Northward Expansion of Oysters with Climate Change

The invasive pacific oyster, Crassostrea gigas, was once limited to the southern Atlantic regions of Europe, and southern Bourgneuf Bay in France marked their northern-most limit. However, over the past 10 years, the oysters have spread north and have established populations throughout northern European Atlantic regions.  They have been causing declines in performances of farmed oysters in these regions.  Michaël Dutertre and his colleagues (2010) of the University of Nantes studied the environmental variables, reproductive physiology, larval development, and post-larval recruitment of C. gigas to investigate the causes of their recent spread into northern temperature environments.  They found that there has been a trend of warming in the northern Atlantic and increased temperatures have allowed successful reproduction, larval development, and recruitment of the oysters into these regions.— Sanami Nakayama 
Dutertre, M., Beninger, P.G., Barillé, L., Papin, M., Haure, J., 2010. Rising Water Temperatures, Reproduction, and Recruitment of an Invasive Oyster, Crassostrea gigas, on the French Atlantic Coast. Marine Environmental Research 69, 1–9.

 The researchers surveyed C. gigas from a northern, high-turbidity mudflat site, and a southern, sandy-muddy bottom site, in Bourgneuf Bay from 2005–2006.  To analyze reproductive physiology of the oysters, slices of visceral tissue from oysters were examined and oocyte diameters were measured.  These diameters were used to determine the gonadal developmental stage.  Water quality probes were installed at each site to record temperature, salinity, suspended particular matter concentration (SPM, the total amount of particles), particulate organic matter concentration (potentially digestible particles) and chlorophyll-a concentration (amount of available phytoplankton) of the water.  Ratios of POM:SPM, which gives an indication of food availability, and of Chl-a:POM, which gives an indication of the quality of food, were calculated.  D-larva and post-larval densities at each site were determined from water samples.  Daily water and atmospheric temperatures from 1970 to 2006 were also calculated. 
The results of this study show that the expansion of C. gigas northward coincided with a marked increase in water temperature.  Optimal larval development requires a period of at least two weeks during which water temperature is higher than 22 oC.  Massive recruitment of C. gigas in the northern site occurred when summer temperatures started reaching over 20 oC.   Oocyte diameters of oysters at both sites showed that reproductive cycles are timed by water temperature thresholds.  The oocyte growing stage begins when spring water temperature reaches 8-10 oC.  The mature stage was reached more quickly in 2006 than in 2005 because 2006 was marked by reduced daily amplitude of water temperatures and greater spring food quality (i.e. higher Chl-a:POM).  Maximum d-larva density was observed during the summer periods in which water temperature was higher than 20–22 oC, and the northern site showed greater numbers of larvae and post-larvae than the southern site.  Post-larval recruitment was much higher in 2006 than 2005 at both sites.  At the northern site, chlorophyll-a levels were found to control post-larval recruitment. 
Reproductive and developmental processes are thus very sensitive to temperature and an increase in water temperature has allowed for successful oocyte growth, larval development, and greater post-larval recruitment in the northern Atlantic regions of Europe.  As temperature continues to increase, it is expected that C. gigas will expand further north, into coastal areas that are used for oyster farming.  Active management of this species should be considered to prevent the invasive oysters from causing further damage to oyster farms.  

Northward Expansion of Crassostrea gigas with Climate Change

The invasive pacific oyster, Crassostrea gigas, was once limited to the southern Atlantic regions of Europe, and southern Bourgneuf Bay in France marked their northern-most limit. However, over the past 10 years, the oysters have spread north and have established populations throughout northern European Atlantic regions.  They have been causing declines in performances of farmed oysters in these regions.  Michaël Dutertre and his colleagues (2010) of the University of Nantes studied the environmental variables, reproductive physiology, larval development, and post-larval recruitment of C. gigas to investigate the causes of their recent spread into northern temperature environments.  They found that there has been a trend of warming in the northern Atlantic and increased temperatures have allowed successful reproduction, larval development, and recruitment of the oysters into these regions.— Sanami Nakayama 
Dutertre, M., Beninger, P.G., Barillé, L., Papin, M., Haure, J., 2010. Rising Water Temperatures, Reproduction, and Recruitment of an Invasive Oyster, Crassostrea gigas, on the French Atlantic Coast. Marine Environmental Research 69, 1–9.

