33 Chapter 33 Adaptation and Extinction


Learning Objectives

  • Identify the early and predicted effects of climate change on biodiversity.
  • Impact of Global Climate Change on Biodiversity
  • The Role of Plants in Carbon Storage and Greenhouse Gas Emissions
  • Adaptations to Climate Change in C3, C4, and CAM Plants

Effects of climate change on biodiversity

Global climate change is  a consequence of human population needs for energy and the use of fossil fuels to meet those needs (Figure 47.10). Environmental issues, such as toxic pollution, have specific targeted effects on species, but they are not generally seen as threats at the magnitude of the others.

Figure 47.10 Atmospheric carbon dioxide levels fluctuate in a cyclical manner. However, the burning of fossil fuels in recent history has caused a dramatic increase in the levels of carbon dioxide in the Earth’s atmosphere, which have now reached levels never before seen in human history. Scientists predict that the addition of this “greenhouse gas” to the atmosphere is resulting in climate change that will significantly impact biodiversity in the coming century.

Climate Change

Climate change, and specifically the anthropogenic (meaning, caused by humans) warming trend presently escalating, is recognized as a major extinction threat, particularly when combined with other threats such as habitat loss and the expansion of disease organisms. Scientists disagree about the likely magnitude of the effects, with extinction rate estimates ranging from 15 percent to 40 percent of species destined for extinction by 2050. Scientists do agree, however, that climate change will alter regional climates, including rainfall and snowfall patterns, making habitats less hospitable to the species living in them, in particular, the endemic species. The warming trend will shift colder climates toward the north and south poles, forcing species to move with their adapted climate norms while facing habitat gaps along the way. The shifting ranges will impose new competitive regimes on species as they find themselves in contact with other species not present in their historic range. One such unexpected species contact is between polar bears and grizzly bears. Previously, these two distinct species had separate ranges. Now, their ranges are overlapping and there are documented cases of these two species mating and producing viable offspring, which may or may not be viable crossing back to either parental species. Changing climates also throw off species’ delicate timed adaptations to seasonal food resources and breeding times. Many contemporary mismatches to shifts in resource availability and timing have already been documented.

Range shifts are already being observed: for example, some European bird species ranges have moved 91 km northward. The same study suggested that the optimal shift based on warming trends was double that distance, suggesting that the populations are not moving quickly enough. Range shifts have also been observed in plants, butterflies, other insects, freshwater fishes, reptiles, and mammals.

Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations. Some climates will completely disappear. The accelerating rate of warming in the arctic significantly reduces snowfall and the formation of sea ice. Without the ice, species like polar bears cannot successfully hunt seals, which are their only reliable source of food. Sea ice coverage has been decreasing since observations began in the mid-twentieth century, and the rate of decline observed in recent years is far greater than previously predicted.

Finally, global warming will raise ocean levels due to meltwater from glaciers and the greater volume of warmer water. Shorelines will be inundated, reducing island size, which will have an effect on some species, and a number of islands will disappear entirely. Additionally, the gradual melting and subsequent refreezing of the poles, glaciers, and higher elevation mountains—a cycle that has provided freshwater to environments for centuries—will also be jeopardized. This could result in an overabundance of salt water and a shortage of fresh water.

Global climate change is resulting in increases in daily, seasonal, and annual mean temperatures, and increases in the intensity, frequency, and duration of abnormally low and high temperatures. Temperature and other environmental variations have a direct impact on plant growth and are major determining factors in plant distribution. Since humans rely on plants—directly and indirectly—a crucial food source, knowing how well they’re able to withstand and/or acclimate to the new environmental order is crucial.

What effect will climate change have on the ranges of animal species?

