- Climate change forces three fates on species: adapt, flee or die. A new meta-analysis compiled data from 27 studies to see how species distributions have changed over timescales of 10-159 years, and included 976 species. Almost half (47 percent) had seen some local populations disappear along the warming edge of their ranges.
- The tropics were especially vulnerable to climate change-driven local extinctions. The data showed that 55 percent of tropical and subtropical species experienced local extinctions, whereas the figure was only 39 percent for temperate species. Though the tropical data set was not large, this higher tropical risk concurs with past studies.
- Tropical species are at greater risk due to climate change because they live in some of the world’s hottest environments, so are already at the upper limit of known temperature adaptation, are restricted to small areas, particular rare habitats, and narrow temperature ranges, or have poor dispersal ability and slow reproductive rates.
- Scientists see multiple solutions to the problem: beyond the curbing of greenhouse gas emissions, they recommend conserving large core areas of habitat, and preserving strong connectivity between those core areas, so plants and animals can move more freely between them as required as the world warms.
A species has just three options in the face of environmental change: move, adapt or die. As global temperatures rise many species are shifting their ranges, particularly towards the cooler poles and upslope to higher elevations. But if they can’t adapt or move, populations can be lost along the warmer edge of their range. These local population extinctions could have major implications for individual species, ecosystems and global biodiversity.
New research, published in PLOS Biology, warns that local extinctions caused by climate change are already widespread. The meta-analysis, by John Wiens of Arizona University, compiled data from 27 studies that resurveyed sites to see how species distributions had changed over time. These studies spanned a range of timescales between 10 and 159 years, and encompassed 976 species. Almost half of these (47 percent) had seen some local populations disappear along the warm edge of their range. Local extinctions were prevalent in all geographic regions and taxonomic groups.
“As Yogi Berra said, it is tough to make predictions, especially about the future,” said Wiens, whose research has contributed to a growing body of work that tries to forecast how biodiversity will respond to climate change. “And it is hard to tell whether your predictions are actually accurate or not. So, for this study, I changed my focus and instead asked: what has happened [due to climate change] already?”
The extent of local extinctions came as a surprise, Wiens said, “given that climate has changed little relative to the much greater changes predicted in the future.”
Stuart Butchart, director of science at BirdLife International expressed alarm over the results: “The fact that evidence of such population extinctions was found for about half of species, and that the pattern held across plants and animals, tropical and temperate regions, and in marine, terrestrial and freshwater systems is striking.”
Local vs. global extinction
It’s important to note that local extinction doesn’t necessarily mean that a species is in danger of global extinction: if its range is expanding elsewhere, and it is able to move in a timely fashion, the species could be successfully responding to the changing climate.
On the other hand, population extinction could indicate trouble for species that are unable to expand their range into areas with more suitable conditions — either because they can’t move quickly enough, or because there is nowhere else for them to go.
So how should we interpret the widespread local extinctions that have been recorded so far?
Wiens agrees that “is the big question: will these local extinctions of populations turn into global extinctions of entire species?”
“The short answer is: I don’t know.” But given their prevalence, “and that global warming is predicted to increase by an additional two- to five-fold, it seems hard to think that there will not be many global extinctions also.”
Trouble in the tropics
When Wiens compared taxonomic groups, he found that the percentage of local extinctions was higher in animals than in plants, and in freshwater over marine and terrestrial species. A geographic trend was also apparent, with 55 percent of tropical and subtropical species experiencing local extinctions, whereas the figure was lower, at 39 percent, for temperate species. This might seem counterintuitive when temperatures are rising fastest at higher latitudes — as seen in the more rapidly warming Arctic — but it is something that scientists have previously predicted based on the biology and ecology of tropical species.
Tropical species live in some of the world’s hottest environments, so are already at the upper limit of known temperature adaptation. More heat means more stress, potentially beyond their ability to adapt.
What’s more, tropical environments tend to be more stable throughout the year, so tropical species are adapted to a much narrower range of temperatures than their temperate counterparts.
The fact that tropical species are already showing higher levels of local extinctions is especially worrying. “I think that the most important implication of this pattern is that climate-related extinctions are most likely in the part of the world that has the most species, the tropics,” said Wiens. “So, the prognosis is worse for global biodiversity than if extinctions were just spread randomly across the planet.”
But others caution that more data is needed to gain a clear picture of geographic variation in local extinction rates. “A major limitation of the Wiens study is the utter lack of data from the tropics,” said Kenneth Feeley, of the University of Miami, who pointed out that once subtropical climatic regions such as Arizona were excluded, only 5 of the 27 studies that Wiens examined were from the true tropics, and only one focused on plants. “Given this lack of data it is premature to draw conclusions about local extinctions of tropical species and especially tropical plant species,” he said.
