Updated: Jul 16, 2020
An Examination of the Implications of Climate Change on Biodiversity
Keywords: Biogeography; Conservation; Climate Change; Impact; Ecology; Macroecology; Biotic Interactions
Among the many achievements of the fearless explorer and naturalist Alexander von Humboldt (1769-1859), one was especially influential to the study of the link between climate change and biodiversity. He noted that changes in vegetation structure coincided with differences in temperature along elevation gradients (1). In other words, he was the first to reveal the deep and complex interconnectivity between climate, geography, biodiversity, and long-term human impact in ecosystems (2-4).
Humboldt pioneered a new scientific legacy and way of perceiving and understanding the natural world. According to the historian Susan Schulten of the University of Denver, he was one of the first scientists to use maps to shape his thinking and test scientific hypotheses (3). In detail, Humboldt designed an emblematic and completely revolutionary view of the Earth as a complementary system, which he named Naturgemälde, or nature painting (figure 2). Through it, he represented the vertical stratification of vegetation dependent on elevation (and temperature), with rainforests in the tropics, deciduous forests and grasslands in temperate zones, and tundra at the higher latitudes (5). It would be the first of a long series of examinations founding the reasoning that species distributions are mostly confined by climate and any alterations to it (6).
Since Humboldt conceptualized the Naturgemälde more than 200 years ago, climate change evolved into one of the greatest threats humanity faces (7). However, it is far from being just a Homo sapiens problem, as ecosystems and biodiversity are on the frontlines of this global emergency (8, 9). In effect, among the many unfortunate consequences from anthropogenic climate change will be an accelerated rate of population decline and extinction of non-human species (4, 10-13).
In one of my previous works, Where the Wild Things Were is Where Humans Are Now (14) I echoed the concerns stated by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services on their latest report (12) and connected them to wildlife decline trends. The IPBES stated:
“In the past 50 years, the human population has doubled, the global economy has grown nearly fourfold, and global trade has grown tenfold, together driving up the demand for energy and materials.”
Undoubtedly, population and economic growth are directly correlated with the progression of climate change, due to the emission of greenhouse gases (15-19). Under these circumstances, scientists from IPBES (12) formulate how climate change is expected to affect the natural world:
"These changes have contributed to widespread impacts in many aspects of biodiversity, including species distribution, phenology, population dynamics, community structure and ecosystem function."
Comparatively, the Living Planet Report from The World Wildlife Fund (20) reiterates the point that:
"Climate change is playing a growing role and is already beginning to have an effect at an ecosystem, species and even genetic level."
Another key point is that the present consensus surrounding the decline and extinction of biodiversity is being mainly motivated by two different sources. On one hand by the overexploitation of plants, animals, and other organisms via the harvest, logging, hunting, and fishing. On the other hand, habitat destruction via agricultural activity, deforestation, urban development, transportation, and energy production (12, 21).
Still, the prediction is that climate change will progressively overtake these causes and become the main threat to the natural world (13, 22). To demonstrate, a recent publication in the journal Nature (23) predicts that entire ecosystems can abruptly collapse due to the gradual increase in temperature, to which individual species are unable to cope with. The scientists declared that a rise in temperature on the order of 4ºC above pre-industrial levels by 2100 (under the Representative Concentration Pathway [RCP] 8.5), will precipitate simultaneous die-offs of co-dependent species and profound 'regime shifts' in ecological assemblages (24, 25). Notably, at least fifteen percent of ecosystems will face an "abrupt exposure event," which means that one-fifth of their integral species will cross a threshold with irreversible consequences (23).
Correspondingly, one of the more conventional examples of species decline linked with climate change has been the pollinators, particularly on honeybees (Apis spp.) (26-28) (figure 3). To be sure, the credible threat of extinction of honeybees has been connected with widespread repercussions on ecosystem services (29-31), which in turn spells out catastrophe for the maintenance of humanity's civilizational project (32-34).
Proportionately, future risks and impacts caused by climate change are the favored expertise of the Intergovernmental Panel on Climate Change (IPCC), specifically their latest work, the Fifth Assessment Report (9).
Although their efforts are commendable, ultimately their concern is mostly centered on how anthropogenic climate change affects humans, even if the effects are felt, first and foremost by non-human species. In detail, even one of the less anthropocentric statements in their latest report mirrors an apprehension over the fate of Homo sapiens (9, p. 80):
"Continued high emissions would lead to mostly negative impacts for biodiversity, ecosystem services and economic development, and amplify risks for livelihoods and for food and human security."
