If we do not reduce our carbon emissions and instead allow global temperatures to rise by 4.5˚C, up to half the animals and plants in some of the world’s most biodiverse areas could go extinct by 2100, according to a new study. In fact, even if we are able to limit global warming to the Paris climate agreement goal of 2˚ C, areas such as the Amazon and the Galapagos could still lose one quarter of their species, say the researchers, who studied the effects of climate change on 80,000 plants and animals in 35 areas. Another study found that local extinctions (when a species goes extinct in a particular area, but still exists elsewhere) are already occurring in 47 percent of the 976 species studied, in every kind of habitat and climatic zone.
With temperatures rising, precipitation patterns changing, and the weather getting less predictable and more extreme, a 2016 study determined that climate change is already significantly disrupting organisms and ecosystems on land and in water. Animals are not only shifting their range and altering the timing of key life stages— they are also exhibiting differences in their sex ratios, tolerance to heat, and in their bodies. Some of these changes may help a species adapt, while others could speed its demise.
Move, Adapt or Die
Animals can react to climate change in only three ways: They can move, adapt or die.
Many animals are moving to higher elevations and latitudes to escape warming temperatures, but climate change may be happening too quickly for most species to outrun it. In any case, moving is not always a simple solution—entering new territory could mean encountering more competition for food, or interacting with unfamiliar species. Some animals, such as the hamster-like American pika, are at the farthest extent of their range. Pikas need the cool moist conditions of the alpine Sierra Nevadas and Western Rockies, but the rocky habitat they require is getting hotter, drier and less snowy. Because they already live so high in the mountains, when their terrain becomes inhabitable, there’s nowhere left to go. Other animals attempting to move to cooler climes may be hemmed in by highways or other manmade structures.
In addition, some impacts of rising temperatures can’t be outrun. Monarch butterflies take their cues from day length and temperature to fly south from Canada to overwinter in Mexico. Lately, the butterflies’ southern migration has been delayed by up to six weeks because warmer than normal temperatures fail to cue them to fly south. Scientists also found that the onset of cooler temperatures in Mexico stimulates the butterflies to return northward to lay their eggs in the spring.
As temperatures warm, their migrations could fall out of sync with the bloom time of the nectar-producing plants they rely on for food. Logging where they overwinter in Mexico and the dwindling of the milkweed habitat, where they breed and their larvae feed, due to drought, heat and herbicides are additional factors in the monarch’s decline. Its numbers have decreased by 95 percent in the last two decades.
As temperatures rise in the Arctic and sea ice melts, polar bears are also losing their food source; they are often unable to find the sea ice they use to hunt seals from, and rest and breed on. Puffins in the Gulf of Maine normally eat white hake and herring, but as oceans warm, those fish are moving farther north. The puffins are trying to feed their young on butterfish instead, but baby puffins are unable to swallow the larger fish, so many are starving to death.
Some Species are Adapting
Some animals, however, seem to be adapting to changing conditions. As spring arrives earlier, insects emerge earlier. Some migrating birds are laying their eggs earlier to match insect availability so their young will have food. Over the past 65 years, the date when female butterflies in southern Australia emerge from their cocoons has shifted 1.6 days earlier per decade as temperatures there have warmed 0.14˚C per decade.
Coral reefs, which are actually colonies of individual animals called polyps, have experienced extensive bleaching as the oceans warm—when overheated, they expel the colorful symbiotic algae that live within them. Scientists studying corals around American Samoa found that many corals in pools of warmer water had not bleached.
When they exposed these corals to even higher temperatures in the lab, they found that just 20 percent of them expelled their algae, whereas 55 percent of corals from cooler pools also exposed to the high heat expelled theirs. And when corals from a cool pool were moved into a hot pool for a year, their heat tolerance improved—only 32.5 percent now ejected their algae. They adapted without any genetic change.
