(1) decomposers refer to bacteria and fungi living in saprophytes. They can decompose organic substances in animal and plant residues into inorganic substances such as carbon dioxide and water, and return them to the inorganic environment for reuse by plants, thus promoting the circulation of substances.

(2) According to the information in the table, * * has six food chains, namely plants → locusts → frogs → birds → wild domestic cats, plants → locusts → birds → wild domestic cats, plants → fish → frogs → wild domestic cats, plants → fish → frogs. It is one-way, acyclic and gradually decreases through the food chain. Therefore, the shorter the food chain, the less energy loss. So the food chain that domestic cats can get the most energy is: plants → fish → wild domestic cats.

(3) The more species in the ecosystem, the more complex the nutritional structure, the stronger the self-regulation ability and the stronger the stability. In recent years, in order to improve the water system landscape, Yuanmingyuan Ruins Park has systematically cleared a variety of wild plants, expanded the planting area of several ornamental plants such as lotus, destroyed species diversity, and is not conducive to improving the stability of Yuanmingyuan wetland ecosystem.

So the answer is: (1) decomposer;

(2) Plants → fish → wild domestic cats;

(3) unfavorable.