Regional and fine-scale local adaptation in salinity tolerance in Daphnia inhabiting contrasting clusters of inland saline waters.

Understanding the spatial scales at which organisms can adapt to strong natural and human-induced environmental gradients is important. Salinization is a key threat to biodiversity, ecosystem functioning and the provision of ecosystem services of freshwater systems. Clusters of naturally saline habitats represent ideal test cases to study the extent and scale of local adaptation to salinization. We studied local adaptation of the water flea Daphnia magna, a key component of pond food webs, to salinity in two contrasting landscapes-a dense cluster of sodic bomb crater ponds and a larger-scale cluster of soda pans. We show regional differentiation in salinity tolerance reflecting the higher salinity levels of soda pans versus bomb crater ponds. We found local adaptation to differences in salinity levels at the scale of tens of metres among bomb crater pond populations but not among geographically more distant soda pan populations. More saline bomb crater ponds showed an upward shift of the minimum salt tolerance observed across clones and a consequent gradual loss of less tolerant clones in a nested pattern. Our results show evolutionary adaptation to salinity gradients at different spatial scales, including fine-tuned local adaptation in neighbouring habitat patches in a natural landscape.

The chytrid insurance hypothesis: integrating parasitic chytrids into a biodiversity-ecosystem functioning framework for phytoplankton-zooplankton population dynamics.

In temperate lakes, eutrophication and warm temperatures can promote cyanobacteria blooms that reduce water quality and impair food-chain support. Although parasitic chytrids of phytoplankton might compete with zooplankton, they also indirectly support zooplankton populations through the "mycoloop", which helps move energy and essential dietary molecules from inedible phytoplankton to zooplankton. Here, we consider how the mycoloop might fit into the biodiversity-ecosystem functioning (BEF) framework. BEF considers how more diverse communities can benefit ecosystem functions like zooplankton production. Chytrids are themselves part of pelagic food webs and they directly contribute to zooplankton diets through spore production and by increasing host edibility. The additional way that chytrids might support BEF is if they engage in "kill-the-winner" dynamics. In contrast to grazers, which result in "eat-the-edible" dynamics, kill-the-winner dynamics can occur for host-specific infectious diseases that control the abundance of dominant (in this case inedible) hosts and thus limit the competitive exclusion of poorer (in this case edible) competitors. Thus, if phytoplankton diversity provides functions, and chytrids support algal diversity, chytrids could indirectly favour edible phytoplankton. All three mechanisms are linked to diversity and therefore provide some "insurance" for zooplankton production against the impacts of eutrophication and warming. In our perspective piece, we explore evidence for the chytrid insurance hypothesis, identify exceptions and knowledge gaps, and outline future research directions.

Spatial insurance against a heatwave differs between trophic levels in experimental aquatic communities.

Climate change-related heatwaves are major threats to biodiversity and ecosystem functioning. However, our current understanding of the mechanisms governing community resistance to and recovery from extreme temperature events is still rudimentary. The spatial insurance hypothesis postulates that diverse regional species pools can buffer ecosystem functioning against local disturbances through the immigration of better-adapted taxa. Yet, experimental evidence for such predictions from multi-trophic communities and pulse-type disturbances, like heatwaves, is largely missing. We performed an experimental mesocosm study to test whether species dispersal from natural lakes prior to a simulated heatwave could increase the resistance and recovery of plankton communities. As the buffering effect of dispersal may differ among trophic groups, we independently manipulated the dispersal of organisms from lower (phytoplankton) and higher (zooplankton) trophic levels. The experimental heatwave suppressed total community biomass by having a strong negative effect on zooplankton biomass, probably due to a heat-induced increase in metabolic costs, resulting in weaker top-down control on phytoplankton. While zooplankton dispersal did not alleviate the negative heatwave effects on zooplankton biomass, phytoplankton dispersal enhanced biomass recovery at the level of primary producers, providing partial evidence for spatial insurance. The differential responses to dispersal may be linked to the much larger regional species pool of phytoplankton than of zooplankton. Our results suggest high recovery capacity of community biomass independent of dispersal. However, community composition and trophic structure remained altered due to the heatwave, implying longer-lasting changes in ecosystem functioning.

