Optimal approaches for balancing invasive species eradication and endangered species management

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Science  30 May 2014:
Vol. 344, Issue 6187, pp. 1028-1031
DOI: 10.1126/science.1250763

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  1. Fig. 1 Changing abundance of invasive Spartina.

    (Left) Abundance of invasive hybrid Spartina before the eradication program (2005–2011). (Right) Abundance of invasive Spartina showing greatly reduced distribution in 2011. Eradication is not permitted in the remaining untreated areas, totaling ~8% of the area covered by invasive Spartina in 2005.

  2. Fig. 2 Invasive species eradication concurrent with endangered species recovery.

    Both invasive Spartina and native Spartina are stage-structured and comprise isolates, which include any form of Spartina in which clapper rail cannot nest (isolated plants, seedlings), and meadows, which include any form of Spartina in which clapper rail can nest (large areas covered by mature stands of Spartina). Bare soil represents area that was eradicated but where native Spartina is still absent. Each Spartina species has its own natural growth rates (dashed arrows). The goal of management is to remove invasive Spartina, while maintaining sufficient clapper rail habitat composed of either invasive or native Spartina meadows (or both) at any given time. We examine how to optimally combine the three management actions (solid arrows: invasive meadows removal; invasive isolates removal; native replanting) over time to achieve the goals in the most cost-effective manner.

  3. Fig. 3 Optimal synthesis between invasive species eradication and endangered species recovery.

    (Top) The fraction covered (unitless) by invasive Spartina (IS) and by native Spartina (NS), when we assume optimal management. (Bottom) Optimal investment levels in eradication and restoration over time given a fixed annual budget (B): First, remove IS as quickly as possible but not below the minimum habitat needed for the clapper rail (dashed line in top panel); next, replant some NS isolates; and finally, use only the necessary amount of budget to remove the remaining IS while keeping enough meadows to provide nesting habitat for clapper rail. The progress continues until there are enough NS meadows to allow complete eradication of IS. The parameters used here are based on field data (table S1).

  4. Fig. 4 Importance of long-term restoration and conservation.

    (Top) Plotted is the present value of the total net costs over the years, including both direct costs due to damages caused by invasive Spartina and costs of treatment. If the manager eradicates invasive Spartina without considering the endangered clapper rail, then eradication could be completed within Tdirect years (dark line). Optimally, the manager would eradicate as fast as possible until eradication is completed within Tdirect years, and therefore, additional years would not make any difference. However, achieving both eradication of invasive Spartina and recovery of clapper rail, using the same annual budget, requires at least Tmin years (light line). Moreover, it would be more cost-effective to divide the budget over several more years, using a smaller fraction of the annual budget each year. Particularly, approaching the minimal net cost is possible if the manager can carry the budget over more years until year Topt (24). (Bottom) Plotted are Tdirect, Tmin, and Topt for various maximal annual budgets, B. The actual annual budget used in the project is Bproj. Parameters are the same as used in Fig. 3 (table S1).

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