Impacts of agriculture on biodiversity and ecosystem functioning

Since its beginnings in the Neolithic period, the expansion of agriculture has been accompanied by technical innovations that have progressively increased agricultural yields and enabled the growth of the world’s human population over the centuries, seemingly pushing back indefinitely the productivity limits of agrosystems and the biosphere. What is the current status of this dynamic? Can we measure the impacts of agriculture on biodiversity and the functioning of (socio-) ecosystems on a global scale? What lessons can we learn for the future?

1. The Ouroboros paradox

In ancient Egypt, more than 3500 years ago, tombs and papyrus were often decorated with a snake biting its own tail, forming a circular loop (Figure 1).

Designated later by the Greeks with the name of Ouroboros -literally, ‘which devours its tail’-, this mythical animal symbolizes the cycle of time, the eternal renewal, but also the duality of the world, for cultures as diverse as the Egyptian, Phoenician, Greek, Nordic, and even Aztec civilizations..

This Ouroboros also symbolizes the idea of paradox, or impossible action, because it feeds on itself. And in the current French language, the expression “un serpent qui se mord la queue” (a snake biting its own tail) designates a vicious circle, “a succession of problems whose end cannot be seen”. This Ouroboros paradox seems to apply today to human societies confronted with agriculture, both nourishing and destructive.

To the three factors of agricultural development: (i) spatial expansion, (ii) technical progress in agricultural practices and (iii) selection-domestication of more productive animal and plant populations, has been added (iv) the transformation and then globalization of agri-food systems, and of the market chains associated with agriculture.

However, these factors of development and evolution of agriculture are all pressures – or impact factors – on dynamic socio-ecosystems, made up of multiple species (including humans) interacting in their living environment (See focus on What are the major agricultural impact factors?), which interact in a ‘ouroborian’ dynamic/spiral.

2. Impacts on plant biomass and the productivity of terrestrial ecosystems

In order to grow and reproduce, all living things require matter and energy that they draw from their environment. Like animals and fungi, many species feed on living matter (from prey, leaves, …) or organic compounds synthesized by other organisms (e.g. nectar from flowers, for bees), which share its habitat. But the activity of these so-called “heterotrophic” species would be impossible without the presence at the base of the food web of so-called “autotrophic” organisms which, such as plants, algae, cyanobacteria (photosynthetic) and chemobacteria, are able to synthesize organic molecules from mineral compounds and energy, either light or chemical (Figure 2).

The annual primary production of an ecosystem is the biomass of autotrophic organisms produced each year by that ecosystem. Its productivity is equal to the rate of biomass production, per unit area and time. Located at the base of food webs, or ‘food chains’, this primary production conditions the global production and functioning of ecosystems.

With the exception of the Nile Valley and a few other small desert regions, the conversion of “natural” or “semi-natural” terrestrial habitats such as forests and marshes into pastures and cultivated fields results in a net reduction in the biomass and plant productivity of the ecosystems concerned.

On a global scale, Erb et al [1] estimate that human land-use changes have reduced by half the total biomass of terrestrial plants  (about 450 Gigatonnes of Carbon (Gt C) less, from about 900 Gt C) since the Neolithic period. More than two thirds of this biomass loss can be attributed to agriculture, the rest to forest exploitation and land artificialisation.

The annual plant production of terrestrial habitats has been reduced by about 10% globally [2] since the beginning of agriculture, due to land use changes. Moreover, about 20% of this terrestrial plant production is taken up annually by humans through agriculture, with large variations between regions (Figure 3).

3. Habitat carrying capacity, abundance of wildlife communities

While forests and other natural habitats support complex and diverse species networks, agrosystems dedicated to the cultivation of a small number of domesticated species inherently contain fewer wildlife individuals than the surrounding natural habitats. Intensive agriculture moreover reduces the carrying capacity of cultivated plots for wild species in many ways: through the physical removal of trees and hedges, soil erosion, elimination of messicolous plants, insecticide, antibiotic or antifungal treatments, surface water drainage, pollution or possible salinization of the soil, etc. Thus, the conversion of (semi-)natural habitats into intensive agricultural systems considerably reduces the biotic capacity of these habitats for wildlife communities, in terms of numbers of individuals.