A process-based metacommunity framework linking local and regional scale community ecology.

The metacommunity concept has the potential to integrate local and regional dynamics within a general community ecology framework. To this end, the concept must move beyond the discrete archetypes that have largely defined it (e.g. neutral vs. species sorting) and better incorporate local scale species interactions and coexistence mechanisms. Here, we present a fundamental reconception of the framework that explicitly links local coexistence theory to the spatial processes inherent to metacommunity theory, allowing for a continuous range of competitive community dynamics. These dynamics emerge from the three underlying processes that shape ecological communities: (1) density-independent responses to abiotic conditions, (2) density-dependent biotic interactions and (3) dispersal. Stochasticity is incorporated in the demographic realisation of each of these processes. We formalise this framework using a simulation model that explores a wide range of competitive metacommunity dynamics by varying the strength of the underlying processes. Using this model and framework, we show how existing theories, including the traditional metacommunity archetypes, are linked by this common set of processes. We then use the model to generate new hypotheses about how the three processes combine to interactively shape diversity, functioning and stability within metacommunities.

Grazing resistance and poor food quality of a widespread mixotroph impair zooplankton secondary production.

Growing evidence suggests that global climate change promotes the dominance of mixotrophic algae especially in oligotrophic aquatic ecosystems. While theory predicts that mixotrophy increases trophic transfer efficiency in aquatic food webs, deleterious effects of some mixotrophs on consumers have also been reported. Here, using a widespread mixotrophic algal genus Dinobryon, we aimed to quantify how colonial taxa contribute to secondary production in lakes. We, therefore, studied the dietary effects of Dinobryon divergens on Cladocera (Daphnia longispina) and Copepoda (Eudiaptomus gracilis), representing two main taxonomic and functional groups of zooplankton. In feeding experiments, we showed that Dinobryon was largely grazing resistant and even inhibited the uptake of the high-quality reference food in Daphnia. Eudiaptomus could to some extent compensate with selective feeding, but a negative long-term food quality effect was also evident. Besides, Eudiaptomus was more sensitive to the pure diet of Dinobryon than Daphnia. Low lipid content and high C:P elemental ratio further supported the low nutritional value of the mixotroph. In a stable isotope approach analysing a natural plankton community, we found further evidence that carbon of Dinobryon was not conveyed efficiently to zooplankton. Our results show that the increasing dominance of colonial mixotrophs can result in reduced dietary energy transfer to consumers at higher trophic levels. In a wider perspective, global climate change favours the dominance of some detrimental mixotrophic algae which may constrain pelagic trophic transfer efficiency in oligotrophic systems, similarly to cyanobacteria in eutrophic lakes.

Habitat loss over six decades accelerates regional and local biodiversity loss via changing landscape connectance.

When habitats are lost, species are lost in the region as a result of the sampling process. However, it is less clear what happens to biodiversity in the habitats that remain. Some have argued that the main influence of habitat loss on biodiversity is simply due to the total amount of habitat being reduced, while others have argued that fragmentation leads to fewer species per site because of altered spatial connectance among extant habitats. Here, we use a unique data set on invertebrate species in ponds spanning six decades of habitat loss to show that both regional and local species richness declined, indicating that species loss is compounded by habitat loss via connectivity loss, and not a result of a sampling process or changes in local environmental conditions. Overall, our work provides some of the clearest evidence to date from a longitudinal study that habitat loss translates into species loss, even within the remaining habitats.

Phytoplankton Species Richness along Coastal and Estuarine Salinity Continua.

The high number of freshwater species at low salinity and the correspondingly high number of marine species at high salinity enveloping a conspicuous richness minimum at intermediate salinities has shaped our basic understanding of biodiversity along a coastal salinity gradient for almost 80 years. Visualized as the Remane curve, this iconic concept was originally based on sedentary macroinvertebrates in the Baltic Sea. To what extent the concept can be generalized, particularly to free-drifting organisms, is currently debated. Here we use approximately 16,000 phytoplankton samples from two large coastal ecosystems-the Baltic Sea and Chesapeake Bay-to analyze the relationship between salinity and phytoplankton species richness. Alpha diversity showed a consistent variation along the salinity gradient, with a minimum at mesohaline salinities of around 7-9. Rarefied species pools at narrow salinity intervals also showed reduced diversity at intermediate salinities, surrounded by high richness toward both ends of the gradient. The cumulative likelihood of species presence validated the minimum at intermediate salinities. Community composition changed abruptly at the α diversity minimum in the Baltic Sea, while it changed gradually along the salinity gradient in Chesapeake Bay. We conclude that the Remane concept is in every respect valid for phytoplankton.

