A major factor influencing species richness is spatial heterogeneity, since the greater the spatial heterogeneity, the greater the number of niches, and the more opportunity for a successful invasion by a species. The hydrologic regime is not the only factor affecting spatial diversity, but it is a major one. First, flooding waters provide a vehicle for the movement of materials, either dissolved or suspended, which is absent in terrestrial environments. This may have the effect of minimizing the spatial diversity because of uniform mixing. Therefore, wetlands subject to sheet flow by flooding waters tend to be quite uniform and to have large areas of monospecific stands. Examples are the large reed (Phragmites communis) marshes at the mouth of the Mississippi River, the sawgrass wetlands of the Florida Everglades, and blanket bogs of Minnesota.

On the other hand, the hydrologic regime can contribute to elevational and substrate differences, which are a chief source of species diversity in wetlands. As rising waters crest over stream banks, current velocity is reduced, resulting in a gradient of elevation and sediment grain size. The secondary effect is typical plant zonation, with different species occurring at different elevations (wetness of the environment). Diversity generally seems to increase with elevation and therefore is a function of flooding duration and depth.

Plant species richness increases with increasing water velocity (and probably renewal rate) in northern Minnesota peatlands. Thus, the hydrologic regime can lead to either uniformity or to diversity, depending on the regime of a specific wetland landscape. Whether diversity increases through time is often determined by whether the developing plants have a reciprocal effect on the hydrologic regime.

The following diagram expresses the relationship between biodiversity total annuall production for three wetland communities each of which has a particular assemby of species dependent upon the wetness of the environment.

Cattail community: Typha angusopholia emerging from water-logged substrate

Sedge swale: Carex laeviconica dominant

Sedge scrub