The majority of marine populations are demographically open; their rep
lenishment is largely or exclusively dependent on a supply of juvenile
s from the plankton. In spite of much recent research, no consensus ha
s yet been reached regarding the importance of recruitment relative to
other demographic processes in determining local population densities
. We argue 1. that demographic theory suggests that, except under rest
rictive and unlikely conditions, recruitment must influence local popu
lation density to some extent. Therefore, 2. the question as to whethe
r the size of a particular population is limited by recruitment is mis
guided. Finally, 3. the effect of recruitment on population size can b
e difficult to detect but is nonetheless real. A major weakness of mos
t existing studies is a lack of attention to the survival of recruits
over appropriate scales of time and space. Acknowledgment of the multi
factorial determination of population density should guide the design
of future experimental studies of the demography of open populations.