Nominator's Summary of Significance: The Porongurups contain a major vegetation remnant of the botanically rich south west Australian biome. The place holds a significant diversity of plants, 709 native species, in a relatively small area (less than 3,200ha).
Notably diverse plant groups include beard-heaths (Leucopogons), flame-peas (Chorizema) and bitter-peas (Daviesia and Bossiaea), orchids, banksias and hakeas and mountain bells (Darwinia). Plant diversity is highest especially in lower and open scrub and open forest areas of jarrah (Eucalyptus marginata) and marri (E. calophylla), which are remnants of the surrounding countryside now extensively cleared. The place is also notable for holding outliers of karri forest (E. diversicolor), which is a function of its 670 metre elevation and granitic substrate, which acts as a refuge for moisture loving species.
The place is one of the largest granite massifs in Western Australia and amongst the oldest in the world. South-west Australian granitic mountain tors, including the Porongurups, are of evolutionary interest supporting relict plant species, and the Porongurup Ranges reflect this in holding outlier karri forest (E. diversicolor), and at least nine endemic plant species, along with Gondwanan relict invertebrates such as trapdoor spider (myglomorphs) species (Neohomogona bolganupsis and Moggridgea sp.), unusual land snails and a giant earthworm (Megascolex sp.).
The ranges' dramatic mountain scenery and spectacularly colourful wildflower displays attract thousands of visitors each year.
The place also holds eight of the thirteen internationally recognised southwest Australian endemic bird species.