Compiling a large number of measurements of bird densities by habitat type, Gaston et al [3] explored the impact of agriculture on terrestrial avifauna abundance. At the local scale, given that a tropical forest (rich in about 2500 birds/km²) supports about five times more adult birds per unit area than a meadow (about 550 birds/km², in the tropics) or an extensively farmed field (about 450 birds/km²), conversion of tropical forests to meadows and cultivated fields reduces the local carrying capacity of these habitats for birds by 4/5^e. In temperate regions, conversion of mixed or deciduous forests (populated by 800-1000 birds/km²) to either meadows (370 birds/km²) or extensive agricultural fields (300 birds/km2) reduces local bird abundance by about two-thirds.

On a global scale, based on 1990 estimates of the area of major terrestrial biome types by Klein-Goldjewijk [4], Gaston et al. [3] estimated the reduction in global ‘terrestrial’ avifauna (excluding seabirds) due to the expansion and intensification of agriculture from the Neolithic period to 1990 to be about 30 billion birds, or 20-25% of pre-agricultural numbers.

Applying these estimates of bird density per habitat to the four land-use change scenarios developed for 2050 at the global scale by the Millennium Ecosystem Assessment (MEA) [5], Teyssèdre and Couvet [6] estimated the loss of global terrestrial avifauna due to agricultural expansion and intensification expected in 2050 to be between 10 and 25% of 1990 numbers, depending on the land-use scenario considered (Figure 5). Unsurprisingly, the worst scenario for biodiversity is the so-called ‘Order from strength‘ scenario of reactive and regionalized ecosystem management, without anticipation or interregional collaboration.

This amounts to a reduction of 27 to 45% in the number of breeding birds, from the beginning of agriculture to 2050, depending on the scenario considered. A rather bleak outlook, considering that current socio-economic development trajectories are similar to the worst-case scenario of the MEA! (see IPCC [7]).

Located near the top of food webs and relatively easy to observe, birds integrate many environmental variations into their population dynamics. Their variations in abundance noteworthily depend on the abundance and distribution of their prey, themselves conditioned by the state of the vegetation cover, and therefore by that of the communities of microorganisms and invertebrates in the soil.

As a result, birds are a good indicator of terrestrial biodiversity, recognized internationally by the Convention for Biological Diversity (CBD) since 2004 [8] (see Figure 7 below). Although in a less systematic, collaborative and standardized way, variations in abundance of other groups of organisms have been monitored at the local scale by many researchers (see below), in many geographical locations, which confirm and reinforce the results obtained on avifauna.

Like all ecosystems, agrosystems are open systems, interacting with their environment and the surrounding landscape. While the direct impacts of ‘extensive’ agriculture on biodiversity are essentially local, i.e. restricted to ecosystems converted for agriculture, those of intensive agriculture go far beyond the boundaries of cultivated fields, via the dispersion of pollutants. Transported by air, surface water or mobile animals (insects, vertebrates…), locally applied chemical compounds reach and affect adjacent or even distant ecosystems. Thus, fertilizers and pesticides spilled on fields and washed off during rainfall, or reaching the water table, are harmful to ecosystems of all types downstream.

Pollution of the coastline by nitrogenous fertilizers used massively in certain regions is well known, which is expressed by recurrent algal blooms (known as ‘green tides’), deleterious for biodiversity, and can transform certain wetlands, estuaries and coastal ecosystems into ‘dead’ zones deprived of aerobic species (Figure 6).

Numerous research and biodiversity monitoring studies point to the impacts of intensive agriculture, and in particular of systemic pesticides -such as neonicotinoids- marketed since the 1990s, on non-target organisms and ecosystems (e.g. [9],[10]). According to a study [11], the abundance of flying insects measured in various protected areas of agricultural regions in Germany has dropped by more than 75% in 27 years (between 1989 and 2016). Monitoring of insects in German grasslands and forests (between 2008 and 2017) [12] shows comparable trends, and indicates that the decline of grassland entomofauna increases with the encroachment of intensive agriculture into the landscape.

On a global scale, compiling data from 166 insect abundance monitors on all continents (1676 sites), van Klink et al. [13] estimated a less severe average decline in terrestrial insect abundance (of about 1% per year), largely attributed to agricultural intensification.

4. Wildlife community composition

In a biotic community of similar species – a local community of ‘herbivorous’ fish, for example, or phytophagous insects, or trees… – not all species have the same ecological aptitudes and requirements. They therefore do not react in the same way to spatial or temporal variations in their habitat (See What responses of Biodiversity to global changes?).

