by DK
In Gaia theory, feedback loops keep Earth in balance. One example is the effect that sea algae called coccolithophores have on keeping the planet’s climate in check. When the algae die, they release a gas, dimethyl sulphide (DMS), that helps to create clouds.
Saving the planet
As Lovelock elaborated on the theme, the scientific establishment gradually began to accept the Gaia hypothesis. In the 1980s, a series of “Gaia conferences” attracted scientists from many different disciplines, willing to explore the mechanisms involved in regulating Earth’s environment to achieve homeostasis. Later, more attention was devoted to looking at the implications of the hypothesis in the face of climate change. Human activity had been shown to disturb Gaia’s system, but the issue was now whether its regulatory mechanisms could withstand further pressure—or whether Earth was facing another irreversible tipping point.
Environmentalists, who had been among the first to embrace Gaia, reacted with dismay to the theory that the human species may precipitate a catastrophic change in Earth’s equilibrium. The rallying cry of Green activists became “Save the planet!” but this was at odds with the fundamental idea of Gaia. Although the destruction of natural habitats, the excessive burning of fossil fuels, the depletion of biodiversity, and other human-made threats were likely to have severe consequences for many species—including humans—the planet, according to the Gaia hypothesis, will survive and find a new equilibrium.
Nuclear power stations produce plentiful “clean” energy, but also toxic waste. James Lovelock believes Earth is able to absorb and overcome the waste’s radioactive effects.
Daisyworld
At first, scientists criticized the Gaia hypothesis for its supposed implication that the ecosystems in the biosphere could collectively influence Earth’s environment. So to enhance the plausibility of the Gaia theory, in 1983 James Lovelock and fellow British scientist Andrew Watson produced “Daisyworld,” a simple explanatory model.
Daisyworld is a barren planet, orbiting a sun. As the intensity of the sun’s rays increases, black daisies start to grow. They absorb heat and warm the planet’s surface to the point where white daisies can thrive. They, in turn, reflect the sun’s energy, so cooling the ground. The two kinds of daisy reach a point of equilibrium, whereby they regulate the temperature of the planet. When the sun’s heat increases further, the white daisies, able to reflect the sunlight and stay cool, replace the black daisies. Finally, the sun heats up so much that even the white daisies can no longer survive.
See also: The ecosystem • Evolutionarily stable state • The biosphere • A holistic view of Earth
IN CONTEXT
KEY FIGURE
Luis Alvarez (1911–88)
BEFORE
1953 American geologists Allan O. Kelly and Frank Dachille suggest in their book Target: Earth that a meteor impact may have been responsible for the extinction of the dinosaurs.
AFTER
1991 The Chicxulub Crater in the north of the Yucatan Peninsula in southeastern Mexico is proposed as the site of a massive comet or meteor impact at the end of the Cretaceous period.
2010 An international panel of scientists agrees that the Chicxulub impact led to the Cretaceous–Paleogene mass extinction, around 65 million years ago.
There have been five periods in Earth’s history when abnormally large numbers of multicellular organisms have died off in a relatively short time. These mass extinctions are defined by the loss of multicellular animals and plants because their fossils are far easier to detect than those of single-celled organisms.
The general (“background”) rate of extinction is between one and five species a year. Fossil records show, for example, the extinction of two to five families of marine animals every million years. This is far exceeded during mass extinctions, which always mark the boundary between two geological periods. Scientists do not understand all the factors responsible for these events, though they are agreed on some. Increased volcanic activity, changes in the composition of the atmosphere and oceans, climate change, sea level rises and falls, tectonic movement of the continents, and meteor impacts are all likely causes. Some scientists suggest we have now entered a sixth mass extinction, this time the result of human activity.
The meteor that hit Earth at the end of the Cretaceous period was traveling at 40,000 mph (64,000 kph). Its power was a billion times greater than the Hiroshima atomic bomb.
“All geologic history is full of the beginnings and the ends of species–of their first and last days.”
Hugh Miller
Scottish geologist
End of the dinosaurs
The mass extinction that scientists understand best is also the most recent, around 65 million years ago. Geologists refer to it as the K-Pg extinction event because it occurred at the end of the Cretaceous and start of the Palaeogene periods. Although an extraterrestrial origin was first suggested for the event in the 1950s, this was not taken seriously until two discoveries, in Europe and North America.
In 1980 a team of scientists working in Italy, including physicist Luis Alvarez and his geologist son, Walter Alvarez, discovered a clay layer between Cretaceous and Paleogene deposits. Examination of the clay revealed that it contained the mineral iridium, which is rare on Earth but common in asteroids. The discovery led to the Alvarez Hypothesis, which proposed that the extinction at the end of the Cretaceous period was caused by a catastrophic meteor strike. The location of the impact was still a mystery, until 11 years later, when a massive crater 106 miles (170 km) across on Mexico’s Yucatan Peninsula was found to date from the time of the extinction.
