Spatio-temporal connectivity of O. aquatica in a dynamic landscape
Our results on spatiotemporal connectivity of O. aquatica across kettle holes are important in two major respects: first, we found genetic differentiation and restricted gene-flow among sampling sites in all cohorts, both in the extant populations and the soil seed bank, and across varying spatial scales from a few hundred meters up to several kilometers. And second, the general connectivity patterns and levels of genetic differentiation in our study system persist over time. This is supported by the pronounced isolation by distance pattern observed in all cohorts and a clear separation of the majority of local populations (across both extant plants and seed banks) into unique clusters. Accordingly, the spatial structuring observed in the viable dormant propagule pool is well reflected in aboveground O. aquatica populations. In particular, long-established local populations exhibit nearly unchanged allele frequencies over the time period studied here. However, variance partitioning, population structure analyses, and inferred migration events suggest some admixture across sites.
STRUCTURE clustering and GENECLASS2 assignment tests showed an increase of connectivity between spatially adjacent patches in a few cases, but also indicate a (re-) colonization of patches with presumably limited or absent seed banks (i.e., those sites where no germinations were observed in any of the soil bank samples). Interestingly, of the limited number of recent migrants inferred, a considerable proportion were long-distance migrants.
Individual sites harbor genetically distinct populations with repeated local recruitment and formation of persistent soil seed banks. At individual sites, genetic diversity stochastically varies among cohorts, presumably reflecting varying degrees of drift due to local environmental fluctuations. However, genetic diversity measures do not exhibit any consistent trend over the time period covered here (presumably several decades). Genetic variation partitioning reveals substantial divergence among local populations, despite occasional outcrossing with variants from nearby sites, as well as rare, but detectable, long-distance dispersal (Table 2). Overall, many populations in the habitat network examined store a long-term viable, sufficiently diverse seed bank and therefore are not primarily driven by recurrent extinction/recolonization dynamics as assumed by metapopulation theory in its strict sense 44,45. However, variation in functional connectivity, including short-term changes in the occurrence of local populations (as, e.g., in P01, P07) and genetically inferred dispersal across some kettle holes comprise features of metapopulation dynamics in this historically scattered landscape. In particular, the occurrence of O. aquatica at patches with no detectable seed bank is pointing to recruitment from transient seedbanks or recent colonization.
These incidences may comprise previously unoccupied patches, where recent immigration could be facilitated by contemporary environmental conditions that increase dispersal along stepping-stone corridors, as shown by the dispersal paths revealed by the resistance analyses. Such colonization would require some recent temporal synchronization of favorable conditions among donor and recipient patches 46. Water regimes among individual kettle holes are typically asynchronous. However, a plausible scenario for a synchronization in the water regime could be an intense recent spring flooding in a range of patches, followed by a simultaneous dry fall in summer. Kettle holes in this region were reported to overflow after snowmelt and to dry up during the summer period 47,48. Among such flooded patches, seeds of O. aquatica may be repeatedly transported by mobile linkers, such as water birds 49,50, and accumulate throughout the season. Such processes would promote regional connectivity in the long term. An alternative scenario is that transient local populations emerge occasionally based on randomly occurring mass effects, given that some patches may provide less suitable conditions for seed longevity 19. This would prevent the formation of a soil seed bank in the long run. Such ephemeral populations (as represented by our grey brown genetic cluster−P13,P14; cf. Figure 2) may emerge as a result of a 'recolonization rescue' as described by Hanski 51. A sudden short-term emergence of O. aquatica is likely since the species can form monotypic stands from only a few individuals if conditions are favorable 52. A community study conducted in 2015 in the same study area suggests mass effect processes to be prevalent in non-flooded patches 53, as these patches may experience (1) temporarily elevated mobile link activity and/or (2) prolonged dry periods associated with higher seed or seedling mortality for amphibic plant species. However, the genetic consequences of these two mass effect scenarios will differ. With elevated mobile link activity after sudden flooding, we may observe a higher genetic diversity in populations emerged from diverse migrant seed variants accumulated in short-term standing waters, as, e.g., at P14 and P16. Contrary, when recovering from high mortality, large populations may have arisen from recent immigration of only a few variants in drier environments. Here, founder effects/bottlenecks may cause reduced genetic diversity 54, as observed in the most spatially isolated populations, i.e., P12 and P10 (Supplementary Table s1). Still, this effect might be mitigated if gene flow via pollen is maintained between patches 55. Any of these scenarios could apply to those local populations where we could not detect soil seed banks.
Both scenarios would be in line with the observed isolation-by-distance (IBD) pattern, i.e., an increase of genetic differentiation with geographic distance. This IBD population structure suggests a stepping-stone dynamic in our study system, which is further corroborated by contemporary dispersal being predominantly inferred among adjacent sites.
With our methodology, we cannot fully exclude that scarce seeds in the soil remained undetected. Specifically, if a population without a detected seed bank forms a unique cluster, it could originate from such scarce seeds bearing a local genotype. However, the high seed production of our studied species and our small-scale sampling of both soil and extant populations make this interpretation less probable. Therefore, we consider it more likely that those populations which form their own genetic cluster but do not hold a detectable seed bank have been colonized by specimens from a population not sampled in our study.
In future studies, it would be worth to monitor the longevity of seed banks in different hydrological regimes to identify factors that limit seed viability. Such information could improve the reliability of predictions about how stable the observed population connectivity may be under altered environmental conditions.
Consequences Of Spatial Isolation And Local Habitat Features
Taking into account that aquatic plants cannot colonize the surrounding agriculturally utilized matrix, kettle holes play a key role as connective landscape structures with stepping-stone functions56, as confirmed by the inferred isolation-by-distance pattern in our study system. Populations with a long-lived seed bank represent starting points for recolonization rescue, from where local variation slowly propagates across the landscape. This is in agreement with the significant latitudinal cline detected by spatially explicit analyses, separating northern and southern populations with an increased admixture at the geographical center. As precipitation is a major driver of kettle hole hydrological conditions, increased levels of connectivity between adjacent habitat patches are likely to be related to local environmental synchrony 57,58. Small-scale patch configuration effects on local diversity were already shown in an earlier study on plant metacommunities reporting a decline in species richness with increasing spatial isolation and decreasing patch area 59,60. We confirmed noticeable effects of isolation on local genetic diversity at the population level (Fig. 6) and found inbreeding to increase with distance from the nearest neighbor patch, pointing towards the relative importance of mobile link movement 61,62. Limited pollinator availability in isolated patches74 may lead to increased levels of selfing and therefore elevated inbreeding at these sites. A reduced primary seed dispersal distance causing clustered occurrences of siblings close to their parent plant would also explain enhanced inbreeding at remote habitat patches likely less frequented by large mobile linkers. The effect of patch size on local genetic diversity was negligible. As a result of increased inter-specific competition, seedling recruitment in O. aquatica is likely to be suppressed at late successional stages when more competitive species emerge. This process is likely more pronounced in larger patches 60,63 and ultimately results in an ephemeral population-built-up. Hence, unlike in populations that undergo progressive fragmentation of continuous habitat, the extent of genetic variation and connectivity in our study system is less a matter of patch size 59 than a matter of small-scale environmental conditions and historical population establishment in the naturally scattered landscape.