This coral research illustrates the difference between evolution through natural selection over the course of many generations, and adaptation through phenotypic plasticity—the ability of an organism to change its developmental, behavioral and physical features during its lifetime in response to changes in the environment. (“Plasticity” here means flexible or malleable. It has nothing to do with the hydrocarbon-based products that are clogging our landfills and oceans.) The corals living in the hot pools had evolved over many generations as natural selection favored survival of the most heat-tolerant corals and enabled them to reproduce. But the corals from the cool pool exposed to the hotter water were also able to adapt because they had phenotypic plasticity.
How Does Phenotypic Plasticity Work?
When some animals (and plants) encounter the impacts of climate change in their environment, they respond by changing behavior and moving to a cooler area, modifying their physical bodies to better deal with the heat, or altering the timing of certain activities to match changes in the seasons. These “plastic” changes occur because some genes can produce more than one effect when exposed to different environments.
Epigenetics—how environmental factors cause genes to be switched on or off—bring about phenotypic plasticity mainly through producing organic compounds that attach to DNA or modifying the proteins that DNA is wound around. This determines whether and how a gene will be expressed, but it does not alter the DNA sequence itself in any way. In some cases, these changes can be passed along to the next generation, but epigenetic changes can also be reversed if the environmental stresses are eliminated.
Scientists don’t know whether all species have the capacity for epigenetic responses. For those that do, epigenetic changes could buy them time to evolve genetic adaptations to changing environmental conditions. And over the long term, phenotypic plasticity could become an evolutionary adaptation if the individuals with the genetic capacity for phenotypic plasticity are better suited to the new environment and survive to reproduce more.
“Like any trait, phenotypic plasticity can undergo natural selection,” emailed Dustin Rubinstein, associate professor in Columbia University’s Department of Ecology, Evolution and Environmental Biology. “This means that when there is a benefit to having a plastic response to the environment, this can be favored by natural selection … Some traits (like behaviors) may be more likely to be plastic than others.”
For species that take a long time to mature and reproduce infrequently, epigenetics may give them the flexibility to be able to adapt to rapidly changing conditions. Species with shorter life spans reproduce more frequently, and the rapid succession of generations helps them evolve genetic adaptations through natural selection much more quickly.
Examples of Epigenetic Changes
Guinea pigs from South America normally mate at a temperature of about 5˚C. After keeping the males at 30˚C for two months, scientists conducting one study found evidence of epigenetic changes on at least ten genes linked to modifying body temperature. The guinea pigs’ offspring also showed epigenetic changes, but these were different from those of their fathers. It seems that that the fathers produced their own epigenetic changes in response to the heat, but passed along to their young a different set of “preparedness” changes.
A population of winter skate fish from the southern Gulf of St. Lawrence have a much smaller body size than other populations of winter skate along the Atlantic coast. Scientists found that these skates had adapted to the gulf’s 10˚C warmer water temperatures by reducing their body size by 45 percent compared with other populations. (Since oxygen content decreases when oceans warm, it is difficult for bigger fish to get enough oxygen.) The scientists detected 3,653 changes in gene expression that reflected changes in body size and some life history and physiology traits. Despite these epigenetic changes, the DNA of these winter skates—which have lived in the southern Gulf of St. Lawrence for 7,000 years—was identical to that of another Atlantic skate population.
When Phenotypic Plasticity is Not Protective
“It is important to not confuse species responses and adaptation as an indicator that everything will be okay,” said ecologist Brett Scheffers, from the University of Florida.
A prime example is the green sea turtle, whose sex is determined by the temperature of the sand around its egg as it develops. Warmer incubation temperatures produce more females.
Scientists examined turtles around the Great Barrier Reef, a large and important turtle breeding area in the Pacific. They found that turtles from the cooler southern nesting beaches were 65 to 69 percent female, while those from the warmer northern nesting beaches were 87 percent female. In juvenile turtles, females now outnumber males by about 116 to 1. Turtles are so sensitive that if temperatures rise a few degrees Celsius more, certain areas could end up producing only females, eventually resulting in local extinctions.
