Search for: All records

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.

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.

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.