Bipolar distributions in vascular plants: A review

Bipolar disjunct distributions are a fascinating biogeographic pattern exhibited by about 30 vascular plants, whose populations reach very high latitudes in the northern and southern hemispheres. In this review, we first propose a new framework for the definition of bipolar disjunctions and then reformulate a list of guiding principles to consider how to study bipolar species. Vicariance and convergent evolution hypotheses have been argued to explain the origin of this fragmented distribution pattern, but we show here that they can be rejected for all bipolar species, except for Carex microglochin. Instead, human introduction and dispersal (either direct or by mountain‐hopping)—facilitated by standard and nonstandard vectors—are the most likely explanations for the origin of bipolar plant disjunctions. Successful establishment after dispersal is key for colonization of the disjunct areas and appear to be related to both intrinsic (e.g., self‐compatibility) and extrinsic (mutualistic and antagonistic interactions) characteristics. Most studies on plant bipolar disjunctions have been conducted in Carex (Cyperaceae), the genus of vascular plants with the largest number of bipolar species. We found a predominant north‐to‐south direction of dispersal, with an estimated time of diversification in agreement with major cooling events during the Pliocene and Pleistocene. Bipolar Carex species do not seem to depend on specialized traits for long‐distance dispersal and could have dispersed through one or multiple stochastic events, with birds as the most likely dispersal vector.     

No need for stepping stones: Direct,joint dispersal of the lichen‐forming fungus Mastodia tessellata (Ascomycota) and its photobiont explains their bipolar distribution

Aim

The hypotheses proposed to explain the high percentage of bipolar lichens in Antarctica have never been explicitly tested. We used the strictly bipolar, coastal lichenized fungus Mastodia tessellata (Verrucariaceae, Ascomycota) and its photobionts (Prasiola, Trebouxiophyceae, Chlorophyta) as model species to discern whether this extraordinary disjunction originated from vicariance or long‐distance dispersal.

Location

Coasts of Antarctica, Tierra del Fuego (Chile), Alaska (USA) and British Columbia (Canada).

Methods

Based on a comprehensive geographical (315 specimens and 16 populations from Antarctica, Tierra del Fuego and North America) and molecular sampling (three and four loci for the fungus and algae respectively), we implemented explicit Bayesian methods to compare alternative hypotheses of speciation and migration, and performed dating analyses for the fungal and algal partner, in order to infer the timing of the colonization events and the direction of gene flow among distant, disjunct areas.

Results

Mastodia tessellata comprises two fungal species which in turn associate with three photobiont lineages along the studied distribution range. Independent estimation of divergence ages for myco‐ and photobionts indicated a middle to latest Miocene species split in the Southern Hemisphere, and a late Miocene to Pleistocene acquisition of the bipolar distribution. Comparison of migration models and genetic diversity patterns suggested an austral origin for the bipolar species.

Main conclusions

The complex evolutionary history of Mastodia tessellata s.l. can be explained by a combination of vicariant and long‐distance dispersal mechanisms. We provide novel evidence of a pre‐Pleistocene long‐term evolution of lichens in Antarctica as well as for bipolar distributions shaped by Southern to Northern Hemisphere migratory routes without the need for stepping stones.     

‘What's larvae got to do with it?’ Disparate patterns of post‐glacial population structure in two benthic marine gastropods with identical dispersal potential