Despite this caveat, Wiens’ overall conclusions tally with what Feeley and colleagues have already been observing in their own research in the forests of Central and South America. “[M]any tropical tree species in Costa Rica, Colombia and Peru are shifting their ranges to higher elevations and [..] in many cases these shifts are due primarily to species dying back, and going locally extinct, from the lower, hotter portions of their ranges.”
The devil is in the details
A great number of variables make a thorough assessment of tropical extinction potential challenging, but many factors added together tip toward higher future local extinction rates. For species in the middle of a vast lowland tropical region, such as the Amazon or Congo basins, for example, cooler temperatures, and suitable habitats, can be many hundreds of kilometers away, making a timely escape along a temperature gradient almost impossible.
Common traits of tropical species don’t bode well either: “In general, species with certain characteristics, such as being restricted to small areas, restricted to particular rare habitats, having poor dispersal ability and slow reproductive rates are most extinction-prone. These are characteristics of many tropical species,” explained Jane Hill, a professor at York University, UK.
“Given the long generation times and narrow niches of many tropical tree species, there is good reason to predict that many […] simply won’t be capable of tolerating hotter temperatures and will go locally extinct,” said Feeley.
Though we might not see the impacts immediately, cautions Naia Morueta-Holme, an ecologist at University of California, Berkeley: species that might look as though their range is stable could be building up an “extinction debt” — a time delay in which the extinction of a species in the future is due to events in the past.
“[L]ong-lived plants like trees can often survive in more extreme environments, long after they have stopped being able to reproduce,” she noted. “In such cases, it will take longer time for us to see climate-driven local extinctions.”
Who’s at risk?
Aside from the trees themselves, lowland forest species at greatest risk of being left behind in the race to reach suitable habitat include understory birds, primates and small mammals.
A study of nearly 500 Western Hemisphere mammal species conducted by University of Washington scientists identified the western Amazon as the region in greatest trouble, with 14.5 percent of species predicted to be unable to keep up with habitat shifts. Primates in Central and South America face average range reductions of 75 percent over the coming century, the study concluded; many of these species are already threatened with extinction, such as the Endangered white-bellied spider monkey (Ateles belzebuth), and white-nosed saki monkey (Chiropotes albinasus) — which raises another point; climate change is just one of many human-caused stressors in the tropics, ranging from soy and oil palm agribusiness expansion, to logging and wildlife trafficking.
On tropical mountains, species may face another problem, even though cooler temperatures are more easily and immediately available to wildlife and plants further upslope: they might simply run out of room at the top. As the world warms, species pushed higher find themselves struggling to survive on smaller and smaller mountaintop islands of habitat.
A study of moths on Borneo’s Mount Kinabalu by Hill and colleagues revealed that most species’ ranges had shifted significantly upwards over 42 years, but cool edge expansion had proceeded faster upslope than warm edge contraction. Although this has kept range sizes stable for the time being, at high elevations movement upwards is likely to be limited by the geology of the mountain, which makes it unsuitable habitat for the moths. As a result, several endemic species could be at risk of extinction with continued warming.
Birds on New Guinea’s Mount Karimui face a similar problem. A study by Benjamin Freeman, of the University of British Columbia, found that 40 out of 64 bird species had seen range contractions at lower elevations, with four upper elevation species likely to be lost from the mountain by 2100.
So far, “[t]here haven’t been many documented cases of such mountaintop extinctions,” he said. “For example, the two bird species that [renowned researcher] Jared Diamond found living only on the top of Mount Karimui in the 1960s were still living only at the top of Mount Karimui in 2012.”
Freeman, whose work was included in Wiens’ study, also emphasized that “[w]hat Wiens terms a local extinction could be a population shifting upslope 50 meters, and on a steep mountain slope this might not be what we typically think of as an ‘extinction.’”
Çağan Şekercioğlu, of the University of Utah, has studied how the elevational range of bird species affects their chances of extinction under climate change projections. In a study of the world’s land birds “[the] already endangered scissor-tailed hummingbird (Hylonympha macrocerca) ended up as one of the most vulnerable species,” he said, “as it is a tropical forest understory resident limited just to the Paria Mountains of Venezuela and to elevations between 530-1,200 meters [1,700-3,900 feet].” The population stands at just 3000-4000 individuals, and habitat conversion to agriculture, coupled with the relatively low height of the mountain at just 1,371 meters (4,500 feet), means the species is rapidly running out of space. Overall, the study predicted that hundreds of bird species would go extinct, and thousands would be at risk of extinction, by 2100 due to the interaction between narrow elevational range, loss of habitat and climate change.
