(d) Soil organisms, such as mycorrhizal fungi, impact the rate of succession in ecosystems. Mycorrhizal fungi have a mutualistic relationship with various species of plants and increase the plant growth and survival. Sand dunes often lack mycorrhizal fungi, and a researcher wanted to determine the effect of adding mycorrhizal species to sand on disturbed beaches on grass growth. The
researcher set up various mycorrhizal communities at disturbed beaches in different locations on the east coast of the United States,
0 Identify the independent variable in the study.