Impacts of dreissenid mussel invasions on chlorophyll and total phosphorus in 25 lakes in the USA

1. Invasions of zebra and quagga mussels have had long‐term, large‐scale impacts on lake ecosystems in the USA as characterised by high abundance, broad‐scale spread and effective adaption to new environmental conditions. Due to their high filtering capacity, decreases in chlorophyll a (Chl) and total phosphorus (TP) concentrations have been reported in many affected lakes. 2. In 25 US lakes, we analysed the effects of dreissenid invasions on changes in Chl and TP concentrations, measured as the probability of a concentration decrease in the post‐invasion period and changes in Chl–TP relationships using Bayesian hierarchical regressions. We also examined whether changes in Chl and TP concentrations and in the Chl–TP relationship differed according to lake hydromorphology, such as mean depth or mixing status (mixed versus stratified lakes). 3. Our results showed that dreissenid invasions were often, but not always, associated with subsequent TP and Chl declines. Dreissenid effects on Chl and TP may be influenced by lake thermal structure. Decreases in Chl and TP were consistently found in mixed lakes where benthic–pelagic coupling is tight, while the effects were less predictable in stratified lakes. Within stratified lakes, Chl and TP reductions were more clearly discernible in deeper lakes with long water residence times. 4. Regression results demonstrated that a joint increase in slope and decrease in intercept and a tighter correlation of the Chl–TP relationship were likely to occur in dreissenid‐invaded lakes; this does not support the idea of a shift from bottom‐up to top‐down control of primary production. These results have important implications for management, suggesting that a relaxation of TP standards would be unwarranted. 5. Across lakes, the slope of the Chl–TP relationship for mixed lakes was substantially higher than that for stratified lakes before mussel invasion, indicating an important role of light in limiting primary production. The slope differences between mixed and stratified lakes decreased in the post‐invasion period, possibly because mussel filtration results in a relaxation of light limitation that is more pronounced in deeper, stratified lakes.     

Genetic patterns across an invasion's history: a test of change versus stasis for the Eurasian round goby in North America

Biological invasions comprise accidental evolutionary experiments, whose genetic compositions underlie relative success, spread and persistence in new habitats. However, little is known about whether, or how, their population genetic patterns change temporally and/or spatially across the invasion's history. Theory predicts that most would undergo founder effect, exhibit low genetic divergence across the new range and gain variation over time via new arriving propagules. To test these predictions, we analyse population genetic diversity and divergence patterns of the Eurasian round goby Neogobius melanostomus across the two decades of its North American invasion in the Laurentian Great Lakes, comparing results from 13 nuclear DNA microsatellite loci and mitochondrial DNA cytochrome b sequences. We test whether ‘genetic stasis’, ‘genetic replacement’ and/or ‘genetic supplement’ scenarios have occurred at the invasion's core and expansion sites, in comparison with its primary native source population in the Dnieper River, Black Sea. Results reveal pronounced genetic divergence across the exotic range, with population areas remaining genetically distinct and statistically consistent across two decades, supporting ‘genetic stasis’ and ‘founder takes most’. The original genotypes continue to predominate, whose high population growth likely outpaced the relative success of later arrivals. The original invasion core has stayed the most similar to the native source. Secondary expansion sites indicate slight allelic composition convergence towards the core population over time, attributable to some early ‘genetic supplementation’. The geographic and temporal coverage of this investigation offers a rare opportunity to discern population dynamics over time and space in context of invasion genetic theory vs. reality.     

A paradox of trout invasions in North America

A paradox of invasion biology is that even though native species are locally adapted to environmental conditions, nonnative species without this advantage often invade. Ecologists have advanced four main theories to explain why invaders are successful in some places and not others: biotic resistance, environmental resistance, human disturbance, and natural enemies. However, none of these theories alone can account for invasions by two trout species outside their native ranges in North America. Brook trout (Salvelinus fontinalis) are able to displace native cutthroat trout (Oncorhynchus clarkii) in the inland western US, but are themselves displaced by nonnative rainbow trout (O. mykiss) in the southeastern US. An alternative hypothesis is that an interaction among zoogeography, evolutionary history, and environmental resistance from the natural flow regimes can account for this paradox. The nonnative species invade successfully at the southern edge of the ranges of the native species, which are farthest from their ancestral origins. Due to their evolutionary history, the native species are poorly adapted to the natural disturbance regime at the southern limit of their ranges, but the nonnative species are preadapted by chance due to theirs. This alternative hypothesis about the interaction between the historical contingency of evolution and environmental resistance should be more widely tested, to inform both invasion biology and the conservation of native trout.     

