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3.1 Setting the Scenario and Marxan Project ConsiderationsBefore execut ing a Marxan model, conservation experts should decide on scenariosettings which may include a) the set of data to be used; and b) the conditions applied tothe objective function for Marxan to use (Ball and Possingham, 2000). Users must feelcomfortable about the data that is going into the model and the conservation goals thathave been assigned. It is a difficult and often abstract task to set goals, decide on costmeasures, planning units, etc., but these are essential decisions that must be made.Remember that Marxan produces flexible solutions that meet specific quantitativerepresentation goals in addition to minimizing threats constraints. As this is an iterativeprocess, a numberof conservation scenarios can be formulated and explored usingMarxan. There are also a number of ways solutions can be tested and found using theheuristics or the annealing algorithm.In addition to setting the conservation scenarios, there are several project considerationsto be made before finalizing the conservation targets, planning units, protected areas, andcost surface. Ideally, these considerations should be made through consultation withconservation experts and some can be determined through experimental model runs. Thefollowing are suggestions from Huggins (2005):a. Analysis Extent: The areas of interest should be defined based on the extent of alltarget and socio-economic data that have been gathered and assessed. The extentshould be defined by a polygon so that all features to be used in the model areclipped to the extent of the analysis area.b. Target Screening: All target distributions to be entered into the analysis shouldbe considered viable occurrences that are robust enough to influence the portfolioselection. Screening biodiversity distribution maps should be considered toimprove the likelihood of only including viable occurrences. Methods previouslyused include screening vegetation by expert opinion, removing all patches belowa threshold size, and screening using threat factors such as an Environmental RiskSurface (ERS) or freshwater flow accumulation models.c. Target Stratification: Targets can be stratified to allow a geographic spread ofrepresentation or to represent biologically distinct target sub-groups. Theportfolio will be greatly influenced by the way the targets are defined. Forexample, if the portfolio is required to hold both upland and lowland portions of aspecific target, it must be defined in a way that includes this information.Examples include stratification by elevation or bathymetry, surface geology, or byother geographic units that define biologically meaningful differences.d. Designing Planning Units (Adapted from Game and Grantham, 2008 andPressey and Logan, 1998): An essential pre-processing step is to divide yourplanning region into a set of planning units. Planning units are areas for whichdata on occurrence, frequency and extent of the targets exists. In their simplestform, planning units may be defined by overlaying your planning region with aTNC Protected Area Tools (PAT) Version 3.0The Nature Conservancy, August 200946