  1. a) Species will not be affected
  2. b) Species will move with their adapted climate norms
  3. c) Species will not move with their adapted climate norms
  4. d) None of the above
Impact of Global Climate Change on Biodiversity

Current Rates and Causes of Extinction

In the fossil record, an individual vertebrate (amphibian, bird, fish, mammal, or reptile) species lasts on average at least one million years before it becomes extinct. Thus, in an average year, no more than one out of one million species should go extinct. The current observed extinction rate since 1,600, for vertebrates, is 2.6 per 10,000 species per year. That is at least 260 times the background rate of extinction. At this rate, it would take less than 15,000 years to equal the extinction event that killed the dinosaurs over several million years. Further, because we know that the primary cause of modern extinctions is the loss, degradation, and fragmentation of habitat, and because we know the response to habitat loss is not linear, we expect that background rate to continue to increase and probably become an order of magnitude greater than it is currently.

The reason for this increased and increasing rate of extinction is not difficult to fathom. Humans have been strongly implicated in global extinctions for tens of thousands of years, but the current mass extinction is due to the fact that in the last 50 years we have used more of Earth’s resources than we have for the entire history of humanity before that point. We are losing topsoil at least ten times faster than it can be replaced, about 10 % of the Earth’s agricultural land has become unfit for agriculture in the past 40 years while the population continues to expand, 80 % of the world’s fish stocks for which assessment information is available are reported as fully exploited or overexploited, we are using more than 20 % of the world’s renewable fresh water just for irrigation, and about 40 % of the world’s rainforests have been lost in the past 50 years. The human population has increased from 3.0 to 6.9 billion during those same 50 years and we are expecting another two billion over the next 40 years.

However, the current rate of extinction might pale compared to what anthropogenic climate change threatens. If we do not do something about climate change then all the money and the effort that has gone into saving species from extinction will likely be lost. This is particularly true because the current threat from habitat destruction and fragmentation interacts with climate change in a nonlinear way so that the negative impacts are greater than expected by looking at the threats independently.

Current and Future Rates of Global Climate Change

Over the past century the Earth has warmed approximately 0.74 oC, averaged over all land and ocean surfaces (IPCC 2007a). This warming of the Earth has clear effects on the species that live there. By the 1990s data started being published showing that 1,700 species had shifted their ranges an average of 6.1 km per decade toward the poles and that the timing of spring activities (e.g., breeding, migration, egg laying, flowering of plants) were occurring several days earlier each decade over the past 50 years. There is now a vast amount of data showing changes in the biology of plants and animals which are in accordance with expectations under a warming climate. Predictions for the next century are for increases in average global temperature of anywhere from 1.1 oC to 6.4 oC  due mostly to unprecedented increases in the atmospheric concentrations of the three most important greenhouse gases, carbon dioxide, methane, and nitrous oxide. However, growth in emissions continues to far exceed expectations and it is possible that predicted increases in temperature are conservative. Further, temperatures are expected to increase for at least several centuries beyond this one, because of the half-life of CO2 in the atmosphere, loss of polar ice which reflects rather than absorbs the sun’s energy (Albedo effect), release of energy from the ocean into the atmosphere, and interactions between atmospheric warming and release of CO2 from sources such as melting permafrost.

Another major impact of climate change includes effects on rainfall patterns and water storage. Rainfall is expected to become more variable with longer droughts and more flooding. Drought areas have more than doubled over the past 40 years . In many parts of the world more precipitation will fall as rain instead of snow, snows at higher elevations will melt earlier, and this will cause deleterious changes in river flow. These changes are already causing a looming water crisis in the western USA. Similarly, the Tibetan plateau and the Himalayas are sources for the five major rivers of Asia, all of which are vitally important to already threatened biodiversity and to 1.4 billion people. Changes in temperature and precipitation are expected to drastically alter snow melt patterns over the next 40 years, so that the Indus, Brahmaputra, and Mekong basins are likely to experience severe negative effects owing to the dependence on irrigated agricul- ture and meltwater in these areas.

In terms of temperature, sea levels, and other climatic factors, we are headed towards conditions that have not existed for millions or probably tens of millions of years. Even the most rapid changes in the Earth’s climate that led to those conditions in the past took at least 100,000 years to occur, not a couple of centuries or less.


