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  1. Abstract

    Adaptation across environmental gradients has been demonstrated in numerous systems with extensive dispersal, despite high gene flow and consequently low genetic structure. The speed and mechanisms by which such adaptation occurs remain poorly resolved, but are critical to understanding species spread and persistence in a changing world. Here, we investigate these mechanisms in the European green crabCarcinus maenas, a globally distributed invader. We focus on a northwestern Pacific population that spread across >12 degrees of latitude in 10 years from a single source, following its introduction <35 years ago. Using six locations spanning >1500 km, we examine genetic structure using 9376 single nucleotide polymorphisms (SNPs). We find high connectivity among five locations, with significant structure between these locations and an enclosed lagoon with limited connectivity to the coast. Among the five highly connected locations, the only structure observed was a cline driven by a handful of SNPs strongly associated with latitude and winter temperature. These SNPs are almost exclusively found in a large cluster of genes in strong linkage disequilibrium that was previously identified as a candidate for cold tolerance adaptation in this species. This region may represent a balanced polymorphism that evolved to promote rapid adaptation in variable environments despite high gene flow, and which now contributes to successful invasion and spread in a novel environment. This research suggests an answer to the paradox of genetically depauperate yet successful invaders: populations may be able to adapt via a few variants of large effect despite low overall diversity.

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  2. Abstract

    Extreme climate events, such as drought, are becoming increasingly important drivers of plant community change, yet little is known about their impacts on invasive plants. Further, drought impacts may be altered by other anthropogenic stressors, such as eutrophication. We found drought dramatically reduced density of invasiveLepidium latifoliumin salt marshes, and this die‐back was mitigated by nutrient addition. In a 3‐yr field experiment (2014–2016) conducted during an unprecedented drought (2012–2015), we tracked the effects of drought and nutrient addition on the plant community. We conducted this research at four salt marshes across a salinity gradient in the San Francisco Bay, California, USA. We manipulated paired native and invaded plots, one‐half of which were treated monthly with N and P for 1.5 yr during the most intense period of the drought and one subsequent wet winter. In addition, we monitored unmanipulatedL. latifolium‐invaded transects within our freshest and most saline sites throughout the three years of our manipulative experiment and one additional wet winter. We documented a dramatic die‐back of invasiveL. latifoliumduring extreme drought, with reductions in stem density (52–100%) and height (17–47%) that were more severe at low salinity sites than high salinity sites. We found nutrient application lessened the effect of drought onL. latifoliumstem density, but not height. In native plots, extreme drought reduced native plant cover (4–24%), but nutrient addition mitigated this impact. Interestingly, native plants in invaded plots did not suffer reductions in cover due to drought, perhaps because they were simultaneously benefiting from the die‐back of the invader. Our results show drought negatively impacted both native and invasive plants and this impact was stronger on the invader, which experienced persistent declines two years after the end of the drought. However, by mitigating the effect of drought on invasive plants, nutrient addition potentially erased the advantage drought provided native plants over invasive plants under ambient nutrient conditions.

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  3. Abstract

    Climate change is projected to increase the frequency of extreme drought events, which can have dramatic consequences for ecosystems. Extreme drought may interact with other stressors such as invasion by non‐native species, yet little research has explored these dynamics. Here, we examine the physical mechanisms and temporal scale underlying a dieback of an invasive non‐native plant,Lepidium latifolium,in tidal salt marshes of the San Francisco Bay, California, USA, during an extreme, multi‐year drought occurring from 2012 to 2015. Using generalized additive mixed models (GAMMs), we explored the relationship between eight years of estuarine salinity data and five years ofL. latifoliumdensity data from three marshes spanning a gradient of salinity across the San Francisco Bay. We found a significant time‐lagged (3 yr) effect of estuarine salinity onL. latifoliumdensity, with high salinities preceding reductions inL. latifoliumdensities and low salinities preceding increases. The most dramatic change in stem density, a 54% reduction in 2015, was preceded by a salinity increase of 43% from 2011 to 2012. We found theL. latifoliumdecline was driven by impacts on mature, rather than young, plants. Additionally, we tested the importance of local precipitation in drivingL. latifoliumdensities in a one‐season rain exclusion experiment. We found 100% exclusion of precipitation during one rainy season (January–mid‐May) did not have a significant impact on densities of mature stands ofL. latifolium. Our finding that estuarine salinity was a key driver ofL. latifoliuminvasion dynamics suggests sea level rise, like extreme drought, may hinderL. latifoliuminvasion, as it will also raise estuarine salinities. Further, our study highlights the importance of temporal lags in understanding climate change impacts on biological invasions, which has received very little study to date.

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  4. Abstract

    We describe the range expansion and first occurrence of the subtropical crabPortunus xantusii(Stimpson, 1860) in northern California during 2016 and link the range expansion to the regional extreme water temperature event during this time. We collectedP. xantusiioccurrence data from crab trapping surveys conducted along the California coast as well as incidental observations by fishermen and SCUBA divers. We then analyzed 10 years of regional offshore temperature patterns using National Data Buoy Center data around the trapping region. We also examined evidence of northern California warm water refugia using sensors monitoring Tomales Bay, Elkhorn Slough, and San Francisco Bay. We found thatP. xantusiiwas present in every major estuary north of Monterey Bay and as far north as Tomales Bay and that the documented range expansion was likely due to the unusual oceanographic event that occurred northern California during this time period. Mean offshore temperatures and mean nearshore temperatures during 2014–2016 were about 2°C (one standard deviation) higher than the 2006–2013 mean, with extreme temperatures reaching three standard deviations above the 2006–2013 mean. We suggest that this unusual warm water event permitted survival of dispersing larvae ofP. xantusiilarvae northward via coastal currents, and that the extended warm water period allowedP. xantusiito complete its development. Long‐term crab trapping programs in place since 1994 within this region provide robust support for the absence ofP. xantusiiprior to 2016. Temperature data indicate that the estuaries in which adultP. xantusiiwas found could allow persistence of adultP. xantusiiin northern California.

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  5. Abstract Aim

    Mesophotic ecosystems, found at the limit of light penetration in the ocean, are rich in biodiversity and harbour unique ecological communities. However, they remain among the least studied habitat zones on earth due to the high costs and technological limitations. Here, we characterize mesophotic communities in two marine reserves across a range of habitat types, depths and temperatures using submersible technologies, with the goal of understanding the processes that structure these communities across biogeographical regions.


    The Bay of La Paz and the Revillagigedo Archipelago, Mexico.


    Fish and algal species.


    We used a small and inexpensive remotely operated vehicle (ROV) to conduct roving‐swim surveys of major habitat types in depths from 12 to 94 m. With the resulting binary data on the presence of fish species, we used generalized linear mixed models and canonical correspondence analysis to determine whether biogenic habitat, depth and/or temperature best explained species richness and community structure across reef and non‐reef substrate.


    We identified 72 species or genera, including new depth records for nine fish species and a new geographical record for one fish species. Our surveys included large undocumented rhodolith beds (free‐living coralline algae) and mesophotic algal communities, in addition to diverse communities of soft corals and sponges. Fish species richness was positively associated with rocky substrate and warmer water, and reef fish communities differed significantly by depth, temperature and biogenic habitat.

    Main conclusion

    Our results highlight the importance of biogenic habitat in structuring communities across gradients of depth and temperature. We also demonstrate the effectiveness of a small and economical ROV for conducting mesophotic surveys in remote regions. Our methods and results provide a framework that can be used to greatly increase the biogeographical and taxonomic scope of mesophotic research, especially for readily identifiable taxa such as fish.

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