 The researchers surveyed C. gigas from a northern, high-turbidity mudflat site, and a southern, sandy-muddy bottom site, in Bourgneuf Bay from 2005–2006.  To analyze reproductive physiology of the oysters, slices of visceral tissue from oysters were examined and oocyte diameters were measured.  These diameters were used to determine the gonadal developmental stage.  Water quality probes were installed at each site to record temperature, salinity, suspended particular matter concentration (SPM, the total amount of particles), particulate organic matter concentration (potentially digestible particles) and chlorophyll-a concentration (amount of available phytoplankton) of the water.  Ratios of POM:SPM, which gives an indication of food availability, and of Chl-a:POM, which gives an indication of the quality of food, were calculated.  D-larva and post-larval densities at each site were determined from water samples.  Daily water and atmospheric temperatures from 1970 to 2006 were also calculated. 
The results of this study show that the expansion of C. gigas northward coincided with a marked increase in water temperature.  Optimal larval development requires a period of at least two weeks during which water temperature is higher than 22 oC.  Massive recruitment of C. gigas in the northern site occurred when summer temperatures started reaching over 20 oC.   Oocyte diameters of oysters at both sites showed that reproductive cycles are timed by water temperature thresholds.  The oocyte growing stage begins when spring water temperature reaches 8-10 oC.  The mature stage was reached more quickly in 2006 than in 2005 because 2006 was marked by reduced daily amplitude of water temperatures and greater spring food quality (i.e. higher Chl-a:POM).  Maximum d-larva density was observed during the summer periods in which water temperature was higher than 20–22 oC, and the northern site showed greater numbers of larvae and post-larvae than the southern site.  Post-larval recruitment was much higher in 2006 than 2005 at both sites.  At the northern site, chlorophyll-a levels were found to control post-larval recruitment. 
Reproductive and developmental processes are thus very sensitive to temperature and an increase in water temperature has allowed for successful oocyte growth, larval development, and greater post-larval recruitment in the northern Atlantic regions of Europe.  As temperature continues to increase, it is expected that C. gigas will expand further north, into coastal areas that are used for oyster farming.  Active management of this species should be considered to prevent the invasive oysters from causing further damage to oyster farms.  

Potential Distribution of the Invasive Pest Tetranychus evansi

Tomato red spider mite, Tetranychus evansi, is a major invasive pest of solanaceous plants that have caused major crop losses throughout the world.  The change in distribution of this species with climate change is becoming a growing concern.  Alain Migeon and his colleagues (2009) developed a model to predict the potential distribution of the species under current climate conditions and to determine the climate factors that limit their distribution.  They found that T. evansi has the potential to survive in areas that they do not currently occupy and various climate factors such as dryness, excess moisture, and cold stress limit their distribution.— Sanami Nakayama 
  
Migeon, A., Ferragut, F., Escudero-Colomar, L.A., Fiaboe, K., Knapp, M., de Moraes G.J., Ueckermann, E., Navajas, M., 2009. Modelling the Potential Distribution of the Invasive Tomato Red Spider Mite, Tetranychus evansi (Acari: Tetranychidae). Experimental and Applied Acarology 48, 199–212.

