1.2. Decline of nature
Human impacts on Earth
The human population has increased 2000-fold over the past 12.000 years, from four million at the start of the Holocene to nearly eight billion today. The enormous increase in human population and its demands for food and other resources has taken its toll on ecosystems and the animals, plants and other organisms on Earth.
The graph below gives a concrete illustration of the increasing impact humans have on Earth. In 2020, the mass of human-made material (buildings, roads, machines and so on) surpassed the mass of all the living material (trees, animals, microbes and so on) on Earth. And this ‘anthropogenic mass’ is increasing rapidly. For an average person on the planet, more than the person’s body weight of new material is produced each week.
Figure. The mass of human-made material exceeded the dry dry weight of all living things on Earth in 2020 and increases rapidly. Data from Elhacham et al (2020). The amount of biomass has not changed greatly between 1900-2020, as deforestation was widespread already before the 20th< century. 1 teratonn = 1000 billion tonns.
Not only do humans fill the world with new human-made materials, we have also significantly reduced the biomass of land vegetation – in particular, trees – at least since the start of agriculture. Due to human land use the biomass of land vegetation today is only about 50% of what it would be without human impact. In large areas in Europe, South and East Asia, as well as in Eastern South and North America, the reduction of biomass is more than 80% (see Figure below). In these areas, much of land has been converted to agricultural land or is covered by cities, roads, and other infrastructure.
Conversion of land to e.g. agricultural fields and cities is however responsible for only half of the total reduction in land vegetation biomass. The other half is due to the use of ecosystems for forestry and grazing. In these managed ecosystems, which comprise about 10% of Earth’s land area, the reduction of biomass is about 60 to 70%.
Figure. Human actions have caused enormous loss of terrestrial biomass in the most densely inhabited areas. Source Erb et al (2018), reprinted with permission from the publisher.
The only places that have not been significantly altered by humans are deserts, tundra, and parts of northern boreal forests in North America and Eurasia. Also tropical rainforests in Amazonia, the Congo basin, and the Southeast Asian archipelago have some areas that are still relatively undisturbed, but these areas are experiencing increasing human impact and are shrinking fast due to deforestation and conversion for agricultural lands.
Recent human impacts on nature can be studied with a 'human footprint' indicator, which incorporates the extent of built environments, cropland, pastureland, human population density, night-time lights, railways, roads, and navigable waterways. This indicator has been changing in the past decades, but with marked regional differences (see Figure below). The footprint has increased especially in the most biodiverse areas of the world: eastern Brazil, sub-Saharan Africa, India, and Southeast Asia.
Figure. The 'human footprint' on nature has increased in the period 1993–2009 especially in tropical areas rich in biodiversity. Source Venter et al (2016).
On the other hand, in regions such as Europe and North America, where there has been widespread destruction of pristine nature in the past, human impacts on nature appear to be lessening to some extent. This is due to the decreasing need for land use in these prosperous, largely urbanised areas. At the global level, however, there has been a decline in the state of nature over the period between 1993 and 2009, the period examined in the Venter et al. study. It is also important to understand that in today's world, where the economy is based on global trade, biodiversity loss in biodiversity-rich areas is linked to consumption across the globe. In Brazil, for example, almost 80% of deforestation-causing soybean production is exported abroad, mainly for feed for farm animals (source: Stockholm Environment Institute report).
The human use and domination of Earth are also reflected in the numbers and biomass of animals (see Figure below). One hundred thousand years ago, when modern humans only lived in Africa, the biomass of wild mammals on Earth was approximately 40 megatonnes (in carbon). Today, the biomass of wild mammals (elephants, whales, monkeys, deer, etc) is less than one-fifth of the pre-human values, and the biomass of humans and livestock (cattle, sheep, pigs, etc) outweighs the biomass of wild mammals by a factor of 23. In total, the biomass of mammals (humans and livestock included) is now four times larger than before human impact. This increase in mammal biomass has been achieved by transforming most of the productive land on Earth to feed the increasing human population (and its livestock).
Figure. The biomass of mammals 100.000 years BCE and at present. Data from Bar-On et al (2018).
Extinctions and extinction risks
Of the many impacts humans have on nature, the extinction of species is of particular importance, as it is irreversible. While ecosystems, like forests, can recover from human disturbance given enough time, extinct species are lost forever. Later on, we will learn about the extinctions humans caused thousands of years ago as they moved to new continents and landmasses, but now we look at the more recent patterns of species extinction.
