Directed dispersal by an abiotic vector : Wetland plants disperse their seeds selectively to suitable sites along the hydrological gradient via water
Soons, Merel B. ; Groot, Arjen de; Cuesta Ramirez, M.T. ; Fraaije, Rob G.A. ; Verhoeven, Jos T.A. ; Jager, Monique de - \ 2017
Functional Ecology 31 (2017)2. - ISSN 0269-8463 - p. 499 - 508.
Anemochory - Biodiversity - Evolutionary adaptations - Hydrochory - Hydrological niche - Plant functional traits - Riparian zones - Seed dispersal - Water dispersal - Wind dispersal
Plant species around the world invest in seed dispersal by producing large numbers of seeds, with a wide range of morphological adaptations that facilitate dispersal. Not all dispersed seeds reach suitable sites, however, and plants can significantly improve their fitness by increasing the proportion of seeds arriving at suitable sites for germination and establishment. Disproportionate dispersal to suitable sites is known as 'directed dispersal'. Yet, mechanisms of directed dispersal are only known for a limited number of animal-dispersed plant species. We tested the hypothesis that directed dispersal can also be driven by abiotic vectors, such as water or wind. We used a tiered approach, combining analyses of experimental, field and literature data on wetland plant species and evaluating the potential for evolution of directed dispersal with a spatially explicit individual-based model. The data collected demonstrate that wetland plants produce seeds with adaptations to promote transportation and deposition by water towards microsites along the hydrological gradient where they germinate and establish best. Aquatic species produce seeds that sink and are transported by water as bed load towards inundated sites. In contrast, shoreline species produce seeds that float for very long periods of time so that they are eventually entrapped by shoreline vegetation or deposited at the waterline. Our model simulations confirm that the patterns we observed in nature can evolve under natural selection through adaptations in seed buoyancy. For wind dispersal, the situation is more complex. Wind does not provide directed dispersal in the strictest sense but, rather, simply appears to be the best available dispersal vector for more terrestrial wetland plant species to reach drier areas in a wet environment. Synthesis. We show that directed dispersal towards specific, suitable microsites is not exclusive to animal-dispersed plant species, but may be far more common in plants - also in species dispersed by abiotic vectors, in particular water. As water and wind are very common dispersal vectors throughout the plant kingdom, directed dispersal (and not just dispersal distance) seems to be of general importance for the ecology of plants.
Gene flow and genetic structure of the aquatic macrophyte Sparganium emersum in a linear unidirectional river
Pollux, B.J.A. ; Luteijn, A. ; Groenendael, J.M. Van; Ouborg, N.J. - \ 2009
Freshwater Biology 54 (2009)1. - ISSN 0046-5070 - p. 64 - 76.
Assignment tests - Asymmetric bidirectional dispersal - Hydrochory - One-dimensional ecosystems - Zoochory
1. River systems offer special environments for the dispersal of aquatic plants because of the unidirectional (downstream) flow and linear arrangement of suitable habitats. 2. To examine the effect of this flow on microevolutionary processes in the unbranched bur-reed (Sparganium emersum) we studied the genetic variation within and among nine (sub)populations along a 103 km stretch of the Niers River (Germany-The Netherlands), using amplified fragment length polymorphisms. 3. Genetic diversity in S. emersum populations increased significantly downstream, suggesting an effect of flow on the pattern of intrapopulation genetic diversity. 4. Gene flow in the Niers River is asymmetrically bidirectional, with gene flow being approximately 3.5 times higher in a downstream direction. The observed asymmetry is probably caused by frequent hydrochoric dispersal towards downstream locations on the one hand, and sporadic zoochoric dispersal in an upstream direction on the other. The spread of vegetative propagules (leaf and stem fragments) is probably not an important mode of dispersal for S. emersum, suggesting that gene flow is mainly via seed dispersal. Realized dispersal distances exceeded 60 km, revealing a potential for long-distance dispersal in S. emersum. 5. There was no correlation between geographical and genetic distances among the nine S. emersum populations (i.e. no isolation by distance), which may be due to the occurrence of long-distance dispersal and/or colonization and extinction dynamics in the Niers River. 6. Overall, the genetic population structure and regional dispersal patterns of S. emersum in the Niers River are best explained by a linear metapopulation model. Our study shows that flow can exert a strong influence on population genetic processes of plants inhabiting stream systems.
Intraspecific variation of seed floating ability in Sparganium emersum suggests a bimodal dispersal strategy
Pollux, B.J.A. ; Verbruggen, E. ; Groenendael, J.M. Van; Ouborg, N.J. - \ 2009
Aquatic Botany 90 (2009)2. - ISSN 0304-3770 - p. 199 - 203.
Buoyancy - Cryptic seed heteromorphism - Germination - Hydrochory - Long-distance dispersal - Seed mass
Water-mediated spread of seeds (hydrochory) plays an important role in the dispersal of aquatic plants. In this study we investigate intraspecific variation in floating ability and germination capacity of Sparganium emersum seeds in relation to seed mass, within three natural populations along the Rur River (the Netherlands-Germany). Our results suggest that S. emersum produces two types of seeds: (i) short-floating seeds (SFS) that sink within 4 weeks (approximately 71% of all seeds), and (ii) long-floating seeds (LFS) that float at least for 6 months (approximately 28% of all seeds). Our study further shows that short-floating seeds display a significantly higher germination (%) (SFS = 89.9% vs LFS = 32.6%), a faster germination rate (SFS = 8.71 ± 3.3 vs LFS = 9.32 ± 3.1 days to germination) and a higher mean seed mass (SFS = 15.17 ± 4.5 vs LFS = 11.25 ± 3.8 mg), compared to long-floating seeds. It is argued that the production of these two types of seeds by S. emersum plants, each type with a different potential for water-mediated dispersal, represents a bimodal hydrochoric dispersal strategy.
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.