The major impact to saltmarsh in our area has been the effort to control saltmarsh mosquitoes (see my Books page for an excellent book on Florida’s war on mosquitoes). Almost no pristine saltmarsh remains, the vast majority of it having been dredged, diked, impounded and/or filled to thwart the hordes of mosquitoes that used to breed here.
The saltmarsh mosquito won’t lay its eggs in standing water. This is a tremendously successful defense mechanism against predators. Instead it lays its eggs in moist mud or sand. When summer rains flood the eggs they hatch and finish their larval stage before predators can disperse in to get at them. Pesticides, including DDT, were used to some effect, but collateral damage as well as the fact that mosquitoes quickly began to evolve a resistance to the pesticides caused people to look elsewhere for an answer. This answer came in a plan called “Source Reduction”.
The idea behind source reduction is straightforward – simply take away the habitat that mosquitoes breed in and it will reduce the ability of the habitat to produce mosquitoes. This has been accomplished in two major ways: diking and impounding. In diking the saltmarsh wetlands are simply dredged up into dikes to remove the low relief areas that stay moist without being underwater during the dry months.
When a marsh is impounded, it is surrounded by a perimeter dike. Then the saltmarsh is literally pumped full of water to keep it flooded during mosquito season. Culverts allow the impoundment to be opened after mosquitoes have finished breeding (usually in November) to facilitate exchange of water with the lagoon until the rainy season brings the next mosquito breeding season, usually in May. Both diking and impounding have been used in the Thousand Islands.
The entire point to mosquito control by source reduction has been to decrease the amount of habitat used by mosquitoes to breed. But what about any other animals that use that habitat to breed, how would they be affected?
The best way to think of the effect source reduction has had in the islands is to imagine saltmarsh in cross-section. In Figure 1 you see a very gently sloped shoreline. The transition from water to land is a gradual change. This is the breeding habitat for saltmarsh mosquitoes, and it is reduced or eliminated by mosquito control.
In Figure 2 you see the result – the shore is now an abrupt change from very wet to very dry with almost no “in between”. The habitat has been spatially compressed, and any plants or animals that depend on that habitat have lost living/breeding space.
Habitat compression is generally a problem along any shoreline. Think of a mangrove-covered shoreline being changed into a seawall as a house is built. All the animals that depend on the mangrove have lost habitat, and all the processes that took place there
Along the beach the back dune area (and all the grasses that live there) has been virtually eliminated, and along with it so has the Beach Mouse. In the lagoon, impoundments have reduced the breeding habitat for horseshoe crabs and the diamondback terrapin. Impoundment dikes also allow raccoons to cover great distances when foraging, giving them access to nests of the few terrapins that do manage to lay eggs.