Climate Change and Biodiversity

There are three possible fates for populations and species making up the current biodiversity of Earth, in a rapidly changing environment: (1) Plants and animals can migrate in order to leave unfavorable environmental conditions in one area and take up residence in an area with environmental conditions that are more conducive to survival and reproduction. The evidence that plants and animals have migrated long distances during past warming and cooling periods over the past 250,000 years is irrefutable. We already see such movement in extant species and can expect to see much more in the future. (2) Adapt to changing conditions through selection on genetic diversity present or arising in the population during the period of climatic change. (3) Go extinct when some combination of the first two are not sufficient to keep the population or species extant. Over hundreds of millions of years, the Earth has experienced innumerable cooling and heating periods of different magnitudes and rates. Periods of rapid change are usually accompanied by increases in extinction rates.

Projections of species extinction rates during the current period of global climate change are controversial because of uncertainty concerning how much the climate will change and how fast, because of methodological challenges especially concerning species ability to migrate through a human-dominated habitat matrix, and because of imperfect data from past extinctions. However, much is known and reasonable estimates can be made by looking at past extinctions under global climate change. An analysis of the fossil record over the past 520 million years provides a consistent relationship between global temperatures and biodiversity levels. During warm phases, extinction rates have been relatively high in both terrestrial and marine environments. Extinction rates may increase approximately 10 % for every 1 oC increase in temperature. The end-Permian event that caused the extinction of approximately 95 % of all species on Earth was accompanied by a 6 oC increase in global temperatures over a few million years.  An increase in temperature of approximately 5 oC over several million years caused a great loss of plant biodiversity in Greenland.

The biodiversity currently extant on Earth faces a much more difficult situation than does biodiversity during past periods of rapid climate change. There are two major reasons for this: (1) Species habitats are smaller than in the past. Smaller habitats support smaller populations which harbor less genetic diversity and have less evolutionary potential. This evolutionary potential is critical for species’ ability to adapt to the changing environmental conditions. (2) Species habitats are more fragmented than in the past. The fragmentation prevents individ- uals from being able to shift their distribution in response to climate-related impacts as easily as in the past. Recall, these are the two fates available to species other than going extinct: adapt to climate change or migrate in response to climate change in order to track environmental conditions favorable to survival. The current rate of climate change is probably unprecedented and would present extreme challenges to the biota of the planet under normal circumstances. How- ever, the combination of the magnitude of change, the extreme fragmentation of habitats, and the fact that there are 6.9 billion people using a very large proportion of the Earth’s resources means that neither evolution nor migration will be sufficient to allow many species to cope with current rates of global climate change. They will go extinct and their value to humans and their beauty lost.

Many of the extinction may not be due directly to global climate change alone. The interactions between climate change and other factors will likely be extremely important. Habitat destruction and conversion continues in Southeast Asia and the Amazon, both places where I have done research and both areas rich in biodiversity. The continued fragmentation and loss of these forested areas alone is cause for grave concern. However, when you factor in that destruction of forests in these areas further limits movement of species trying to track a changing environment and that this continues to fuel further climate change by dumping more CO2 into the atmosphere, it starts to appear catastrophic. Warmer temperatures will cause novel diseases to spread into naïve populations and will act as a general stress causing species to be susceptible to diseases their immune systems once were able to fight. The interaction between global climate change and diseases is already manifest in the widespread extinction of amphibians. Increases in the frequency of drought and the introduction of novel diseases form an almost perfect combina- tion for driving populations extinct, as these are the two biggest causes for popula- tion collapse among vertebrates. Temperature stress, and other forms of environmental stress incurred through climate change, will interact with declining population size and loss of genetic diversity in a way that makes populations more vulnerable than either factor independently.

