BIODIVERSITY CRISIS

"Nous ne sommes pas face à un problème scientifique, 

nous sommes face à un problème de vision du monde..." 

(Aurélien Barreau) 

BIODIVERSITY

Biodiversity reflects the number, variety and variability of living organisms on Earth from all sources, including terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part (ICPB 1992; Millennium Ecosystem Assessment).

It includes:

• diversity within species (genetic diversity), 

• diversity between species (species diversity), 

• diversity between ecosystems (ecosystem diversity).

Biodiversity remains difficult to quantify precisely (Millennium Ecosystem Assessment 2005). 

"Ideally, to assess the conditions and trends of biodiversity either globally or sub-globally, it is necessary to measure:

•  the abundance of all organisms over space and time, using taxonomy (such as the number of species), 

• functional traits,

• the interactions among species that affect their dynamics and function (predation, parasitism, competition, and facilitation), and how strongly such interactions affect ecosystems. 

Even more important would be to estimate turnover of biodiversity, not just point estimates in space or time. Currently, it is not possible to do this with much accuracy because the data are lacking." 

"Because the multidimensionality of biodiversity poses formidable challenges to its measurement, a variety of surrogate or proxy measures are often used." 

"Species richness represents a single but important metric that is valuable as the common currency of the diversity of life, but it must be integrated with other metrics to fully capture biodiversity." "Species- or other taxon-based measures of biodiversity, indeed, rarely capture key attributes such as variability, function, quantity, and distribution—all of which provide insight into the roles of biodiversity."

Estimates of the total number of species on Earth range from 5 million to 30 million (Millenium Ecosystem Assessment 2005), 8.7 million species according to Mora et al. (2011). 

Irrespective of actual global species richness, however, it is clear that the 1.7–2.0 million species that have been formally identified represent only a small portion of total species richness. About 10,000 new species are found every year (mostly insects and other invertebrates). New vertebrate species are still being discovered (about 1‐5 birds and 1‐5 mammals per year).

Biodiversity is not evenly distributed on Earth. 

The latitudinal gradient describes the general pattern observed of increase in species diversity from the poles to the tropics. 

"With notable exceptions, the pattern generally holds true, regardless of the biota’s taxonomic affiliation (e.g., mammals, fishes, insects, and plants), geographic context (e.g., all continents and oceans), or time domain (e.g., recent and 70 Mya)."  (Willig et al. 2003)

Many hypotheses have been advanced to explain this latitudinal pattern in species diversity (Pianka 1966; Willig et al. 2003), including climatic stability, competition, predation, productivity, spatial heterogeneity, and time hypotheses. 

"But latitudinal gradients of diversity are ultimately dependent on the historical, geographic, biotic, abiotic, and stochastic forces (Schemske 2002) affecting the geometry, internal structure, and location of species ranges in ecological or evolutionary time. Indeed, latitude is a surrogate for a number of primary environmental gradients (e.g., temperature, insolation, seasonality) that interact and are correlated to each other, making direct tests of hypotheses difficult and controvertible." (Willig et al. 2003)

According to the elevation gradient, species richness also tends to increase with elevation until a certain threshold and then decreases. 

The theory of island biogeography makes a couple of straightforward predictions based on an island's size and how isolated it is.

"Islands which are easy to reach will be colonized by many species, while those that are more difficult to get to will find themselves home to fewer guests. Isolated islands, on the other hand, will only be colonized by a few species, and, as a result, all of the species on those islands will be descendants of the handful of original settlers. 

The more isolated an island is, the lower its species richness will be. 

An island's size also affects its biodiversity, since larger islands will have a wider variety of habitats, so species which arrive on the island will diversify to fill up the available niches. 

All in all, the theory predicts that an island's size sets a maximum for how many species it can host, while its isolation and local speciation on the island will decide how many species it actually has. 

