In the last few centuries, human activity has led to the extinction of a considerable number of animal species. This ongoing event has recently been called “Anthropocene defaunation”, which includes several human activities such as the destruction of habitats, the exploitation of natural resources and their impact on climate. In this article, the current status of global animal biodiversity will be explored, with the aim of understanding how humans have threatened, and are still threatening, the Earth’s fauna.
“Anthropocene defaunation” is a term commonly used to describe the ongoing loss of animal species that is caused by human activities. These human activities include: the destruction of natural habitats, the exploitation of natural resources, the introduction of invasive species in new environmental areas and unsettling climate change trends across the globe.
Historically, Earth encountered five events – the so-called “big five” – mass extinction phenomena, namely the Ordovician event (∼443 million years ago), the Devonian event (∼359 million years ago), the Permian event (∼251 million years ago), the Triassic event (∼200 million years ago) and the Cretaceus event (∼65 million years ago).* Following each of these events, at least 75% of species present at the time had disappeared (1).
This observation clearly indicates that biodiversity has emerged multiple times and in countless directions since the appearance of life on Earth.
Extinction can be described by two independent parameters, namely rate and magnitude. While rate represents the number of lost species in a certain period of time, magnitude is the percentage of extinct species from the initial number of species. Barnosky and colleges aimed at answering a fundamental question: are current rates going to cause a mass extinction phenomenon whose magnitude is comparable to one of the “big-five” events? When calculated over the last five centuries, current extinction rates of birds, mammals and amphibians are faster than the ones that led to mass extinction during the “big-five” events. If this continues, planet Earth could meet the sixth mass extinction in just a few centuries. Considering that the evolution of new species happens – at the very least – in the order of hundreds of thousands of years, humanity will experience a long period of inadequate biodiversity, beyond which it might not survive (1).
Human impact on biodiversity
Human threats to ecosystems are difficult to quantify, given that civilization can either negatively or positively affects the environment (2). Nevertheless, a few major events can be identified: the devastation and fragmentation of habitats; the use of pesticide and pollution; the introduction of alien species and climate change.
Habitat loss due to human activities can cause a reduction in the availability of nutrients and other vital resources, while fragmentation of natural environments can render them unreachable. However, as already mentioned, human behaviour can also be beneficial for the habitat: urbanization and agriculture can mobilize resources or generate novel liveable habitats for certain species (2).
Intensive farming involves the use of agrochemicals, which are potentially harmful to the agricultural habitat. While insecticides can directly poison the pollinators, herbicides and fertilizers can reduce the abundance and biodiversity of flowers (2). As we will further discuss in this article, pollinators are essential to maintain agricultural productivity at reasonable rates for human livelihood worldwide.
Alien species are non-native species intentionally or accidentally introduced to a certain habitat by humans. These populations can become invasive: their growth rate increases exponentially with the risk of damaging the environment they populate in a permanent fashion. They act by either physically damaging other species, or by outcompeting them for access to vital resources (3). One emblematic example is represented by Eichornia crassipes, commonly known as “common water hyacinth”. Native to the Amazon basin, it was introduced to several regions across the globe as an ornamental plant. Unfortunately, it can completely cover lakes and ponds, as happened on the shores of lake Victoria’s Winam Gulf in Kenya’s Kisumu county. The proliferation of the plant is so pervasive, that fishermen are having trouble navigating the waters and accessing the dry land. This plague is not only clogging the fishing routes vital for local trade, but it is also killing fishing communities. Furthermore, the abundance of water hyacinth slows down water current, creating an ideal habitat for disease-carrying mosquitoes (4).
Climate change is a complex plethora of phenomena which we will not discuss in detail here. The effects of global climate change on biodiversity are still controversial, although few critical projections have been proposed. For instance, a major anthropogenic contribution to global warming comes from the sustained emission of greenhouse gases – including carbon dioxide, methane and nitrous oxide – in response to fossil fuel burning.