 The main hosts of T. evansi are solanaceous plants, including tobacco, tomato, and eggplant.    The mites cause serious damage to these crops throughout the world, including North America, Indian Ocean Islands, countries in sub-Sarahan Africa, the Mediterranean, Hawaii, and Taiwan.  Because T. evansi is a cause of major economic loss, it is crucial for us to prevent their expansion.  Migeon and his colleagues used a CLIMEX model to predict the potential distribution of T. evansi.  This model uses conditions that favor the growth and four stress indices (cold, heat, dry and wet) and their interactions (cold-wet, cold-dry, heat-wet, heat-dry) of a species to determine its ability to persist in a certain location.  This model also uses climatic parameters of environments that a species is currently found in to determine the areas that are climatically suitable for the species.  Minimum and maximum temperature thresholds of 10 oC and 38 oC and a dry stress threshold and wet stress threshold, in terms of proportion f soil moisture holding capacity, of 0.15 and 1.5 were used in the model.  A cold-wet stress temperature threshold of 12 oC and a hot-wet stress temperature threshold of 32 oC were also used.
The researchers found that in addition to the areas that T. evansi currently occupies, they have the potential to occupy additional areas of land.  The model shows that areas that are not currently occupied by the mites but are climatically suitable include parts of Asia, all of the Australian coasts, and many parts of Central America and the Caribbean.  Cold stress, dry stress, wet stress and hot-wet stress are the limiting factors of the distribution of T. evansi in South America.  In temperate zones, dry stress, wet stress, and cold-stress limit their distribution.
In the Mediterranean Basin and Japan, the distribution of T. evansi is limited by cold climatic conditions and cold stress.  In the Mediterranean Basin, the mites are restricted to coastal areas where minimum temperature is relatively high.  In Japan, the mites are restricted to climatically mild areas.  These areas represent their northern-most limit because north of this limit, temperatures become too low for successful development of T. evansi. 
In most areas, temperature stress plays an important role in determining the distribution of T. evansi.  As the climate continues to warm, it is likely that more areas will become more climatically suitable for the mites.  It is thus becoming more pertinent for the management of T. evansi and the prevention of their expansion into novel areas.  The researchers suggest that we should determine the pathways by which T. evansi disperse as a first step in the management of the species.    

Introduction of Non-Native Species through Aquarium Trade

A growing vector for the introduction of non-native aquatic species to new environments in the US is the aquarium trade.  Andrew L. Chang and his colleagues (2009) studied this vector by surveying pet stores in the San Francisco Bay-Delta region to determine which fish species being sold have the potential to become invasive if introduced to the Bay-Delta.  Using a conservative model, the researchers found that 5 species sold throughout the region could survive in the Bay-Delta if introduced.  Using a less conservative model, they found that up to 27 species could survive in the Bay-Delta. .— Sanami Nakayama 
Chang, A.L., Grossman, J.D., Spezio, T.S., Weiskel, H.W., Blum, J.C., Bury, J.W., Muir, A.A., Piovia-Scott, J., Veblen, K.E., Grosholz, E.D., 2009. Tackling Aquatic Invasions: Risks and Opportunities for the Aquarium Fish Industry. Biological Invasions 11, 773–785.

  The researchers first identified 168 stores in the San Francisco Bay-Delta area that sold aquarium fish.  They then separated these stores into three regions based on the store’s proximity to freshwater or saltwater regions of the Bay-Delta and they classified each store as either independent (individually owned and operated) or a chain store (stores that are part of a chain with multiple retail stores and centralized management).  They then randomly selected nine independent and nine chain stores in each region to survey.  At each store, they conducted an inventory of all of the fish being sold and for each tank, they recorded the species listed on the tank, the species actually present in the tank, and any additional labeling information that was available.  In addition, they conducted telephone surveys to ask store representatives about their knowledge of invasive species and their willingness to address threats posed by these species. 
To determine whether a species has the potential to survive in, and invade, the Bay-Delta region, the researchers separated the region into marine and freshwater subregions and developed a model to compare temperature and salinity tolerances of each species to environmental data for each salinity region.  Because minimum temperature tolerance is a major limiting factor in fish survival, they chose to use the warmest minimum temperature in each salinity region as the criterion for determining survival potential.  Because preliminary models gave results that underestimated invasion potential of aquarium fish, the researchers came up with two models: a more conservative, colder, scenario, and a less conservative, warmer, scenario.    Under the colder scenario, they compared lowest recorded temperature tolerances of each species to the warmest minimum temperature of the Bay-Delta in each salinity region.  Under the warmer scenario, warmest minimum temperatures of the regions were increased by 3 oC and fish temperature tolerances were reduced by 3oC. 
Of the 867 fish species that the researchers identified, under the colder scenario, three freshwater and two saltwater species had the potential to become invasive if introduced to the Bay Delta.  Under the warmer scenario, nine freshwater and eighteen saltwater species had the potential to become invasive.  Nearly all independent and chain stores sold at least one of these species.  Under the colder scenario, chain stores had a significantly greater number of potentially invasive species compared to independent stores.  Under the warmer scenario, there was no difference.  It was also found that a greater percentage of fish species were labeled correctly in chain stores compared to independent stores. A majority of the respondents of the telephone surveys had some knowledge of invasive species and most were willing to sell different species in place of those that are potentially invasive. 
The results of this study suggest that as global temperatures continue to increase, an increasing number of fish species in the aquarium trade are likely to have the potential to survive and become invasive if introduced to the San Francisco Bay-Delta Region.  It is thus becoming more imperative for management of potentially invasive species in the trade to reduce the risk of introducing these species to the Bay-Delta.  The researchers suggest that two management actions should be taken to reduce the risk of invasion: the implementation of programs to enhance invasive species awareness and education among store representatives, and improvement in fish labeling practices.  If store representatives are knowledgeable about invasive species, they can relay important information to their customers and advise them of the risks of their purchases.  Improving labels by including information such as life history traits and warnings about the potential hazards of releasing pets can help customers avoid buying fish that they will be unhappy with and reduce inappropriate disposal of unwanted fish into local waterways.