Species occupy ecological niches* of varying sizes. Equipped to exploit certain resources, specialist species are more competitive than generalists in their preferred habitat(s); generalist species, on the other hand, thrive in variable habitats or in a mosaic of different habitats, and are more tolerant of unexpected changes in their living conditions.

Therefore, conversion of habitats to agriculture not only generally reduces the abundance of local biotic communities, but also affects their composition. For example, conversion of forests to fields and pastures replaces the original forest communities, dominated by forest specialists, with ‘agricultural’ communities dominated by grassland (and other open habitats) specialists. The intensification of agriculture, as well as the fragmentation of (semi)natural habitats for agriculture, disadvantages specialist species and increases the proportion of generalists.

Expected on a theoretical level, these impacts of habitat transformation on local and regional community composition have been verified, explored, and modeled for more than 15 years. In the mid-2000s, a team from the Muséum National d’Histoire Naturelle developed a method for estimating the degree of specialization of species and biological communities, in order to monitor and understand variations in their ‘specialization index‘ over time and space. Researchers have shown that the specialization index of bird communities varies inversely with the rate of disturbance of their habitats, particularly agricultural habitats: the more disturbed a habitat is, the more generalist the species it hosts [14] (Figure 7).

Whether for birds, insects (Figure 8) or other taxa, numerous biodiversity monitoring studies highlight the depletion of specialist species in the sites studied, attributed mainly to agricultural intensification, pollution and global warming (e.g. [15] and [10]).

At the scale of landscapes and regions, the expansion of generalist species (possibly originating from other regions of the globe) or/and commensals of humans, associated with the decline of specialist species, results in an increasing resemblance of ecological communities, known as ‘functional homogenization‘, current in regions of intensive agriculture ([15], [16]).

5. Impact on the number of local species

If we assume that the number of species – or ‘species richness‘ – of a biotic community increases with its abundance in number of individuals [17], we can expect that the expansion and intensification of agriculture, by reducing the abundance of local communities of wild species, will also reduce their (local, so-called ‘alpha’) diversity in number of species.

This reasoning would be correct if biotic communities were neutral, i.e. formed of ecologically equivalent species, and responded in the same way to changes in their living conditions [17]. But as seen above, this is not the case: agricultural intensification penalizes species that are specialists of open habitats, but benefits -to some extent- generalist species. Ecological modelling shows that, in the case of progressive modification of habitats (e.g. by pollution, increasing salification or aridification), involving a reduction in their carrying capacity, the number of local species first increases – despite a decrease in the total number of individuals – with the degree of habitat disturbance, and then decreases beyond a certain threshold [18] (Figure 9).

This can be explained simply by considering that the decline of specialist species, in number of individuals, does not mean their immediate local disappearance. On the contrary, the colonization of a transformed habitat by new generalist species implies both an increase in the number of species and individuals (generalists). Schematically, the total number of local species increases with habitat disturbance as long as colonization by new generalist species exceeds the local loss of specialist species; then it decreases when the latter exceeds the contribution of generalist species.

In short, unlike their specialization index, the species richness of local communities cannot be a good indicator of the stability or ‘quality’ of habitats, especially agricultural ones, since it increases or decreases with the degree of disturbance. This theoretical prediction is confirmed by avifauna monitoring [16] and by other biodiversity monitoring, in terrestrial or aquatic environments. A meta-analysis of biodiversity monitoring conducted at local or regional scales for more than 30 years [19] shows a change in community composition over time and confirms the erosion of biodiversity at the global scale, but does not observe any general decreasing trend in the number of local species (see also Teyssèdre and Robert [20]).

6. Impact on global biodiversity, in number of species

On a global scale, the total number of species depends on the rate of appearance of new species (through speciation) and the rate of extinction. The current rate of habitat transformation and fragmentation is such that only those species and populations that are most tolerant to these variations, and mainly abundant species with high rates of multiplication, seem likely to diversify in response to current global changes (see Biodiversity responses to global changes). The massive decline of specialist, low fecundity and/or high trophic level species, on the other hand, should result in a net reduction in the number of these species -and in the average number of trophic levels- at the global scale.

Given the lack of knowledge about the global diversity of bacteria, archaea, protists, ‘algae’ and fungi, it is difficult to estimate the impact of agriculture on the total number of species in these vast kingdoms of Life. The same is true for most of the major animal groups, with the exception of three more widely studied groups, insects, mammals and birds.