Simply put, an increased air concentration of greenhouse gases is leading to two alarming events:
- The so-called “greenhouse effect” used to be a natural phenomenon that allows life on Earth. However, lately it has become a massive and alarming process. In principle, sunlight is adsorbed and then reflected by terrestrial objects. The reflected light tries to move out from the atmosphere, where it encounters several gases. If their abundance is too high, most of the solar energy gets trapped and remains in proximity to the Earth’s surface, heating the planet. As a consequence, pessimistic scenarios predict radical changes in the structure of habitats, like the desiccation of large lake volumes or the replacement of rainforests by savannahs in the near future (5).
- An excessive concentration of carbon dioxide in the atmosphere leads to increased carbon dioxide dissolution on the surface of oceans. Carbon dioxide reacts with water molecules, generating carbonic acid, which in turns sequesters carbonate ions, bricks that are essential for organisms in order to build shells and skeletons (5). Thus, a significant increase in this carbon cycle can lead to a loss of species. In addition, the elevated concentration of greenhouse gases in the atmosphere that causes temperature increases in turn leads to evaporation of water from oceans, lakes, rivers and other terrestrial bodies of water. When water vapour concentrations increase, they cause a further increase in temperature, triggering a dangerous and likely never-ending feedback loop.
Some numbers to show the ongoing process of species extinction
Let us explore some statistics in order to realize the seriousness of the situation.
Our planet hosts an estimated number of species ranging between 5 and 9 million, of which we lose between 11 and 55 thousand every year (6).
Vertebrates, among all other species, are the most threatened. To note: vertebrates include mammals, birds, reptiles and amphibians. Therefore, they comprise animals that we empathize with pretty well: elephants, rhinos, tigers and lions (mammals), pigeons and parrots (birds), snakes and lizards (reptiles), frogs and salamanders (amphibians), just to mention a few. Since the sixteenth century, more than three hundred vertebrate species faced extinction, and lots of them are currently in danger and on the verge of being extinct (2).
While human communities are exponentially increasing, communities of other mammals are quickly disappearing.
As of today, the global human population consists of over 7 and half billion people, and it is dramatically growing: every day nearly one-hundred and seventy thousand people are added to the world population, once deaths are subtracted.
During the Winter of 2018, the WWF (World Wild Fund for Nature) declared that only 415000 elephants were populating Africa, while in 1930 the continent could still count 10 million of them.
How many tigers are left, considering the entire “family” of tiger species? Fewer than 4000 in their natural environment. However, in the United States alone over 5000 individuals are kept in captivity. Simply put, there are more tigers kept in captivity by humans than in the wild.
Similar fates might be reserved also for innumerable invertebrate species (6).
Historically, there is strong evidence correlating megafaunal extinction to the spread of Homo sapiens in different regions of the planet. The core explanation is that humans are predators capable of hunting a wide range of animals, including large-bodied species. Often times, megafaunal species display long generation times and low reproductive rates, meaning that individuals do not reproduce so often during their lifespan, and when they do, it takes many years until their offspring become sexually mature. It is therefore likely that the arrival of humans in different regions of the planet applied a predation pressure on megafauna, likely triggering the extinction of key species of large herbivores. This event alone can alter vegetation distribution and composition, fire regimes and the abundance of prey for megacarnivores, causing a subsequent cascade of extinction phenomena (7).
This cascade effect is indeed the most tremendous issue related to species loss. As a general rule, every species on Earth does not simply proliferate and populate a certain habitat as an independent unit without affecting other species or being affected by them. Simply put, species are a part of ecosystems, where their survivability is dependent on the co-existence of many other species (Read Culturico’s “Are humans still under the pressure of natural laws? ” to get more insights on the matter). A key example is represented by the loss of top carnivores, predators that are found at the apex of the food chain and are not themselves predated. When their abundance in the habitat is severely reduced, species belonging to inferior levels of the food chain can disproportionally proliferate. Often times, these species are omnivores whose dietary habits can in turn affect the distribution and abundance of both animal and plant species (7). Intriguingly, a fourteen-year study showed that the re-introduction of gray wolves to Yellowstone National Park was sufficient to limit the size of elk populations, which in turn led to an increased area covered by the plant species they eat. This phenomenon is called “trophic cascade”, and it clearly illustrates how a successful interaction between carnivores, herbivores and plants is essential for the prosperity and well-being of entire ecosystems (8).