Table 2 Several million species of plant and animals face extinction due to global climate change, perhaps as early as 2050. Species most affected would be poor dispersers or those for which anthropogenic habitat fragmentation prevents dispersal and those with very narrow thermal toler- ances and/or low genetic variation that prevent(s) evolution of new tolerances. The table below is based on the results of Thomas et al. (2007), who predicted extinction under different warming and movement scenarios. Migration rates are certainly somewhere between the extreme of no move- ment and the ability to track moving habitats perfectly, and current data suggests that warming trends will be close to the warmest predictions of Thomas et al. (2007). Consequently, :::33 % of known species may be doomed to extinction by 2050

Warming level No movement (%) Perfect movement (%)
Low 34 extinction 11 extinction
Medium 45 extinction 19 extinction
High 58 extinction 33 extinction


Exactly how bad will it get? The prediction from a team of experts is that if temperatures increase by >3.5 oC, 40–70 % of plant and animal species will face extinction (Rosenzweig et al. 2008). One prominent study suggests that about 25 % of known plant and animal species will be committed to extinction by 2050 under current warming predictions (Table 2) (Thomas et al. 2004). A recent study suggests that 20 % of lizards may be extinct by 2080 as a direct result of increasing global temperatures (Sinervo et al. 2010). Exact predictions are of course impossible, but the combination of extremely rapid climate change, reduced population sizes, and fragmented habitats suggest that a very large proportion of Earth’s biodiversity (>50 % of plants and animals) suffers a very high risk of extinction, particularly if definitive measures are not taken rapidly to ameliorate climate change. This level of species extinctions will be associated with an even larger percentage loss of populations and genetic diversity. This represents a very large proportion of the biodiversity and biological resources that humans need now more than ever.

Future Direction

In order to stave off huge losses in biodiversity and to save vast amounts of human suffering, humans will have to change the way they live and use their boundless innovation to produce a high quality of life in a way that does not endanger the planet. The following are some urgent and important general suggestions for directions human societies need to head in to mitigate climate change:

•       Limit land-use change and make intelligent choices in land-use changes that balance agriculture against biodiversity loss. This is especially urgent in tropical and subtropical regions, but also in boreal and temperate forests. Zero-loss of old-growth forests must be the immediate goal and this be combined with reforestation. Manage forests more efficiently and create/enforce laws that protect forests from illegal logging and pillaging from corporations at the expense of the general populace.

  • Aggressive climate change mitigation, including immediate implementation of existing cleaner energy technologies, increased research to develop cleaner energy sources, and immediate legislation to increase fuel efficiency for motor vehicles.
  • Place a global cap on greenhouse gas emissions and sell permits up to that cap in a

global auction. Use the profits to finance the other mitigation measures discussed. Include the economic price for land-use emissions. Eco- nomic models must begin to include all costs associated with production of goods, governments must regulate rather than subsidize corporations and their impacts on the public, and people must hold their governments accountable for lack of enforcement or collusion with corporations.

  • Stabilize and eventually reduce human population size through intense family planning, education, and a change in societal norms.
  • Make human-dominated landscapes more hospitable to biodiversity. Reclamation of degraded lands and reintroduction of extirpated species.
  • Education of people, especially in the rural tropics, concerning biodiversity and the value of nature. Shift in the education and reward systems of developed nations away from short-term profiteering and financial markets toward science, engineering, innovation, and longer-term good to society.
  • Create space and opportunities for ecosystems to self-adapt and reorganize because novel climates without current analogs will appear.

The Role of Plants in Carbon Storage and Greenhouse Gas Emissions

When it comes to the global carbon cycle, plants are the ultimate carbon-capturing champions. While artificial carbon capture technologies are being developed to address climate change, photosynthesis can be thought of as a “technology” that has been perfected and deployed for over 2 billion years. The rate of atmospheric carbon uptake by plants via photosynthesis dwarfs all other forms of carbon capture. Land plants pull an estimated 120 petagrams of carbon from the atmosphere each year-over 15% of the total atmospheric carbon pool.

