The noxious weed grows more than the other plants, crowding them out of the space, blocking other plants from getting sunlight, and taking up nutrients that other plants could have used. The result is that instead of a field with a lot of biodiversity, or many different kinds of wildflowers, there is only one kind of wildflower growing, the noxious weed. Sometimes the native plant that is crowded out of an area is food for another animal. This creates a chain reaction in the area - if the native plant is no longer there to feed the animal, then the animal will leave too. Not all noxious weeds are dangerous everywhere, a wildflower that is invasive in one area or environment might not be invasive in another.
Noxious weeds come from other places such as different countries or areas.
When something comes from another place, it is called an exotic. Usually people get them from other places and plant them in their gardens or use them to stop erosion without knowing that they will be harmful. Others are carried to different places by animals, ships, cars, bugs or even wind and storms. Keep in mind that not all exotic plants are invasive, or noxious. If you're planting new plants or seeds, make sure you find out which plants are invasive for your area or environment.