In low-density populations of Ambrosia trifida plants grown in monoculture in the field all individuals survived to maturity. The plants had high rates of leaf birth and death. However, under high densities there was death of whole plants and a decline in leaf birth - and death-rates. Furthermore, under high densities leaves lived longer than did leaves under low density. Thus, regulation of populations in response to density occurs at two levels: whole plants and individual leaves.