Abundance
In July 2010, we set an observation point every 1.5 km along National Road 218 from Zeketai Town (43°37′–43°40′N, 83°10′–83°39′E) to Nalati Town (43°15′–43°37′N, 83°85′–84°56′E) in Xinyuan County; observation points were laid out within 0-10 km on both sides of the road, and each observation point covered 10 m × 10 m. The observation points included farmland ecotones, residential areas, grasslands, and roadside habitats. There were 25 points in each habitat (100 points in total). From 2010 to 2017, we investigated the incidence of the two Ambrosia species and calculated their distribution and abundance in each plot using Eqs. (1) and (2), respectively:
Abundance in habitat = (number of occurrences in a habitat / 25) × 100%. (1)
Total abundance = (number of occurrences in all habitats / 100) × 100%. (2)