Invasion genetics of vendace (Coregonus albula (L.)) in the Inari‐Pasvik watercourse: revealing the origin and expansion pattern of a rapid colonization event

Species invasions can have wide‐ranging biological and socio‐economic effects and are generally unwanted by legislation. Identification of the source population as well as the ecology and genetics of both the invader population and the receiving community is of crucial importance. The rapid invasion of a small coregonid fish vendace (Coregonus albula) in a major northern European subarctic watercourse has resulted in a labile ecological situation in the receiving community. The ecological impact of the invasion has been thoroughly documented, but the genetics of the invasion remains to be explored. We analyzed the genetic diversity and divergence patterns among the two possible source populations from southern Finnish Lapland and three colonists populations within the Inari‐Pasvik watercourse using ten microsatellite loci in order to (i) identify the most likely source of the invasion, (ii) reveal the dispersal pattern and genetic structure of the secondary expansion, and (iii) to investigate whether the initial introduction and the secondary expansion were associated with founder effects. We revealed that repeated translocation of vendace from Lake Sinettäjärvi into a tributary lake of L. Inari in 1964–1966 is the most plausible source for the invasion. Both the initial introduction and the secondary expansion were found not to be associated with significant founder effects. The secondary expansion followed a stepping stone pattern and the source and colonist populations of this expansion have undergone rapid genetic divergence within a period of 15–35 years (ca. 8–17 generations). The rapid divergence may be contributed to lack of gene flow among the source and colonist populations due to the extensive hydroelectric damming in the watercourse. Multiple introductions and substantial genetic variation in combination with the boom‐and‐bust population development of the species thus likely counteracted the founder effects as well as fueled the rapid establishment and expansion of this species within the Inari‐Pasvik watercourse.     

Invasion impacts local species turnover in a successional system

Exotic plant invasions are often associated with declines in diversity within invaded communities. However, few studies have examined the local community dynamics underlying these impacts. Changes in species richness associated with plant invasions must occur through local changes in extinction and/or colonization rates within the community. We used long‐term, permanent plot data to evaluate the impacts of the exotic vine Lonicera japonica. Over time, species richness declined with increasing L. japonica cover. L. japonica reduced local colonization rates but had no effect on extinction rates. Furthermore, we detected significant reductions in the immigration of individual species as invasion severity increased, showing that some species are more susceptible to invasion than others. These findings suggest that declines in species richness associated with L. japonica invasion resulted from effects on local colonization rates only and not through the competitive displacement of established species.     

Land‐use history and fragmentation of traditionally managed grasslands in Scandinavia

Abstract. Plants associated with traditional agricultural landscapes in northern Europe and Scandinavia are subjected to drastic habitat fragmentation. In this paper we discuss species response to fragmentation, against a background of vegetation and land‐use history. Recent evidence suggests that grassland‐forest mosaics have been prevalent long before the onset of human agriculture. We suggest that the creation of infield meadows and outland grazing (during the Iron Age) increased the amount and spatial predictability of grasslands, resulting in plant communities with exceptionally high species densities. Thus, distribution of plant species in the present‐day landscape reflects historical land‐use. This holds also when traditional management has ceased, due to a slow response by many species to abandonment and fragmentation. The distribution patterns are thus not in equilibrium with the present habitat distribution. Fragmentation influences remaining semi‐natural grasslands such that species density is likely to decline as a result of local extinctions and invasion by habitat generalists. However, species that for a long time have been subjected to changing mosaic landscapes may be more resistant to fragmentation than is usually believed. Conservation should focus not only on ‘hot‐spots’ with high species richness, but also consider species dynamics in a landscape context.     

Invasive grass litter facilitates native shrubs through abiotic effects

Questions: Plant invasions are considered one of the top threats to the biodiversity of native taxa, but clearly documenting the causal links between invasions and the decline of native species remains a major challenge of invasion biology. Most studies have focused on impacts of invaders' living biomass, rather than on mechanisms mediated by litter. However, invasive plant litter, which is often of a very different type and quantity than a system's native plant litter, can have multiple important effects on ecosystem processes – such as nitrogen cycling and soil microclimate – that may influence native plants. Location: We studied effects of litter of invasive grass species that are widespread throughout western North America on native shrubs in southern California's semi‐arid habitat of coastal sage scrub. Methods: We combined a 3‐year field manipulation of non‐native litter with structural equation modeling to understand interacting effects on non‐native grasses, native shrubs, soil nitrogen (available and total), and soil moisture. Results: Litter addition facilitated non‐native grass growth, revealing a positive feedback likely to enhance invasion success. Contrary to a major paradigm of invasion biology – that competition with invasive plant species causes declines of native plants – we found that litter also facilitated growth of the native dominant shrub, a result supported by observational trends. Structural equation models indicated that enhanced soil moisture mediated the positive effects of litter on shrub growth. Conclusions: We demonstrate that invasive plants, via their litter, can facilitate dominant native plants by altering soil moisture. Our results highlight that understanding the impacts and mechanisms of plant invasions may be enhanced by considering the role of invasive plant litter on native plants and ecosystem properties.     