In marine environments, many species have apparently colonized high latitude regions following the last glacial maximum (LGM) yet lack a life-history stage, such as a free-living larva, that is clearly capable of long-distance dispersal. Two hypotheses can explain the modern high latitude distributions of these marine taxa: (1) survival in northern refugia during the LGM or (2) rapid post-glacial dispersal by nonlarval stages. To distinguish these two scenarios, I characterized the genetic structure of two closely related northeastern Pacific gastropods that lack planktonic larvae but which have distributions extending more than 1000 km north of the southern limit of glaciers at the LGM. Despite having identical larval dispersal potential, these closely related species exhibit fundamentally different patterns of genetic structure. In Nucella ostrina, haplotype diversity among northern populations (British Columbia and Alaska) is low, no pattern of isolation by distance exists and a coalescent-based model of population growth indicates that during the LGM population size was reduced to less than 35% of its current size. In the congeneric and often sympatric N. lamellosa, northern populations harbour a diversity of ancient private haplotypes, significant evidence of isolation by distance exists and regional subdivision was found between northern (Alaska) and southern (southern British Columbia, Washington and Oregon) populations. Estimates of coalescent parameters indicate only a modest reduction in population size during the LGM and that northern and southern populations of N. lamellosa split approximately 50 Kyr before the LGM. The patterns are consistent with the hypothesis that N. ostrina recently reinvaded the northeastern Pacific but N. lamellosa survived the LGM in a northern refuge. A comparison of similar studies in this region indicates that depleted levels of genetic variation at high latitudes--evidence suggestive of recent colonization from a southern refuge--is more common among intertidal species that live relatively high on the shore, where exposure times to cold stress in air are longer than for species living lower on the shore. These data suggest that for some faunas, ecological differences between taxa may be more important than larval dispersal potential in determining species' long-term biogeographical responses to climate change.     

Pleistocene expansion of the bipolar lichen Cetraria aculeata into the Southern hemisphere

Many boreal and polar lichens occupy bipolar distributional ranges that frequently extend into high mountains at lower latitudes. Although such disjunctions are more common among lichens than in other groups of organisms, the geographic origin of bipolar lichen taxa, and the way and time frame in which they colonized their ranges have not been studied in detail. We used the predominantly vegetative, widespread lichen Cetraria aculeata as a model species. We surveyed the origin and history of its bipolar pattern using population genetics, phylogenetic and genealogical reconstruction methods. Cetraria aculeata originated in the Northern Hemisphere and dispersed southwards during the Pleistocene. The genetic signal suggests a Pleistocene dispersive burst in which a population size expansion concurred with the acquisition of a South‐American range that culminated in the colonization of the Antarctic.     

Driven by the West Wind Drift? A synthesis of southern temperate marine biogeography,with new directions for dispersalism

Aim Twentieth century biogeographers developed intriguing hypotheses involving West Wind Drift dispersal of Southern Hemisphere biota, but such models were largely abandoned in favour of vicariance following the development of plate tectonic theory. Here I present a synthesis of southern temperate marine biogeography, and suggest some new directions for phylogeographic research. Location The southern continents, formerly contiguous components of Gondwana, are now linked only by ocean currents driven by the West Wind Drift. Methods While vicariance theory certainly facilitates the development of testable hypotheses, it does not necessarily follow that vicariance explains much of contemporary southern marine biogeography. To overcome the limitations of narratives that simply assume vicariance or dispersal, it is essential for analyses to test biogeographic hypotheses by incorporating genetic, ecological and geological data. Results Recent molecular studies have provided strong evidence for dispersal, but relatively little evidence for the biogeographic role of plate tectonics in distributing southern marine taxa. Despite confident panbiogeographic claims to the contrary, molecular and ecological studies of buoyant macroalgae, such as Macrocystis, indicate that dispersal predominates. Ironically, some of the better supported evidence for marine vicariance in southern waters has little or nothing to do with plate tectonics. Rather, it involves far more localized and recent vicariant models, such as the isolating effect of the Bassian Isthmus during Pleistocene low sea‐level stands (Nerita). Main conclusions Recent phylogeographic studies of southern marine taxa (e.g. Diloma and Parvulastra) imply that passive rafting cannot be ignored as an important mechanism of long‐distance dispersal. I outline a new direction for southern hemisphere phylogeography, involving genetic analyses of bull‐kelp (Durvillaea) and its associated holdfast invertebrate communities.     