Biogeographic patterns of soil diazotrophic communities across six forests in the North America

Soil diazotrophs play important roles in ecosystem functioning by converting atmospheric N₂ into biologically available ammonium. However, the diversity and distribution of soil diazotrophic communities in different forests and whether they follow biogeographic patterns similar to macroorganisms still remain unclear. By sequencing nifH gene amplicons, we surveyed the diversity, structure and biogeographic patterns of soil diazotrophic communities across six North American forests (126 nested samples). Our results showed that each forest harboured markedly different soil diazotrophic communities and that these communities followed traditional biogeographic patterns similar to plant and animal communities, including the taxa–area relationship (TAR) and latitudinal diversity gradient. Significantly higher community diversity and lower microbial spatial turnover rates (i.e. z‐values) were found for rainforests (~0.06) than temperate forests (~0.1). The gradient pattern of TARs and community diversity was strongly correlated (r² > 0.5) with latitude, annual mean temperature, plant species richness and precipitation, and weakly correlated (r² < 0.25) with pH and soil moisture. This study suggests that even microbial subcommunities (e.g. soil diazotrophs) follow general biogeographic patterns (e.g. TAR, latitudinal diversity gradient), and indicates that the metabolic theory of ecology and habitat heterogeneity may be the major underlying ecological mechanisms shaping the biogeographic patterns of soil diazotrophic communities.     

Population genetic structure of the malaria mosquito Anopheles arabiensis across Nigeria suggests range expansion

Ten microsatellite loci, four located within and six outside chromosome inversions, were employed to study the genetic structure of Anopheles arabiensis across the ecological zones of Nigeria (arid savannah in the north gradually turns into humid forest in the south). Regardless of location within or outside inversions, genetic variability at all loci was characterized by a reduction in both the number of alleles per locus and heterozygosity from savannah to forest. Across all loci, all but one allele in the forest also occurred in the savannah, whereas at least 78 alleles in the savannah were missing in the forest. Genetic differentiation increased with geographical distance; consequently, genetic distances between zones exceeded those within zones. The largest genetic distances were between localities at the extremes of the transect (range F(ST) = 0.196-0.258 and R(ST) = 0.183-0.468) and were as large as those between A. arabiensis and Anopheles gambiae s.s. Gene flow across the country was very low, so that Nm between the extremes of the transect was < 1. These data suggest that A. arabiensis has extended its range from the savannah into the forest during which it experienced a reduction in effective population size due to sequential founder effects. Gene flow post range expansion appears too restricted by geographical distance to homogenize the gene pool of A. arabiensis across Nigeria.     

Tracing recent invasions of the Ponto-Caspian mysid shrimp Hemimysis anomala across Europe and to North America with mitochondrial DNA

The mysid crustacean Hemimysis anomala ('bloody-red shrimp') is one of the most recent participants in the invasion of European inland waters by Ponto-Caspian species. Recently the species also became established in England and the Laurentian Great Lakes of North America. Using information from mitochondrial cytochrome oxidase I (COI) gene sequences, we traced the invasion pathways of H. anomala ; the inferences were enabled by the observed phylogeographical subdivision among the source area populations in the estuaries of the Ponto-Caspian basin. The data distinguish two routes to northern and western Europe used by distinct lineages. One route has been to and through the Baltic Sea and further to the Rhine delta, probably from a population intentionally introduced to a Lithuanian water reservoir from the lower Dnieper River (NW Black Sea area) in 1960. The other lineage is derived from the Danube delta and has spread across the continent up the Danube River and further through the Main–Danube canal down to the Rhine River delta. Only the Danube lineage was found in England and in North America. The two lineages appear to have met secondarily and are now found intermixed at several sites in NW Europe, including the Rhine and waters linked with the man-made Mittellandkanal that interconnects the Rhine and Baltic drainage systems.     

