This is a post by an ecology PHD student on Reddit:
"Ecology" is such a huge field that covers a lot of different systems and scales, so I don't know if it's possible to identify the biggest misconception, but a few come to mind:
People not understanding that honeybees are (at least in North America) basically livestock, not native pollinators. And that if you want to "save the bees", focusing on honeybees is one of the worst ways to do it
Thinking that "forest" is the default healthiest ecosystem and that we should plant trees everywhere to save the planet
The impact of cats on small bird and mammal populations. Cats kill BILLIONS of animals every year, mostly just for fun. But many people seem to think that it's "natural" to let their cat roam outdoors and hunt whatever it comes across.
Other small and more specific misconceptions include:
Being upset about culling overabundant deer populations and thinking that we should just "relocate" them
Extremes of opinion on non-native plant species; either that all non-native plants are invasive and have to be removed, or that there's no such thing as an invasive plant and that we should just leave them all alone