Aspen succession and nitrogen loading: a case for epiphytic lichens as bioindicators in the Rocky Mountains,USA

Question: Can lichen communities be used to assess short‐ and long‐term factors affecting seral quaking aspen (Populus tremuloides) communities at the landscape scale? Location: Bear River Range, within the Rocky Mountains, in northern Utah and southern Idaho, USA. Method: Forty‐seven randomly selected mid‐elevation aspen stands were sampled for lichens and stand conditions. Plots were characterized according to tree species cover, basal area, stand age, bole scarring, tree damage, and presence of lichen species. We also recorded ammonia emissions with passive sensors at 25 urban and agricultural sites throughout an adjacent populated valley upwind of the forest stands. Nonmetric multidimensional scaling (NMS) ordination was used to evaluate an array of 20 variables suspected to influence lichen communities. Results: In NMS, forest succession explained most variance in lichen composition and abundance, although atmospheric nitrogen from local agricultural and urban sources also significantly influenced the lichen communities. Abundance of nitrophilous lichen species decreased with distance from peak ammonia sources and the urban center in all aspen succession classes. One lichen, Phaeophyscia nigricans, was found to be an effective bioindicator of nitrogen loading. Conclusions: Lichen communities in this landscape assessment of aspen forests showed clear responses to long‐term (stand succession) and short‐term (nitrogen deposition) influences. At the same time, several environmental factors (e.g. tree damage and scarring, distance to valley, topography, and stand age) had little influence on these same lichen communities. We recommend further use of epiphytic lichens as bioindicators of dynamic forest conditions.     

Historical biogeography and phenotype‐phylogeny of Chroodiscus (lichenized Ascomycota: Ostropales: Graphidaceae)

Aim To analyse the historical biogeography of the lichen genus Chroodiscus using a phenotype‐based phylogeny in the context of continental drift and evolution of tropical rain forest vegetation. Location All tropical regions (Central and South America, Africa, India, Southeast Asia, north‐east Australia). Methods We performed a phenotype‐based phylogenetic analysis and ancestral character state reconstruction of 14 species of the lichen genus Chroodiscus, using paup * and mesquite ; dispersal–vicariance analysis (DIVA) and dispersal–extinction–cladogenesis (DEC) modelling to trace the geographical origin of individual clades; and ordination and clustering by means of pc‐ord , based on a novel similarity index, to visualize the biogeographical relationships of floristic regions in which Chroodiscus occurs. Results The 14 species of Chroodiscus show distinctive distribution patterns, with one pantropical and one amphi‐Pacific taxon and 12 species each restricted to a single continent. The genus comprises four clades. DIVA and DEC modelling suggest a South American origin of Chroodiscus in the mid to late Cretaceous (120–100 Ma), with subsequent expansion through a South American–African–Indian–Southeast Asian–Australian dispersal route and late diversification of the argillaceus clade in Southeast Asia. Based on the abundance of extant taxa, the probability of speciation events in Chroodiscus is shown to be extremely low. Slow dispersal of foliicolous rain forest understorey lichens is consistent with estimated phylogenetic ages of individual species and with average lengths of biological species intervals in fungi (10–20 Myr). Main conclusions The present‐day distribution of Chroodiscus can be explained by vicariance and mid‐distance dispersal through the interconnection or proximity of continental shelves, without the need for recent, trans‐oceanic long‐distance dispersal. Phylogenetic reconstruction and age estimation for Chroodiscus are consistent with the ‘biotic ferry’ hypothesis: a South American origin and subsequent eastward expansion through Africa towards Southeast Asia and north‐eastern Australia via the Indian subcontinent. The present‐day pantropical distributions of many clades and species of foliicolous lichens might thus be explained by eastward expansion through continental drift, along with the evolution of modern rain forests starting 120 Ma, rather than by the existence of a hypothetical continuous area of pre‐modern rain forest spanning South America, Africa and Southeast Asia during the mid and late Cretaceous.     

