title: r gbm and dismo for present available data
body: <p>I want to use the r gbm (and dismo) package for present and available data 
  with the functions gbm.step, predict.gbm, gbm.interactions, gbm.plot and gbm.perspec.
  My presences are coded as 1 and random available points are coded as 0.
  The number of presence observations is the same as the number of random available points.
  Will the results be useful as the packages are designed for present/absent data and if so, how must predicted values and the plotted fitted values be read?</p>