Some species have been around so long that we forget that they are not from around here. Colonization by European settlers and shipping trade introduced many new species of animals, plants and pests to North America. Earthworms are a good example. They were brought over to North America by European settlers. Northern forests have no native earthworms. Much of the fallen litter in pre-earthworm forests would not get broken down year to year. Now, earthworms can break down that litter in a matter of months, drastically changing the soil and available nutrients. Soil properties are usually the first filter for plant germination and growth, dictating if new plants will move in or if current species will be pushed out. In extreme cases, forest undergrowth is considerably less than in non-earthworm forests and allows for other non-native species to move in.
So, what about the oxeye daisy? It’s hard to imagine fields and ditches without them. It is one of those hitchhikers that came from Europe, brought along by gardeners who admire their beauty. It outcompetes native species of grass and flowers because it multiplies into dense mats through its underground “stem” (rhizome). It is among the most common weeds in North America but flies under the radar since it has been around before any of us were born.