Changes in Growth Rates of Invasive and Native Plant Species in Response to Elevated CO2 Levels

Rapidly increasing global CO2 levels will have profound effects on the growth of many plant species by directly impacting photosynthetic processes.  Purnima Raizada and her colleagues investigated whether invasive and native dry deciduous species of India respond differently to elevated CO2 levels.  They grew seedlings of two invasive and four native plant species under ambient and elevated CO2 levels and compared the growths of each species.  They found that growth response to elevated CO2 levels varied among species but biomass, relative growth rate, and net assimilation rate of invasive species were higher than those of native species. .— Sanami Nakayama 
Raizada, P., Singh, A., Raghubanshi, A.S., 2009. Comparative Responses of Seedlings of Selected Native Dry Tropical and Alien Invasive Species to CO2 Enrichment. Journal of Plant Ecology 2, 69–75.
The researchers used the invasive species Lantana camara and Hyptis suaveolens, the two most important invaders in dry deciduous forests of India.  They used the four native species Acacia catechu, Bauhinia variegate, Dalbergia latifolia, and Tectona grandis.  They planted seedlings of each species under ambient (375–395 µmol/mol) or elevated CO2 levels (700–750 µmol/mol).  To expose plants to elevated CO2, the researchers used decomposed manure as a source of CO2 and grew the seedling in trenches that were covered with polythene frames.  Seedlings grown under ambient conditions were planted in trenches that were left uncovered, without organic matter.  The researchers used three plants of each species under each treatment.
Initially, before being grown under a treatment, randomly chosen plants of each species were harvested to measure initial growth data.  After 60 days of exposure to ambient or elevated CO2, the researchers measured biomass partition parameters (root shoot ratio, root mass fraction, stem mass fraction, leaf mass fraction, and leaf area ratio) and growth parameters (specific leaf area, relative growth rate, and net assimilation rate) of each plant.  These parameters were then used to compare biomass accumulation and growth of each species. 
Growth performance of seedlings in response to elevated CO2 levels differed across species.  Elevated CO2 significantly promoted growth of seedlings of all six species, but increase in height was greater for the invasive species than the natives.  The invasives also accumulated more biomass under elevated CO2 compared to the natives.  Net assimilation rates and relative growth rates were also higher in the invasives.  The researchers suggest that higher relative growth rates in the invasive species will give them relative advantage over the natives and may help them become competitively dominant under different environmental stresses. 
The results of this study show that as global CO2 levels increase, we can expect the invasive species L. camara and H. suaveolens to perform better than many native species in dry deciduous forests of India.  The invasive plants may become competitively dominant, changing competitive hierarchies and the structures and functions of these forests.  Management of the invasives may become more difficult as CO2 levels increase because of their rapid growth and adaptability under these conditions.  

