Resumo. Disturbed ecosystems do not present original floristic composition. Their soils are depleted, shallow, stony, with low infiltration and present erosion with different levels of geodynamics. The study region has areas of pasture with sparse herbaceous vegetation which is weakened in every rain and fire, covering less ground. The individuals that colonized and settled in these environments were considered rustic species. This study raised and identified by census, the rustic species and their structure in the most degraded part (lower third) of the watershed of Cacaria’s river at the base of the Serra do Mar, Piraí, Rio de Janeiro state, and evaluated the influence of ecological exposure, slope, elevation, topography and rock outcrops in the establishment and growth of these species. For the vegetation survey it was conducted census in an area of 22 hectares, where it was measured, geo-referenced and identified all spontaneous tree species that were isolated in a pasture area. Ecological factors exposure, elevation and slope were determined with a compass, altimeter and clinometer, respectively. There were identified 131 individuals, representing 14 species, grouped into nine families. Tabernaemontana laeta Mart., Sparattosperma leucanthum (Vell.) Schum., Machaerium hirtum (Vell.) Stellfeld, Tabebuia chrysotricha (Mart. ex DC.) Stan., Cecropia pachystachya Trec., Peltophorum dubium (Spreng.) Taub., Guarea guidonia (L.) Sleumer, Acacia polyphylla DC. and Psidium guajava L.were present in portions of the slope with exposure to the north, with altitudes from 60 to 80m and with slope strongly corrugated (20-45%), indicating a preference of these species for microhabitats with those characteristics.