Two required parameters
name and id.
Optional parameters
anything accepted by {{cite web}}

Examples:

{{WWF ecoregion|name=Southeastern Indochina Dry Evergreen Forests|id=IM0210}}
produces
"Southeastern Indochina Dry Evergreen Forests". Terrestrial Ecoregions. World Wildlife Fund.


{{WWF ecoregion|id=na0504|name=Atlantic coastal pine barrens|accessdate=2010-12-21}}
produces
"Atlantic coastal pine barrens". Terrestrial Ecoregions. World Wildlife Fund. Retrieved 2010-12-21.


{{WWF ecoregion |mode=cs2 |id=nt0705 |name=Eastern Pacific Ocean, southeast of Mexico |accessdate=2018-01-17}}
produces
"Eastern Pacific Ocean, southeast of Mexico", Terrestrial Ecoregions, World Wildlife Fund, retrieved 2018-01-17