Despite last month’s cold snap in Texas and Louisiana, climate change is driving warmer winter temperatures across the southern United States, creating a golden opportunity for many tropical plants and animals to move north, according to a new study published this week in the journal. Biology of global change.
Some of these species may be welcome, such as sea turtles and the Florida manatee, which extend their range northward along the Atlantic coast. Others, like the invasive Burmese python – in the Florida Everglades the largest was 18 feet from end to end – perhaps less.
Insects, including mosquitoes that carry diseases such as West Nile virus, Zika virus, dengue and yellow fever, and beetles that destroy native trees, are also unwelcome and among the fastest to spread in the areas. warming zones.
“Many species of mosquitoes grow north, as well as many forest pests: bark beetles, the southern mountain pine beetle,” said Caroline Williams, associate professor of integrative biology at the University of California, Berkeley, and a co-author of the article. “In our study, we really focused on that border in the United States, where we get this rapid transition between tropical and temperate regions. One of the main, if not the main, drivers of the shift in distributions are changes in winter conditions.
This transition zone, north of which frosts occur every winter, has always been a hindrance for species that have evolved at more stable temperatures, said Williams, who specializes in insect metabolism – in particular, How winter frosts and snow affect the survival of species.
“For the vast majority of organisms, if they freeze, they die,” she said. “Cold snap like the one that happened recently in Texas might not happen for another 30, 50, or even 100 years, and then you see these widespread death events where tropical species that have crawled north are suddenly pushed back. But as the return times get longer and longer for these episodes of extreme cold, it allows tropical species to establish themselves more and more, and perhaps even populations to adapt in situ to. allow them to tolerate more extreme cold in the future. “
The study, conducted by a team of 16 scientists led by the US Geological Survey (USGS), focused on the effects of warming winters on the movement of a wide range of sensitive tropical plants and animals. cold towards the southern United States, in particular towards the eight subtropical states of the American continent: Florida, Alabama, Mississippi, Louisiana, Texas, New Mexico, Arizona and California. Williams and Katie Marshall of the University of British Columbia in Vancouver co-authored the section on insects for the study.
The team found that a number of tropical species, including insects, fish, reptiles, amphibians, mammals, grasses, shrubs, and trees, are expanding their range northward. Among them are species native to the United States, such as mangroves, which are salt tolerant tropical trees; and snook, a warm-water coastal sport fish; and invasive species such as Burmese pythons, Cuban tree frogs, Brazilian pepper trees and buffalo.
“We don’t expect this to be an ongoing process,” said USGS research ecologist Michael Osland, lead author of the study. “There’s going to be an expansion north, then a contraction with extreme cold events, like the one that just happened in Texas, then a movement again. But by the end of this century, we expect for tropicalization to occur. “
The authors document several decades of changes in the frequency and intensity of extreme cold spells in San Francisco, Tucson, New Orleans, and Tampa – all cities with temperature records dating back to at least 1948. In each city, they found, average winter temperatures have risen over time, the colder winter temperatures have warmed up, and there are fewer days each winter when the mercury drops below freezing.
Temperature readings from San Francisco International Airport, for example, show that prior to 1980, each winter typically experienced several days below freezing. In the past 20 years, there has only been one day with below freezing temperatures.
Changes already underway or planned in the home ranges of 22 plant and animal species from California to Florida include:
- Continuous displacement of plants from temperate salt marshes through cold-sensitive mangrove forests along the Gulf and southern Atlantic coasts. Although this encroachment has occurred over the past 30 years, with sea level rise, mangroves can also move inland, displacing temperate and freshwater forests.
- Buffelgrass and other annual grasses moving through the deserts of the southwest, fueling forest fires in native plant communities that have not evolved in conjunction with frequent fires.
- The likelihood that tropical mosquitoes that can transmit encephalitis, West Nile virus and other diseases will further expand their range, exposing millions of people and wildlife to these diseases.
- Likely northward movement, with warming winters, of the southern pine beetle, a pest that can damage commercially valuable pine forests in the southeast.
- Disruption of recreational and commercial fisheries by altering migration patterns and northward movement of inshore fish.
The changes are expected to lead to the replacement of some plant and animal communities in the temperate zones found in the southern United States with tropical communities.
“Unfortunately, the general story is that the species that are going to do really well are the more generalist species – their host plants or food sources are quite varied or widely distributed, and they have a relatively broad thermal tolerance, so that ‘they can tolerate a wide range of conditions,’ said Williams. “And, by definition, these species tend to be the pest species – which is why they are pests: they are adaptable, widespread and relatively unhindered by changing conditions, whereas the more specialized or the most boutique tend to decline as they become displaced from their relatively narrow niche. “
She warned that insect populations on the whole are declining around the world.
“We are seeing an alarming decrease in the total number of natural areas, managed areas, national parks, tropical rainforests – globally,” she said. “So while we are seeing some widespread pest species increase, the general trend is for insects to decline extremely quickly.”
The authors suggest extensive laboratory studies to learn how tropical species can adapt to extreme conditions and modeling to show how lengthening intervals between cold snaps will affect plant and animal communities.
“On a hopeful note, it is not that we are heading towards extinction of absolutely everything, but we must prepare for widespread changes in the distribution of biodiversity as the climate, including the winter climate is changing, ”said Williams. “The actions we take over the next 20 years will be critical in determining our course. Along with obvious changes, like reducing our carbon footprint, we need to protect and restore insect habitat. Individuals can create their own habitat. backyards for insects by growing native plants that support pollinators and other native insects. These are small things that people can do that can be important in providing corridors for species to move through our highly fragmented habitats. “