An invasive fish species within its native range: community effects and population dynamics of Gambusia affinis in the central United States

1. Management of invasive species benefits from detailed information on the biology of the invaders, both from where they have already invaded, and from within their areas of origin. Western mosquitofish, Gambusia affinis, is a widely invasive and destructive freshwater fish. However, within its native range, G. affinis co‐exists with many other fish species in a wide variety of habitats without obvious harm. 2. In this study, we used data on fish communities within the native range of G. affinis at 154 sites across a broad spatial scale to examine the effects of G. affinis on species richness and diversity of residual (species other than G. affinis) fish assemblages. We further used data based on annual samples at eight fixed river sites over 18 summers to examine temporal population dynamics of G. affinis and to test factors associated with population fluctuations. 3. Higher residual species richness occurred in the presence of G. affinis, but residual diversity did not differ. We found an inverse relationship between relative abundance of G. affinis and residual species richness (although effect size was extremely small), but no effect on residual diversity. 4. Gambusia affinis populations fluctuated markedly across summers at all eight fixed sites, but population sizes at a site over time were not autocorrelated. However, population fluctuations were highly correlated among sites across all years, suggesting that regional factors influenced population size. Regional abundance of G. affinis did not correlate with drought, rainfall or winter temperature, but varied with spring temperature. We suggest earlier onset of reproduction in warmer springs resulted in larger summer populations. 5. Overall, within its native range, G. affinis does not appear to impact negatively on the assemblages in which it occurs, possibly due to fluctuations in its density. These findings suggest that introduced Gambusia populations, and those of other invasive species, warrant careful monitoring over long periods of time where they have invaded. Long‐term monitoring of new populations can establish if they are prone to ‘boom and bust’ dynamics, in which case the invader may be less a threat than sometimes assumed. Population information from long‐term studies, either in their native ranges or at invaded sites, can thus help to form the basis of prudent, cost‐effective management strategies for invasive organisms.     

Stronger regional biosecurity is essential to prevent hundreds of harmful biological invasions

Biological invasions often transcend political boundaries, but the capacity of countries to prevent invasions varies. How this variation in biosecurity affects the invasion risks posed to the countries involved is unclear. We aimed to improve the understanding of how the biosecurity of a country influences that of its neighbours. We developed six scenarios that describe biological invasions in regions with contiguous countries. Using data from alien species databases, socio‐economic and biodiversity data and species distribution models, we determined where 86 of 100 of the world's worst invasive species are likely to invade and have a negative impact in the future. Information on the capacity of countries to prevent invasions was used to determine whether such invasions could be avoided. For the selected species, we predicted 2,523 discrete invasions, most of which would have significant negative impacts and are unlikely to be prevented. Of these invasions, approximately a third were predicted to spread from the country in which the species first establishes to neighbouring countries where they would cause significant negative impacts. Most of these invasions are unlikely to be prevented as the country of first establishment has a low capacity to prevent invasions or has little incentive to do so as there will be no impact in that country. Regional biosecurity is therefore essential to prevent future harmful biological invasions. In consequence, we propose that the need for increased regional co‐operation to combat biological invasions be incorporated in global biodiversity targets.     

Acquired phototrophy stabilises coexistence and shapes intrinsic dynamics of an intraguild predator and its prey

In marine ecosystems, acquired phototrophs – organisms that obtain their photosynthetic ability by hosting endosymbionts or stealing plastids from their prey – are omnipresent. Such taxa function as intraguild predators yet depend on their prey to periodically obtain chloroplasts. We present a new theory for the effects of acquired phototrophy on community dynamics by analysing a mathematical model of this predator–prey interaction and experimentally verifying its predictions with a laboratory model system. We show that acquired phototrophy stabilises coexistence, but that the nature of this coexistence exhibits a ‘paradox of enrichment’: as light increases, the coexistence between the acquired phototroph and its prey transitions from a stable equilibrium to boom‐bust cycles whose amplitude increases with light availability. In contrast, heterotrophs and mixotrophic acquired phototrophs (that obtain  < 30% of their carbon from photosynthesis) do not exhibit such cycles. This prediction matches field observations, in which only strict ( > 95% of carbon from photosynthesis) acquired phototrophs form blooms.     

