Zombitse-Vohibasia is not one continuous wilderness but three separate parcels. The Zombitse forest, at 16,845 hectares, straddles the RN7 highway 147 kilometres northeast of Toliara. To its northwest lies the Vohibasia forest at 16,170 hectares. Between them sits the smaller Isoky-Vohimena fragment at 3,293 hectares. The Zombitse section rests on sandstone, while the other parcels sit on Jurassic limestone -- a geological divide that creates subtle differences in soil, drainage, and the communities of plants and animals each fragment supports. Isoky-Vohimena perches on the shoulder of the Isalo Massif, sharing some species with nearby Isalo National Park, including Pachypodium rosulatum and several aloe species. The park lies in the homeland of the Bara and Mahafaly peoples, who have lived in this landscape for centuries.

BirdLife International recognizes Zombitse-Vohibasia as an Important Bird Area, describing it as a "biological reservoir of primary importance, due to its location straddling the western and southern domains of Madagascar." That overlap is the park's ecological signature. Nearly 300 plant species in 84 families grow here, with over 80% of the vascular plants endemic to Madagascar and only six non-native species recorded. Two plant families are found nowhere outside the island. Two species exist only within these boundaries: Ampelosycios bosseri, a cucurbit, and Ivodea trichocarpa, from the citrus family. A critically endangered orchid, Grammangis spectabilis, also clings to life here. Baobabs of two species -- Adansonia za and Adansonia grandidieri -- punch through the canopy, their pale trunks visible from a distance against the darker forest below.

The park shelters eight lemur species across day and night shifts. By day, ring-tailed lemurs, Verreaux's sifakas, and red-fronted lemurs move through the canopy -- all three now classified as Endangered or Near Threatened. After dark, a different cast emerges: gray mouse lemurs, fat-tailed dwarf lemurs, Coquerel's giant mouse lemurs, pale fork-marked lemurs, and the park's most irreplaceable resident, Hubbard's sportive lemur. Thirty-three reptile species have been documented, including Standing's day gecko, classified as Vulnerable. The park is also rich in invertebrates, with over 40 ant species catalogued across more than 20 genera. Nasolo's shrew tenrec, a Vulnerable insectivore known from only three sites worldwide, adds to the weight of rarity concentrated in these fragments.

Botanical inventory of Zombitse-Vohibasia began with collections by Perrier de la Bathie in 1910, more than eight decades before the area became a national park in the late 1990s. Comparisons between 1949 and 1994 aerial photographs show significant forest loss. Slash-and-burn agriculture -- called tavy in Malagasy -- continues to eat into the park's edges, and fire, grazing, and selective browsing have thinned what remains. The vegetation is thought to be floristically reduced from its historical richness. Where forest gives way to grassland, a transition zone of scattered shrubs and small trees forms a woodland belt before opening into savannah. In the Vohibasia section, the rainy season brings a brief explosion of diversity as annual plants carpet the understory, only to disappear when the dry months return. The contrast between that fleeting green and the burned margins beyond the park's borders is stark and sobering.