the world > the earth > earth sciences > geography > map-making > map > [noun] > line on map > biological phenomena (3)
A line (imaginary or on a map) linking places in which seasonal biological phenomena (the flowering of plants, etc.) occur at the same time.
A line (either imaginary or on a map) connecting points which migrating birds reach at the same time.
A line (imaginary or on a map) linking areas containing plants showing similar numbers of distinguishing characteristics.