Alien Beetle Species Takes Over Native Beetle Species in a Mountain-Top Environment

High-elevation mountain environments are one of the most sensitive ecosystems to the warming climate because they are characterized by low temperatures.  Because plants and animals that live in these environments must be able to tolerate harsh conditions created by low temperatures, it was believed that these environments are not likely to experience biological invasions.  However, with the modern world’s increasing temperatures, researchers have become concerned that high-elevation mountain environments are becoming more prone to invasions.  Marco A. Molina-Montenegro and his colleagues investigated the effects of global warming on two ladybird species, Eriopis connexa and Hippodamia variegata, in the Andes of central Chile (Montenegro et al. 2009).  E. connexa is a native species to Chile, while H. variegata is an alien species.  The results of this study show that as the climate warms, the alien species will increase in abundance and drive the native species out of their mountain-top habitats.— Sanami Nakayama 
Molina-Montenegro, M.A., Briones, R., Cavieres, L.A., 2009. Does Global Warming Induce Segregation Among Alien and Native Beetle Species in a Mountain-Top? Ecol Res 24, 31–26.

 The researchers looked specifically at the differences in abundances of the two ladybird species in two different habitats- one at exposed sites on the mountain-top and one in open-top chambers (OTCs).  OTCs are chambers that prevent heat loss, and the temperature inside a chamber is 4-5oC higher than the surrounding environment.  Twelve cushions plants, plants on which the ladybirds live, were randomly chosen. OTCs were placed around six of them; the other six were left exposed to the mountain-top environment.  The abundance of the two ladybird species were estimated once a month, from November to March.  The soil moisture and abundance of other arthropods at each site were also estimated. 
The researchers found that the abundance of ladybirds were higher at sites with OTCs compared to sites without, for both species.  At sites without OTCs, the abundance of H. variegata and E. connexa did not differ.  However, at sites with OTCs, the abundance of the alien species was 13 times greater than that of the native species.  Soil moisture at sites with and without OTCs did not differ, suggesting that the difference in abundance of the two species at sites with OTCs is due to the temperature inside the chambers.  The researchers also found a total of eight arthropod species at sites with OTCs, most of which are alien species.  They found that the abundances of all of these species were higher at sites with OTCs compared to those without.  Only five of these species were found at the sites without OTCs. 
The results of this study suggest that global warming will have an effect on the abundance of both H. variegata and E. connexa.  With increasing temperature, the alien species will increase in abundance, while the native species are driven to possible extinction.  The researchers believe that the alien species are benefiting from the warmer temperature because they can be successful in a range of temperatures.  These beetles have been found to be more adaptive in behavior and metabolism compared to the native species, allowing them to adjust quickly to changing temperatures.  Thus while the native species struggles to adapt to the change in their environment, the alien species may out compete the native species and drive them out of their habitats.  The results of this study also suggest that we should be cautious about possible invasion by other alien, arthropod species in high-elevation mountain environments with the warming climate.   

Projected Ranges of Invasive Hawkweeds in Australia with Climate Change

Management of invasive species is crucial within conservation reserves in Australia because many species listed under the Threatened Species Conservation Act of 1995 are threatened by invasive species.  Beaumont and his colleagues investigated the projected distributions of three invasive hawkweeds (Hieracium spp.) under current and future climate conditions using ecological niche models (Beaumont et al. 2009).  They found that the hawkweeds still have the potential to increase their ranges under current climate condition but as the climate warms, their ranges are likely to contract overall.  Though the hawkweeds have not established in the Australian Alps, much of the conservation reserves in the Alps are currently climatically suitable for the weeds and they will remain suitable until at least 2070. — Sanami Nakayama 
Beaumont, L.J., Gallagher, R.V., Downey, P.O., Thuiller, W., Leishman, M.R., Hughes, L., 2009.  Modelling the Impact of Hieracium spp. on Protected Areas in Australia Under Future Climates. Ecography 32, 757–764.