Plants, in addition to being the primary conduits for CO2 removal from the atmosphere, can sequester carbon. As mentioned above, the global terrestrial plant carbon pool stores approximately 560 petagrams of carbon. It is easier to estimate aboveground plant carbon pools than it is to estimate carbon in soils. Plants, and more specifically the re­flectance of the chlorophyll in leaves, can be measured from satellites in space. This technique is called remote sensing and is a powerful tool for assessing the amount of plant biomass on the land surface. Changes in reflectance over time are correlated with changes in leaf area, which in turn is directly related to rates of plant growth. Thus, satellite imagery repeated over time can be used to estimate how much plant growth is occurring on land. Below ground plant parts, namely roots, are much harder to estimate than above ground plant parts, as roots are hidden from view. Roots can extend deep into soils. Roots can be large, like the structural roots of trees, or small and ephemeral, like the main absorptive roots of grasses and herbs. Although measuring root biomass is difficult, it is important for understanding the carbon cycle. In some ecosystems there is as much root carbon below ground as is stored in plant biomass above ground. Roots are also important because they are the main conduits for soil organic matter formation.

Roots are generally assumed to be a greater contributor to soil carbon stocks than aboveground plant tissues because roots are already buried below ground and can be more easily captured and sequestered in the soil than aboveground tissues. Thus, roots play a key role as transmitters of carbon into soils.

What is the main point of the text?

  1. a) Plants are the ultimate carbon-capturing champions
  2. b) Artificial carbon capture technologies are more efficient than photosynthesis
  3. c) The rate of atmospheric carbon uptake by plants is decreasing
  4. d) None of the above

What percentage of the total atmospheric carbon pool is pulled by land plants each year?

  1. a) 5%
  2. b) 10%
  3. c) 15%
  4. d) 20%

Forests as carbon sinks and sources

Overall, trees store more carbon than any other plant type. Forests cover about 30% of the total land area on Earth and account for approximately 80% of the terrestrial plant biomass. This means that forests store an estimated 350 petagrams of carbon in their tissues, most of which is in wood. Forests are vulnerable to natural and human-caused disturbance events such as fire, logging, pests, and weather-related disturbances. Some estimates suggest that 60% of the world’s forests are in some stage of recovery from the last disturbance event. Forest disturbance often leads to the emissions of greenhouse gases. Deforestation is a big contributor to global greenhouse gas emissions, especially in the tropics. At a global scale, tropical deforestation accounts for about 10% of all greenhouse gas emissions annually. Some of the greenhouse gas emis­sions from deforestation result from disturbance to soils. Tree cutting and removal can break up soil aggregates, exposing previously trapped carbon to microbial decomposition and providing fuel to microbes that produce CO2, methane, and nitrous oxide. Greenhouse gases are also released during the decomposition of the plant litter produced from deforestation.

Forest fire and biomass burning is another large source of green­ house gas emissions. Fires consume biomass and produce CO2, meth­ ane, and nitrous oxide, among other gases. Globally, 2 to 3 petagrams of carbon are emitted to the atmosphere annually from fires. Over 80% of this comes from tropical regions, with approximately 1 petagram of carbon per year coming from savannas (wooded grasslands). Climate change is increasing the frequency and severity of drought in some re­gions, and this can in turn increase the occurrence of fires.

Emission Reduction via Agricultural Management

Management of agricultural lands has historically been a major con­tributor to climate change, amounting to approximately 25% of global greenhouse gas emissions. When plants and animals are harvested from working lands, the associated carbon and nutrients are harvested as well. Fertilizer can replace some of the nutrients harvested, and plant growth can bring new carbon into ecosystems, but rarely do we replace all the carbon and nutrients that are lost. Fertilization, irrigation, and biomass burning, as well as practices that disturb soils such as plowing and tillage, can increase emissions of all three of the major greenhouse gases. Human land use over the last 12,000 years has resulted in the loss of an estimated 116 petagrams of SOC in the top 2 meters of soil, globally. Deforestation, primarily in the tropics, results in the loss of approximately 1.7 petagrams organic carbon per year from ecosys­tems. In order to slow climate change, and bend the curve, greenhouse gas emissions from working lands must be reduced. There are several possible approaches for reducing emissions that can yield significant greenhouse gas savings, including improved fertilizer, tillage, water, and residue management, as well as matching crops to appropriate soils and climates, and incorporating fallow periods. Improved grazing land and livestock management, together with better manure management, are additional practices that are known to reduce emissions. Taken together at a global scale, these practices have been estimated to have the poten­tial to save over 3 petagrams in CO2 equivalents (CO2e) per year. Below, we detail two examples of approaches that can reduce emissions and offer valuable co-benefits.