Knowing the past to predict the future: land‐use change and the distribution of invasive bullfrogs

Biological invasions and land‐use changes are two major causes of the global modifications of biodiversity. Habitat suitability models are the tools of choice to predict potential distributions of invasive species. Although land‐use is a key driver of alien species invasions, it is often assumed that land‐use is constant in time. Here we combine historical and present day information, to evaluate whether land‐use changes could explain the dynamic of invasion of the American bullfrog Rana catesbeiana (=Lithobathes catesbeianus) in Northern Italy, from the 1950s to present‐day. We used maxent to build habitat suitability models, on the basis of past (1960s, 1980s) and present‐day data on land‐uses and species distribution. For example, we used models built using the 1960s data to predict distribution in the 1980s, and so on. Furthermore, we used land‐use scenarios to project suitability in the future. Habitat suitability models predicted well the spread of bullfrogs in the subsequent temporal step. Models considering land‐use changes predicted invasion dynamics better than models assuming constant land‐use over the last 50 years. Scenarios of future land‐use suggest that suitability will remain similar in the next years. Habitat suitability models can help to understand and predict the dynamics of invasions; however, land‐use is not constant in time: land‐use modifications can strongly affect invasions; furthermore, both land management and the suitability of a given land‐use class may vary in time. An integration of land‐use changes in studies of biological invasions can help to improve management strategies.     

Catastrophic events and recovery from low densities in populations of otariids: implications for risk of extinction

Two key factors in a population's risk of extinction are major population declines induced by natural or anthropogenic events (catastrophes) and whether the population's rate of growth increases or decreases at very low abundance levels. These two elements should be included in any population viability analysis (PVA), but estimates of the frequency and intensity of catastrophic events and data on the dynamics of low population densities are difficult to obtain. We examined the literature on population dynamics of otariids (fur seals and sea lions), to determine how frequently populations are subjected to major population declines, and to what extent depleted populations recover from low population size. We present frequency distributions for percentage declines for otariid life‐stages (pup, juvenile, adult female and male), and describe eight examples of events leading to a population decline of 50% or greater among otariids. We found that numerous otariid populations have been reduced to very low densities by exploitation (low enough to be thought extinct) and have recovered to levels where they are no longer at risk of extinction. This suggests that the reduction in population rate of increase at low densities in otariid populations may not be strong.     

Marine invasion history and vector analysis of California: a hotspot for western North America

Aim We examine the regional dominance of California as a beachhead for marine biological invasions in western North America and assess the relative contribution of different transfer mechanisms to invasions over time. Location Western North America (California to Alaska, excluding Mexico). Methods We undertook extensive analysis of literature and collections records to characterize the invasion history of non‐native species (invertebrates, microalgae and microorganisms) with established populations in coastal marine (tidal) waters of western North America through 2006. Using these data, we estimated (1) the proportion of first regional records of non‐native species that occurred in California and (2) the relative contribution of transfer mechanisms to California invasions (or vector strength) over time. Results Excluding vascular plants and vertebrates, we identified 290 non‐native marine species with established populations in western North America, and 79% had first regional records from California. Many (40–64%) of the non‐native species in adjacent states and provinces were first reported in California, suggesting northward spread. California also drives the increasing regional rate of detected invasions. Of 257 non‐native species established in California, 59% had first regional records in San Francisco Bay; 57% are known from multiple estuaries, suggesting secondary spread; and a majority were attributed to vessels (ballast water or hull fouling) or oysters, in some combination, but their relative contributions are not clear. For California, more than one vector was possible for 56% of species, and the potential contribution of ballast water, hull fouling and live trade increased over time, unlike other vectors. Main conclusions California, especially San Francisco Bay, plays a pivotal role for marine invasion dynamics for western North America, providing an entry point from which many species spread. This pattern is associated historically with high propagule supply and salinity. Any effective strategies to minimize new invasions throughout this region must (1) focus attention on California and (2) address current uncertainty and future shifts in vector strength.     

