Effects of population size and isolation on reproductive output in Aquilegia canadensis (Ranunculaceae)

It is generally expected that small, isolated populations win suffer reduce d fitness due to inbreeding, yet few studies have investigated the relation between population characteristics, inbreeding and fitness. Among Ontario populations of the short-lived, perennial plant Aquilegia canadensis, large populations (N > 90 flowering plants) outcross twice as frequently as smal l populations (N = 30-40), and inbreeding depression is extremely strong. W e tested the prediction that reproductive output, a major component of popu lation fitness, should be positively associated with population size. Data from a survey of 33 populations located on small islands in the St. Lawrenc e River, Canada and 23 populations on adjacent mainland areas supported thi s prediction. Population size correlated positively with reproductive outpu t, measured as the number of seedlings produced per plant in 1995 (average r = +0.39 pooled P = 0.019), and the number of fruits per plant in 1997 (r = +0.30, P = 0.056). We also tested the prediction that fitness should decl ine with increasing spatial isolation between populations by measuring the distance separating all island populations. However, reproductive output di d not correlate with isolation in either year. We compared island and mainl and populations to test the prediction that reproductive output should be l ower for populations on small islands than those occurring in more continuo us mainland habitat. In contrast to our predictions, island populations exh ibited, if anything, higher reproductive output than mainland populations. We also found no support for the prediction that the positive association b etween population size and reproductive output should be stronger for presu mably isolated populations on small islands than for those on adjacent main land areas. While the mechanisms underlying the association between populat ion size and fitness are impossible to identify with correlations alone, ou r results are consistent with the hypothesis that inbreeding can significan tly reduce the fitness of natural populations.