A close analysis of the IPCC's Working Group II contribution to the Fifth Assessment Report (WGII AR5) delivers a thorough inquiry into expected impacts from climate change on for example Natural and Managed Resources and Systems, and Their Uses; Human Settlements, Industry and Infrastructure and Human Health, Well-Being, and Security (35). Colloquially speaking, the IPCC's function is to highlight the connection of "how does climate change affect biodiversity, and how does that in turn potentially influence us?"
With that in mind, this essay aims to bring together the science of climate change and focus it specifically on conservation, biogeography, and ecology. For this to happen, I deem it necessary to go beyond the IPCC's focus on human welfare and review and interpret what the scientific community has been desperately calling attention to as being the biological extinction of non-human species, exacerbated by climatic changes. Still, to clarify, this work does not discard the important contributions from the IPCC, since these are the foundation for the climate data used by the scientific community studying the impacts of climate on biodiversity. It only hopes to merge both the IPCC's highly condensed knowledge with other scientific perspectives.
The paleoclimatic record clearly shows periods of accentuated variation, with that all the major extinction episodes known to have taken place on this planet being correlated with the rise of carbon dioxide levels (figure 4) (36-42).
Humanity is currently living through a unique fragment of time in Earth's history, a geological epoch dominated by its presence - correctly dubbed as Anthropocene (44-46). H. sapiens are indeed unique in many regards, particularly in precipitating that which is now commonly understood as the Sixth Mass Extinction (47-50), responsible for a "background" extinction rate of species, exceptionally higher when compared with historical records (49). Albeit others argue that the problem is not species extinction but the contraction and fragmentation of populations till the point where they become unviable (51).
Nonetheless, it continues to be an exercise of inference to assert that the celerity related to the accumulation of carbon dioxide in the atmosphere is now faster compared with the paleoclimatic record. This is mainly due to the uncertainty and error (52, 53) associated with studying events in the deep past or making rigorous prognostications for the future, which according to the IPCC, are part and parcel of the scientific method connected to climate change (54-56).
Although we can't get a full picture of past major extinction events (57, 58), it appears that the modern rise in CO2 (roughly 2-3 ppm/year) is still significantly higher when compared with the end-Cretaceous (KPg) extinction event, roughly 66 million years ago (not shown above), and superior to the Thermal Maximum 55 million years ago (about 0.11 parts per million CO2 per year) (42, 59), making the Anthropocene a singular event in Earth's history.
By all means, the Earth's climate has undergone profound changes, without any anthropogenic assistance, and the fossil record attests to the scale of extinction (60, 61). As a result, it is known that biodiversity has been profoundly affected in the past by changes to the composition of the atmosphere, in particular by recurrent elevated concentrations of CO2 and even prolonged rebound effects (41).
For this reason, it can be inferred that our present-day anomalous changes in temperature (compared at least with the previous 11,000 years) (62) can signal the onset of an upcoming mass extinction event driven by climate disruption (47-50, 63, 64).
To point out, global climate change (GCC) is expected to have multiple components interacting with all levels of biodiversity, from the organism to the level of biomes (figure 5) (13, 65). For the most part, fitness decrease, expressed at different levels of biological organization, will be the most evident impact of climatic changes. In detail, from the most elemental level of organization, climate change will clash with genetic diversity, reverberating through the higher organization levels (66, 67).
There are no lack of examples of how biodiversity is expected to undergo loss of genetic diversity due to climate change, even though it is still one of its most underexplored impacts (68, 69). In detail, genetic erosion due to GCC has been reported in the timber rattlesnake (Crotalus horridus) (70), the lycaenid butterfly Lycaena helle (71), the reindeer (Rangifer tarandus) (68), alpine mammals (72), various northern plant species (73) and over many populations and species (74).
Climate change also impinges on physiological components, mainly through the abiotic factor of temperature, which ends up determining the distribution of the planet's biota (75). To point out, the responses of endotherms are likely to deeply contrast those of ectotherms, or the mobile species from sedentary ones (76).
To be sure, these changes have been shown to increase mortality and disease susceptibility, with the most well-known case being coral bleaching (77, 78). Other eco-physiological responses to warming have, for example, been reported in Heretoptera species (79), with the water strider Aquarius paludum reporting a loss of sensitivity to gradual decreases in natural day-length, because they tended to grow faster under warmer conditions (80).
Additionally, the repercussions of GCC on the physiology of organisms become even more crucial when taking into account that non-indigenous species can more easily extend their geographical ranges and become invasive with changes in temperature (81).