Eight questions about invasions and ecosystem functioning

I pose eight questions central to understanding how biological invasions affect ecosystems, assess progress towards answering those questions and suggest ways in which progress might be made. The questions concern the frequency with which invasions affect ecosystems; the circumstances under which ecosystem change is most likely; the functions that are most often affected by invaders; the relationships between changes to ecosystems, communities, and populations; the long-term responses of ecosystems to invasions; interactions between biological invasions and other anthropogenic activities and the difficulty of managing undesirable impacts of non-native species. Some questions have been answered satisfactorily, others require more data and thought, and others might benefit from being reformulated or abandoned. Actions that might speed progress include careful development of trait-based approaches; strategic collection and publication of new data, including more frequent publication of negative results; replacement of expert opinion with hard data where needed; careful consideration of whether questions really need to be answered, especially in cases where answers are being provided for managers and policy-makers; explicit attention to and testing of the domains of theories; integrating invasions better into an ecosystem context; and remembering that our predictive ability is limited and will remain so for the foreseeable future.     

Different mechanisms drive the maintenance of polymorphism at loci subject to strong versus weak fluctuating selection

The long‐running debate about the role of selection in maintaining genetic variation has been given new impetus by the discovery of hundreds of seasonally oscillating polymorphisms in wild Drosophila, possibly stabilized by an alternating summer‐winter selection regime. Historically, there has been skepticism about the potential of temporal variation to balance polymorphism, because selection must be strong to have a meaningful stabilizing effect—unless dominance also varies over time (“reversal of dominance”). Here, we develop a simplified model of seasonally variable selection that simultaneously incorporates four different stabilizing mechanisms, including two genetic mechanisms (“cumulative overdominance” and reversal of dominance), as well as ecological “storage” (“protection from selection” and boom‐bust demography). We use our model to compare the stabilizing effects of these mechanisms. Although reversal of dominance has by far the greatest stabilizing effect, we argue that the three other mechanisms could also stabilize polymorphism under plausible conditions, particularly when all three are present. With many loci subject to diminishing returns epistasis, reversal of dominance stabilizes many alleles of small effect. This makes the combination of the other three mechanisms, which are incapable of stabilizing small effect alleles, a better candidate for stabilizing the detectable frequency oscillations of large effect alleles.     

Range expansion of the invasive house mouse Mus musculus domesticus in Senegal,West Africa: a synthesis of trapping data over three decades, 1983–2014

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Avian ecological succession in the Amazon: A long‐term case study following experimental deforestation

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Range‐wide genetic structure of a cooperative mouse in a semi‐arid zone: Evidence for panmixia

Landscape topography and the mobility of individuals will have fundamental impacts on a species’ population structure, for example by enhancing or reducing gene flow and therefore influencing the effective size and genetic diversity of the population. However, social organization will also influence population genetic structure. For example, species that live and breed in cooperative groups may experience high levels of inbreeding and strong genetic drift. The western pebble‐mound mouse (Pseudomys chapmani), which occupies a highly heterogeneous, semi‐arid landscape in Australia, is an enigmatic social mammal that has the intriguing behaviour of working cooperatively in groups to build permanent pebble mounds above a subterranean burrow system. Here, we used both nuclear (microsatellite) and mitochondrial (mtDNA) markers to analyse the range‐wide population structure of western pebble‐mound mice sourced from multiple social groups. We observed high levels of genetic diversity at the broad scale, very weak genetic differentiation at a finer scale and low levels of inbreeding. Our genetic analyses suggest that the western pebble‐mound mouse population is both panmictic and highly viable. We conclude that high genetic connectivity across the complex landscape is a consequence of the species’ ability to permeate their environment, which may be enhanced by “boom‐bust” population dynamics driven by the semi‐arid climate. More broadly, our results highlight the importance of sampling strategies to infer social structure and demonstrate that sociality is an important component of population genetic structure.     

