Summary
It is generally expected that small, isolated populations will suffer reduced 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 small populations (N=30–40), and inbreeding depression is extremely strong. We tested the prediction that reproductive output, a major component of population fitness, should be positively associated with population size. Data from a survey of 33 populations located on small islands in the St. Lawrence River, Canada and 23 populations on adjacent mainland areas supported this prediction. Population size correlated positively with reproductive output, 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 decline with increasing spatial isolation between populations by measuring the distance separating all island populations. However, reproductive output did not correlate with isolation in either year. We compared island and mainland populations to test the prediction that reproductive output should be lower for populations on small islands than those occurring in more continuous mainland habitat. In contrast to our predictions, island populations exhibited, if anything, higher reproductive output than mainland populations. We also found no support for the prediction that the positive association between population size and reproductive output should be stronger for presumably isolated populations on small islands than for those on adjacent mainland areas. While the mechanisms underlying the association between population size and fitness are impossible to identify with correlations alone, our results are consistent with the hypothesis that inbreeding can significantly reduce the fitness of natural populations.
Methodology
We sampled 56 natural populations of A. canadensis on both the Admiralty Islands of the St. Lawrence River and adjacent mainland areas in eastern Ontario, Canada. The Admiralty archipelago consists of 79 islands ranging in size from about 100 m2 to several ha. Most islands have a Precambrian Canadian Shield granite base and are characterized by open, dry forest (Rowe 1972), small cliffs and areas of bare rock, with shallow, acidic soil (Crowder 1983). We thoroughly surveyed every island during the flowering period in both 1995 and 1997 and located a total of 57 populations on 28 islands. Components of population fitness were estimated for 29 of these populations in 1995. Twenty-five of these populations plus an additional four populations were studied in 1997. A population was defined as a discrete group of plants separated from other such groups by at least 100 m. Of the 29 populations sampled, 23 consisted of distinct, well-defined patches of plants, while six consisted of several patches separated from each other by at least 10 m but no more than 100 m. Our use of 100 m as the minimum criterion for population separation reflects the way in which plants are patchily distributed across the islands. As yet, we do not have the data required for a more objective, genetic definition of population.
We also sampled 23 populations in three areas on the adjacent mainland at: (1) the Queen’s University Biological Station on Lake Opinicon at Chaffey’s Locks (10 populations); (2) around and including Charleston Lake Provincial Park near Lansdowne (10 populations); and (3) the north shore of the St. Lawrence River between Gananoque and Mallorytown Landing (3 populations). Components of population fitness were estimated for 20 mainland populations in 1995. Fourteen of these populations plus an additional three populations were studied in 1997. Because the mainland is a much larger, continuous area than the islands, a comprehensive survey of all populations in each area was not possible. However, as on the islands, all populations were discrete groups of plants separated from other groups by at least 100 m, usually much more. Population locations are in Mavraganis (1998).