For the most part, transgenic species are betterment for humans; but the impact they have/can have on genetic diversity is one of its disadvantages.
The potential impact of these species depends on how well the species competes. Their ability to survive, compared to other organisms, could harm other useful species and poorly disturb the food web. If the genes of the transgenic species are surviving better in their environment, the gene pool will include a greater amount of these genes. The transgenic species affecting a gene pool in a certain population is a severe impact in itself; but genes from genetically engineered organisms have the potential to move from their original release point to affect the gene pool of other plants and animals.
In the short term of things, genetic engineering can increase genetic diversity, because it allows genes to be moved from one species to another to produce new combinations; this is a good impact that these transgenic species have on genetic diversity. However, in the long term, creating these transgenic species can have an adverse impact on genetic diversity. Selecting desired genes and creating new genetic combinations can/will reduce the original genetic material of the organisms involved; possibly to the very adverse impact on genetic diversity where the original genes may be lost