Ecologists have warned that Melbourne is at risk of losing more than half its native plant species over the next century, with grasslands in Melbourne's west most vulnerable to the city's urban sprawl.

A study comparing 22 cities around the world found Melbourne had the largest "extinction debt" in terms of the proportion of native plant species likely to go extinct.

Ecologist Mark McDonnell has warned that Melbourne is at risk of losing more than half its native plants species in the next century. Credit:Justin McManus

Results of a study published in the journal Ecological Management and Restoration, found that 55 per cent of the 1200 Indigenous plant species found in greater Melbourne are threatened with extinction over the next 100 years.

Grassland species alone are forecast to be reduced by 21 per cent, or 184 species, over the same period.