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Ecological and genetic processes underlying Allee effects among tree populations in the context of divergent population histories

Ecological and genetic processes underlying Allee effects among tree populations in the context of divergent population histories


Centre for Biological Sciences, ETH Zurich, 8092 Zürich, SWITZERLAND.


Rare plant species are vulnerable to genetic erosion and inbreeding associated with small population size and isolation due to increasing fragmentation, though the degree to which these problems undermine population viability remains debated. The critically endangered long-lived tropical tree Medusagyne oppositifolia is an endemic to the Seychelles with a naturally patchy distribution. The species comprises 90 extant trees in four populations, with only the largest (78 trees) having successful recruitment. Whether recruitment failure in the three small populations is linked to genetic problems associated with fragmentation is not known. It could be shown that genetic diversity is high in three populations, with only the smallest having relatively low diversity. All populations have high genetic differentiation and low pollen and seed dispersal distances (< 100 m). Inter-population pollination crosses from the large donor population to a small recipient population resulted in higher reproductive success relative to within-population crosses. These results demonstrate the potential for genetic and ecological rescue to support conservation of plant species with limited gene flow.

Historically widespread species with limited gene flow may be particularly vulnerable to the negative genetic effects of forest fragmentation and small population size. Vateriopsis seychellarum (Dipterocarpaceae) is a formerly widespread canopy tree of the Seychelles, but is now reduced to 132 adult individuals distributed in eleven sites. A genetic inventory of all adult trees and 317 sampled progeny shows that despite its restricted range, overall genetic diversity was relatively high but the juvenile cohort had significantly lower genetic diversity than adults. Overall low historical (< 150m) and contemporary gene flow (seeds disperse < 25 m, pollen disperse < 50 m) was detected. The molecular data confirm that two populations were derived from self-fertilised offspring from a single tree. These selfed progeny produce viable offspring. Despite extreme genetic bottlenecks self-compatibility may provide V. seychellarum with some resistance to the genetic consequences of habitat fragmentation, at least in the short term.

Glionnetia sericea is an endemic and vulnerable tree species restricted to habitats between 400m and 900m in the Seychelles. Due to highly mobile and specialized pollinators (Agrius convolvuli and Cenophodes tamsi) the potential for the species persistence in a fragmented landscape is high. The results show no decrease in genetic diversity from adult to seedling stage and no increase in genetic structuring despite habitat fragmentation. Historical and contemporary gene flow between populations is apparent despite low general seed dispersal abilities. Artificial cross pollinations between populations did not lead to an enhanced fruit or seed set compared to within-population crosses. Thus, the current and historical gene flow due to a mobile pollinator seems to have maintained genetic diversity and population connectivity in a rare and restricted endemic of the Seychelles.

A literature review indicated that widespread species are more prone to negative genetic and ecological consequences of habitat fragmentation compared to historically fragmented species. Furthermore, negative effects to habitat fragmentation could also be associated with short distance gene flow in formerly widespread species. These finding could, in part, also be observed for Medusagyne oppositifolia, Vateriopsis seychellarum and Glionnetia sericea. The formerly widespread species, Vateriopsis seychellarum showed a significant loss in genetic diversity and increased genetic differentiation from adult to seedling stage due to short distance gene dispersal. The historically patchily distributed species Glionnetia sericea showed no significant negative response to habitat fragmentation but long-distance pollen flow. The historically fragmented species Medusagyne oppositifolia showed no negative response at least in its largest population. These findings demonstrate that population histories are important factors that need to be considered when exploring trees’ responses to habitat fragmentation. The positive effects of long distance gene flow, underlines the need to protect whole ecosystems and not only species, especially for insect pollinated trees.