Observations on bipolar disjunctions of moonwort ferns (Botrychium,Ophioglossaceae)

Peter Raven, in 1963, included two fern taxa of the genus Botrychium in his list of plant species exhibiting American amphitropical bipolar disjunctions. He attributed the southern hemisphere occurrences to post‐Pleistocene long‐distance dispersal from counterparts in the northern hemisphere, probably assisted by annual bird migrations between the disjunct areas. Using genetic evidence gathered through worldwide analyses of phylogenetic relationship in Botrychium, we now review and reconsider Raven's conclusions. Genetic similarities indicate that South American Botrychium dusenii is an allotetraploid taxon closely related to B. spathulatum, a North American endemic, and that B. lunaria in New Zealand possesses a genotype identical to that of a taxon in North America derived through introgressive hybridization between B. lunaria and an endemic North American species, B. neolunaria. Both North American counterparts exhibit Raven's characteristics of bipolar disjuncts in their occurrence in mountain and coastal meadows, copious production of small propagules (spores in Botrychium), occurrence in habitats frequented by transpolar bird migrants, and ability to found new colonies through inbreeding. We discuss these characteristics in Botrychium and relative to other ferns and suggest further studies on Botrychium and related taxa to address questions of time, number, and mode of bipolar dispersals.     

Altered fire regimes cause long‐term lichen diversity losses

Many global ecosystems have undergone shifts in fire regimes in recent decades, such as changes in fire size, frequency, and/or severity. Recent research shows that increases in fire size, frequency, and severity can lead to long‐persisting deforestation, but the consequences of shifting fire regimes for biodiversity of other vegetative organisms (such as understory plants, fungi, and lichens) remain poorly understood. Understanding lichen responses to wildfire is particularly important because lichens play crucial roles in nutrient cycling and supporting wildlife in many ecosystems. Lichen responses to fire have been little studied, and most previous research has been limited to small geographic areas (e.g. studies of a single fire), making it difficult to establish generalizable patterns. To investigate long‐term effects of fire severity on lichen communities, we sampled epiphytic lichen communities in 104 study plots across California's greater Sierra Nevada region in areas that burned in five wildfires, ranging from 4 to 16 years prior to sampling. The conifer forest ecosystems we studied have undergone a notable increase in fire severity in recent decades, and we sample across the full gradient of fire severity to infer how shifting fire regimes may influence landscape‐level biodiversity. We find that low‐severity fire has little to no effect on lichen communities. Areas that burned at moderate and high severities, however, have significantly and progressively lower lichen richness and abundance. Importantly, we observe very little postfire lichen recolonization on burned substrates even more than 15 years after fire. Our multivariate model suggests that the hotter, drier microclimates that occur after fire removes forest canopies may prevent lichen reestablishment, meaning that lichens are not likely to recolonize until mature trees regenerate. These findings suggest that altered fire regimes may cause broad and long‐persisting landscape‐scale biodiversity losses that could ultimately impact multiple trophic levels.     

Phylogeography and divergence date estimates of a lichen species complex with a disjunct distribution pattern

Disjunct species distributions may result from a combination of geologic events and long-distance dispersal. The foliose lichen species complex Leptogium furfuraceum-L. pseudofurfuraceum has an intercontinental disjunction pattern. Populations of this species complex are found in western North America, southern South America, Africa, and southern Europe. We conducted a phylogenetic study to reconstruct the biogeographic history of this species complex using two ribosomal genes (ITS and LSU) and a protein-coding gene (partial RPB2). Results indicated that the complex comprises four geographically restricted genetic lineages. A sister relationship was found between populations from the same hemispheres, incongruent with previous data derived from morphological characteristics and geographical classification schemes. Incorporating Bayesian ancestral area reconstruction and Bayesian divergence time estimation, we proposed an evolutionary hypothesis for the species complex. The results suggested that processes of biotic expansion via transoceanic dispersal were responsible for the species divergence and distribution patterns observed today. This study also expands the view that cryptic speciation is not a rare phenomenon among fungi and lichens.     

