The destruction of wetlands would indeed lead to less biodiversity. The first reason is because wetlands are home to a dizzying array of lifeforms, many of which do not live anywhere else. Insects, plants, fish, and amphibians in particular flourish in wetlands. Within these habitats, which are often isolated from each other, we find species uniquely adapted, that cannot survive outside that particular set of conditions. One example is the famed Venus flytrap, which lives only in a small swath of wetlands in North and South Carolina. The destruction of wetlands would lead to the destruction of this species. This is only one example of thousands of species that make wetlands their home. So the destruction of these habitats, whether drained for farmland or development, would naturally lead to a reduction in biodiversity.
Another reason that biodiversity might be affected by the destruction or pollution of wetlands is that many species that do not make such habitats their home for their entire lives use wetlands for spawning and for growth at the larval stage. Many varieties of fish, crustaceans and mollusks, including many that represent a major part of human diets, spend this crucial period in coastal wetlands. The decline in Chesapeake Bay blue crabs, for instance, formerly crucial to that region's economy, has been traced to drained and polluted wetlands upstream, where the animals spawn, as well as to the increased pollution of the bay, itself related to wetlands destruction, and to overfishing. So a loss of wetlands results in, or at least contributes to, a loss of biodiversity.