section_path: Importance to humans/Negative effects
section_name: Negative effects
paragraph_text: Negative effects
  
  Weeds are commercially or aesthetically undesirable plants growing in managed environments such as in agriculture and gardens. People have spread many plants beyond their native ranges; some of these plants have become invasive, damaging existing ecosystems by displacing native species, and sometimes becoming serious weeds of cultivation.
  
  Some plants that produce windblown pollen, including grasses, invoke allergic reactions in people who suffer from hay fever. Many plants produce toxins to protect themselves from herbivores. Major classes of plant toxins include alkaloids, terpenoids, and phenolics. These can be harmful to humans and livestock by ingestion or, as with poison ivy, by contact. Some plants have negative effects on other plants, preventing seedling growth or the growth of nearby plants by releasing allopathic chemicals.