A Population History of North America

Population History of North America. Michael R. Haines and Richard H. Steckel. New York: Cambridge University Press, 2000. 736 pp.     

Colonization history and genetic diversity: adaptive potential in early stage invasions

The introduction of Anolis cristatellus from the multiple species anole community of Puerto Rico in the Greater Antilles to the island of Dominica in the Lesser Antilles, with its solitary endemic anole, provides an example of a very recent, timed, single colonization. We investigate the geographic origin and adaptive potential of the Dominican population using a range of methods including mtDNA phylogeography, nuclear microsatellite variation and multiple paternity studies, as well as heritability estimates, common garden experiments and comparative geographic studies of quantitative scalation traits. Phylogeographic analysis of NADH2 and microsatellite studies suggests that the Dominican population arose from a set of individuals from the central west area of Puerto Rico within their endemic range. The multiple‐individual inoculation, together with sperm storage and evidence of multiple paternity indicate genetic variability and suggest the potential for adaptation by natural selection. Estimates of heritability, common garden experiments and broad sense Q_ST/F_ST ratios, linked to replicated comparisons along elevational transects go some way to suggesting that the invasive populations may be adapting by natural selection, in parallel with the endemic anole, in the brief period since their introduction.     

A hierarchical classification of freshwater mussel diversity in North America

Aim North America harbours the most diverse freshwater mussel fauna on Earth. This fauna has high endemism at the continental scale and within individual river systems. Previous faunal classifications for North America were based on intuitive, subjective assessments of species distributions, primarily the occurrence of endemic species, and do not portray continent‐wide patterns of faunal similarity. The aim of this study is to provide an analytical portrayal of patterns of mussel diversity in a hierarchical framework that informs the biogeographical history of the fauna. Location The study considered the mussel fauna of North America from the Rio Grande system northwards. Methods Patterns of mussel faunal similarity in 126 river systems or lake watersheds across North America were examined. The dataset was developed from the literature and consisted of recent species presence/absence (282 species) in each drainage unit; subspecies were not included. Patterns of mussel diversity were examined with hierarchical cluster analysis, based on a pairwise distance matrix between all drainage units. Results Cluster analysis revealed 17 faunal provinces within four major faunal regions: Mississippian, Atlantic, Eastern Gulf and Pacific. The Mississippian Region dominates the North American fauna with 11 provinces, including five not recognized by previous classifications: Mississippi Embayment, Upper Mississippi, Great Plains, Ohioan and Pontchartrain–Pearl–Pascagoula. Within the Eastern Gulf Region (containing three provinces), the Escambia–Choctawhatchee Province is distinctive from the Apalachicolan Province, under which it was previously subsumed. Patterns of diversity in the Atlantic Region (two provinces) and Pacific Region (one province) were similar to previous classifications. Main conclusions The classification proposed in this study largely corroborates earlier schemes based on the occurrence of endemic species but identifies additional heterogeneity that reflects unique assemblages of widely distributed species. The study proposes a hierarchical structure that illustrates relationships among these provinces. Although some provinces in the Mississippian Region have high endemism, all Mississippian provinces share a group of widely distributed species. The Atlantic and Eastern Gulf regions have distinctive, endemic faunas suggesting limited past connectivity with the Mississippian Region. The Pacific Region is the most distinct fauna in North America and bears close affinity to the Eurasian mussel fauna.     

Population history and natural selection shape patterns of genetic variation in 132 genes

Identifying regions of the human genome that have been targets of natural selection will provide important insights into human evolutionary history and may facilitate the identification of complex disease genes. Although the signature that natural selection imparts on DNA sequence variation is difficult to disentangle from the effects of neutral processes such as population demographic history, selective and demographic forces can be distinguished by analyzing multiple loci dispersed throughout the genome. We studied the molecular evolution of 132 genes by comprehensively resequencing them in 24 African-Americans and 23 European-Americans. We developed a rigorous computational approach for taking into account multiple hypothesis tests and demographic history and found that while many apparent selective events can instead be explained by demography, there is also strong evidence for positive or balancing selection at eight genes in the European-American population, but none in the African-American population. Our results suggest that the migration of modern humans out of Africa into new environments was accompanied by genetic adaptations to emergent selective forces. In addition, a region containing four contiguous genes on Chromosome 7 showed striking evidence of a recent selective sweep in European-Americans. More generally, our results have important implications for mapping genes underlying complex human diseases.     