How loss of biodiversity impacts on humans
The multifaceted benefits that humans obtain from the proper functioning of a natural environment are generally termed “ecosystem services”. The quality and efficiency of ecosystem services rely on several environmental parameters, which are at least partially dependent on human activity.
Humans can be responsible for multiple environmental alterations, such as a change in the use and cover of lands, lack of resources – including loss of wildlife species – and climate change. These events contribute to deteriorating ecosystem services, which affect proper nutrition, disposition of clean water, protection from natural catastrophic events, regulation of atmospheric temperature, protection against infectious diseases, natural pollination of crops and nutrient cycling.
An analysis on the status of all ecosystem services goes beyond the scope of this article. However, the following examples will hopefully provide some hints on how loss of biodiversity can affect human health and activity.
Many diseases – including the infamous malaria – are transmitted to humans via an intermediate host. In simple terms, an animal gets infected, and only subsequently can the pathogen reach humans. An emerging concept in the field of disease ecology is the so-called “dilution effect”: when the habitat is populated by numerous species that display variable efficiencies in transmitting the pathogen, statistically speaking the pathogen itself infects competent hosts less frequently, reducing the likelihood of human infection (9, 10). Thus, a loss of host biodiversity might account for higher infection of competent hosts, favouring the host-mediated transmission of the pathogen to humans. A study on malaria transmission reports that increasing the proportion of incompetent hosts could prevent malaria outbreaks in Brazilian tropical rainforests (11). Similar associations between biodiversity loss and increased exposure to pathogens have been proposed for other diseases, such as the schistosomiasis, West Nile fever, hantaravirus infections or Lyme disease. If we consider that in 2017 there were 219 million cases of malaria worldwide, and at least 200 million cases of schistosomiasis are reported annually (12), finding strategies to reinforce the “dilution effect” in critical areas could help to fight numerous infectious diseases.
The eradication of communities of pollinators – such as bees – can destabilize the presence of wild plant species and decrease agricultural productivity (13). Pollinators are required to transport pollen – the plant “sperm” – from one plant to another for plant reproduction. If the number of pollinators decreases, the number of plant species will be also affected. Statistically speaking, 75% of crops that are relevant for human nutrition require insect pollinators. In case of definitive eradication of pollinators, the worldwide production of fruits would decrease by 12%, and the production of vegetables by 6% (14). Although these projections appear harmless, when applied to some regions of the planet – such as North Africa, as well as Central, East and South Asia – they are alarming. In 2005, the production of fruits in East Asia was 19% higher than the consumption. Extinction of all pollinator species would drastically invert the proportion, making the region in urgent need of fruit imports. This observation is relevant if we consider that East Asia produces about 20% of the fruits worldwide. This dramatic scenario would also hit Europe, where the consumption of fruits continuously exceeds their production. A drop in global fruit production would dangerously reduce the European potential to import fruits (14).
Let us now come back to our beloved megafauna to explore another example. A historical digression will help us understand how the loss of biodiversity can have a long-term impact on human behaviour, for thousands of years. Formally, urbanization developed around 5000 years ago in fertile river floodplains, located in Mesopotamia, Egypt, the Indus Valley and China. Those regions were characterized by constant fertility, thanks to erosion and precipitation of primary rocks surrounding the regions. Those areas were also inhabited by wild large animals like elephants, which are essential to redistribute nutrients from the floodplains. These animals contribute in two ways: by being able to walk long distances, they either transport dead bodies of organisms they eat, or they simply defecate, fertilising the soil. Although contemporary touristic tours portray elephants living in harmony with humans, history tells us something different. Elephants can damage human agriculture, and humans have intensively hunted them since the reign of Thutmose III in ancient Egypt (15). Scientists assume that entire populations of the local megafauna have gone extinct, significantly impairing the redistribution of nutrients outside the fertile floodplains (15). At first, this event alone forced human communities to decrease agricultural activities in regions located outside the fertile floodplains, and later on to eventually leave them (15). It is therefore likely that local megafaunal extinction limited population growth and its interest to geographically expand and prosper far away from the fertile floodplains.