Since the theory is framed in terms of isolation and area, it's also a useful tool for addressing questions in conservation biology, where a species' habitat may be reduced to distant patches, which are, effectively, islands." 

(see https://www.nature.com/scitable/blog/accumulating-glitches/island_biogeography_in_the_era/)

Species richness is commonly thought to increase with habitat diversity or environmental heterogeneity (Hortal et al. 2009). 

However, a recent theoretical model aiming to unify niche and island biogeography theories predicted a hump-shaped relationship between species richness and habitat diversity. 

BIODIVERSITY CRISIS

Species extinction is common and is normally balanced by speciation. Of the four billion species estimated to have evolved on the Earth over the last 3.5 billion years, some 99% are gone (Novacek 2001). 

But 5 times in life’s history of the last ∼540 million years (called the ‘Big Five’ mass extinctions), extinction rates appeared particularly elevated, spiking higher than in any other geological interval and exhibiting a loss of over 75% of estimated species: near the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods (Raup & Sepkoski 1982). Different causes are thought to have precipitated the extinctions: basaltic super-eruptions, impacts of asteroids, global climate changes, continent drifts, etc.

Biologists now suggest that a sixth mass extinction may be under way, given the known rate and magnitude of species losses over the past few centuries and millennia, and that human activities are responsible of it (Barnosky et al. 2011; Ceballos et al. 2015; Payne et al. 2016; Briggs 2017). Between 17,000 and 100,000 species disappear each year (Leakey & Lewin 1995), a rate which is 100 to 1000 times higher than what is admitted for the periods preceding the advent of Humans. There is no general agreement on when this sixth mass extinction has started. But increases in global rates of extinction have been elevated above background rates since at least 1500, and appear to have accelerated in the 19th century and further since. 

Humans have been impacting Earth for thousands of years. According to https://en.wikipedia.org/wiki/Holocene_extinction, "Some have suggested that anthropogenic extinctions may have begun as early as when the first modern humans spread out of Africa between 200,000 and 100,000 years ago; this is supported by rapid megafaunal extinction following recent human colonisation in Australia, New Zealand and Madagascar, as might be expected when any large, adaptable predator (invasive species) moves into a new ecosystem (Kolbert 2014). In many cases, it is suggested that even minimal hunting pressure was enough to wipe out large fauna, particularly on geographically isolated islands (Crowley & Brooke 2010; Perry et al. 2014)." No less than 65% of large mammals have disappeared between -50'000 years and -12'500 years, in a context of significant climatic variations (glaciations, deglaciations). Hunting and deforestation by burning could be the most likely causes. 

But a more recent start dates is usually proposed for the beginning of the sixth mass extinction: the beginning of the Agricultural and Neolithic Revolution 12,000–15,000 years ago, which characterized the start of the Holocene. Holocene is the current geological epoch, which forms with the preceding Pleistocene the Quaternary period. It began approximately 11,650 cal years before present, after the last glacial period, which concluded with the Holocene glacial retreat. The Holocene has been identified with the current warm period, known as MIS 1. About 11'600 years ago, the Neolithic revolution allowed agriculture to spread, with as corollaries deforestation (and thus CO2 emissions), as well as the first rice crops (about 5000 years ago), both CO2 sinks and methane emitting sources. The Holocene corresponds with the rapid proliferation, growth and impacts of the human species worldwide, including all of its written history, technological revolutions, development of major civilizations, and overall significant transition towards urban living in the present. 

Anthropocene (which means the “recent age of man") is a proposed geological epoch dating from the commencement of significant human impact on Earth's surface, atmosphere, oceans (Zalasiewicz et al. 2019). In other words, “Anthropocene” means that Homo sapiens has become a dominant evolutionary force (Palumbi 2001). It makes up the third worldwide division of the Quaternary Period (2.6 million years ago to the present) after Holocene. 

