Some functions tend to be compatible, such as flood control and water purification. Other functions tend to be incompatible, such as flood control and food chain support. In addition, wetlands of a given type do not have the same effectiveness in performing a given function. For example, the effectiveness of a given forested wetland for flood control depends on its size, shape, location in the watershed, and so forth. Because wetlands are constantly being affected by disturbance, their effectiveness in performing functions constantly changes.
Uplands can and do perform some of the functions performed by wetlands, such as sediment trapping.
But because wetlands are situated in the low points of the landscape or are adjacent to streams, rivers, lakes, and oceans, they are more able to perform these functions. In many cases, wetlands are the last line of defense for the protection of surface water quality. Other wetland functions, however, cannot be performed by uplands or replaced by artificial substitutes. An especially important function of wetlands is supporting rich plant diversity. Of these plants, half are restricted to, or usually occur in, wetlands.