This case study clearly shows the impact humans have on the ecosystems and how this impact can influence and eventually guide future behaviour of entire civilizations to come.
What can we do?
According to Myers and colleagues (10), there are several limitations to a deep comprehension of the effects that environmental alterations have on human health.
- Literature generally lacks research that focuses on multiple consequences of environmental change. For instance, deforestation might represent a huge loss of plant and animal biodiversity, leading to a global reduction of carbon storage and of wild meat availability. However, at the same time it would give room for agriculture, urbanization and industrialization, which could potentially benefit local and global productivity.
- Literature generally lacks research that investigates the combined interaction of multiple environmental parameters. For example, climate change combined with food shortage might lead to completely different outcomes in certain regions of the planet when compared to others.
- Humans should think more seriously about solutions to adapt to irreversible environmental modifications. In a hypothetic scenario in which the ongoing process of defaunation led to an unrecoverable loss of white, red and fish meat, how would humanity deal with it?
- Environmental changes can have different effects at different scales. Contemporary humanity could decide to consume all the planet’s resources, and live better in the short-term, instead of being long-sighted and taking into account the health of future generations. Nowhere is it stated how humanity should behave, and thinking about short-term versus long-term impacts of environmental modifications could make a dramatic difference.
Overall, it is evident that humanity is in need of a more concerted effort to properly understand how the Earth is changing.
*[More information on the timeline of the events and their causes can be found in (1).]
- Barnosky, A.D., et al., “Has the Earth’s sixth mass extinction already arrived?”, Nature, 2011.
- Potts, S.G., et al., “Global pollinator declines: trends, impacts and drivers”, Trends in Ecology and Evolution, 2010.
- Stout, J.C., & Morales, C.L., “Ecological impact of invasive alien species on bees”, Apidologie, 2009.
- Honlah, E., et al., “Effects of water hyacinth invasion on the health of the communities, and the education of children along River Tano and Abby-Tano Lagoon in Ghana”, Cogent Social Sciences, 2019.
- Bellard, C., et al., “Impacts of climate change on the future of biodiversity”, Ecology Letters, 2012.
- Dirzo, D., et al., “Defaunation in the Anthropocene”, Science, 2014.
- Malhi, Y., et al., “Megafauna and ecosystem function from the Pleistocene to the Anthropocene”, PNAS, 2016.
- Ripple, W.J., & Beschta, R.L., “Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction”, Biological Conservation, 2012.
- Schmidt, A.K., & Ostfeld, R.S., “Biodiversity and the Dilution Effect in Disease Ecology”, Ecology, 2001.
- Myers S.S., et al., “Human health impacts of ecosystem alteration”, PNAS, 2013.
- Laporta, G.Z., et al., “Biodiversity Can Help Prevent Malaria Outbreaks in Tropical Forests”, PLOS Neglected Tropical Diseases, 2013.
- Hajissa, K., et al., “Prevalence of schistosomiasis and associated risk factors among school children in Um-Asher Area, Khartoum, Sudan”, BMC Res Notes, 2018.
- Potts, S.G., et al., “Global pollinator declines: trends, impacts and drivers”, Trends in Ecology and Evolution, 2010.
- Gallai, N., et al., “Economic valuation of the vulnerability of world agriculture confronted with pollinator decline”, Ecological Economics, 2009.
- Doughty, C.E., et al., “The impact of large animal extinctions on nutrient fluxes in early river valley civilizations”, Ecosphere, 2013.