Functional diversity has emerged as a relevant aspect in the ecosystem studies; several indexes have been used for its evaluation. However, these indexes have not considered the functional redundancy among species, even when its implication in forest resilience facing natural and anthropogenic disturbances have been recognized. In this study an index for functional diversity, considering functional redundancy, is proposed and applied to know changes in functional diversity types according to its disturbance responses, in sites under different disturbance levels, in a microphyll semi-deciduous forest. The probability theory was used to derive a functional biodiversity index from the Simpson biodiversity index. The sensitivity of the index in relation to changes in the individual, species and functional type number was analyzed and compared with other indexes proposed in literature. The less functional diversity value was found in highly disturbed sites, and the highest value in the less disturbed sites. Results confirmed that in less disturbed sites, individuals were more uniformly distributed inside the species and these inside the functional types, than those individuals found in highly disturbed areas; it confers the system a superior functional redundancy. It is concluded that the proposed index is, besides, a good reference of the resilience in a forest ecosystem, and the forest management oriented to keep the functional diversity, as it is interpreted, is a guaranty for the conservation of the forest resilience and integrity.