Insects are a diverse and abundant group of small (external skeletal) animals, often with good fecundity and dispersal abilities, inhabiting all kinds of terrestrial and freshwater habitats. According to estimates, they now number between 5 and 20 million species -of which one million are listed and described- for a total biomass of about 200 Mt C [21]. Compiling the results of 73 insect surveys conducted on all continents over several decades, Sanchez-Bayo and Wyckhuys [10] estimate that about 40% of species could disappear in the next few decades on a global scale, under the main pressure of changes in land use, agricultural intensification and climate change, in interaction with biological factors (pathogenic species, invasive exotic species, etc.) (see Figure 11). Thus, the high fecundity of most insects does not protect them from pesticides and the massive transformation of their habitats.

For vertebrates, the IUCN [22] estimates that 26% of mammal species, 14% of bird species and 41% of amphibian species are currently threatened with extinction, under pressure from the same anthropogenic factors identified above for insects.

7. Impacts on the structure and functioning of ecological networks

Whether it is a forest, a savanna, a pond or an agrosystem (or even a holobiont, read Dialogue and cooperation among bacteria), any ecosystem is formed by a dynamic network of living beings interacting with each other and with their physical environment (or ‘biotope’), the non-living component of the ecosystem.

In a minimally anthropized ecosystem, the interactions between species are usually multiple, ancient and diversified: trophic relations, of course, between predators and prey or between host species and parasites (or parasitoids), from the base to the top of the ‘food chains’; but also relations of competition for resources, of facilitation (to the installation of other species) or of mutual aid, between species of varied trophic levels (e.g. Couvet and Teyssèdre [23], and Figure 12).

Among the cooperative relationships, or mutual aid, we can cite the nutritional associations between plants and bacteria (Rhizobium in the roots) or between plants and fungi (e.g. mycorrhizae), the services exchanged between flowering plants and pollinating insects (feeding vs. reproduction), between parasitized species and predators of parasites (feeding or sheltering vs. protection), the dispersal of seeds by insects, birds and mammals (granivores or frugivores), etc. The repetitiveness of interactions contributes over time to the co-adaptation (and synchronization) of species sharing the same habitats, while their diversity contributes to the stability of ecological networks [24] (Figure 13).[25]

In an agrosystem, the diversity of interactions between species decreases, or even collapses, depending on the intensity of agricultural treatments -physical, chemical and biological- imposed. The simplification of ecological networks culminates in the vast monocultures of plants (wheat, rapeseed, maize, rice, soya…), which grow ‘sheltered’ from messicolous plants, most insects, birds and other organisms -thanks in particular to the use of pesticides- before being mowed and transformed into agro-industrial ‘products’, often preserved in cold storage and/or in plastic packaging before being marketed (Figure 14). At a second trophic step, these plant products are consumed either directly by humans, or by a small number of domestic animal species such as pigs and chickens, or by machines and engines as agrofuels. The third trophic level, that of carnivores, is almost monopolized by humans (see Focus on the impacts of agriculture on the proportions of wild and domestic species?).

The diversity of detritivorous, coprophagous, saprophytic and recycling organisms is in turn reduced and largely relocated outside the fields, in the slurry of livestock buildings, sewers, wastewater treatment plants and downstream water bodies or coastlines.

8. Impacts on the stability of agrosystems

Research over the past 50 years has verified that, at least on a local scale, the functioning and stability of terrestrial ecosystems increases with the diversity of plants they harbor [26].

Thus, the reduced biodiversity of agrosystems decreases their resistance to disturbances, especially to pest invasions. More precisely, the increased stability of diversified agrosystems, relative to the highly impoverished ecological networks of intensive monocultures, refers to several components of biodiversity:

• Genetic diversity of cultivated species (e.g. [27]),
• Specific and functional diversity of cultivated (polyculture vs. monoculture) and non-cultivated species (e.g. [27] and [28]),
• Abundance of pollinators (e.g. [29]),
• Diversity of trophic levels, including the local presence of natural enemies (predators, parasitoids, etc.) of pests and other crop ‘pests’, possibly from other habitats in the area (e.g. [30]),
• Diversity of non-trophic interactions (including facilitation, e.g. [30], [31]),
• Age of interactions (coadaptation, e.g. [31]),
• Local versus regional diversity, and crop diversity at the national scale (e.g. [29]),
• Diversity of farmer-crop variety networks (e.g. [32]).