Nitrogen fertilizer

Nitrogen fertilizer comes in organic and inorganic forms and is widely used in agriculture to enhance plant growth. Inorganic nitrogen fertilizer can be a large source of greenhouse gas emissions, from production to field application. The manufacturing of inorganic nitrogen fertilizer is a carbon-intensive activity. A lot of energy is required to convert dinitro­gen gas to ammonia during fertilizer production. In 2004, the fertilizer industry used approximately 1% of the world’s energy, with 90% of that used to produce ammonia. Producing the 119 million metric tons (MMT) of nitrogen fertilizer applied to soils globally in 2018 resulted in at least 492 MMT of CO2 emissions (values calculated using Statista 2014). This is assuming that natural gas was used in the manufacturing process; if coal was used, the energy cost was higher. To compound the problem, nitrogen is often applied to fields in excess of plant requirements. This extra nitrogen fertilizer stimulates microorganisms in the soils that make nitrous oxide gas, and nitrous oxide emissions increase exponentially with the amount of nitrogen fertilizer added.

At the field scale, there are several approaches that can lower green­ house gas emissions from fertilizer use. Careful monitoring of plant re­ quirements could significantly lower the amount of nitrogen fertilizer needed for agriculture. There are important co-benefits from this rela­ tively simple action. Less fertilizer applied means that less fertilizer will need to be produced, lowering the carbon footprint of fertilizer manu­ facturing. Lower fertilizer application rates will also lower nitrous oxide emissions. More efficient fertilization application could save the farmer money, helping to support a more financially sustainable agricultural industry. And finally, less fertilizer use can help reduce nitrogen runoff and pollution of waterways. Some additional ways to lower greenhouse gas emissions associated with fertilizer use include the following:

►        Use low-carbon or no-carbon fuels in fertilizer manufacturing.

►        Capture biosolids and wastewaters and convert them to nitrogen amendments: this also helps remove nitrogen pollution from waterways.

►        Use organic nitrogen and slow-release fertilizer. If the fertilizer is released slowly, it can result in lower emissions and have a lower overall carbon footprint.

►        Use buried or drip irrigation. Supplying only the amount of water the plant needs can minimize overwatering that can stimulate nitrous oxide emissions.

►        Use nitrogen-fixing cover crops. Nitrogen-fixing plants are species.

that can pull nitrogen from the atmosphere and supply it to soils. Nitrogen-fixing plants in the legume (pea) family are often used as cover crops during fallow periods (see below). Nitrogen-fixing cover crops can also stimulate nitrous oxide emissions but do not result in the energy costs associated with inorganic nitrogen fertil­ izer production.

The total greenhouse gas savings from these improved practices have not yet been estimated at a global scale, but models suggest the results will be very promising.

Environmental Impact on Photosynthesis

All plants ingest atmospheric carbon dioxide and convert it into sugars and starches through the process of photosynthesis but they do it in different ways. The specific photosynthesis method (or pathway) used by each plant class is a variation of a set of chemical reactions called the Calvin Cycle. These reactions impact the number and type of carbon molecules a plant creates, the places where those molecules are stored, and, most importantly for the study of climate change, a plant’s ability to withstand low carbon atmospheres, higher temperatures, and reduced water and nitrogen.

These processes of photosynthesis—designated by botanists as C3, C4, and CAM,—are directly relevant to global climate change studies because C3 and C4 plants respond differently to changes in atmospheric carbon dioxide concentration and changes in temperature and water availability.