 The two main goals of this study were to 1) assess the potential distributions of three hawkweed species Hiernacium pilosella, Hiernacium aurantiacum, and Hiernacium murorum under current and future climate conditions and 2) determine whether the potential ranges of these invasive species coincide with conservation reserves.  The researchers used eight ecological niche models to assess the potential distributions of the hawkweeds under current and future climate conditions in 2030 and 2070.  Ecological niche models are commonly used to generate projections of ranges of exotic species by determining the areas that are likely to remain climatically suitable for a species.  The researchers obtained information about areas that are set aside for conservation in every Australian state and territory, and they used ArcGIS to determine the extent to which the projected distributions of hawkweeds coincide with these reserves. 
Results show that the three hawkweed species have the potential to increase in range and occupy larger areas than those they currently occupy.  The researchers suggest that the weeds have not expanded into areas that are currently climatically suitable for them because they have not had enough time since they were introduced to Australia.  All three species were introduced to Australia less than 20 years ago, and within the next few years, the weeds may realize their invasive potential and expand into larger areas.  The researchers also suggest that the ability of the weeds to expand into suitable areas may be limited by their poor dispersal. 
 As the climate warms, it is projected that the ranges of the hawkweeds will contract overall.  However, about a fifth of the areas that are projected to be currently suitable for the weeds are contained within conservation reserves in the Australian Alps, which are home to many endemic species.  As temperature increases, a larger fraction of suitable areas are projected to be contained within reserves.  The results of this study emphasize the need for control and management of the hawkweeds to minimize the possibility of these species moving into conservation reserves.

Possible Restoration of Native Plant Species with Climate Change

It is widely expected that with global warming, invasive plant species will expand in range and pose further threats to native species.  Bethany Bradley and her colleagues at Princeton University, however, examined possible reductions in invasive plant competitiveness with climate change because conditions may become climatically unsuitable for some invasive species (Bradley et al. 2009).  The researchers used bioclimatic envelope modeling to predict the range shifts of five invasive plant species in the western US by 2100.  From these models, they found that two of the species will expand in range, two will shift in range, and one will contract.  Shifts and contraction in ranges of invasives may create opportunities for restoration of native species.— Sanami Nakayama
Bradley, B.A., Oppenheimer, M., Wilcove, D.S., 2009. Climate Change and Plant Invasions: Restoration Opportunities Ahead? Global Change Biology 15, 1511–1521.

 The researchers used bioclimatic envelope modeling, an approach used to predict species distributions based on geographical relationships between current distributions of the species and future climate conditions, to predict the ranges of five invasive plant species: Bromus tectorum, Centaurea biebersteinii, Centaurnea solstitialis, Tamarix spp., and Euphorbia esula.  They used these five species because they are some of the most problematic invasive species in the western US.  They are currently widespread, they outcompete native species, and they have been able to dominate and alter ecosystems.  In their models, the researchers used climate conditions that best predict the presence of a species, and these climate variables were determined by identifying climate conditions that most constrained a species distribution.  Future climate conditions were derived from a compilation of 10 atmosphere-ocean general circulation models (AOGCMs) of precipitation, minimum temperature, and maximum temperature change by 2100.  Using four climatic variables, bioclimatic envelopes were created to predict the distributions of the five invasive plant species by 2100. 
Based on predictions made by these models, by 2100 only two of the species will expand in range.  One will contract, and two will expand in some places and contract in others.  The two likely to expand, C. solstitialis and Tamarix spp., also are likely to prevent restoration in areas they currently occupy.  B. tectorum and C. biebersteinii are likely to shift in range because climatically suitable areas for these species are likely to move northwards, thus they will expand into some regions while they contract from others.  E. esula are likely decrease in range and contract from the regions it currently occupies.  
There are restoration opportunities for native plant species in regions from which invasive species contract.  The researchers suggest that modeling and experimental work should be done to determine whether it is possible for native species to occupy these areas.  They may not be able to re-establish because of climate change but if restoration of native species is possible, we should attempt to re-introduce the species soon after the invasive species have retreated, to prevent new invasive species from establishing in these areas.