Meadow voles born in autumn are born with a thicker coat than those born in spring, thanks to environmental cues the mother relays through her hormones while the pup is in the womb. These predictive adaptive responses, believed to be controlled by epigenetics, guide the animal’s metabolism and physiology to enable it to adapt to the environment it will supposedly be born into. But if it’s suited to life in a certain kind of environment, it could end up being maladapted when conditions change—for instance, if winters become warmer.
Phenotypic plasticity can even limit adaptive evolution. A butterfly from Malawi speeds up its growth and reproduction and lives a short life when it is born at a warm, wet time of year; if born in a cool dry season, it leads an inactive long life with delayed reproduction. While the butterfly has a lot of variety in gene expression, scientists have found very little actual gene variation for this plasticity. The butterflies adapted to very specific, predictable and consistent environmental cues. Natural selection furthered these carefully tuned reactions because any deviation from these precise responses would have been maladaptive. Consequently, over time, natural selection eliminated the genetic variation that would have allowed for more plasticity. So, paradoxically, phenotypic plasticity in seasonal habitats may produce species that are specialists in their particular environments, but are then more vulnerable to climate change.
It’s also believed that species in regions with a very consistent climate will have a harder time adapting to climate change. For example, because the tropics have had little climatic variability over thousands of years, it’s thought that tropical species have less diversity in their genes to deal with changing conditions.
Evolution to the Rescue?
Scott Mills, a professor of wildlife biology at the University of Montana, has been researching global patterns of coat color changes in eight species of hares, weasels and foxes. He has found that individuals who turn white in the winter are more common at higher latitudes, but for some animals, the mismatch of their white coats with less snowfall has led to a reduction in their range.
“We know that whether or not an animal is brown in the winter or white in the winter has a very strong genetic component,” said Mills. “And the coat color change trait doesn’t have much plasticity. There doesn’t seem to be any obvious capacity for them to have behavioral plasticity either—to behave so as to reduce mismatch or reduce being killed by the mismatch.” As snowfall decreases, there will be more and more mismatches, so if these species are to survive, they will have to evolve.
Mills’ research identified some populations of these animals with individuals that turn white and others that stay brown in winter. Because these groups have that genetic variability, they have the best chance to adapt, since evolution operates the fastest when there’s ample variation within a population for natural selection to act upon.
Both phenotypic plasticity and evolutionary change are more likely to occur in larger populations of animals and those connected to other populations. A large, diverse group will have more individuals with genes that allow for phenotypic plasticity, which can ultimately be favored by natural selection. In addition, “generalist” species—those that can live in environments with a wide variety of conditions—usually have more variation in their traits that can be inherited.
“One of the biggest discoveries over the last 20 years in biology,” said Mills, “is that meaningful evolutionary changes can happen fast. Evolution isn’t just for fossils—evolution can happen on ecological time scales in five to 10 generations. That’s led to more anticipation that evolutionary change might be able to play a role in rescuing species…With the right work and focus, this can become another tool in the conservation tool kit.”
What Needs to be Done
Human beings rely on biodiversity—the variety of life on Earth—and functioning ecosystems for food, clean water and our health. If other species are unable to adapt to climate change, the consequences for humans could be dire. Society needs to implement strategies to help wildlife adapt to the impacts of climate. This means identifying and protecting zones where species exhibit genetic variability and preserving natural marine and land-based ecosystems.
It means purposefully increasing connectivity between natural areas, and providing stretches of land that animals can migrate along and to. These measures would enable species to travel to cooler areas and allow for larger, more connected populations that can promote the genetic diversity needed for phenotypic plasticity and adaptive evolution.
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) just released four reports on biodiversity. Written by more than 550 experts from 100 countries, the reports found that biodiversity is declining in every region of the world, endangering “economies, livelihoods, food security and the quality of life everywhere.” In the words of IPBES chair Robert Watson: “The time for action was yesterday or the day before.”