Developing a comprehensive database of transboundary conservation complexes is not an easy task to accomplish. Several efforts have been undertaken in creating an inventory, the latest being performed by UNEP’s World Conservation Monitoring Centre (WCMC) in 2007:

   1.   1988. WCMC (Jeremy Harrison) and IUCN (Jim Thorsell) identified 70 transfrontier nature reserves.
   2.   1994/1996. IUCN (‘Parks for Life’) listed 42 transboundary protected areas in Europe
   3.   1997. Dorothy Zbicz and Michael Green identified 136 transfrontier protected area complexes.
   4.   1999. IUCN (‘Parks for Life’) identified 89 transboundary cooperation areas in Europe.
   5.   2001. Dorothy Zbicz lists 169 internationally adjoining protected areas.
   6.   2005. UNEP WCMC identified 188 internationally adjoining protected areas.
   7.   2007. UNEP WCMC listed 227 transboundary protected areas.