined to a small area. Invasions occupying large areas at a low density can potentially be eradicated if individual infestations can be found at affordable cost. The development of low cost aerial surveillance methods allows for larger areas to be monitored but such methods often have lower sensitivity than conventional surveillance methods, making their cost-effectiveness uncertain. Here, we consider the cost-effectiveness of including a new aerial monitoring method in Australia’s largest eradication program, the campaign to eradicate red imported fire ants (Solenopsis invicta). The program previously relied on higher sensitivity ground surveillance and broadcast treatment. The high cost of those methods restricted the total area that could be managed with available resources below the level required to prevent ongoing expansion of the invasion. By increasing the area that can be monitored and thereby improving the targeting of treatment and ground surveillance, we estimate that remote sensing could substantially reduce eradication costs despite the method’s low sensitivity. The development of low cost monitoring methods could potentially lead to substantially improved management of invasive species.

Institutional impediments to conservation of freshwater dependent ecosystems

When freshwater resources become scarce there is a trade-off between human resource demands and environmental sustainability. The cost of conserving freshwater ecosystems can potentially be reduced by implementing institutional reforms that endow environmental water managers with a permanent water entitlement and the capacity to store, trade and release water. Australias Murray Darling Basin Plan (MDBP) includes one of the worlds most ambitious programs to recover water for the environment, supported by institutional reforms that allow environmental water managers to operate in water markets. One of the anticipated benefits of the Plan is to improve the health of flood-dependent forests, which are among the most endangered ecosystems globally because of river regulation and land clearance. However, periodic flooding to conserve floodplain ecosystems in the MDB creates losses to riparian landowners such as damage to fencing and temporary loss of access to flooded land. To reduce these losses reservoir operators restrict daily water release volumes. Using a model of optimal water management in Australias southern MDB we estimate that current reservoir operating restrictions will substantially reduce the ecological benefits of investments made to recover water for the environment. The reduction in benefits is largest if floodplain forests decline rapidly without periodic inundation. In the latter circumstances, ecological losses cannot significantly be reduced by allowing environmental water managers to operate in water markets. Our findings demonstrate that the recovery of large volumes of water for environmental purposes and water market reforms are insufficient for conserving flood-dependent ecosystems without coordination and cooperation among multiple stakeholders responsible for water and land management.