As you continue following the highway through the park you will descend from an elevation of more than 1500 meters to less than 500 meters above sea level on the Caribbean side of the Barva Volcano massif. Although perhaps not immediately apparent to the average visitor, there is a nearly complete change of flora and fauna between the two ends of this 1000 meter elevational transect. Trails at both the upper and lower ranger stations allow access to the rain forest, however, be careful as they have some steep and slippery sections. Also, caution should be used if stopping anywhere along the highway other than at the ranger stations as, unfortunately, there have been numerous cases during the past few years of tourists being robbed at gunpoint while attempting to use trails where there is no park service vigilance.

When crossing the Río Sucio bridge, be sure to look upstream (on your right if traveling from San José towards Limón). Here the Río Sucio (literally "Dirty River") joins the Río Hondura which comes in from the right. Unless it has been raining very heavily, the difference between these two streams is striking -- the Hondura is a clear mountain stream, while the Sucio can vary from grayish to reddish-orange due to its origins on the ash-covered upper slopes of Irazú Volcano.

Among the more than 400 species of birds known from Braulio Carrillo National Park, a few of the more sought after species by inveterate birdwatchers are: Bare-necked Umbrellabird, Snowcap, Sharpbill, Brown-billed Scythebill, Black-crowned Antpitta, Yellow-eared Toucanet, and Latticed-tailed Trogon. Tanagers and hummingbirds, in particular, abound in the lower and middle elevations of the park. The mammals are similarly diverse, but usually not easily seen. Several of the relatively more common species are: Deppe's Squirrel, White-nosed Coati, White-throated Capuchin, and Northern Tamandua.