Wartime scars or reservoirs of biodiversity? The value of bomb crater ponds in aquatic conservation.

Considering the ongoing loss of aquatic habitats, anthropogenic ponds are gaining importance as substitute habitats. It is therefore important to assess their functioning in comparison to their natural precursors. Here we assess the biodiversity value of sodic bomb crater ponds by comparing their gamma diversity to that of natural reference habitats, astatic soda pans, and assess their importance on the landscape level by studying alpha and beta diversity. We studied aquatic organisms ranging from algae to vertebrates in a dense cluster of 54 sodic bomb crater ponds in Central Europe. Despite the overall small area of the pond cluster, gamma diversity was comparable to that found in surveys of natural habitats that encompassed much wider spatial and temporal scales. We also found a considerable number of species shared with reference habitats, indicating that these anthropogenic habitats function as important refuge sites for several species that are associated with the endangered soda pans. Moreover, we found a number of regionally or worldwide rare species. Among the components of beta diversity, species replacement dominated community assembly. Individual ponds contributed similarly to beta diversity in terms of replacement, being equally important for maintaining high gamma diversity and emphasising the role of the pond network rather than individual ponds. This pattern was seen in all studied groups. Bomb crater ponds therefore acted as important contributors to aquatic biodiversity. Considering the tremendous losses of ponds throughout Europe, anthropogenic ponds should be taken into consideration in nature conservation, especially when occurring in pond networks.

Wind dispersal results in a gradient of dispersal limitation and environmental match among discrete aquatic habitats.

Directional dispersal by wind and other dispersal agents may generate spatial patterns in passively dispersing metacommunities which cannot be detected by classical eigenvector methods based on Euclidean distances. We analysed zooplankton communities (Rotifera, Cladocera, Copepoda) in a cluster of soda pans distributed over a short spatial scale of 18 km and tested explicitly for directional signals in their spatial configuration. The study area is exposed to a prevailing northwestern wind direction. By applying asymmetric eigenvector maps (AEM), we were able to identify corresponding directionality in the spatial structure of communities. Furthermore, the match between community composition and environmental conditions exhibited a spatial pattern consistent with the prevailing wind corridor, with best match found downwind the dominant wind direction. We also found that classical eigenvector methods based on Euclidean distances underestimated the role of spatial processes in our data. Our study furthermore shows that dispersal limitation may constrain community assembly in highly mobile organisms even at spatial scales below 5 km.

Mucilage protects the planktonic desmid Staurodesmus sp. against parasite attack by a chytrid fungus.

Zoosporic fungi of the phylum Chytridiomycota are ubiquitous parasites of phytoplankton in aquatic ecosystems, but little is known about phytoplankton defense strategies against parasitic chytrid attacks. Using a model chytrid-phytoplankton pathosystem, we experimentally tested the hypothesis that the mucilage envelope of a mucilage-forming desmid species provides protection against the parasitic chytrid Staurastromyces oculus. Mucilage-forming Staurodesmus cells were not accessible to the chytrid, whereas physical removal of the mucilage envelope rendered the same Staurodesmus sp. strain equally susceptible to chytrid infections as the original non-mucilage-forming host Staurastrum sp. Epidemic spread of the parasite only occurred in Staurastrum sp., whereas non-mucilage-bearing Staurodesmus sp. allowed for co-existence of host and parasite, and mucilage-bearing Staurodesmus sp. caused parasite extinction. In addition to the mucilage defense barrier, we also demonstrate the ability of both Staurastrum sp. and Staurodesmus sp. to resist infection by preventing chytrid development while still remaining viable and being able to reproduce and thus recover from an infection. This study extends our knowledge on phytoplankton defense traits and the functional role of mucilage in phytoplankton as a physical barrier against fungal parasites.