The scientific consensus is that a massive comet or asteroid struck Earth, producing a blast of radiation and a destructive megatsunami more than 328 ft (100 m) high. The radiation would have killed animals nearby, and the megatsunami would have obliterated coastal regions around the Gulf of Mexico. The main damage, however, would be more gradual. A vast cloud of soot and dust would have spread through the atmosphere, blocking out sunlight for several years. Plants died because they could no longer photosynthesize, and algae in coral reefs also succumbed, disrupting food chains worldwide. The impact would have also released sulfuric acid into the atmosphere, which produced acid rain, acidifying the oceans and killing off marine life. Around the same time, a huge amount of volcanic activity flooded 193,000 sq miles (500,000 sq km) of southern India with lava, forming the Deccan Plateau and further changing the climate and atmosphere.
The K-Pg event is best known for the extinction of all nonflying dinosaurs. It was also responsible for the death of nearly all four-legged animals (tetrapods) that weighed more than 55 lb (25 kg). An exception were crocodiles, which may have survived because they are ectotherms (cold-blooded animals), able to survive for a long time without food. Dinosaurs were endotherms (warm-blooded animals), with a fast metabolism that demanded regular meals. Many plant species died because they could not photosynthesize, leaving herbivorous dinosaurs with little vegetation to eat, while predatory species starved for lack of prey. In contrast, fungi, which do not depend on photosynthesis, proliferated.
In the oceans, phytoplankton, a vital food source that also relied on photosynthesis, died out. Creatures that fed on phytoplankton then faced extinction. These included cephalopods, such as belemnites and ammonites, and the marine reptiles known as the mosasaurs and the sauropterygians.
Although many flying dinosaurs survived the K-Pg mass extinction at the end of the Cretaceous period, all pterosaurs perished, ending their 162-million-year stay on Earth.
“We have very strong physical and chemical evidence for a large impact … the extinction coincides with the impact to a precision of a centimetre or better.”
Walter Alvarez
LUIS ALVAREZ
Considered one of the greatest physicists of the 20th century, Luis Alvarez was born in San Francisco in 1911. He graduated from the University of Chicago in 1936 and went on to work at the Radiation Laboratory in the Universit
y of California, Berkeley. There he helped develop nuclear reactors and, during World War II, nuclear weapons. He witnessed the atomic bombing of Hiroshima and helped build a plutonium bomb.
After the war, Alvarez developed the liquid hydrogen bubble chamber, used to discover new subatomic particles. For this, in 1968 he was awarded the Nobel Prize for Physics. Later he provided the calculations to back up the Alvarez Hypothesis of mass extinction caused by a meteor strike. He died in 1988.
Key works
1980 “Extraterrestrial Cause for the Cretaceous–Tertiary Extinction,” Science
1985 “The Hydrogen Bubble Chamber and the Strange Resonances”
1987 Alvarez: Adventures of a Physicist
Marine annihilation
The earliest mass extinction, and the second-most catastrophic, occurred when our planet cooled dramatically, toward the end of the Ordovician period, around 444 million years ago. At this time, most organisms on Earth lived in the oceans. As the supercontinent Gondwana drifted slowly over the South Pole, a giant ice cap formed, lowering global temperatures. Much of the planet’s water became “locked up” as ice, depressing sea levels and reducing the area of Earth’s surface covered by ocean.
As a result, marine organisms living in shallow continental-shelf water suffered particularly high rates of extinction. In at least two peak die-off periods, separated by hundreds of thousands of years, nearly 85 percent of marine species died out, including brachiopods, bryozoans, trilobites, graptolites, and echinoderms.
Slow extinction
By the Late Devonian period, around 359 million years ago, the continents had been colonized by plants and insects, and massive organic reefs thrived in the oceans. The continents of Euramerica and Gondwana were converging into what would become Pangaea—the last of the supercontinents. In this period, a succession of extinctions—possibly as many as seven—took place over a longer timescale than any other mass extinction event, possibly up to 25 million years.
The extinctions may have had many causes, including reduced oxygen in the oceans, falling sea levels, atmospheric changes, the draining of water produced by the spread of plants, and asteroid impacts. Most organisms lived in the oceans, and shallow seas were worst affected, with many reef-building organisms, brachiopods, trilobites, and the last of the graptolite species dying off. Around 75 percent of marine species died, and it would be another 100 million years before corals re-established themselves on a large scale.
“The current extinction has its own novel cause: not an asteroid or a massive volcanic eruption but “one weedy species.””
Elizabeth Kolbert
American journalist
“The Great Dying”
The most dramatic mass extinction took place at the end of the Permian period, 252 million years ago. Also known as “The Great Dying,” it resulted in the loss of 96 percent of marine species and 70 percent of land-living vertebrates. Insects suffered the only mass extinction in their history, and the last of the trilobites, which had been in decline for millions of years, disappeared from the fossil record.
Potential causes for the mass extinction include asteroid impact and oxygen depletion in the oceans. The extinction also coincided with one of the biggest periods of volcanic activity in Earth’s history. The eruptions, which lasted nearly 1 million years, flooded more than 0.8 million sq miles (2 million sq km) of ancient Siberia with basalt lava. The resulting buildup of greenhouse gases would have transformed the atmosphere of Earth, likely resulting in severe global warming and contributing to species extinction.
