Spatial complexity can be included in SSM in any of 3 ways.
Contiguity: For a conservation portfolio of a given size, the shorter the total boundary length around selected planning units, the more compact will be the portfolio. One objective of the SSM, therefore, is to minimize the total length of the boundary of the portfolio.
Minimum Area: It is not enough that the portfolio itself be contiguous and compact but the areas for some species must be adequately large. In SSM, the minimum area that is considered adequate for a sub-population or sub-region can be specified.
Separation Distance: It may also be important to ensure that occurrences of some elements are sufficiently separated to mitigate the effects of localized catastrophes. If a separation distance is specified, then the program will favor solutions where there are at least 3 planning units that are separated by that minimum distance.