Phylogenetic and biogeographical relationships of the Sander pikeperches (Percidae: Perciformes): patterns across North America and Eurasia

North America and Eurasia share several closely related taxa that diverged either from the breakup of the Laurasian supercontinent or later closures of land bridges. Their modern population structures were shaped in Pleistocene glacial refugia and via later expansion patterns, which are continuing. The pikeperch genus Sander contains five species – two in North America (S. canadensis and S. vitreus) and three in Eurasia (S. lucioperca, S. marinus, and S. volgensis) – whose evolutionary relationships and relative genetic diversities were previously unresolved, despite their fishery importance. This is the first analysis to include the enigmatic and rare sea pikeperch S. marinus, nuclear DNA sequences, and multiple mitochondrial DNA regions. Bayesian and maximum‐likelihood trees from three mitochondrial and three nuclear gene regions support the hypothesis that Sander diverged from its sister group Romanichthys/Zingel ～24.6 Mya. North American and Eurasian Sander then differentiated ～20.8 Mya, with the former diverging ～15.4 Mya, congruent with North American fossils dating to ～16.3–13.6 Mya. Modern Eurasian species date to ～13.8 Mya, with S. volgensis being basal and comprising the sister group to S. lucioperca and S. marinus, which diverged ～9.1 Mya. Genetic diversities of the North American species are higher than those in Eurasia, suggesting fewer Pleistocene glaciation bottlenecks. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 156–179.     

Population genetic structure of the seed pathogen Pyrenophora semeniperda on Bromus tectorum in western North America

We examined genetic variation in the ascomycete pathogen Pyrenophora semeniperda cultured from seeds of the invasive grass Bromus tectorum in the Intermountain West of North America. We sequenced the internal transcribed spacer (ITS) region of the nuclear ribosomal RNA genome in 417 monoconidial cultures collected from 20 sites in Washington, Idaho, Utah and Colorado, USA. ITS sequence diversity was surprisingly high; 12 unique haplotypes were identified, averaging 1.3% pairwise sequence divergence. All sites had at least two haplotypes present, and three sites had seven or more. One haplotype composed 60% of the isolates and occurred at all 20 locations; the remaining haplotypes generally occurred at low frequencies within sites but at multiple sites throughout the region. Sites in Washington and Idaho were more diverse than those in Utah and Colorado, averaging two more haplotypes and 67% more pairwise differences among haplotypes at a site. Analysis of molecular variance (AMOVA) indicated that more than 80% of the genetic variation was found within sampling locations, while 7-11% of the variation can be attributed to differences between northern (Washington and Idaho) and southern (Utah and Colorado) populations. The wide distribution of even uncommon haplotypes among sampling sites and weak correlations between genetic and geographic distances among populations (< 0.2) suggested that these populations recently were established from a common source. We hypothesize that the strains of P. semeniperda infecting B. tectorum in western North America probably arrived with the invasive grass from its native Eurasian range.     

Nestedness in riverine mussel communities: patterns across sites and fish hosts

The pattern of nestedness, where species present in depauperate locations are subsets of species present in locations with higher species diversity, is often found in ecological communities. Mussel communities examined in four rivers in the upper Tennessee River basin appeared significantly nested. Mussel species distributions were mostly unrelated to differences in immigration and only weakly related to downstream direction, giving some indication of structuring by differences in extinction. Mussel species distributions were not related to the number of fish species used as hosts for mussel larvae. Mussel species were more likely to overlap on common fish hosts; however, the host‐use matrix was not nested – groups of mussel species used different sets of host fish species in a pattern that appeared phylogenetically related. Sites with high fish host abundance may support high mussel diversity by promoting the survival of mussel species that are less able to attract and infect hosts. Thus, nestedness in freshwater mussel communities may be driven by the array of host fish resources, combined with differences in species’ abilities to use fish hosts. An understanding of the nested pattern in this region can aid conservation of this imperiled fauna.