The protected area or NIPAS area in the municipality is the Bulusan Volcano Natural Park located at barangay Cogon with an area of 1,045.71 hectares. The BVNP consists of montane and lowland forest in a mountain complex covering the municipalities of Irosin, Juban, Casiguran, Barcelona and Bulusan with a total area of 3,673.29 hectares.  Biodiversity profile of the BVNP includes threatened bird species of Philippine Duck, Philippine Hawk-eagle, and Flame-breasted Fruit-Dove as well as restricted-range bird like Luzon Bleeding-Heart, Luzon Hornbill and others. Mammals that have been recorded include Philippine Brown Deer, Philippine Warty Pig and the threatened Giant Cloud Rat. Endemic herpetofauna include forest frogs, tree skinks and the threatened Sailfin Water Lizard.

The following data presented is the result of quadrant samplings in the barangays of Cogon, Monbon, Bagsangan and Patag during the Rapid Site Assessment conducted for BVNP. Of the 38 tree species identified 26 were of hardwood and 12 of softwood variety. The presence of high-density tree population can be accounted to the fact that they are located in higher elevation level with extremely steep slope range. Another important finding is that there is a predominance of tree saplings in the undergrowth of which they have a 27% presence value rating.  Of the        46% of the faunal inventory, the avifaunal population has the highest presence value of 52% while the amphibians have the lowest with 6%. Common: Few Abundance Ratio per specie is as follows: 42:58 for birds, 33:67 for mammals and 80:20 for reptiles. However, when Sighting Frequency is inter-faced with Abundance Level, the ratio is reversed to 46:54. Nonetheless, the faunal inventory is still of good account taking into consideration that the presence value of birds is high and they occupy the top-most position in the food-chain ladder and they are sensitive to ecological changes and thus useful as an ecological indicator.

Adjacent to the protected area are forest areas in alienable and disposable lands located in Cogon, Monbon, Bagsangan, Patag and Mapaso with an area of 313.41.25 hectares. Forest areas can also be found in Mt. Jormajan located in Bolos and Cogon and in the Mt. Maraot Banwa mountain ranges in Cawayan and Tabon-tabon in timberlands and alienable and disposable lands. The Mt. Jormajam forest cover is about 224.75 hectares while the Mt. Maraot Banwa forest is 790.14 hectares. The total area of non-NIPAS forest areas is 1,290.99 hectares. The old growth diptherocarp forests are nearly gone in these areas except for the residual regenerating second growth forests and forests under rehabilitation planted to mahogany, gemelina, acacia, narra and other tree species. Threatened endemics that occur in BVNP can be possibly found in these areas. The existing primary and residual forest vegetation in these non-NIPAS areas rationalizes forest protection.

The remaining forests are already at a risk from illegal tree cutting, slash and burn farming, conversion of forest lands into agricultural uses and indiscriminate extraction of minor forest products. These degenerate practices hasten the forest denudation and loss of biological diversity. It is most important that these practices must be curtailed or stopped to prevent further degradation of the flora, fauna and soil composition of the municipality. Likewise, this upland ecosystem plays a great part in the air and water quality of the lowland ecosystem of the municipality.

Cadac-an River is the namesake of the Cadac-an Watershed which traverses Irosin, Juban and portions of Bulusan, Bulan and Casiguran with an estimated total area of 21,567 hectares. The Cadac-an watershed in Irosin covers 10,356 hectares or 69% of Irosin’s land area. The watershed starts at the slopes of Bulusan Volcano and Mt. Maraot Banwa. The slopes to the east and south of Mt. Maraot Banwa as well as the hills of Casini and Salvacion drains to the watersheds of the municipalities of Sta. Magdalena, Matnog and Bulan. The easements of Cadac-an River, other major rivers, creeks and lakes with an approximate area of 257.75 hectares are also considered as protection lands.

The Irosin Caldera dominates the lowland ecosystem where various creeks, streams and rivers crisscross the sprawling valley floor. This ecosystem is classified into agricultural areas, built-up/urban areas and inland waters. The agricultural areas are divided into areas of sustainable land use, under-utilized land and over used lands. The agricultural production areas are mostly areas of sustainable land use planted to palay, coconut and other crops. The under-utilized lands are situated in the upper and lower river terraces suitable for irrigated rice lands but are planted to perennial trees and areas suitable to annual crops but are planted to coconut. The most common over-used lands are those areas with more than 18% slope suitable for production forests or should be maintained as forest but are planted with perennial crops.

The present attitude of farmers towards crop production is already depleting the soil quality. High yielding variety seeds that are dependent on a large quantity of inorganic fertilizers and pesticides are utilized thereby upsetting the soil pH, fouling the air quality and contaminating the creeks and streams due to the over reliance to pesticides. Thus, even endemic fishes in the rice paddies, rivers and other inland waters are already gone. The mono-cropping practices and inappropriate farming technologies have aggravated further the already deteriorating state of the agricultural production areas.

In the built-up/urban areas the concentrations of human settlements, commerce, industry and public and private services are found. It is also the educational centers are situated as well as the seat of the governance of the municipality. This concentration of the human activity in the valley floor affects the lowland environment. It is a must that proper and appropriate waste disposal, farming practices and use of environmentally friendly products must be affected to negate the accumulation of pollution in the soil and water.

Coconut, woodland and rice are the prominent vegetation cover occupying 62%, 15% and 14% of the gross land area of the municipality respectively. Other agricultural crop is planted in 1% while grass/shrubs cover 2% of the total land area. A total of 94.52% of the municipality’s land area or 14,168.04 hectares has vegetation cover.

Rice occupies the broad alluvial plains with an area of 2,194.25 hectares and is planted in 18 barangays and is mostly confined to elevation 19 to 70 meters in slope range 0 to 8%. Coconuts cover 9,426.20 hectares and can be found over the four points of the compass in 23 barangays up to elevation of 520 meters and in slope ranges of 0 to 50% and above. It must be noted that coconut is interspersed with other crops such as abaca, citrus, fruit trees, pili and other crops. Abaca, which was planted to 3,502 hectares in 2010 was decimated by bunchy top and mosaic virus infestation but patches of this crop are still present.

Woodlands cover 2,082.52 hectares and can be found in elevations from 90 to 1,060 meters in slope range 18 to 50% and above. Woodland occupies a significantly lesser area than documented forest areas considering the presence of grasslands and barren lands at the slope of Bulusan Volcano. Grass/shrubs are those found at the slopes of Bulusan Volcano, in areas razed by forest fires and in abandoned slash-and-burn lots with a total area of 356.07 hectares in elevations 80 to 1,250 meters and in slope range 18 to 50% and above.  Atop Bulusan Volcano are barren rocky lands near the crater located at elevation from 1,250 to 1,500 meters with an estimated area of 80 hectares.