Top-down structuring of freshwater bacterial communities by mixotrophic flagellates.

Mixotrophic and heterotrophic protists hold a key position in aquatic microbial food webs. Whereas they can account for the bulk of bacterivory in pelagic systems, the potential structuring effect of these consumers on bacterial communities is far from clear. We conducted short-term grazing experiments to test for the overall impact on bacterial community structure and possible prey preferences of phagotrophic protists. The protist taxa selected for this study include three mixotrophic flagellates, comprising two obligate- and one facultative mixotroph, and one phagoheterotrophic flagellate lacking phototrophic capacity. Bacterioplankton from seven different lakes were enriched and used to represent semi-natural prey communities. Our study demonstrated protist strain specific impacts on bacterial community composition linked to grazing. The three mixotrophs had variable impacts on bacterial communities where the two obligate mixotrophs exhibited lower grazing rates, while showing a tendency to promote higher bacterial diversity. The phagoheterotroph displayed the highest grazing rates and structured the bacterial communities via apparent selective grazing. Consistent selectivity trends were observed throughout the experiments, such as the apparent avoidance of all flagellates of Actinobacteria, and high grazing on dominant Burkholderiales taxa. However, there was no consistent "fingerprint" of mixotrophic grazing on prey communities, but the structuring impact rather seemed to depend on the trophic mode of the individual protist taxa, i.e. their dependence on phototrophy vs. phagotrophy. Our findings highlight the differential structuring impact of protist taxa on bacterial communities which may have important ecological implications, for example during periodic dominance of obligate mixotrophic bacterivores in changing lake ecosystems.

Chytrids enhance Daphnia fitness by selectively retained chytrid-synthesised stearidonic acid and conversion of short-chain to long-chain polyunsaturated fatty acids.

Chytrid fungal parasites convert dietary energy and essential dietary molecules, such as long-chain (LC) polyunsaturated fatty acids (PUFA), from inedible algal/cyanobacteria hosts into edible zoospores. How the improved biochemical PUFA composition of chytrid-infected diet may extend to zooplankton, linking diet quality to consumer fitness, remains unexplored.Here, we assessed the trophic role of chytrids in supporting dietary energy and PUFA requirements of the crustacean zooplankton Daphnia, when feeding on the filamentous cyanobacterium Planktothrix.Only Daphnia feeding on chytrid-infected Planktothrix reproduced successfully and had significantly higher survival and growth rates compared with Daphnia feeding on the sole Planktothrix diet. While the presence of chytrids resulted in a two-fold increase of carbon ingested by Daphnia, carbon assimilation increased by a factor of four, clearly indicating enhanced carbon transfer efficiency with chytrid presence.Bulk carbon (δ 13C) and nitrogen (δ 15N) stable isotopes did not indicate any treatment-specific dietary effects on Daphnia, nor differences in trophic position among diet sources and the consumer. Compound-specific carbon isotopes of fatty acids (δ 13CFA), however, revealed that chytrids bioconverted short-chain to LC-PUFA, making it available for Daphnia. Chytrids synthesised the ω-3 PUFA stearidonic acid de novo, which was selectively retained by Daphnia. Values of δ 13CFA demonstrated that Daphnia also bioconverted short-chain to LC-PUFA.We provide isotopic evidence that chytrids improved the dietary provision of LC-PUFA for Daphnia and enhanced their fitness. We argue for the existence of a positive feedback loop between enhanced Daphnia growth and herbivory in response to chytrid-mediated improved diet quality. Chytrids upgrade carbon from the primary producer and facilitate energy and PUFA transfer to primary consumers, potentially also benefitting upper trophic levels of pelagic food webs.

Chytrids alleviate the harmful effect of heat and cyanobacteria diet on Daphnia via PUFA-upgrading.

