Human disturbance is causing many parts of Guam to convert from their natural state into areas characterized by scrub vegetation. Many previously forested parts of Guam are now open fields or mowed areas. Places once covered by canopy are now wide open areas such as parks, fields, military areas, etc. Scrub vegetation includes brush-type plants and grasses like Paspalum, Panicum, Chrysopogon, and Sorghum, and represents the ideal habitat for invasive weeds to take hold.

The most notorious invasive plant that thrives in scrub areas is tangantangan (Leucaena leucocephala). After being seeded on the island by U.S. military following WWII, it out-competed many native plants and covered large parts of the island. Similarly aggressive srub plants are chain-of-love (Antigonon leptopus) and dodder (Cuscuta campestris), both of which are vines that block the sunlight and compete for water with native plants. Finally, there is agalondi (Vitex parviflora), which is a non-native tree but now one of the most dominant trees on island.