They arise also through complex interactions with substratum conditions in which contributions to the ground cover by subordinate plants may be expected to have both positive and negative effects (Cavers & Harper 1967; Ross & Harper 1972; Grubb 1977; Connell & Slatyer 1977; Noble & Slatyer 1979; Pickett & White 1985; Bazzaz 1986; Maguire & Forman 1983; Burke & Grime 1996).