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Distribution or abundance?

Once it is clear that you are aiming for predictive mapping rather than direct, you must decide whether you need maps of the distribution (presence vs. absence) or the abundance (number of animals per area, cover of algae or biomass) of the type of biodiversity of interest. Or both!

  • Maps of abundance are more informative than those of distribution because they contain quantitative information. On the other hand, they may be more uncertain because of large seasonal variability.
  • Maps of distribution (usually probability of presence) are less informative but still very useful in marine spatial planning and environmental impact assessment. Although the comparison is not straightforward, they are probably more robust than maps of abundance.

Data on quantities or qualities?

The choice between these two options has some consequences for the selection of modelling technique, but most of all it influences the sampling:

  • Models and maps of abundance require that quantitative data are collected.
  • Maps of distribution require only data on presences and absences (some techniques require only records of presence).

The latter is usually less expensive and easier to accomodate with existing data.

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