Furthermore, for many species, the dominant threat associated with climate change may emerge through alterations to obligatory food and habitat requirements (13). Indeed, climate change is highly correlated with phenological change, which in turn, for example, leads to time mismatches between flowering plants and insect pollinators, with profound consequences for the structure of plant-pollinator networks (82-84).
A precise and accurate illustration of a climate-driven population decline has been properly documented in a Puerto Rican forest, where arthropod biomass has dwindled by up to 99 percent over 4 decades (85). The scientists found that as the biomass of insects and other arthropods decreased, anole lizard numbers fell by half and the Puerto Rican tody, a bird that eats only insects, contracted its populations by 90 percent between 1990 and 2015. The worst part detailed in the study is that all of these biodiversity recessions - that also lead to a decline of ecological function - were found and studied in a national forest and protected area (86). As the researchers point out, climate change might be having a much wider and prevalent effect than previously anticipated (85).
Comparatively, as one moves beyond the realm of the organisms and into those of the populations, climate change is foreseen to reshape the 'web of interactions' at the community level (86, 87). In other words, as organisms independently react to climate change, indirectly, those adjustments can impinge on others that are contingent on them (13). Indeed, a study of almost 10000 interspecific systems, containing pollinators and parasites, indicates that roughly 6300 species could vanish in response to the extinction of their conditional species (88).
On the population level, climate change is expected to alter dynamics via the different abiotic components described in the image above, with temperature being particularly relevant (90). Markedly, the impacts are described to influence the recruitment of new individuals (91); age structure (for example delayed maturity) (92); the abundance and activity of predators (93), and modify development rates, fecundity, voltinism or dispersal (90). By all means, the Living Planet Report from the World Wildlife Fund (20) has written that:
"Climate change was most commonly reported as a threat for bird and fish populations – at 12% and 8% respectively and less frequently for other groups (94). It also reveals a strong association between the warming climate and declines of bird and mammal populations globally. This shows that population declines have already been greatest in areas that have experienced the most rapid warming (95)."
On the distribution component, Charles Darwin (96) was already aware of the relevance of the ecological niche, which he referred to as the group's 'place in the economy of nature' (97). Curiously enough, it is in this component that ecologists and biogeographers meet (both study biological differentiation) and also diverge since they use different scales of analysis (97-99).
The debate rages on (97, 100), however, here I side with the version that species distributions are generally limited by climate and are altered when the abiotic factors change (6, 101), which is to say: "A central tenet of biogeography is that the broad outlines of species ranges are determined by climate (102). In detail, climate change is affecting the ecological niche of species (103), their range size, range localization, and dispersal corridors (104, 105); the habitat quality (106, 107) and size (108). One clear consequence of climate change altering population distribution is the fact that the ecological niche of vectors for infectious diseases is rapidly being adjusted (109).
At the level of the species, climate change is taking its toll on interspecific relationships by producing unexpected changes in ecological patterns. With the Earth's climate rapidly changing and the planet warming, interactions such as competition, predation, host/parasite, and mutualism are reporting major shifts, which in turn impinges on the interactions, ecosystem functioning, and community structures (110-113).
As a result, climate change may provoke dramatic disruptions to food webs if the interacting species respond uniquely and discordantly to shifts in environmental conditions (114). For instance, increasingly warmer springs have been shown to unsettle the trophic linkages between phytoplankton and zooplankton, with severe consequences for resource flow to upper trophic levels (115).
At the community level, predictions in the increase of temperatures are foreseen to cause reductions in biodiversity, for example on the above - and below-ground productivity of grassland communities (116, 117). Changes in precipitation, which are to be expected in a warmer world are also linked to the hydrology of forest communities (118). On the other hand, energy fluxes are effectively being adjusted (119) which can have implications on the level of communities and ecosystems, by disrupting soil temperature and evapotranspiration in boreal peatlands, for example, ultimately unsettling the atmospheric methane flux (120).
When reaching the level of ecosystem services (ES), we must be reminded that this is a term that was created to highlight the dependence of humans on nature (121) and determine its economic value to - potentially - solve market failures (122). There is a long-standing debate if the term is in itself too anthropocentric or if it is crucial for biological conservation (123-125) and that goes beyond the aim of this work. What I will say, however, is that I couldn't find any literature attesting to how the degradation of a given ES would, in turn, also affect biodiversity.
Regardless, all of the abiotic factors so far described are foreseen to cause negative changes, for example, rises in temperature and unpredictable rainfall patterns will present deep impacts on these ES, such as water provisioning, or the regulation of erosion (126). So far, I have made no mention of the impacts of sea-level rise and more severe extreme events, however, when it comes to ES coastal habitats, these are particularly vulnerable, with the loss of wetlands and coral reefs raising concern (127).
