The influence of planning unit characteristics on the efficiency and spatial pattern of systematic conservation planning assessments

Systematic conservation planning is a widely used approach for designing protected area systems and ecological networks. This generally involves dividing the planning region into a series of planning units and using computer software to select portfolios of these units that meet specified conservation targets whilst minimising conservation costs. Previous research has shown that changing the size and shape of these planning units can alter the apparent spatial characteristics of the underlying data and thus influence conservation assessment results. However, this may be less problematic when using newer software that can account for additional constraints based on portfolio costs and fragmentation levels. Here we investigate these issues using a dataset from southern Africa and measure the extent to which changing planning unit shape, size and baseline affects the results of conservation planning assessments. We show that using hexagonal planning units instead of squares produces more efficient and less fragmented portfolios and that using larger planning units produces portfolios that are less efficient but more likely to identify the same priority areas. We also show that using real-world constraints in the analysis, based on reducing socio-economic costs and minimising fragmentation levels, reduces the influence of planning unit characteristics on the results and so argue that future studies should adopt a similar approach when investigating factors that influence conservation assessments.