Humans are currently dependent on plant species that do not thrive in hotter, dryer, and more erratic conditions. As the planet continues to warm up, researchers have begun exploring ways in which plants can be adapted to the changing environment. Modifying the photosynthesis processes may be one way to do that.

C3 Plants

The vast majority of land plants we rely on for human food and energy use the C3 pathway, which is the oldest of the pathways for carbon fixation, and it is found in plants of all taxonomies. Almost all extant nonhuman primates across all body sizes, including prosimians, new and old world monkeys, and all the apes—even those who live in regions with C4 and CAM plants—depend on C3 plants for sustenance.

  • Species: Grain cereals such as rice, wheat, soybeans, rye, and barley; vegetables such as cassava, potatoes, spinach, tomatoes, and yams; trees such as apple, peach, and eucalyptus
  • Enzyme: Ribulose bisphosphate (RuBP or Rubisco) carboxylase oxygenase (Rubisco)
  • Process: Convert CO2 into a 3-carbon compound 3-phosphoglyceric acid (or PGA)
  • Where Carbon Is Fixed: All leaf mesophyll cells
  • Biomass Rates: -22% to -35%, with a mean of -26.5%

While the C3 pathway is the most common, it is also inefficient. Rubisco reacts not only with CO2 but also O2, leading to photorespiration, a process that wastes assimilated carbon. Under current atmospheric conditions, potential photosynthesis in C3 plants is suppressed by oxygen as much as 40%. The extent of that suppression increases under stress conditions such as drought, high light, and high temperatures. As global temperatures rise, C3 plants will struggle to survive—and since we’re reliant on them, so will we.

C4 Plants

Only about 3% of all land plant species use the C4 pathway, but they dominate nearly all grasslands in the tropics, subtropics, and warm temperate zones. C4 plants also include highly productive crops such as maize, sorghum, and sugar cane. While these crops lead the field for bioenergy, they aren’t entirely suitable for human consumption. Maize is the exception, however, it’s not truly digestible unless ground into a powder. Maize and other crop plants are also used as animal feed, converting the energy to meat—another inefficient use of plants.

  • Species: Common in forage grasses of lower latitudes, maize, sorghum, sugarcane, fonio, tef, and papyrus
  • Enzyme: Phosphoenolpyruvate (PEP) carboxylase
  • Process: Convert CO2 into 4-carbon intermediate
  • Where Carbon Is Fixed: The mesophyll cells (MC) and the bundle sheath cells (BSC). C4s have a ring of BSCs surrounding each vein and an outer ring of MCs surrounding the bundle sheath, known as the Kranz anatomy.
  • Biomass Rates: -9 to -16%, with a mean of -12.5%.

C4 photosynthesis is a biochemical modification of the C3 photosynthesis process in which the C3 style cycle only occurs in the interior cells within the leaf. Surrounding the leaves are mesophyll cells that contain a much more active enzyme called phosphoenolpyruvate (PEP) carboxylase. As a result, C4 plants thrive on long growing seasons with lots of access to sunlight. Some are even saline-tolerant, allowing researchers to consider whether areas that have experienced salinization resulting from past irrigation efforts can be restored by planting salt-tolerant C4 species.

CAM Plants

CAM photosynthesis was named in honor of the plant family in which Crassulacean, the stonecrop family or the orpine family, was first documented. This type of photosynthesis is an adaptation to low water availability and occurs in orchids and succulent plant species from arid regions.

In plants employing full CAM photosynthesis, the stomata in the leaves are closed during daylight hours to lessen evapotranspiration and open at night in order to take in carbon dioxide. Some C4 plants also function at least partially in C3 or C4 mode. In fact, there’s even a plant called Agave Angustifolia that switches back and forth between modes as the local system dictates.

  • Species: Cactuses and other succulents, Clusia, tequila agave, pineapple.
  • Enzyme: Phosphoenolpyruvate (PEP) carboxylase
  • Process: Four phases that are tied to available sunlight, CAM plants collect CO2 during the day and then fix CO2 at night as a 4 carbon intermediate.
  • Where Carbon Is Fixed: Vacuoles
  • Biomass Rates: Rates can fall into either C3 or C4 ranges.