In the Peruvian Andes, the picture for some bird species is more hopeful, thanks to the availability of high quality, protected habitat, said German Forero-Medina, of the Wildlife Conservation Society Colombia.
Forero-Medina’s study of birds in the Cerros del Sira of central Peru, also featured in Wiens’ analysis. He found that although upwards shifts were evident, they were smaller than predicted due to warming. “The area is protected by the Reserva Comunal El Sira and the vegetation is in good condition, so [most birds] should have space to move.” However, he highlighted the endemic Sira Tanager (Tangara phillipsi), with a narrow elevational range, as a species in need of close monitoring for population declines.
The only African data in Wiens’ study was for frogs and reptiles on the highest mountain in Madagascar. For these species, living on the Tsaratanana massif in the north of the country, things are not looking good. “They have very little available cold edge habitat left for them to expand into,” explained Christopher Raxworthy, curator of herpetology at the American Museum of Natural History, who led the research. “Essentially warming could push them up and off the top of the mountain,” with the same issue facing endemic species on at least 9 other mountain systems in Madagascar, he said.
It’s not just the potential loss of individual tropical species that’s a cause for great concern. Species interactions will also be disrupted as animals and plants move, adapt or die, which will irrevocably alter complex interrelationships within habitats, ecosystems, and even biomes. “It is likely that we will see considerable disruption to ecological communities, with changes in the dynamics between predators and prey, competitors, diseases and parasites and their hosts,” said Butchart.
“The other worry is: what happens to tropical forests at sea level?” said Freeman. Without species coming into the lowlands from somewhere even hotter, these biodiverse regions may undergo a depopulating transformation. “Do lowland plants and animals shift up and leave sea level tropical forests impoverished (termed “biotic attrition”) or not? We really don’t know the answer yet.”
Local extinctions, with or without biotic attrition, “will lead to changes in forest composition, structure and function,” said Feeley. “Given the extreme interconnectedness of tropical forest species and systems, these changes can in turn lead to more and more extinctions.”
Adding to all these hazards, tropical animals will likely find their movements in response to climate change blocked by all things human: long built and newly built fences, roads, railways, soy and oil palm plantations, cities and towns all offer impediments to movement and could add to extinctions.
Whether in the mountains or the lowlands, ever-growing barriers to movement — the result of habitat loss and deforestation, infrastructure development and industrial agriculture — will make keeping up with climate change even harder.
When it comes to taking action to mitigate these impacts, there is a lot of agreement among scientists: make sure that habitat stays connected. “Overall, I would say that the highest priority is to protect corridors of intact habitat that span from the lowlands to the highest elevations,” said Wiens.
Hill agrees: “[i]n more connected landscapes species can reach new areas and hence maintain their overall range size (even though the range location has shifted),” she said. “Intact rainforest plays an important role in buffering forest species from the detrimental impacts of climate change. So conserving large tracts of well-connected rainforest is key in this context.”
Protecting habitat will benefit people as well as biodiversity: “Local extinctions of plant species could have devastating impacts for human populations in the developing world,” said Wiens. “Many people rely on just a few grass species to prevent starvation.”
“In some cases, conserving montane forest may make a ton of sense for people-focused conservation as well,” agreed Freeman, who added that “watershed protection and forest conservation go hand-in-hand.”
Morueta-Holme thinks these arguments are especially relevant in the tropics where “millions of people are highly dependent on local natural resources.”
Other scientists emphasize the importance of getting a better handle on the specific ways in which species are responding to climate change, especially because not all species respond in the same way, or to the same environmental triggers: for some, rainfall determines habitat suitability more than temperature, for example. Sensitivity to different environmental factors may explain why, counter intuitively, some species have been recorded moving downslope rather than up, or are maintaining stable ranges, despite climate change.
“The first thing is establishing monitoring programs, so that elevational shifts can be detected if they are occurring,” said Raxworthy.
Feeley concurs: “In my opinion, the number one conservation priority for the tropics is collecting and collating more data. We cannot hope to protect forests from future climate change if we don’t know how species are already responding to current climate change.”
“[T]here is an urgent need to better understand the mechanisms involved” in how the most threatened species respond to climate change, said Forero-Medina. “It is time to move from patterns to mechanisms, this will help guide conservation decisions for those species.”
“I think that the highest priority is to reduce global warming in addition to mitigating its effects. The potential consequences for global biodiversity and humans are just too severe,” concluded Wiens. “Two of the biggest threats to global biodiversity are habitat destruction and climate change, and they seem to have synergistic effects,” he said.
But this synergism could also be beneficial if appropriate action is taken in time. “Preserving habitats can help reduce the negative impacts of climate change,” Wiens explained. “And intact forests and other habitats can help suck up the carbon that causes global warming in the first place.”
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