Integrated planning of linear infrastructure and conservation offsets

Linear infrastructure (e.g. roads, railways, utility corridors) is critical to the functioning of modern industrialised societies, but has profound impacts on ecological systems and their biological diversity. The ecological impacts of linear infrastructure may be exacerbated by the failure to anticipate and plan for them early on. Moreover, poor planning can lead to the costly redesign or abandonment of projects to avert unforeseen ecological impacts. Explicitly considering trade-offs between linear infrastructure development and conservation requirements at an early planning stage provides the greatest opportunities for achieving both routing and offsetting objectives cost-effectively.

Here, we combine route planning for linear infrastructure with strategic conservation offset decision making in a unified framework to identify efficient opportunities for both routing and impact mitigation. The routing component is spatially explicit and links two or more target nodes via a network of connected planning units with fixed costs while accounting for impacts on features of conservation concern. The offsetting component identifies efficient solutions for mitigating the unavoidable impacts of linear infrastructure. We demonstrate the benefits of solving both planning objectives simultaneously by contrasting the cost-effectiveness of the integrated solutions with those found by solving the problems sequentially.

We formulated these planning problems in a mathematical optimisation framework in which space is discretised into planning units. Four sets of attributes were quantified for each planning unit as inputs to the optimisation problem: (i) the expected loss of conservation value resulting from linear infrastructure development for each of the conservation features (e.g. species); (ii) the cost of linear infrastructure development; (iii)the cost of offsetting; and (iv) the estimated benefit of offsetting with respect to the conservation features. The decision variables determine whether each planning unit is part of the routing solution, the offsetting solution,or neither. The mitigation effort is required to be sufficient to offset all of the impacts on biodiversity (no net loss) generated by the linear infrastructure development.

We describe an integer linear programming formulation of the optimisation problem and evaluate its efficacy through simulation. We find that solving both problems simultaneously can dramatically improve efficiency compared to solving the routing and offsetting problems sequentially. Furthermore, a sequential approach resulted in infeasible offsetting problems in 25% of cases. This work suggests both linear infrastructure and conservation offset planning could benefit from greater coordination in the early stages of the planning process.