Himachal Pradesh situated in the lap of northwestern Himalayas is one of the richest repositories of plant diversity in India. However, during the past three decades, because of the increased pace of development and interference of humans through introduction of invasive exotics, the ecology of the state has changed tremendously. Ragweed parthenium, billy goat weed, and lantana—the three exotics from South America—have caused much harm in the state because of their invasive potential. A study conducted to assess the changes in the structural composition and dynamics of vegetation shows that density and diversity of native flora were adversely affected because of invasion by these three exotics. Because all the three exotic weeds are known to exhibit allelopathy, it might be one of the major nonresource–based hypothesis for the successful invasion by these exotic species.

Nomenclature: Billy goat weed, Ageratum conyzoides L.; lantana, Lantana camara L.; ragweed parthenium, Parthenium hysterophorus L.

Additional index words: Diversity, dominance, ecological status, Importance Value Index, invasive weeds, species richness, vegetation analysis.

Abbreviation: IVI, Importance Value Index.