A method for the direct detection of airborne dispersal in lichens

This study sets out a novel method to determine dispersal distances in lichens. Direct measurement of dispersal often remains difficult for lichens and other small inconspicuous species because of the need to track microscopic reproductive propagules, which even if they can be captured, cannot be identified using traditional morphological approaches. A low‐cost device (<£200) was developed to trap the reproductive propagules of lichens, capable of sampling around 0.1 m³ of air per minute. In parallel, molecular techniques were developed to enable species‐specific detection of propagules caught by the devices, with identification using novel species‐specific primers and optimization of a standard DNA extraction and nested PCR protocol. The methods were tested for both their sensitivity and specificity against a suite of lichen epiphytes, differing in their reproductive mechanisms, dispersal structures and rarity. Sensitivity tests showed that the molecular techniques could detect a single asexual propagule (soredium or isidium), or as few as 10 sexual spores. As proof of concept, propagule traps were deployed into a wooded landscape where the target epiphytes were present. Extractions from deployed propagule traps were sequenced, showing that the method was able to detect the presence of the target species in the atmosphere. As far as we are aware, this is the first attempt to use mechanized propagule traps in combination with DNA diagnostics to detect dispersal of lichens. The tests carried out here point the way for future dispersal studies of lichen epiphytes and other passively dispersed microscopic organisms including fungi or bryophytes.     

Postglacial recolonization shaped the genetic diversity of the winter moth (Operophtera brumata) in Europe

Changes in climate conditions, particularly during the Quaternary climatic oscillations, have long been recognized to be important for shaping patterns of species diversity. For species residing in the western Palearctic, two commonly observed genetic patterns resulting from these cycles are as follows: (1) that the numbers and distributions of genetic lineages correspond with the use of geographically distinct glacial refugia and (2) that southern populations are generally more diverse than northern populations (the “southern richness, northern purity” paradigm). To determine whether these patterns hold true for the widespread pest species the winter moth (Operophtera brumata), we genotyped 699 individual winter moths collected from 15 Eurasian countries with 24 polymorphic microsatellite loci. We find strong evidence for the presence of two major genetic clusters that diverged ~18 to ~22 ka, with evidence that secondary contact (i.e., hybridization) resumed ~ 5 ka along a well‐established hybrid zone in Central Europe. This pattern supports the hypothesis that contemporary populations descend from populations that resided in distinct glacial refugia. However, unlike many previous studies of postglacial recolonization, we found no evidence for the “southern richness, northern purity” paradigm. We also find evidence for ongoing gene flow between populations in adjacent Eurasian countries, suggesting that long‐distance dispersal plays an important part in shaping winter moth genetic diversity. In addition, we find that this gene flow is predominantly in a west‐to‐east direction, suggesting that recently debated reports of cyclical outbreaks of winter moth spreading from east to west across Europe are not the result of dispersal.     

Climatic flowering requirements of bipolar sedges, Carex spp., and the feasibility of their trans-equatorial migration by mountain-hopping

A selection of northern and southern hemisphere populations of a range of bipolar Carex species was grown in controlled environments for comparison of their temperature and daylength requirements for flowering. All studied species have basically the same dual induction requirements for flowering but differ quantitatively in their respective critical temperatures and daylengths for primary and secondary floral induction. Usually, low temperature or short days (SD), over a wider range of temperatures, were required for initiation of inflorescence primordia, whereas long days (LD) advanced but were not obligatory for heading and flower development. It is concluded that no change or adaptation in the flowering requirements would have been necessary for these plants to migrate across the tropics and their penetration into the southern hemisphere by mountain-hopping. The flowering requirements of these and a range of other bipolar plant species are compatible with the hypothesis that absence of an obligatory long day floral induction requirement has facilitated or been a prerequisite for trans-equatorial migration of plants by mountain-hopping and the establishment of bipolar species distributions.     