Over the past 500 years – since the year 1500 – 186 species of birds and 115 species of mammals have gone extinct, and the extinction rate is increasing. Also other vertebrates – fishes, amphibians, and reptiles – show accelerating extinction rates that are at least tens to hundreds of times higher than the natural extinction rate over the past 10 million years.
EXTINCTION OF THE PASSENGER PIGEON
An example of extinction in recent past is the case of the passenger pigeon (Ectopistes migratorius), which was once the most numerous species of bird in North America, and possibly the world. In the early 19th century the population numbered several billion individuals, and approximately every third bird in North America was a passenger pigeon. Due to its high abundance and its habit of living in large colonies, it was very easy to hunt. Although the indigenous peoples of North America had also hunted the species to eat, it was not before the European settlers started intensive, commercial hunting in the 19th century that the species started to decline. Birds were shot, netted, and even captured by felling trees with nests and nestling. Trapped birds were also used as live targets in shooting tournaments.
In the 1870s the species was noticed to be declining, but hunting continued. By the 1890s the species had practically disappeared. The last certain observation of the bird in the wild was an individual shot in 1902, and the last survivor of the species died in captivity in 1914. In just a few decades the most numerous bird in the continent, and possibly in the world, was driven to extinction by large-scale commercial hunting.
Due to its abundance, the passenger pigeon had an important role to play in the local forests. The birds distributed seeds of plants and trees and modified the habitats at nesting sites by damaging trees and fertilizing the ground with excrement. However, the rapid changes in land use and the spread of introduced fungal diseases that killed local tree species largely masked the impact of the extinction of passenger pigeon on local ecosystems.
The International Union for Conservation of Nature (IUCN) uses rigorous scientific criteria to assess the conservation status of species, and the results are published in the IUCN Red List of Threatened Species (https://www.iucnredlist.org/). By 2023, the status of more than 150.000 species (mostly vertebrates and plants) has been assessed, and 28% of these species are classified as threatened, meaning that they are facing (at minimum) a high risk of extinction in the wild. Altogether, up to a million species may be at risk of extinction. Most of these species would be tropical insects that have not yet been described.
In the interactive graph below, you can study the global extinction threat of best-known animal groups. Numbers in the graph refer to the number of species in each category. The 'data deficient' category refers to species that, despite having been researched, are not known in sufficient detail for their extinction risk to be determined.
Figure. Numbers of species that have gone extinct since year 1500 and numbers of still extant species in different threatedness categories as classified by the IUCN in 2021.
Direct drivers of Nature's decline
Before looking at the importance of nature on human well-being, it is good to have a grasp of the most important drivers of nature's decline. In 2019, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services published a global assessment on biodiversity and ecosystem services and their relations to human well-being. The assessment found that there are five key reasons for the human-caused decline in nature: land and sea use change, direct exploitation (e.g. fishing and hunting), pollution in its various forms, climate change, and spread of invasive species. All these contribute to extinctions, population declines, and overall ecosystem deterioration.
Land and sea use (change/intensification)
The most important driver of nature decline in terrestrial and freshwater ecosystems is human land use. In 2017, 51% of the ice-free land area was under intensive and 31% under less intensive human use. Only 19% of the land was sufficiently free of human disturbance to be considered ‘wilderness’. It is thus no surprise that many species have had their habitats disappear, fragment, and deteriorate, decimating population sizes of animals and plants and increasing the extinction threat of species.
Currently, the most rapid destruction of intact ecosystems is taking place in tropical forests that are cleared to make space for cattle ranching in Latin America and for palm oil plantations in South-East Asia. In the previous section, we already learned that most natural forests of e.g. Europe have been destroyed already a long time ago.
Mining for the extraction of underground resources has increased continuously, and while still using less than 1% of Earth’s land area, mining has large negative impacts on biodiversity. Besides deforestation and modification of landscapes, mining creates vast quantities of waste and uses large quantities of fresh water. Mining also pollutes waters, land, and air with chemicals and heavy metals like mercury, harming both nature and human health.
Wetlands are some of the most affected land ecosystems. Inland wetlands, which include floodplains, marshes, bogs, swamps, and peatlands, have been drained for conversion to agricultural land and for the development of infrastructure. The use of water for irrigation and the construction of dams for hydroelectricity has reduced the availability of water, changing the nature of many wetlands. Currently, the rate of loss and degradation is highest in coastal wetlands: estuaries, deltas, and mangrove forests. Coastal wetlands are increasingly cleared for the development of ports and cities, and for aquaculture, such as shrimp farming.