"It has been suggested that human activity has made the period starting from the mid-20th century different enough from the rest of the Holocene to consider it a new geological epoch, known as the Anthropocene, a term which was considered for inclusion in the timeline of Earth's history by the International Commission on Stratigraphy in 2016. In order to constitute the Holocene as an extinction event, scientists must determine exactly when anthropogenic greenhouse gas emissions began to measurably alter natural atmospheric levels on a global scale, and when these alterations caused changes to global climate. Using chemical proxies from Antarctic ice cores, researchers have estimated the fluctuations of carbon dioxide (CO2) and methane (CH4) gases in the Earth's atmosphere during the late Pleistocene and Holocene epochs. Estimates of the fluctuations of these two gases in the atmosphere, using chemical proxies from Antarctic ice cores, generally indicate that the peak of the Anthropocene occurred within the previous two centuries." (see https://en.wikipedia.org/wiki/Holocene_extinction) 

Various start dates for the Anthropocene have been proposed, ranging from the beginning of the Agricultural Revolution 12,000–15,000 years ago, to as recent as the 1960s (e.g., the beginning of the Agricultural and Neolithic Revolution 12,000–15,000 years ago; the Bronze Age (-5,000 years) and the Iron Age (-3,000 years), which allowed civilizations to develop through the exploitation of minerals while causing considerable soil erosion and sedimentation in rivers ; the start of the Industrial Revolution c. 1780, with the invention of the steam engine when the highest greenhouse gas levels were recorded). "The ratification process is still ongoing, and thus a date remains to be decided definitively, but the peak in radionuclides fallout consequential to atomic bomb testing during the 1950s has been more favoured than others" (see https://en.wikipedia.org/wiki/Anthropocene).

Major drivers of recent biodiversity loss include habitat alteration, destruction and fragmentation, spread of exotic species, overexploitation, pollution and climate change. Virtually all of Earth’s ecosystems have now been dramatically transformed through human actions. 

Main threats on biodiversity are overexploitation and habitat changes. It is estimated that 60% of the world’s ecosystems are degraded or used unsustainably; 75% of fish stocks are over-exploited or significantly depleted and 13 million hectares of tropical forests are cleared each year (MA, 2005; UN FAO, 2011). According to the Millennium Ecosystem Assessment, "Over half of the 14 biomes that the Millennium Ecosystem Assessment assessed have experienced a 20–50% conversion to human use, with temperate and Mediterranean forests and temperate grasslands being the most affected (approximately three quarters of these biome’s native habitat has been replaced by cultivated lands). In the last 50 years, rates of conversion have been highest in tropical and sub-tropical dry forests."

Between 10% and 50% of well-studied higher taxonomic groups (mammals, birds, amphibians, conifers, and cycads) are currently threatened with extinction, based on IUCN–World Conservation Union criteria for threats of extinction. Some 12% of bird species, 23% of mammals, and 25% of conifers are currently threatened with extinction (https://www.greenfacts.org/en/biodiversity/l-3/8-millennium-ecosystem-assessment.htm). In addition, 32% of amphibians are threatened with extinction. Higher levels of threat (52%) have been found in the cycads, a group of evergreen palm-like plants.

CONSEQUENCES IN TERMS OF ECOSYSTEM FUNCTIONING AND SERVICES

Biodiversity plays a vital role in ecosystem functioning. Hence, when biodiversity disappears, this affects and fragilizes ecosystem processes and functions.    

As explained in Hooper et al. (2005): 

"Ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors. (...)

Species' functional characteristics strongly influence ecosystem properties. Functional characteristics operate in a variety of contexts, including effects of dominant species, keystone species, ecological engineers, and interactions among species (e.g., competition, facilitation, mutualism, disease, and predation). Relative abundance alone is not always a good predictor of the ecosystem‐level importance of a species, as even relatively rare species (e.g., a keystone predator) can strongly influence pathways of energy and material flows. (...) 