Recent and rapid population growth and range expansion of the Lyme disease tick vector,Ixodes scapularis,in North America

Migration is a primary force of biological evolution that alters allele frequencies and introduces novel genetic variants into populations. Recent migration has been proposed as the cause of the emergence of many infectious diseases, including those carried by blacklegged ticks in North America. Populations of blacklegged ticks have established and flourished in areas of North America previously thought to be devoid of this species. The recent discovery of these populations of blacklegged ticks may have resulted from either in situ growth of long‐established populations that were maintained at very low densities or by migration and colonization from established populations. These alternative evolutionary hypotheses were investigated using Bayesian phylogeographic approaches to infer the origin and migratory history of recently detected blacklegged tick populations in the Northeastern United States. The data and results indicate that newly detected tick populations are not the product of in situ population growth from a previously established population but from recent colonization resulting in a geographic range expansion. This expansion in the geographic range proceeded primarily through progressive and local migration events from southern populations to proximate northern locations although long‐distance migration events were also detected.     

The Middle Asian Element in the Southern Rocky Mountain Flora of the western United States: a critical biogeographical review

Aim Presentation of an hypothesis suggesting that the extraordinarily similarity of the Russian Altai and the American Southern Rocky Mountain Flora represents an Oroboreal Flora; that had to have had an essential continuity across the northern part of the world in the Tertiary period, constituting a highland and steppe component of the better‐known Arcto‐Tertiary Flora of eastern and far‐western North America and eastern Asia. Location North America and Middle (Altai) Asia. Methods Summarization of the author's field and herbarium studies of whole floras over a period of over 60 years, consisting of successive specializations in vascular plants, lichens, and bryophytes. Main conclusions (1) The modern alpine and associated marginal steppe and montane floras contain taxa of Tertiary age. (2) The floras of the southern mountains antedate those of the present‐day Arctic. (3) The Middle Asiatic and the North American floras once enjoyed a contiguous existence over a broad area involving connections between North America and Asia across the North Pole by way of Greenland. Their present disjunctions are products of extinction and attrition of ranges, not of long‐distance migration or dispersal mechanisms. (4) North‐eastern North American disjunctions of so‐called Cordilleran species (the Nunatak hypothesis) need not require explanations involving long‐distance dispersal or migration, but represent relictual populations of the once widely distributed Oroboreal flora.     

Northern genetic richness and southern purity,but just one species in the Chelonoidis chilensis complex

Fritz, U., Alcalde, L., Vargas‐Ramírez, M., Goode, E.V., Fabius‐Turoblin, D.U. & Praschag, P. (2012). Northern genetic richness and southern purity, but just one species in the Chelonoidis chilensis complex. —Zoologica Scripta, 41, 220–232. The Chelonoidis chilensis complex, the sister group of the famous Galápagos tortoises, is a widely distributed group of South American land tortoises, ranging from the dry Chaco of Bolivia, Paraguay and northern Argentina to northern Patagonia. Within this complex, up to three distinct species have been recognized. Using sequence data of the mitochondrial cytochrome b gene and length polymorphisms of 10 microsatellite loci, we investigate genetic differentiation among all three nominal species. We find only negligible differentiation, with decreasing genetic diversity from north to south. We conclude that only one species, Chelonoidis chilensis (Gray, 1870), is valid, with C. donosobarrosi ( Freiberg, 1973 ) and C. petersi ( Freiberg, 1973 ) as its junior synonyms. Morphological variation within C. chilensis sensu lato is in accord with the observation that size variation in chelonians follows Bergmann’s rule, with body size increasing with latitude. The observed phylogeographic differentiation inverses the well‐known pattern of southern genetic richness and northern purity from the northern hemisphere, resulting from dispersal from glacial refugia. This implies that in higher latitudes of both hemispheres genetic diversity may decrease with increasing distance from the refugium. For C. chilensis sensu lato, it seems likely that long‐distance dispersal via rafting on the Desaguadero River led to the foundation of the southernmost populations in northern Patagonia during the Holocene.     

