to another. Some water bodies, like taxi cabs, are mobile; some are stationary, like an ‘off-duty’ cab. And that’s where the changes from lake to pond to wetland, or the line between adjacent ecological communities, can get a little fuzzy to someone standing on the curb, er, the edge of the water.
Over what period of time do lakes become ponds? How long does it take for ponds to become wetlands? For wetlands to become meadows? The short answer is several thousand years, if nothing has interrupted (or accelerated) the natural evolution of these waters. This is called succession. Biology students learning about wetland succession in a classroom can experiment with an aquarium—starting with a mini pond or wetland habitat. For a biology teaching guide written by BioMedia (Russell) that outlines the key ingredients to such an experiment for a year-long study,click here. Limnologists say, “lakes are destined to die,” whereas ponds are the “death of a lake” and the “birth of a marsh.” For an explanation on pond succession, click here.
So how does a pond become a wetland? The first stage, called the ‘pioneer’ stage of wetland succession, starts with the pond without plant life at the bottom. Plankton, which inhabit the pond, and carry miniscule plant and animal life, arrive on the winds or wings of insects. Over time, plankton die on the pond bottom and create a mucky layer, which is rich enough for water emergent plants to grow, such as water lilies, ancient wetland plants. As water lilies form a blanket over the surface of the water, they cut off the sunlight to the bottom, killing off the submergent plants. These processes can take a variety of timeframes from a matter of years to a matter of millennia. Trees, shrubs and grasses move into the space that was once the pond and a wetland takes shape. This is a dynamic process with many variables. Some wetland ecologists have argued against the idea of wetland succession because of these variables.
Succession is not a sure thing. It does not occur with all lakes in the U.S.. (For instance, there is no scientific concern that the Great Lakes will eventually turn into ponds, or meadows.) There are many factors that can interrupt a “natural” succession process such as a changing climate, soils, drainage, land development, introduction of invasive plants or other aquatic species, phosphorus run-off (causing dissolved oxygen) or other factors.
In addition to the possible succession pattern of pond to wetland, some wetlands can be turned into ponds. In the U.S. Fish & Wildlife Service’s Status & Trends of Wetlands in the Conterminous United States 2004-2009, ponds are recognized as a type of freshwater wetland. The report indicates a net increase of 207,200 acres of ponds between 2004-2009, an increase of 3.2% in ponds nationally (FWS). The trouble with ponds, for example, farm ponds, being created while another type of freshwater wetland is lost, is that there is a difference between constructed ponds and wetlands—including natural ponds, in terms of their ecological functions. According to the Status & Trends Report, the majority of ponds in the U.S. are constructed farm ponds. Only 31% of the ponds in the lower 48 states are natural.
Mankind has a dramatic impact on natural landscapes frequently disrupting succession. This means it’s an uncertain destiny for our lakes, ponds, streams, rivers and wetlands. For those working to protect wetlands, and to harness the power of wetlands to sequester carbon and provide unique and solvent ways to fight climate change’s impact on our planet, this is cause for concern. Save wetlands, save ourselves.
Wetland Ecosystems by William J. Mitsch, James G. Gosselink et. al. (2009)
Wetland Ecology: Principles and Conservation, 2nd Edition by Paul Keddy, (2010)
Other recent blogs on wetland succession:
Conservation Maven: Study finds post-restoration wetland succession highly variable
Ian Lunt’s Ecological Research Site: There’s a wetland in my grassland
Constantine Alexander’s blog: Artificial wetlands can provide benefits over the long haul(on Bill Mi