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Project
Ecological and genetic founder events during community assembly of semi-natural grasslands.
Progressing assembly of the above ground vegetationwas found to consist of a sequential replacement of generalist species with specialist species, which was reflected by a directional assembly at the functional trait level. Landscape configuration significantly affected this assembly, as grassland isolation slowed down assembly at both the species and the trait level. More interestingly, spatial isolation was found to act as a trait filter, independent of assembly age. We founda proportionally higher occurrence of species with light seeds and a high seed attachment potential in more isolated restoration patches, whichcould indicate that dispersal is likely more limited in isolated grasslands.
Furthermore, we compared differentiation among these restored grasslands based on the species and functional trait composition. These analyses showed that trait similarity among grasslands clearly increased with the amount of time since restoration, indicating trait convergence through time. At the species level, we found no evidence of convergence through time, with even a trend towards divergence. These results support the idea that only limited niches occur, which are only filled by species that have the appropriate functional traits, resulting in clear deterministic assembly at the trait level. Species identity, on the contrary, has no role in this niche filling. The first appropriate species to reach a restoration site will be the ones that get established, resulting in divergence of the species composition among restored grasslands. Whencomparing the genetic diversity of recent populations and old, putativesource populations of Origanum vulgare, we did not observe decreased genetic diversity in recent populations, nor inflated genetic differentiation among them.
Nevertheless, a significantly higher inbreeding coefficient was observed in recent populations, although this was not associated with negative effects on two measured proxies related to reproductive success. Our analyses indicated that colonization occurred from several source populations, with sufficient gene flow overcoming any large genetic founder effects, which likely increased the overall metapopulation viability of O. vulgare. Gene flow was nonetheless affected by the spatial configuration of the grasslands as gene flow into the recent populations mainly originated from nearby source populations.
Comparing the soil seed bank composition of restored and ancient grassland, we observedthat the species richness decreased through time. This was reflected atthe trait level by a replacement of traits associated with generalist therophytes by traits typical for chamaephytes and grassland specialists.While species differentiation remained relatively constant, trait differentiation was observed to decrease through time. Only the species composition of ancient grasslands was affected by spatial isolation. The seedbank composition of ancient grasslands was furthermore observed to be anested subset of that of young grasslands. These results suggest that community disassembly occurs in the seed bank. This implicates that directly following restoration, a large and diverse seed bank is formed, followed by a gradual net loss of species. Although theory predicts this species loss to be driven by seed persistence traits, we found that this was not the case in our system, but that species loss was likely governed by functional changes in the above ground community. This disassembly process results in one deterministic end state at the trait level, but notat the species level.
Our results suggest that several parallels in assembly patterns exist among the different organizational levels of diversity, most notably among both the species and functional trait level of the above ground community and the soil seed bank. Nevertheless, cleardifferences among the different organizational levels also remain, illustrating the importance of a multi-level approach to gain in-depth insight in community assembly following restoration. More specifically, restoration monitoring should evaluate the genetic viability of colonizing species in parallel with community assembly since colonization itself is not a guarantee for successful establishment. The soil seed bank can furthermore significantly affect above ground assembly and should for this reason be taken into account. Finally, we observed that the spatial configuration of the study area and priority effects significantly affect assembly patterns, and should therefore be included when designing restoration projects.
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Furthermore, we compared differentiation among these restored grasslands based on the species and functional trait composition. These analyses showed that trait similarity among grasslands clearly increased with the amount of time since restoration, indicating trait convergence through time. At the species level, we found no evidence of convergence through time, with even a trend towards divergence. These results support the idea that only limited niches occur, which are only filled by species that have the appropriate functional traits, resulting in clear deterministic assembly at the trait level. Species identity, on the contrary, has no role in this niche filling. The first appropriate species to reach a restoration site will be the ones that get established, resulting in divergence of the species composition among restored grasslands. Whencomparing the genetic diversity of recent populations and old, putativesource populations of Origanum vulgare, we did not observe decreased genetic diversity in recent populations, nor inflated genetic differentiation among them.
Nevertheless, a significantly higher inbreeding coefficient was observed in recent populations, although this was not associated with negative effects on two measured proxies related to reproductive success. Our analyses indicated that colonization occurred from several source populations, with sufficient gene flow overcoming any large genetic founder effects, which likely increased the overall metapopulation viability of O. vulgare. Gene flow was nonetheless affected by the spatial configuration of the grasslands as gene flow into the recent populations mainly originated from nearby source populations.
Comparing the soil seed bank composition of restored and ancient grassland, we observedthat the species richness decreased through time. This was reflected atthe trait level by a replacement of traits associated with generalist therophytes by traits typical for chamaephytes and grassland specialists.While species differentiation remained relatively constant, trait differentiation was observed to decrease through time. Only the species composition of ancient grasslands was affected by spatial isolation. The seedbank composition of ancient grasslands was furthermore observed to be anested subset of that of young grasslands. These results suggest that community disassembly occurs in the seed bank. This implicates that directly following restoration, a large and diverse seed bank is formed, followed by a gradual net loss of species. Although theory predicts this species loss to be driven by seed persistence traits, we found that this was not the case in our system, but that species loss was likely governed by functional changes in the above ground community. This disassembly process results in one deterministic end state at the trait level, but notat the species level.
Our results suggest that several parallels in assembly patterns exist among the different organizational levels of diversity, most notably among both the species and functional trait level of the above ground community and the soil seed bank. Nevertheless, cleardifferences among the different organizational levels also remain, illustrating the importance of a multi-level approach to gain in-depth insight in community assembly following restoration. More specifically, restoration monitoring should evaluate the genetic viability of colonizing species in parallel with community assembly since colonization itself is not a guarantee for successful establishment. The soil seed bank can furthermore significantly affect above ground assembly and should for this reason be taken into account. Finally, we observed that the spatial configuration of the study area and priority effects significantly affect assembly patterns, and should therefore be included when designing restoration projects.
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Date:1 Oct 2009 → 18 Oct 2013
Keywords:Ecology, Grasslands
Disciplines:Plant biology
Project type:PhD project