Scopelophila cataractae is a rare species that grows in environments with high concentration of copper. We analyzed the relationship between plant growth and copper ion concentration in the rainwater in a field study and the relationship between photosynthetic activity and copper ion concentration in a laboratory. We surveyed six sites with S. cataractae; these sites had rainwater copper ion concentrations ranging from 16.8 to 29.2 ppm. We found that the growth area, gametophore size, and leaf size of S. cataractae decreased as copper ion concentration increased. We observed reduced photosynthetic activity at copper ion concentrations of 20.0 ppm or higher. We discovered that the growth of S. cataractae gametophores was adversely affected by copper ions at concentrations commonly observed under copper roofs. Thus, S. cataractae may inhabit areas with high concentrations of copper ion (∼30 ppm) partly because of a selective advantage when interspecific competition is high. We recommend that copper roofs should be renewed partly rather than all at once, as aged copper roofs create environments with copper ion concentrations suitable for S. cataractae.