At a higher level of biodiversity, climate change can generate adjustments in vegetation communities that are anticipated to be of substantial size to disrupt biome integrity. The Millennium Ecosystem Assessment forecasts shifts for about 5–20% of Earth’s terrestrial ecosystems, including cool conifer forests, tundra, scrubland, savannahs, and boreal forest (128). Of particular concern are the ‘tipping points’ where ecosystem thresholds can provoke permanent shifts in biomes (129).
Considering all the extensive work that has been done to understand the impacts of climate change on biodiversity, many scientists are still worried that conservation management is mostly underestimating the repercussions connected to it (130-138). In effect, conservation plans continue to painfully disregard climate change in their groundwork and prospects as Lesley Hughes of Macquarie University in Sydney explains in New Scientist (130):
“It’s a classic case of the ‘knowing-doing’ gap”. The reasons we may fail to act even when we know what needs to be done include a lack of resources, an inability to believe that things could get as bad as forecast, a reluctance to intervene and a focus on short-term threats such as invasive species."
To comprehend the scale of the danger of climate change to wildlife, I have selected two species under direct threat of GCC, for a brief examination. The first one is the Emperor penguin (A. forsteri).
If our civilization continues to act as the "heat machine" (or thermodynamic system) (139, 140) that it is and pumping out GHGs in the process, the course of the Emperor penguins (A. forsteri) becomes almost entirely predetermined to court extinction by the end of the century (141). Given that rapid global warming is causing the disappearance of ice which is vital for the breeding practices of the species, as well as providing a place for resting and getaway from predators, the expression "walking on thin ice" acquires a whole new meaning for the A. forsteri (142).
By all means, the graph below illustrates how the future of this species hinges almost entirely on how humanity tightens the belt on its economic activity (143-146). Considering climate projections already perceive containing temperatures below 2ºC to be extremely unlikely (147), and a business-as-usual scenario throwing temperatures way above 3ºC (148), the currently estimated total 595,000 emperor penguins (and the 250,000 breeding pairs) are foreseen to undergo an 81-86% decline in their populations (figure 6).
The consensus that has prevailed in the scientific community is that West Antarctica and the Antarctic Peninsula have both been losing mass, while East Antarctica has remained stable or even gaining mass (149). Scientists have already explicitly stated that any ice recovered in East Antarctic wouldn't offset the rapid losses in the Artic (150), which is envisioned to be iceless through September each summer if temperature rises by as much as 2ºC above pre-industrial levels (151).
Nevertheless, a recent paper in the Proceedings of the National Academy of Sciences (PNAS) has challenged the consensus and found mass loss in all of Antarctica’s ice sheets (152). The destabilization of the ice shelves will mean a massive rise in sea-level, even though, once again there is uncertainty about how much, as the IPCC's Fifth Assessment Report (AR5) states (153, 154). Regardless, climate change is prompting ice loss which affects humans indirectly (155), and emperor penguins directly, and excessively.
Given these points, scientists from the Australian Antarctic Program (stationed in East Antarctica) have recorded the first documented heatwave on the continent, with their findings having been published in the journal of Global Change Biology (156).
In particular, they recorded for more than three consecutive days (between January 23rd and 26th) very high maximum and minimum temperatures, specifically minimums above zero degrees Celsius and maximums peaking above 7.5ºC. Moreover, on January 24, the team registered a record high temperature of 9.2ºC, 6.9ºC above the recorded mean maximum (156). On top of this, it has to be remembered that record high temperatures were observed on the other side of the continent, on the Antarctic Peninsula, early this year (157).
It is unclear what all this might mean for the emperor-penguin, but it is conceivable that the species might be acting as our "canary in the coal mine" by serving as an indicator species of future implications of climate change (141), especially for those that depend on rare and diminishing habitats (130).
According to the International Union for the Conservation of Nature (IUCN), in the Iberian peninsula therein lies the most endangered feline in the world, the Iberian Linx (Lynx pardinus) (158). A major campaign to overturn the demise of the species has been operating between the Iberian countries for more than 15 years (159), and there haven't been as many individuals in Iberia in the previous fifty years as there are today (roughly 600) (160, 161). This has led the Portuguese government to proclaim that the lynx "has been saved from extinction" (162).
Notwithstanding all of these efforts, the threat of climate change still looms for the L. pardinus (163, 164), even though its implication continues to be downplayed or downright neglected in conservation projects (165, 166).