CAM plants exhibit the highest water-use efficiencies in plants which enable them to do well in water-limited environments, such as semi-arid deserts. With the exceptions of pineapple and a few agave species, such as the tequila agave, CAM plants are relatively unexploited in terms of human use for food and energy resources.

Evolution and Possible Engineering

Global food insecurity is already an extremely acute problem, rendering the continued reliance on inefficient food and energy sources a dangerous course, especially when we don’t know how plant cycles will be affected as our atmosphere becomes more carbon-rich. The reduction in atmospheric CO2 and the drying of the Earth’s climate are thought to have promoted C4 and CAM evolution, which raises the alarming possibility that elevated CO2 may reverse the conditions that favored these alternatives to C3 photosynthesis.

Evidence from our ancestors shows that hominids can adapt their diet to climate change. Ardipithecus ramidus and Ar anamensis were both reliant on C3 plants but when a climate change altered eastern Africa from wooded regions to savannah about four million years ago, the species that survived—Australopithecus afarensis and Kenyanthropus platyops—were mixed C3/C4 consumers. By 2.5 million years ago, two new species had evolved: Paranthropus, whose focus shifted to C4/CAM food sources, and early Homo sapiens that consumed both C3 and C4 plant varieties.

C3 to C4 Adaptation

The evolutionary process that changed C3 plants into C4 species has occurred not once but at least 66 times in the past 35 million years. This evolutionary step led to enhanced photosynthetic performance and increased water- and nitrogen-use efficiency.

As a result, C4 plants have twice as the photosynthetic capacity as C3 plants and can cope with higher temperatures, less water, and available nitrogen. It’s for these reasons, biochemists are currently trying to find ways to move C4 and CAM traits (process efficiency, tolerance of high temperatures, higher yields, and resistance to drought and salinity) into C3 plants as a way to offset environmental changes faced by global warming.

At least some C3 modifications are believed possible because comparative studies have shown these plants already possess some rudimentary genes similar in function to those of C4 plants. While hybrids of C3 and C4 have been pursued more than five decades, due to chromosome mismatching and hybrid sterility success has remained out of reach.

What is the benefit of C4 plants compared to C3 plants?

a) C4 plants are more resistant to cold temperatures

b) C4 plants require less water and nitrogen than C3 plants

c) C4 plants have less photosynthetic capacity than C3 plants

d) C4 plants are more susceptible to diseases than C3 plants

The Future of Photosynthesis

The potential to enhance food and energy security has led to marked increases in research on photosynthesis. Photosynthesis provides our food and fiber supply, as well as most of our sources of energy. Even the bank of hydrocarbons that reside in the Earth’s crust was originally created by photosynthesis.

As fossil fuels are depleted—or should humans limit the use of fossil fuel to forestall global warming—the world will face the challenge of replacing that energy supply with renewable resources. Expecting the evolution of humans to keep up with the rate of climate change over the next 50 years is not practical. Scientists are hoping that with the use of enhanced genomics, plants will be another story.

What is the potential benefit of enhancing photosynthesis?

  1. a) Enhancing photosynthesis can help in reducing the atmospheric CO2 levels.
  2. b) Enhancing photosynthesis can provide more water to plants.
  3. c) Enhancing photosynthesis can enhance food and energy security.
  4. d) Enhancing photosynthesis can increase the pollution levels.

Reference Text


Ramanathan, Veerabhadran, Roger Aines, Max Auffhammer, Jonathan Cole, Fonna Forman, Hahrie Han, Mark Jacobsen et al. “Learning Companion to Bending the Curve: Climate Change Solutions.” (2019).



Additional Reading

Beer, C., et al. 2010. Terrestrial gross carbon dioxide uptake: global distribution and covariation with climate. Science 329, 834-838.

FAO. 2010. Global Forest Resources Assessment 2010. FAO Forestry Paper 163. Food and Agricultural Organization of the United Nations, Rome, Italy.

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