Alteration of biota in ecosystems via species invasions and extinctions caused by human activities has altered ecosystem functioning. Many of these changes are difficult, expensive, or impossible to reverse or fix with technological solutions. The effects of species loss or changes in composition, and the mechanisms by which the effects manifest themselves, can differ among ecosystem properties, ecosystem types, and pathways of potential community change. (...)

Some ecosystem properties are initially insensitive to species loss because :

(a) ecosystems may have multiple species that carry out similar functional roles (redundancy), 

(b) some species may contribute relatively little to ecosystem properties, or 

(c) properties may be primarily controlled by abiotic environmental conditions."

As also mentioned in the Millennium Ecosystem Assessment (2005):

"Ecosystem functioning is strongly influenced by the ecological characteristics of the most abundant species, not by the number of species. The relative importance of a species to ecosystem functioning is determined by its traits and its relative abundance. (...) Thus conserving or restoring the composition of biological communities, rather than simply maximizing species numbers, is critical to maintaining ecosystem services. (...)

Changes in biotic interactions among species—predation, parasitism, competition, and facilitation—can lead to disproportionately large, irreversible, and often negative alterations of ecosystem processes. In addition to these direct interactions, the maintenance of ecosystem processes depends on indirect interactions as well, such as a predator preying on a dominant competitor such that the dominant is suppressed, which permits subordinate species to coexist. 

Interactions with important consequences for ecosystem functioning include:

• pollination; 

• links between plants and soil communities, including mycorrhizal fungi and nitrogen-fixing microorganisms; 

• links between plants and herbivores and seed dispersers; 

• interactions involving organisms that modify habitat conditions (beavers that build ponds, for instance, or tussock grasses that increase fire frequency); 

• indirect interactions involving more than two species (such as top predators, parasites, or pathogens that control herbivores and thus avoid overgrazing of plants or algal communities)."

Ecosystem services are defined as the benefits obtained by people from ecosystems that improve and sustain human wellbeing (Daily 1997).

Four categories of ecosystem services are distinguished (Millennium Ecosystem Assessment 2005): 

• provisioning services, which are products obtained from ecosystems, such as food, clean water, timber, fiber, and genetic resources;

• regulating services, which are benefits obtained from the regulation of ecosystems, including services such as as purification of water, flood control, or regulation of the climate via carbon sequestration or pollination;

• cultural services, which are benefits obtained from ecosystems through spiritual enrichment, cognitive development, reflection, recreation, and aesthetic experience;

• supporting services, which are services that are needed for the production of all other services, such as soil formation, and nutrient cycling.

Human well-being is the result of many factors, many directly or indirectly linked to biodiversity and ecosystem services while others are independent of these, and they consists of five main components (Millennium Ecosystem Assessment report 2005): 

• the basic material needs for a good life, 

• health, 

• good social relations, 

• security, 

• freedom of choice and action. 

Change in biodiversity, and in particular in individual species and the way they interact with other organisms and their habitats, alters the structure and function of ecosystems and in turns the goods and services that natural systems provide to society (ecosystem services) and human-well-being, with potential feedbacks on ecosystems and biodiversity (Weiskopf et al. 2020). 

"Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity" (Hooper et al. 2005).

"Changes in drivers that indirectly affect biodiversity, such as population, technology, and lifestyle, can lead to changes in drivers directly affecting biodiversity, such as the catch of fish or the application of fertilizers. These result in changes to ecosystems and the services they provide, thereby affecting human well-being" (Millennium Ecosystem Assessment). 

As highlighted by Everard (2017), "Biodiversity decline represents not only an irreversible loss to the planet but also threatens humanity’s life support system: the services that nature provides represent everything from the food we eat to the air we breathe (Díaz et al., 2006; Cardinale et al., 2012; Hooper et al., 2012)." 

Conserving biodiversity for the sustainable management of natural resources, the maintenance of ecosystem services or simply for its own sake has thus become one of the today's biggest human challenges.