A fascinating ecosystems in the world as it actually exists. The one where elk chose to give birth next to highways because of the lesser number of bears.
All the difficulties in the classic model of an ecosystem that stays the same forever with interactions. Even the addition of "disturbance" with the theory that the forest could, say, have regions regrowing from forest fire was not enough. There are forests that look primordial, but have trees that could grow for a millennium, where none of them are older than 700 years -- when the climate changed.
Whites radically changed the environment of the Americas without even trying to settle. The diseases they brought killed maybe 95% of the population, and that produced enormous impacts. The orchards they grew of nut trees went back to the wild. The enormous herd of buffalo that Lewis and Clark reported were probably there because their biggest predator had been taken out. Not that the Americas had not suffered massive changes from them before. Their arrival brought about the extinction of the megafauna. Most of those we still have came from Asia at the same time. Forest managed to grow much more widely without . Then, most other arrivals of humans saw the megafauna died out. New Zealand -- the ancestors gave up their chickens to hunt such birds as the flightless and enormous moa. Then they worked their way through the other hapless birds, and in due course found themselves facing serious hunger issues. Giving up chickens had not been wise.
This produces interesting arguments: ecologists who argue that feral burros and mustangs are wonderful additions because they revert the Southwest to the pre-human ecosystem, which did have equines. At least as well as can be done. Some want to introduce camels, elephants, cheetahs as substitutes, too, and if people object to cheetahs -- why, Africans have to live with large, dangerous predators for ecological reasons. Why should North Americans be exempt?
Goes into the subject of those innocuous islands that get so wrecked when they get contact. Birds that have lost the ability to fly. Roses and raspberries that lose their thorns. Mammals, like those in Australia, that got stupid because Australia was so harsh that being smart was too much a drain on the system. (This woke the world-building theorist in me. I wonder how large they could be. New Zealand with all its flightless birds was pretty big. Is there a theoretical bound before the ecosystem turns up nature red in tooth and claw? Could you have an innocuous idyllic planet? Especially with terraforming?)
And the marvelous novel ecosystems of invasive plants. Ther was one Pacific island where two birds and a bat were saved from extinction, spurred by the removal of native trees, by people planting exotic trees. Which grew faster than native ones, which probably would not have grown fast enough to save them. How exotic birds in Hawaii are doing most of the fruit seed dispersal, and its pine forests often have more diversity and richer understories than the native trees. An ecologist arguing for tantan trees on the grounds that the native understory needs the canopy they provide.
And many more fascinating ecological topics.