What is a Plant?
Plants are essential for any ecosystem. They provide all the energy for the ecosystem, because they can get energy directly from sunlight. They use a process called photosynthesis to use energy from the sun to grow and reproduce. They also must get nutrients from the soil. Those nutrients get into the soil when decomposers break down waste and dead materials. Plants require space to grow and reproduce. The size of your ecodome will influence how much space your plants have.
All other organisms in the food chain get energy from plants, either by directly eating them as herbivores do, or by eating plant eaters, like carnivores do. Omnivores can get energy either by eating plants directly or by eating herbivores. Likewise, decomposers get energy either from plants or from the animals that eat them. Since all the energy in your ecosystem comes from plants, you'd better have a lot of them.
There are several different kinds of plants, and not all animals can eat all kinds of plants.
Grasses are only edible to herbivores. That is because the plants contain kinds of fiber that many omnivores cannot digest efficiently. Many herbivores have specially adapted stomachs that allow them to digest these plants.
Fruit-Bearing Plants make fruit. Herbivores and omnivores can both eat fruit or vegetables from plants, however. Fruit and seeds and sometimes vegetables are part of the plant's reproduction, and generally the presence of pollinators will help these fruit-bearing plants survive better and make more fruit.
Finally, there are a kind of plants called grains which make seeds that can be eaten by certain kinds of omnivores but not all. Humans and chickens can eat grain seeds. Herbivores can eat the whole plant.