Wetlands are critical habitats for many species and sequester large amounts of carbon dioxide. Wetlands also slow and regulate the flow of waters. Preventing further degradation of wetlands, and restoration of already degraded wetlands, is important for protecting biodiversity, mitigating climate change, and buffering coastal and other vulnerable settlements against floods and storm surges.
The biodiversity of inland waters – rivers, lakes, streams, and ponds – has also deteriorated greatly. More than half of all large river systems in the world are affected by dams. There are approximately 2.8 million dams in the world, built for hydropower and storing and diverting water for agriculture, industry, and human consumption. Dams block the movement of fish and other water-living species and reduce the deposition of sediment in river deltas and estuaries, causing coastal erosion.
The geographic pattern of human impacts on rivers mirrors the pattern of terrestrial impacts. In densely populated and industrialised areas, a great majority of rivers have been dammed, with strong negative effects on ecosystems and species. In Europe, for example, 40% of freshwater fishes and 59% of freshwater mollusks are threatened with extinction. The last remaining free-running rivers – for the time being – are restricted to the northern parts of North America and Eurasia, the Amazon and Orinoco basins in South America, the Congo Basin in Africa, and to only a few areas in Southeast Asia.
Direct exploitation (especially fishing)
In marine ecosystems, fishing has been the most important driver of ecosystem decline. In 2017, more than one-third of marine fish stocks were overfished, meaning that they are exploited at a higher rate than they can regenerate, leading to smaller populations and reduced future production. All the rest of the stocks are also very heavily exploited.
Figure. The UN Food and Agriculture Organization assessment of the status of marine fisheries 1974–2017 (FAO 2020). 'Overfished' stocks are exploited at such a high rate that the stock renewal is compromised. 'Maximally sustainably fished' stocks will be overfished if the rate of exploitation increases any further. 'Underfished' stocks are such that they could be exploited more without the catches falling. *Underfished' stocks can however be much smaller than unfished stocks would be.
Exploiting fisheries at a ‘maximally biologically sustainable’ rate can cause substantial negative impacts to other species and to ecosystem structure. In fisheries parlance, ‘maximally biologically sustainable’ refers to stocks that are exploited at a rate that continuously provides maximal fish catches (maximum sustainable yield, MSY). These stocks are much smaller than the unexploited stocks would be because reduced competition increases the growth rate of fish biomass. Furthermore, the concept of ‘biologically sustainable’ fish stock does not take account of the effects of by-catch, fish that are not the main target but are fished alongside the target species. A study looking at the effects of fisheries exploitation found that in a multispecies fishery fished at a ‘maximally biologically sustainable’ level, up to 50% of the stocks could actually have collapsed (Worm et al 2009). Collapsed species have less than 10% of biomass left, their reproduction is compromised, and they cease to play a substantial ecological role. To protect species, ecosystem structure, and ecological functions, much lower exploitation rates than those resulting in so-called biologically sustainable catches would be necessary.
Due to overfishing, catches of wild fish have remained practically static for the past few decades despite the increased fishing effort. Aquaculture production has instead boomed and produces today more seafood than wild fisheries. In the graph below, you can study changes in fisheries and aquaculture production also at the level of individual countries.
Figure. Changes in seafood production from capture fisheries and from aquaculture.
Long-lived and slowly reproducing species are particularly seriously threatened by the extensive and intensive exploitation of marine fisheries. Dramatic examples are oceanic sharks and rays. In the early 2000s, catches of sharks and rays peaked at 63–273 million individuals per year, before declining owing to overfishing. In 2018, 24 out of 31 (77%) oceanic species of sharks and rays were classified as threatened with extinction due to steep reductions in population sizes.
Pollution
Pollution refers to harmful emissions to environments and occurs in many different forms: plastics in oceans, nutrients, and pesticides in waters, greenhouse gases in the atmosphere. Also harmful emissions of different types of energy to the environment count as pollution: noise in urban areas increases human stress levels and underwater noise from shipping has the potential to impact marine ecosystems on a global scale. Also light can be pollution: night lighting disorients migrating birds and causes innumerable deadly collisions with buildings (Loss et al 2014).