Conservation experts who contemplate the repercussions of climate change have continuously called for an expansion of the biogeographic range of the species in Portugal, from the South (which will become too dry) to the Northern regions. As the biogeographer Miguel Bastos Araújo argues in New Scientist (130):
"It's inevitable that a population crash will happen unless they are able to move."
To put it differently, if the warming trajectory of the planet continues unabated, whole habitats, and the species that depend on them might be swallowed up by climate change (130, 167).
To demonstrate this principle, Fordham and colleagues (164) make clear how climate change is foreseen to have a tremendously negative toll on Iberian lynx abundance. The researcher’s estimate (figure 7) than in less than 50 years the lynx will be extinct, even with swift and aggressive cuts to anthropogenic greenhouse gas emissions. In other words, under a scenario with "No Changes" in temperature and precipitation the predictions are for a slow decline of the numbers but no extinction during this century. With a "Policy" mitigation strategy there is a slightly faster population decline than with a "Reference" high CO2 concentration. As the scientists explain, this seemingly conflicting result is due to the closure of coal-fired power stations that have contributed to the dimming effect by emitting atmospheric aerosols that reduce incoming solar radiation from reaching the Earth's soil (168)1.
Considering the evidence and the risk of extinction, the researchers call for a planned relocation strategy which accounts for climate factors, prey availability dependent on disease as well as habitat connectivity, regardless of the scale of commitment to diminish greenhouse gas emissions (164).
As it happens, it wouldn't just be the Iberian lynx that would benefit from conservation plans contemplating a climate change component. By all means, a recent paper published in Nature (131) asserts that of the 459 animals listed as endangered in the US, 99.8% of species are sensitive to what the researchers denominate as eight sensitivity factors (e.g. temperature, disturbance, isolation, obligate relationships), but only 64% of conservation agencies include climate as a threat, and plan for only 18% of species. Another study by (137), concluded that from 100 Australian conservation plans, only 60% listed climate change as a current threat, and of those, only 22% identified any specific action to ameliorate the risk, while even fewer (9%), recommended any interventionist action. As it is portrayed in the graph below, there is a gap between the expected threat and the level of action currently put in place.
In the long run, appropriate conservation actions will demand a combination of (134,169,170):
1. large-scale protection of ecosystems (for example with the ‘Nature Needs Half’ initiative)
2. actively transforming and adapting social-ecological systems
3. building the capacity of communities to cope with change
4. government assistance focused on de-coupling communities from dependence on natural resources
5. cease human population growth
The initial premise of this essay was that the science presented by the IPCC would be insufficient to comprehend how climate change is impacting biodiversity. To understand how non-human life is being affected I considered it compelling to bring on board other areas of scientific knowledge, including ecology, biogeography, and conservation, to name a few. Still and all, the IPCC does display a less anthropocentric analysis when the Working Group III argues in the AR5 (171 p. 238) that:
“It may be impossible to weigh the value of biodiversity against human well-being […] An increasing number of philosophers have argued in recent years that nature also has value in its own right, independently of its benefits to human beings (172,173). They have argued that we should recognize animal values, the value of life itself, and even the value of natural systems and nature itself.”
With this in mind, and on top of the conservation plans which incorporate the threat of climate change, what else is the scientific community’s advice to help biodiversity build resilience?
Several approaches have been suggested, such as evolutionary and forest resilience (174,175), agricultural biodiversity such as agroforestry and High Nature Value Farming Systems (176), and more recently the restoration of ecosystems, the reintroduction, and rewilding of species (177-179), with the curious and fascinating possibility of megafaunal return (180,181).
Specifically, trophic cascades offer an inestimable theoretical framework for rewilding, and trophic (or megafaunal) rewilding is an ecological restoration approach that uses species introductions to restore top-down trophic interactions to bolster self-regulating biodiverse ecosystems. Likewise, restored megafaunas and associated trophic cascades may encourage heightened ecological resilience against climate change (182). For example, restored megafauna may increase other species’ ability to track climate change by augmenting dispersal distances. Conversely, deficiency, or ongoing loss of megafauna may enhance the extinction risk of such associated species. In some cases, restored megafauna may also confer greater resistance toward ecosystem variation under climate change (181).
All in all, the threat from climate change to the natural world is serious and requires immediate and well-informed action. The good news is that there is much we can do about it, but don't be surprised if they take the initiative in moving out.
1- The ecological impact of alterations in solar radiation is poorly understood, but it has been established that reductions in sunlight due to global dimming have been having a non-negligible effect at least since the 1970's in biodiversity (King, 2005; Hulme, 2005).
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