Well it depends on what you want to archieve or show to the viewer. For a still its easy to trick around with alpha planes, you can have a full forest as an alpha plane for background. For the ecosystem there is generally not much you can do, the more different species you have, the higher the computing power needed to handle these. I made an ecosystem that consisted of over 250 million polygons, but it was just one terrain (with the ecosystem) and the ecosystem consisted of just one plant. Having it this way vue handles and renders the scene very fast, also lighting/shadow options can influence the rendering time a lot. In your case as the preview render is ultra slow, the problem is definitely not the render options or resolution, its lighting or ecosystem complexity. It also helps a lot to make plants or objects in the ecosystem less complex, you can reduce polygon size and save the plant as a special species for this scene. This is especially usefull if you have many plants in a far background and you use the high polygon versions for very far plants or ecosystems. You can also reduce texture resolution for your "far away" plant versions. Other tricks are scaling up a bit - bigger plants means less needed to cover the ecosystem. Also give a try rendering with cheap athmosphere, standard lighting model without radiosity/volumetric/spectral effects, so you know how much rendering time your lighting costs.

For a still just cut anything out that cant be seen, try covering computing and polygon expensive party with plants/objects and delete them. render with less anti-aliasing and do that in postwork.

Vue content creator
www.renderarmy.com