[4] The southern islands are older, and are composed of volcanic rocks and marine limestone uplifted by the colliding plates.

Easterly trade winds are fairly constant, with an occasional weak westerly monsoon influence during the summer months.

Older and lower-elevation lava flows are colonized by the tree Casuarina equisetifolia, known locally as gagu, and the fern Nephrolepsis hirsutula.

Yonga is a large tree with a spreading crown, although the Marianas' frequent typhoons often limits their growth to 15 meters.

[2] Most of the natural vegetation on the older southern islands has been cleared or altered by humans, but areas of primary and secondary forest remain.

Merrilliodendron-Ficus forest is dominated by Merrilliodendron megacarpum and Ficus prolixa which form a canopy 30 to 45 meters high.

Ravine forest trees include Pandanus tectorius, P. dubuis, Hibiscus tiliaceus, Ficus prolixa, Glochidion marianum, and Premna serratifolia.

Dense thickets of Leucaena leucocephala, a South American tree widely planted after World War II, are common.

Guam's native bird and lizard species have been decimated by the Australasian brown tree snake (Boiga irregularis), which was introduced in the 1940s.

The Marianas flying fox (Pteropus mariannus) lives on inaccessible vegetated limestone cliffs and volcanic ravine forests.