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Estimation of gene flow into fragmented populations of Bursera simaruba (Burseraceae) in the dry-forest life zone of Puerto Rico.

We examined the impact of habitat fragmentation on gene flow in populations of the neotropical tree Bursera simaruba. In particular, we compared the effectiveness of three common techniques to estimate gene flow in the context of a highly disturbed system. Paternity analysis on emerging seedlings from eight small (N = 3 to 9) stands of trees showed that between 45% and 100% of seedlings were sired from outside their stand, indicating pollen moved readily over the isolation distances examined. Based on six populations of 21-24 trees each, estimates of allozyme genetic diversity (P(s) = 73.3%; H(e) = 0.244) were higher than those reported for species with similar life history traits. Indirect, F(ST)-based gene flow estimates for these six populations yielded an estimate of 3.57 migrants per generation, although possible violations of model assumptions limit the reliability of the estimate. A twogener analysis showed pollen moved either 320 m or 361 m and that there were only 2.46 effective pollen donors per maternal tree. Despite the potential for long-distance pollen movement, seed abortion was high, especially in stands with fewer than four trees. Population size, rather than isolation distance, appears to limit reproduction in the populations examined.