Chytrid fungal parasites increase herbivory and dietary access to essential molecules, such as polyunsaturated fatty acids (PUFA), at the phytoplankton-zooplankton interface. Warming enhances cyanobacteria blooms and decreases algae-derived PUFA for zooplankton. Whether chytrids could support zooplankton with PUFA under global warming scenarios remains unknown. We tested the combined effect of water temperature (ambient: 18°C, heat: +6°C) and the presence of chytrids with Daphnia magna as the consumer, and Planktothrix rubescens as the main diet. We hypothesized that chytrids would support Daphnia fitness with PUFA, irrespective of water temperature. Heating was detrimental to the fitness of Daphnia when feeding solely on the Planktothrix diet. Chytrid-infected Planktothrix diet alleviated the negative impact of heat and could support Daphnia survival, somatic growth and reproduction. Carbon stable isotopes of fatty acids highlighted a ~3x more efficient n-3 than n-6 PUFA conversion by Daphnia feeding on the chytrid-infected diet, irrespective of temperature. The chytrid diet significantly increased eicosapentaenoic acid (EPA; 20:5n-3) and arachidonic acid (ARA; 20:4n-6) retention in Daphnia. The EPA retention remained unaffected, while ARA retention increased in response to heat. We conclude that chytrids support pelagic ecosystem functioning under cyanobacteria blooms and global warming via chytrids-conveyed PUFA toward higher trophic levels.

Life-history omnivory in the fairy shrimp Branchinecta orientalis (Branchiopoda: Anostraca).

Very little is known about the feeding of naupliar and juvenile life stages of omnivorous fairy shrimps (Crustacea: Anostraca). Here, we aim to reveal whether the fairy shrimp Branchinecta orientalis is an ontogenetic omnivore and at which age and ontogenetic stage they gain the ability to feed on zooplankton. We assess how food uptake rates change with age until reaching maturity by providing algae (pico- and nanoplanktonic unicellular algae) and zooplankton (rotifers and copepod nauplii) as food in individual experiments. We found that the fairy shrimp B. orientalis started to feed on both types of algal prey immediately after hatching. Nanoplanktonic algae likely represented the most important food source until reaching maturity. Moreover, fairy shrimps started to feed on zooplankton already when they were 7 days old. Slow-moving rotifers gradually gained importance in the fairy shrimp diet with time. Our results reveal an ontogenetic change in the prey spectrum of fairy shrimp. The systematic shift towards omnivory likely affects both phyto- and zooplankton community composition, possibly contributing to temporal changes in food web dynamics in fairy shrimp habitats, and temporary ponds, which may warrant more detailed investigations in future studies. Supplementary Information: The online version contains supplementary material available at 10.1007/s10750-022-05132-z.

Light-dependent niche differentiation in two mixotrophic bacterivores.

Mixotrophy usually is considered with respect to the advantages gained and the associated trade-offs of this form of nutrition, compared to specialized competitors, strict photoautotrophs and heterotrophs. However, we currently have an incomplete understanding of the functional diversity of mixotrophs and the factors controlling niche differentiation in different mixotrophic species. Here we experimentally studied the light-dependent niche differentiation in two chrysophyte species. We show that the newly isolated Ochromonas sp. is an obligate phototroph and possibly an obligate mixotroph. In contrast, Poterioochromonas malhamensis is a facultative mixotroph; photosynthesis and heterotrophy in this species represent substitutable routes of resource acquisition. We further hypothesize that the variable plasticity in the considered traits of the here tested species may result in different niche differentiation with regard to a vertical light gradient. Ochromonas sp. should perform better in stable stratified surface water layers, where light is available, but prey abundances might be low. However, P. malhamensis should be able to also successfully grow in deeper water layers, benefiting from higher bacterial production. This study represents a first step towards understanding competition between mixotrophs engaging in different physiological strategies, and consequently their potential co-occurrence due to niche differentiation.

Accounting for temporal change in multiple biodiversity patterns improves the inference of metacommunity processes.

