High Genetic Diversity in a Potentially Vulnerable Tropical Tree Species Despite Extreme Habitat Loss

Abstract

Over the last 150 years, Singapore’s primary forest has been reduced to less than 0.2% of its previous area, resulting in extinctions of native flora and fauna. Remaining species may be threatened by genetic erosion and inbreeding. We surveyed >95% of the remaining primary forest in Singapore and used eight highly polymorphic microsatellite loci to assess genetic diversity indices of 179 adults (>30 cm stem diameter), 193 saplings (>1 yr), and 1,822 seedlings (<1 yr) of the canopy tree Koompassia malaccensis (Fabaceae). We tested hypotheses relevant to the genetic consequences of habitat loss: (1) that the K. malaccensis population in Singapore experienced a genetic bottleneck and a reduction in effective population size, and (2) K. malaccensis recruits would exhibit genetic erosion and inbreeding compared to adults. Contrary to expectations, we detected neither a population bottleneck nor a reduction in effective population size, and high genetic diversity in all age classes. Genetic diversity indices among age classes were not significantly different: we detected overall high expected heterozygosity (He = 0.843–0.854), high allelic richness (R = 16.7–19.5), low inbreeding co-efficients (FIS = 0.013–0.076), and a large proportion (30.1%) of rare alleles (i.e. frequency <1%). However, spatial genetic structure (SGS) analyses showed significant differences between the adults and the recruits. We detected significantly greater SGS intensity, as well as higher relatedness in the 0–10 m distance class, for seedlings and saplings compared to the adults. Demographic factors for this population (i.e. <200 adult trees) are a cause for concern, as rare alleles could be lost due to stochastic factors. The high outcrossing rate (tm = 0.961), calculated from seedlings, may be instrumental in maintaining genetic diversity and suggests that pollination by highly mobile bee species in the genus Apis may provide resilience to acute habitat loss.

Conservation genetics of Magnolia acuminata, an endangered species in Canada: Can genetic diversity be maintained in fragmented, peripheral populations?

“…Collectively these results suggests that gene flow is relatively low among the fragmented populations of Ontario, particularly between Niagara and Norfolk, and this will likely impact the future genetic diversity of northerly populations of M. acuminata. High genetic diversity can be maintained in fragmented tree populations if there is high outcrossing and if pollinators are highly mobile (Noreen and Webb 2013); in other words, forest fragmentation should have deleterious effects on genetic diversity only if pollen or seed dispersal curves conform to the boundaries of forest fragments (Craft and Ashley 2007). In this way, trees that experience regular long-distance seed or pollen dispersal may not be greatly impacted by habitat fragmentation.…”

Persistence of long-distance, insect-mediated pollen movement for a tropical canopy tree species in remnant forest patches in an urban landscape

“…(Fabaceae). There was no evidence of a population bottleneck, suggesting that overall the Singapore population of K. malaccensis appeared to have resisted genetic erosion (Noreen and Webb, 2013). However, understanding connectivity between patches is necessary to obtain a more complete picture of the dynamics of gene flow and of the possible fate of remnant rainforest populations in urban landscapes; genetic analyses provide information on the reproductive patterns of the adults and the final outcome of pollen and seed dispersal patterns as shown by population genetics of established recruits.…”

“…In a previous paper, Noreen and Webb (2013) showed that despite a dramatic reduction of habitat area in Singapore in the past~150 years (to o0.2% of the original primary forest area; Corlett, 1992), genetic diversity was still high in adult, juvenile and seedling cohorts of the large canopy tree species Koompassia malaccensis Maingay ex Benth. (Fabaceae).…”

Ecological genomics of tropical trees: how local population size and allelic diversity of resistance genes relate to immune responses, cosusceptibility to pathogens, and negative density dependence

“…Previous studies of polymorphism at putatively neutral loci have shown that tropical trees tend to have reduced genetic variation in small local populations (Hamrick & Murawski 1991;Finkeldey & Hattemer 2007;Noreen & Webb 2013) and strongly increased seedlingto-adult survival for outcrossed progeny (Hufford & Hamrick 2003). Over a scale of a few tens of metres, tropical trees tend to be related, corresponding with seed dispersal kernels, with most pollen originating withiñ 100 to 1000 m of the mother tree, and detectable levels of long-distance pollen flow (Loveless 1992;Boshier et al 1995;Hardy et al 2006;Noreen & Webb 2013).…”