No second chances : Demography from the forest floor to the canopy and back again
Balcázar-Vargas, M.P. ; Salguero-Gómez, Roberto ; Zuidema, P.A. - \ 2015
Journal of Ecology 103 (2015)6. - ISSN 0022-0477 - p. 1498 - 1508.
Amazon forest - Elasticity analysis - Heteropsis - Loop analysis - Matrix population models - Plant demography - Plant population and community dynamics - Secondary hemiepiphytes - Source-sink dynamics - Vegetative reproduction
Secondary hemiepiphytes have one of the most bizarre life histories among vascular plants. They germinate on the forest floor, climb onto a host tree, gradually grow up and start producing seeds if the host tree is taller than the minimum height required for reproductive onset. Because most of the host trees in the forest are too small to allow reproduction, most seedlings of secondary hemiepiphytes of the genus Heteropsis climb onto unsuitable hosts. This problem may be remediated by moving from unsuitable to suitable hosts by vegetative propagation, providing a second chance to contribute to recruitment. Still, such a strategy comes at a demographic risk of increased mortality and may thus contribute little to population growth. Here we ask what are the relative contributions of Heteropsis individuals on suitable vs. unsuitable hosts to their population growth rates (λ). We studied the demography of three species of the genus Heteropsis in the Colombian Amazon, distinguishing subpopulations on suitable and unsuitable hosts. We constructed multistate matrix population models and performed elasticity and loop analyses. For all three Heteropsis species, λ≥1, indicating that studied populations are viable. Vital rates and loop elasticities showed that individuals on suitable hosts contributed 85-98% and 80-96%, respectively. Consequently, individuals that climb directly to suitable hosts predominantly drive the population dynamics of the three studied Heteropsis species and second chances (vegetative reproduction) do not matter for demographic viability. Our results are evidence of a strong source-sink dynamic for the studied species, whereby individuals on unsuitable hosts act as sink subpopulations and individuals on suitable hosts as source subpopulations. The population growth rates of the examined Heteropsis species were strongly limited by recruitment of individuals on suitable hosts and the high proportion of seedlings climbing onto unsuitable hosts, which are demographic 'lost cases'. Synthesis. Our study illustrates a strong environmental filtering and multiple bottlenecks in the life cycle of secondary hemiepiphytes. Our findings contribute with an understudied plant life-history strategy to the extensive repertoire of better-known ecological plant strategies such as tree gap dynamics, clonal reproduction and permanent seedbanks. Secondary hemiepiphytes in the Heteropsis genus germinate on the tropical forest floor and then climb onto a host tree. At first attempt, only few seedlings find a tree that is sufficiently tall to allow development into a reproductive plant. We found that these initially successful individuals drive population growth of three Heteropsis species: second chances hardly contributed to population growth.
Reproductive strategy, clonal structure and genetic diversity in populations of the aquatic macrophyte Sparganium emersum in river systems
Pollux, B.J.A. ; Jong, M.D.E. ; Steegh, A. ; Verbruggen, E. ; Groenendael, J.M. Van; Ouborg, N.J. - \ 2007
Molecular Ecology 16 (2007)2. - ISSN 0962-1083 - p. 313 - 325.
Dispersal - Hydrochory - Sexual reproduction - Vegetative reproduction - Waterfowl - Zoochory
Many aquatic and riparian plant species are characterized by the ability to reproduce both sexually and asexually. Yet, little is known about how spatial variation in sexual and asexual reproduction affects the genotypic diversity within populations of aquatic and riparian plants. We used six polymorphic microsatellites to examine the genetic diversity within and differentiation among 17 populations (606 individuals) of Sparganium emersum, in two Dutch-German rivers. Our study revealed a striking difference between rivers in the mode of reproduction (sexual vs. asexual) within S. emersum populations. The mode of reproduction was strongly related to locally reigning hydrodynamic conditions. Sexually reproducing populations exhibited a greater number of multilocus genotypes compared to asexual populations. The regional population structure suggested higher levels of gene flow among sexually reproducing populations compared to clonal populations. Gene flow was mainly mediated via hydrochoric dispersal of generative propagules (seeds), impeding genetic differentiation among populations even over river distances up to 50 km. Although evidence for hydrochoric dispersal of vegetative propagules (clonal plant fragments) was found, this mechanism appeared to be relatively less important. Bayesian-based assignment procedures revealed a number of immigrants, originating from outside our study area, suggesting intercatchment plant dispersal, possibly the result of waterfowl-mediated seed dispersal. This study demonstrates how variation in local environmental conditions in river systems, resulting in shifting balances of sexual vs. asexual reproduction within populations, will affect the genotypic diversity within populations. This study furthermore cautions against generalizations about dispersal of riparian plant species in river systems.