“Modelled future extinction rates are projected to be 10,000 times Earth’s historical geological background rate.”
Ron Wagler
American academic
Phased losses
All life today is descended from the small minority of species that remained at the start of the Triassic period. During the period’s final 18 million years, ending about 201 million years ago, at least half of all animal species known to be living at that time were wiped out in two or three extinction phases. Climate change caused by more basalt eruptions and an asteroid impact have been cited as causes. In the seas, many reptiles, cephalopods, mollusks, and reef-building organisms died out. On land, most of the reptilelike archosaurs and many large amphibians became extinct. The loss of the archosaurs, in particular, opened up ecological niches that the dinosaurs would fill.
The sixth extinction
Sudan, the last male northern white rhinocerous, died in 2018 (two females remain). Poaching has taken the species to the edge of extinction.
Some ecologists have estimated the current rate of extinction of animals and plants at 100–1,000 times the natural background rate, with most of the increase due directly or indirectly to human activities. They argue that this is evidence the world is already in the middle of the Holocene extinction, named for the present geological epoch. Many species of animals and plants have been lost since the start of the Industrial Revolution in the 18th century. These losses have been driven by habitat change, climate change, overfishing, overhunting, ocean acidification, air pollution, and the introduction of animals that disrupt food chains. American ecologist E.O. Wilson, known as “the father of biodiversity,” believes that if the species die-off continues at the present rate, half of all higher life forms will be extinct by 2100. Stuart Pimm, a British–American biologist and modern extinctions expert, is more cautious, claiming that we are on the cusp of such an event and can still act to stop it.
See also: Ancient ice ages • Moving continents and evolution • The Gaia hypothesis • Ocean acidification
IN CONTEXT
KEY FIGURE
James Hansen (1941–)
BEFORE
1875 In the book Climate and Change, Scottish scientist James Croll describes the climate-warming feedback effect of melting ice.
1965 Canadian biologist Charles Krebs discovers the “fence effect,” showing vole populations protected from foxes rocketing, then crashing.
1969 American planetary scientist Andrew Ingersoll highlights the “runaway greenhouse effect” that caused the planet Venus to heat up.
AFTER
2018 Ecologists in Alaska predict that the accelerating release of methane from formerly frozen lakes will increase global warming.
All parts of an ecosystem are interdependent. Any species or habitat change will feed back into the system, and affect the whole of that system, including the part where it all started. In other words, the feedback travels around in a loop.
In some situations, change is kept in check by the loop. For example, if aphids suddenly multiply, they provide more food for ladybugs, leading to an increase in the number of ladybugs. But with more ladybugs feeding on the aphids, aphid numbers drop again. This is negative feedback and it helps maintain the status quo.
In other cases, feedback can accelerate change. Shrubs, for example, may begin to take over from grass on newly colonized land, casting their shade over the grass, depriving it of sunlight, and slowing its growth. The shrubs now have more water and nutrients, so they prosper at the expense of the grass. This is positive feedback and it is inherently destabilizing.
Ideas about feedback loops first developed early in the 20th century. They were based on the work of two mathematicians—Alfred Lotka (1880–1949) in the US and Italian Vito Volterra (1860–1940)—who independently devised equations based on the interaction between predators and prey. Their equations showed that a prey population will grow rapidly when the number of predators drops, while the predator population will drop when prey numbers drop, because the predators go hungry. The result is a constant cycle of falling and rising predator and prey populations.
Balancing the system
The predator–prey cycles identified by Lotka and Volterra were focused on the interaction between single predator and prey species. Since their studies, the theory of feedback loops has developed to embrace entire ecosystems. Ecologists now think that negative feedb
ack loops are of central importance for the functioning of all ecosystems, keeping every part of them naturally within the bounds of sustainability. Populations can never swell for long beyond the carrying capacity of the rest of the system to support them. Thus, negative feedback regulates an ecosystem and keeps it stable.
Positive feedback interferes with a balanced ecosystem. If there is a surplus of resources, or a lack of predators, a population can grow freely. A bigger population leads to more births, and so an acceleration of the growth in population.
Equally, positive feedback can result in an accelerated contraction of a population. If fish stocks decline in a lagoon, for instance, local people may resort to importing canned food. Pollution from the dumps where the cans are thrown away can seep into the lagoon, killing the fish—and encouraging the locals to import even more of the damaging cans. And yet, positive feedback loops can sometimes set off a chain of events that becomes a “virtuous” circle. For example, if shrubs are planted in unstable soil, their roots may stabilize the soil, allowing both the shrubs and soil to thrive.
In a healthy ecosystem, a repeating fluctuation in numbers between prey, such as rabbits, and predators, such as foxes, is an example of a negative feedback loop balancing the system.
Feedback loops and climate change
Arctic areas such as Greenland have seen a reduction in summer ice of 72 percent since 1980. The warming of the atmosphere and rising sea levels are part of the resulting positive feedback loop.