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article imageTropical plants and animals moving north as planet warms

By Karen Graham     Mar 31, 2021 in Environment
As climate change reduces the frequency and intensity of killing freezes, tropical plants and animals that once could survive in only a few parts of the U.S. mainland are expanding their ranges northward, a new U.S. Geological Survey-led study has found.
The study, published in the journal Global Change Biology looks at the northward trek of a variety of tropical species in North America, from mangrove trees to Burmese pythons.
The authors, led by Michael Osland, an ecologist with the U.S. Geological Survey, focus on the “tropical-temperate transition zones” - areas where occasional intense freezes restrict the range of tropical plants and animals.
While some northward expansion of species such as the Florida manatee, or sea turtles may be welcomed, it is safe to say that many more animals and plants would be unwelcome, like the invasive Burmese python in Florida, or insects like mosquitoes or bark-destroying beetles.
"These extreme freeze events play a very important role in controlling the northern limits of a broad suite of tropical cold-sensitive organisms,” says Osland, according to Popular Science.
As winters continue to warm up thanks to human-caused climate change, that boundary is becoming more porous and some of these creatures are moving north, producing an ecosystem shift that researchers are calling “tropicalization.”
“Changes in winter conditions are one of the major, if not the major, drivers of shifting distributions,” coauthor Caroline Williams, associate professor of integrative biology at UC Berkeley, said in a statement. “For the vast majority of organisms, if they freeze, they die,” but as extreme cold snaps become less common, “it enables tropical species to get more and more of a foothold.”
A map showing North America s tropical-to-temperate transition zone. Red  orange  and yellow depict ...
A map showing North America's tropical-to-temperate transition zone. Red, orange, and yellow depict the more tropical zones, and blues depict the more temperate zones, based on to the coldest recorded temperature for each area between 1980 and 2009. Photos show some cold-sensitive plants and animals with northern range limits governed by winter cold temperature extremes.
Williams went on to explain: "In our study, we were really focusing on that boundary in the U.S. where we get that quick tropical-temperate transition." The transition zone has always provided a barrier to species that evolved in more stable temperatures.
Many readers will remember buying seed packets in the spring and noting on the back of the package when it was safe to plant the seeds, depending on what zone you lived in across the U.S. This also works for determining the transition zone between a temperate and tropical environment.
However, as the Earth warms tropical animals and plants are already invading eight subtropical U.S. mainland states: Florida, Alabama, Mississippi, Louisiana, Texas, New Mexico, Arizona, and California.
Mangrove forest in Key Largo  Florida. With sea-level rise  mangroves may also move inland  displaci...
Mangrove forest in Key Largo, Florida. With sea-level rise, mangroves may also move inland, displacing temperate and freshwater forests.
Lisa Jacobs (CC BY-ND 2.0)
The authors of the study include scientists from the Florida Fish and Wildlife Conservation Commission, National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, University of Arizona, University of California Berkeley, University of California Santa Cruz, University of British Columbia, Louisiana State University and the Bonefish and Tarpon Trust.
The study points out that changes are already underway or close to changing in the home ranges of at least 22 plant and animal species, from California to Florida, reports Science Daily.
Besides the cold-sensitive mangroves, which have already been displacing saltwater marsh plants along the Gulf and southern Atlantic coasts for 30 years, the research team also cites buffalograss and other annual grasses that are moving into Southwestern deserts, as well as Cuban tree frogs, iguanas, and Brazillian pepper trees.
And we must mention recreational and commercial fisheries - already being disrupted by changing migration patterns and the northward movement of coastal fishes.
"We don't expect it to be a continuous process," says Osland. "There's going to be northward expansion, then contraction with extreme cold events, like the one that just occurred in Texas, and then movement again. But by the end of this century, we are expecting tropicalization to occur."
"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 relatively wide thermal tolerance, so they can tolerate a wide range of conditions," Williams said.
"And, by definition, these tend to be the pest species -- that is why they are pests: They are adaptable, widespread and relatively unbothered by changes in conditions, whereas, the more specialized or boutique species are tending to decline as they get displaced from their relatively narrow niche."
More about Tropicalization, Global warming, United States, tropicaltemperate transition zones, temperate ecosystems
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