Analysis of test questions: A: Freshwater ecosystem refers to the ecosystem formed by the interaction between freshwater biota and freshwater bodies, which is divided into still water and running water. The former refers to freshwater lakes, swamps, ponds and reservoirs. The latter refers to rivers, streams and canals. There is no characteristic of water and soil conservation, so this option does not meet the meaning of the question.

B: The marine ecosystem consists of marine biological community and marine environment. There are many kinds of animals, most of which can swim in the water and have no function of conserving water and soil, so this option does not meet the meaning of the question;

C: Forest ecosystems are mainly distributed in humid and semi-humid areas, and there are many kinds of animals and plants. It is the most biodiversity area in the earth's environment. Forests play an important role in water conservation and soil and water conservation, and are called green reservoirs. So this option fits the question.

D: Wetland ecosystem is an ecosystem formed under the condition of excessive moisture and humidity. Swamp is a typical wetland ecosystem, which is dominated by swamp plants and has a wide variety of animals. Wetlands have the functions of purifying water sources, storing floods and fighting drought. It has no function of conserving water and soil, so this option does not meet the meaning of the question.