Genetic diversity of algal and fungal partners in four species of Umbilicaria (Lichenized Ascomycetes) along a transect of the Antarctic peninsula

Lichens from the genus Umbilicaria were collected across a 5,000-km transect through Antarctica and investigated for DNA sequence polymorphism in a region of 480-660 bp of the nuclear internal transcribed spacer region of ribosomal DNA. Sequences from both fungal (16 ascomycetes) and photosynthetic partners (22 chlorophytes from the genus Trebouxia) were determined and compared with homologs from lichens inhabiting more temperate, continental climates. The phylogenetic analyses reveal that Antarctic lichens have colonized their current habitats both through multiple independent colonization events from temperate embarkation zones and through recent long-range dispersal in the Antarctic of successful preexisting colonizers. Furthermore, the results suggest that relichenization-de novo establishment of the fungus-photosynthesizer symbiosis from nonlichenized algal and fungal cells-has occurred during the process of Antarctic lichen dispersal. Independent dispersal of algal and fungal cultures therefore can lead to a successful establishment of the lichen symbiosis even under harsh Antarctic conditions.     

Living in a tilted world: climate change and geography limit speciation in Old World anchovies (Engraulis; Engraulidae)

Historical changes in the distributions of temperate species in response to Milankovitch climate cycles have been well documented in palaeontological studies and recently evaluated with phylogeographical methods. How these cycles influence biological diversity remains a matter of debate. Molecular surveys of terrestrial and freshwater fauna demonstrate glacial refugia in low latitudes and range expansions into high latitudes, but few genetic studies have assessed the corresponding impact on marine fauna. In the present study, mtDNA sequences (N = 84) are surveyed to understand the impact of long‐term climate oscillations on ‘Old World’ anchovies (genus Engraulis), a monophyletic group occurring in north and south temperate zones of the eastern Atlantic and the western Pacific. The analysis of a 521‐bp sequence of mtDNA cytochrome b indicates a late Miocene or Pliocene dispersal from the north‐eastern Pacific (California–Mexico) to the north‐western Pacific (Japan), followed by Pleistocene dispersal from the north‐western Pacific to Europe. Geography mandates that populations in southern Africa and Australia were stepping‐stones for this dispersal. However, neither population occupies an intermediate position in the mtDNA genealogy; both populations are more recently derived from their northern neighbours. Haplotype diversity is high (h = 0.93–0.97) in European, Australian, and Japanese anchovies, but low (h = 0.22) in the southern African population, where all haplotypes are more closely related to European specimens than to each other. These southern populations occupy a precarious position, lacking north–south coastlines that allow range shifts during climatic extremes. Recurring extinctions and episodic recolonizations from northern hemisphere populations are the likely results. In this case, ocean‐climatic changes retard rather than enhance opportunities for evolutionary radiations. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 88 , 673–689.     

Lichen colonization patterns show minor effects of dispersal distance at landscape scale

The role of dispersal in controlling the distribution of species at landscape scale (10²–10⁴ m) is still a matter of dispute. Here, we use the early colonization pattern of 23 epiphytic lichen species in a former tree‐less heathland landscape (170 km²) to test three hypotheses on how a landscape is colonized: A) mainly by long‐distance dispersal (LDD), B) by rare LDD events followed by limited local dispersal, and C) mainly by limited dispersal, resulting in a colonization front. The study system consisted of a chronosequence of 94 habitat patches constituting 0.4% of the landscape area, with a minimum inter‐site distance of 0.2 km. We used generalized linear mixed models with Bayesian inference to test predictions from the hypotheses. When age of sites and habitat area were accounted for, additional effects of geographical position of sites (distance from old sites, distance‐dependent relative propagule pressure, and distance from border of study area) on the probability of colonization by lichen species were small. Furthermore, species richness of sites did not depend on geographical position, either. Our results support a colonization process mainly governed by LDD at landscape scale, and that local stepwise colonization was not important. We argue that passively dispersed species with numerous small propagules tend to exhibit patchy populations with extensive dispersal at the landscape scale, rather than behaving like classical metapopulations.