Biodiversity is the number of species in an ecosystem. Genetic diversity is the variation of genes within a species or a population.
Biodiversity is the number of species in an ecosystem. This helps drive conservation efforts as the more species in the ecosystem, the more resilient it is. If one species becomes extinct (locally or globally), the chances of another species fulfilling its function increases with a more diverse community. Also, since species interactions is so complicated, it is best to ensure that no species is lost.
On the other hand, genetic diversity refers to the variation of genes (and their alleles) within one species or even a population of a species. The principle is the same: the more genetic diversity in a species or population, the more resilient it is to change. If there is a new source of predation, disease or environmental change, hopefully some individuals will have a trait that will allow the species to overcome it.
The loss of genetic variation is very important. The smaller the population, as with extremely diminished populations of tigers and rhinos, the higher the chance of inbreeding. With only a few individuals left, they will have no choice but to inbreed – mate with close relatives. So genetic variation decreases, the population is not resistant to change, and there is an accumulation of recessive genes. More often than not, recessive traits cause less fitness than dominant traits.