RAD sequencing reveals genomewide divergence between independent invasions of the European green crab (Carcinus maenas) in the Northwest Atlantic

Genomic studies of invasive species can reveal both invasive pathways and functional differences underpinning patterns of colonization success. The European green crab (Carcinus maenas) was initially introduced to eastern North America nearly 200 years ago where it expanded northwards to eastern Nova Scotia. A subsequent invasion to Nova Scotia from a northern European source allowed further range expansion, providing a unique opportunity to study the invasion genomics of a species with multiple invasions. Here, we use restriction‐site‐associated DNA sequencing‐derived SNPs to explore fine‐scale genomewide differentiation between these two invasions. We identified 9137 loci from green crab sampled from 11 locations along eastern North America and compared spatial variation to mitochondrial COI sequence variation used previously to characterize these invasions. Overall spatial divergence among invasions was high (pairwise F_ST ~0.001 to 0.15) and spread across many loci, with a mean F_ST ~0.052 and 52% of loci examined characterized by F_ST values >0.05. The majority of the most divergent loci (i.e., outliers, ~1.2%) displayed latitudinal clines in allele frequency highlighting extensive genomic divergence among the invasions. Discriminant analysis of principal components (both neutral and outlier loci) clearly resolved the two invasions spatially and was highly correlated with mitochondrial divergence. Our results reveal extensive cryptic intraspecific genomic diversity associated with differing patterns of colonization success and demonstrates clear utility for genomic approaches to delineating the distribution and colonization success of aquatic invasive species.     

Poverty and Inequality in the Rural Brazilian Amazon: A Multidimensional Approach

This paper analyses poverty and inequality dynamics among smallholders along the Transamazon Highway. We measure changes in poverty and inequality for original settlers and new owners, contrasting income-based with multidimensional indices of well-being. Our results show an overall reduction in both poverty and inequality among smallholders, although poverty decline was more pronounced among new owners, while inequality reduction was larger among original settlers. This trend suggests that families have an initial improvement in livelihood and well-being which tends to reach a limit later—a sign of structural limitations common to rural areas and maybe a replication of boom and bust trends in local economies among Amazonian municipalities. In addition, our multidimensional estimates of well-being reveal that some economically viable land use strategies of smallholders (e.g., pasture) may have important ecological implications for the regional landscape. These findings highlight the public policy challenges for fostering sustainable development among rural populations.     

Overlooking the smallest matter: viruses impact biological invasions

Parasites and pathogens have recently received considerable attention for their ability to affect biological invasions, however, researchers have largely overlooked the distinct role of viruses afforded by their unique ability to rapidly mutate and adapt to new hosts. With high mutation and genomic substitution rates, RNA and single‐stranded DNA (ssDNA) viruses may be important constituents of invaded ecosystems, and could potentially behave quite differently from other pathogens. We review evidence suggesting that rapidly evolving viruses impact invasion dynamics in three key ways: (1) Rapidly evolving viruses may prevent exotic species from establishing self‐sustaining populations. (2) Viruses can cause population collapses of exotic species in the introduced range. (3) Viruses can alter the consequences of biological invasions by causing population collapses and extinctions of native species. The ubiquity and frequent host shifting of viruses make their ability to influence invasion events likely. Eludicating the viral ecology of biological invasions will lead to an improved understanding of the causes and consequences of invasions, particularly as regards establishment success and changes to community structure that cannot be explained by direct interspecific interactions among native and exotic species.     

Is Niagara Falls a barrier to gene flow in riverine fishes? A test using genome‐wide SNP data from seven native species

Since the early Holocene, fish population genetics in the Laurentian Great Lakes have been shaped by the dual influences of habitat structure and post‐glacial dispersal. Riverscape genetics theory predicts that longitudinal habitat corridors and unidirectional downstream water‐flow drive the downstream accumulation of genetic diversity, whereas post‐glacial dispersal theory predicts that fish genetic diversity should decrease with increasing distance from glacial refugia. This study examines populations of seven native fish species codistributed above and below the 58 m high Niagara Falls – a hypothesized barrier to gene flow in aquatic species. A better understanding of Niagara Falls’ role as a barrier to gene flow and dispersal is needed to identify drivers of Great Lakes genetic diversity and guide strategies to limit exotic species invasions. We used genome‐wide SNPs and coalescent models to test whether populations are: (a) genetically distinct, consistent with the Niagara Falls barrier hypothesis; (b) more genetically diverse upstream, consistent with post‐glacial expansion theory, or downstream, consistent with the riverscape habitat theory; and (c) have migrated either upstream or downstream past Niagara Falls. We found that genetic diversity is consistently greater below Niagara Falls and the falls are an effective barrier to migration, but two species have probably dispersed upstream past the falls after glacial retreat yet before opening of the Welland Canal. Models restricting migration to after opening of the Welland Canal were generally rejected. These results help explain how river habitat features affect aquatic species’ genetic diversity and highlight the need to better understand post‐glacial dispersal pathways.