Effects of Decadal Climate Oscillations on Distribution of an Invasive Mussel Species

There have been shifts in biogeographic ranges of many plant and animal species due to global warming.  For some of these species, scientists have been able to predict the direction in which they will shift with increasing temperature.  Thomas J. Hilbish and his colleagues at the University of South Carolina investigated whether changes in distribution of two mussel species, Mytilus galloprovincialis and Mytilus trossulus, and their hybrids along the coast of California follow the predictions made by the global warming hypothesis (Hilbish et al. 2010).  M. trossulus is native to the coast of California while M. galloprovincialis is a warm-water species that was introduced to this region via ballast water.  The two species have hybridized and formed a hybrid zone throughout the coast of California.  The global warming hypothesis predicts that M. galloprovincialis are moving poleward while M. trossulus are moving towards the equator with the warming climate.  The researchers found that shifts in ranges of these species are in directions opposite of those predicted by the global warming hypothesis, but these shifts can be explained by the effects of decadal climate oscillations.— Sanami Nakayama
 
Hilbish, T.J., Brannock, P.M., Jones, K.R., Smith, A.B., Bullock, B.N., Wethey, D.S., 2010. Historical Changes in the Distributions of Invasive and Endemic Marine Invertebrates are Contrary to Global Warming Predictions: The Effects of Decadal Climate Oscillations. Journal of Biogeography 37, 423–431.
To determine whether there have been changes in the biogeographic ranges of the two mussel species, the researchers sampled mussels from various open-coast locations in California, running from south of Monterey Bay to North of Humboldt Bay.  They compared their findings to those of an earlier study, conducted 10 years earlier by Paul D. Rawson and his colleagues.  All mussels collected were genetically analyzed at three nuclear gene loci and were classified as either M. galloprovincialis or M. trossulus, depending on which species-specific allele they were homozygous for.  All other mussels were classified as hybrids.  The researchers also looked at changes in upwellings and sea surface temperatures (SST) at the study sites within the last 10 years, using upwelling indices and data from the National Data Buoy Center from the National Oceanic and Atmospheric Administration (NOAA).
Contrary to predictions made by the global warming hypothesis, M. galloprovincialis and the hybrid zone have shifted their geographic ranges south, towards the equator.   They are almost completely absent from the northern-most sites, where they were abundantly present 10 years ago.  At every site north of Monterey Bay, M. galloprovincialis are less abundant than M. trossulus.  South of Monterey Bay, the sites are still dominated by the invasive species.  The geographic range of M. trossulus, however, has remained unchanged over the past 10 years.
 The researchers believe that the shift in the range of M. galloprovincialis and the hybrid zone to the south is due to decadal climate oscillations.  In the 1990s, the western coast of the US was experiencing a warm phase of the Pacific Decadal Oscillation (PDO) and strong El Niño activity.  Recently, this region is experiencing a cool phase of the PDO and mild El Niño activity.  At most of the buoy sites, SST were lower by about 1 oC from 1999-2005 compared to SST from 1982-1987.  Upwellings, which get stronger during cold phases of a PDO, have also been greater in central and northern California from 1999-2005 compared with 1982-1987.  Upwellings are events that cause loss of warmer surface waters from on-shore regions, which are then replaced by colder, deeper waters from offshore.  Consequently, SST of near-shore regions decrease during times of great upwellings. 
The researchers believe that the most probable explanation for the shift in the ranges of M. galloprovincialis and the hybrid zone is due to the declining SST of the northern and central coast due to the current phase of the PDO.  SST of the study sites south of Monterey Bay have not significantly changed over the past 10 years, thus the abundance of M. galloprovincialis at these sites has remained relatively unchanged.  A decrease in SST by 1 oC was not significant enough to have an effect on M. trossulus.  The researchers also suggest that other explanations may explain the shift in range of M. galloprovincialis, such as increased larval mortality at lower temperatures, low larval recruitment due to stronger upwellings, and northwards dispersal of larvae during a strong El Niño event that occurred 10 years ago.  None the less, this study shows that it is important to take into account decadal climate oscillations when trying to understand the long-term effects of global warming on ecological communities.