In metacommunity ecology, a major focus has been on combining observational and analytical approaches to identify the role of critical assembly processes, such as dispersal limitation and environmental filtering, but this work has largely ignored temporal community dynamics. Here, we develop a "virtual ecologist" approach to evaluate assembly processes by simulating metacommunities varying in three main processes: density-independent responses to abiotic conditions, density-dependent biotic interactions, and dispersal. We then calculate a number of commonly used summary statistics of community structure in space and time and use random forests to evaluate their utility for inferring the strength of these three processes. We find that (i) both spatial and temporal data are necessary to disentangle metacommunity processes based on the summary statistics we test, and including statistics that are measured through time increases the explanatory power of random forests by up to 59% compared to cases where only spatial variation is considered; (ii) the three studied processes can be distinguished with different descriptors; and (iii) each summary statistic is differently sensitive to temporal and spatial sampling effort. Including repeated observations of metacommunities over time was essential for inferring the metacommunity processes, particularly dispersal. Some of the most useful statistics include the coefficient of variation of species abundances through time and metrics that incorporate variation in the relative abundances (evenness) of species. We conclude that a combination of methods and summary statistics is probably necessary to understand the processes that underlie metacommunity assembly through space and time, but we recognize that these results will be modified when other processes or summary statistics are used.

Freshwater salinisation: a research agenda for a saltier world.

The widespread salinisation of freshwater ecosystems poses a major threat to the biodiversity, functioning, and services that they provide. Human activities promote freshwater salinisation through multiple drivers (e.g., agriculture, resource extraction, urbanisation) that are amplified by climate change. Due to its complexity, we are still far from fully understanding the ecological and evolutionary consequences of freshwater salinisation. Here, we assess current research gaps and present a research agenda to guide future studies. We identified different gaps in taxonomic groups, levels of biological organisation, and geographic regions. We suggest focusing on global- and landscape-scale processes, functional approaches, genetic and molecular levels, and eco-evolutionary dynamics as key future avenues to predict the consequences of freshwater salinisation for ecosystems and human societies.

Diversity predicts stability and resource use efficiency in natural phytoplankton communities.

The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the "macroscopic" realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute approximately 50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

Freshwater phytoplankton diversity: models, drivers and implications for ecosystem properties.

Our understanding on phytoplankton diversity has largely been progressing since the publication of Hutchinson on the paradox of the plankton. In this paper, we summarise some major steps in phytoplankton ecology in the context of mechanisms underlying phytoplankton diversity. Here, we provide a framework for phytoplankton community assembly and an overview of measures on taxonomic and functional diversity. We show how ecological theories on species competition together with modelling approaches and laboratory experiments helped understand species coexistence and maintenance of diversity in phytoplankton. The non-equilibrium nature of phytoplankton and the role of disturbances in shaping diversity are also discussed. Furthermore, we discuss the role of water body size, productivity of habitats and temperature on phytoplankton species richness, and how diversity may affect the functioning of lake ecosystems. At last, we give an insight into molecular tools that have emerged in the last decades and argue how it has broadened our perspective on microbial diversity. Besides historical backgrounds, some critical comments have also been made.

Pleistocene allopatric differentiation followed by recent range expansion explains the distribution and molecular diversity of two congeneric crustacean species in the Palaearctic.

Pleistocene glaciations had a tremendous impact on the biota across the Palaearctic, resulting in strong phylogeographic signals of range contraction and rapid postglacial recolonization of the deglaciated areas. Here, we explore the diversity patterns and history of two sibling species of passively dispersing taxa typical of temporary ponds, fairy shrimps (Anostraca). We combine mitochondrial (COI) and nuclear (ITS2 and 18S) markers to conduct a range-wide phylogeographic study including 56 populations of Branchinecta ferox and Branchinecta orientalis in the Palaearctic. Specifically, we investigate whether their largely overlapping ranges in Europe resulted from allopatric differentiation in separate glacial refugia followed by a secondary contact and reconstruct their postglacial recolonization from the inhabited refugia. Our results suggest the existence of distinct refugia for the two species, with genetic divergence among intraspecific lineages consistent with late Pleistocene glacial cycles. While B. ferox lineages originated from Mediterranean refugia, the origin of B. orientalis lineages was possibly located on the Pannonian Plain. We showed that most dispersal events predominantly happened within 100 km, coupled with several recent long-distance events (> 1000 km). Hence the regional habitat density of suitable habitats in Central Europe is possibly a key to the co-existence of the two species. Overall, our study illustrates how isolation in combination with stochastic effects linked to glacial periods are important drivers of the allopatric differentiation of Palaearctic taxa.