Air pollution is a major health issue especially in poorer countries, where energy for cooking and heating comes from the burning of wood and other biomass, and in large cities where transportation, energy and industry emit large quantities of harmful particles. Air pollution causes respiratory and other diseases, and results in millions of deaths every year.
Pollution of waters has increased over the last decades. Major sources of water pollution include untreated urban sewage, industrial and agricultural runoff, oil spills, and plastic waste.
It is estimated that over 80% of urban and industrial wastewater in the world is released to freshwater systems without adequate treatment. However, there is a large regional variance in wastewater treatment. In Europe, 70% of wastewaters are treated, but in Latin America only 20%. Untreated urban wastewaters contain fecal coliforms, harmful organic chemicals from industrial processes, heavy metals, and pharmaceutical residues, which impair organisms in rivers and in estuarine and marine waters. About a quarter of the rivers in Latin America, 10–25% in Africa, and up to 50% in Asia have severe pathogen pollution, largely caused by untreated wastewater.
Agriculture causes soil erosion and nutrient runoff to freshwaters. The flux of nitrogen from fertilizer use has increased manifold, stimulating excessive plant growth in waters and, in extreme conditions, hypoxia or oxygen-depleted 'dead zones' and harmful algal blooms. Also, toxic chemicals used to control insect pests and weeds in intensive farming cause serious harm to freshwater ecosystems by killing invertebrates and algae.
In marine systems, the impacts of oil spills and dumping of toxic compounds have been decreasing, thanks to improved technologies and environmental policies, but the amount of plastic pollution is skyrocketing. Plastic fragments are a particular concern, as they are difficult to remove from the environment and can be ingested, affecting at least hundreds of species including marine turtles, seabirds, and marine mammals. It has been estimated that for each kilogram of fish there is 200 grams of plastic in the ocean.
Invasive species
Global traffic and trade are moving not only people and goods between continents but also other species. Some species introductions have been intentional, as new species of plants or animals are introduced to new areas for agricultural or ornamental purposes, but many introductions have been accidental. The rate of species introductions has been increasing over the past decades and does not seem to be slowing down.
Some of the introduced species become invasive, meaning that they cause harmful effects to local biodiversity, food security, and human health and well-being. Many crop pests and pathogens – especially fungal pathogens – have become widespread, tracking the regional expansion of their host crops. The invasive species are able to thrive in human dominated-ecosystems and reproduce very efficiently. Among the most widespread invaders are the black rat (Rattus rattus), water hyacinth (Eichhornia crassipes), Eastern mosquitofish (Gambusia holbrooki), purple nutsedge (Cyperus rotundus), and cottony cushion scale insects (Icerya purchasi).
By introducing certain species to very wide geographic areas and by causing extinctions of other species that occur only in certain locations, humans are making the ecological communities more similar across the globe, a phenomenon termed the ‘anthropogenic blender’. Introduced species can also contribute to the extinction of native species, and this effect is often particularly severe on islands because island species may have evolved in the absence of strong competition, predation, or pathogens.
Climate change
The release of greenhouse gases has warmed the global mean temperature by approximately 1.2°C above pre-industrial levels and is rising by 0.2°C per decade. The temperature increase is not the same everywhere. Northern latitudes have experienced greater than average warming, for example. Climate change also changes the timing of seasons, increases droughts in some places and rainfall in others, and leads to more frequent and severe heat waves and storms. The release of carbon dioxide also results in ocean acidification, as excess carbon dioxide dissolves in the waters. Ocean acidification causes very serious problems for marine life, especially for corals, and can result in fundamental changes to marine ecosystems with yet unknown consequences.
Climate change has contributed to widespread impacts in many aspects of biodiversity, including species distribution, phenology, population dynamics, community structure, and ecosystem function. The effects are accelerating in marine, terrestrial and freshwater ecosystems and are already impacting agriculture, aquaculture, fisheries, and human well-being. Many species are unable to cope locally with the rapid pace of climate change, and that their continued existence will also depend on the extent to which they are able to disperse, track suitable climatic conditions, and preserve their capacity to evolve.
The impacts of climate change are projected to become more pronounced in the next decades. Even for global warming of 1.5°C to 2°C, the majority of terrestrial species ranges are projected to shrink dramatically. Changes in ranges can adversely affect the capacity of terrestrial protected areas to conserve species, greatly increase local species turnover and substantially increase the risk of global extinctions.
A question to consider: how does knowledge of the state of the environment affect who can be held responsible for the loss of nature? You can share your reflections and discuss with other course participants on the page linked below.
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