In a vicious cycle, the vulnerability of large fields of intensive monoculture to pathogen invasions motivates increased pesticide use by farmers, which further reduces species diversity and their interactions…

9. Impacts on ecosystem functioning

The impacts of agriculture on community structure and dynamics, species numbers and ecological networks at different scales affect the functioning of ecosystems and socio-ecosystems (see e.g. [5]). In addition to the simplification of local ecological networks and ecosystems, another effect of agriculture is to specialize them in the production of biomass for human use, to the detriment of other functions (Figure 16).

Among the multiple ecosystem functions hindered by the expansion and intensification of agriculture are:

• Air purification by forests and other terrestrial (semi-)natural ecosystems
• Carbon capture and sequestration by these same ecosystems, and thus regulation of the global climate,
• Regulation of local climate by forests and wetlands,
• The moderation of floods and droughts,
• The renewal of water tables,
• Freshwater purification, with an impact downstream on the entire watershed,
• The maintenance and productivity of soils now reduced by erosion, pollution, salinization, …,
• The pollination of wild and cultivated plants by bees and other pollinating insects in decline,
• The resistance of wild and cultivated communities to invasions of parasites and other pathogens,
• The carrying capacity of ecosystems for wild species, especially specialists,
• Their physical and psychological capacity to accommodate domestic and farm animals (confined in large numbers in farm buildings),
• Their capacity to recycle waste, food (including feces) and agro-industrial (including plastics, metals …),
• Their capacity to accommodate humans.

Without exploring here the impacts of technical progress, production and consumption patterns, taxation/subsidy systems or international trade on the evolution of agri-food systems, let us emphasize that the multiple impacts on ecosystem functioning outlined above affect not only non-human biodiversity, but also human populations to a very large extent.

10. What future for Ouroboros?

According to the Millennium Ecosystem Assessment (MEA, 2005 [5]), 60% of ecosystem “regulating services” were in decline or lost at the turn of the millennium, a large part of which was due to the expansion and intensification of agriculture. These ecological losses and imbalances have worsened since then, particularly to the detriment of health and food for an increasing number of humans since 2010 (FA0 2020). Other research indicates that the “anthropogenic” modification of certain physical, chemical and biological variables descriptive of the ecosystems (e.g., atmospheric concentrations of GHGs or nitrates in soils), approaches critical thresholds beyond which the biosphere as a whole is expected to shift to a different operating regime, very unfavorable – among other species – to humans [33].

Thus, it seems that the “Uroborian” dynamic of expansion-intensification of agriculture has reached its limits. To summarize, it can be described as an increasing diversion of the primary productivity (i.e. vegetal) of terrestrial ecosystems – and thus their massive transformation – by certain human societies, to the “benefit” of little diversified plant, animal (including human) and microbial communities, whose fragility increases with that of the impoverished and polluted ecosystems that host them… thus threatening the collapse of the societies that depend on them!

To survive, our old Ouroboros must imperatively review its diet and reduce its appetite (for animal meat and agrofuels, noteworthily). Faced with the multiple impacts of this agricultural dynamic, our societies must question the dominant agricultural systems and develop alternative, sustainable policies and practices that are likely to get them out of this destructive loop for ecosystems and biodiversity. New agricultural policies [34] that, in order to be approved and implemented in a coherent manner, must be part of a general change in the conception, objectives and organization of societies within socio-ecosystems, valuing ecological diversity (regulating), sobriety (sustainable) and environmental equity, rather than the intensive – and unsustainable – production and consumption of agricultural “resources”.

11. Messages to remember

• Since the Neolithic period, the expansion and intensification of agriculture has reduced the plant biomass of terrestrial habitats by about a third. Moreover, nowadays, about 20% of their primary productivity (plant biomass produced per unit area and time) is annually diverted by human societies (for food and other uses), with large variations between regions.
• The transformation of habitats for agriculture reduces the abundance and changes the composition of wildlife communities: the proportion of generalist species increases with habitat disturbance.
• Through its direct and indirect effects, livestock farming has a strong impact on biodiversity and ecosystems. Thus, only 3% of the biomass of mammals is wild today.
• In an agrosystem, the diversity of interactions between species decreases with the increasing intensity of agricultural practices (chemical inputs, mechanisation).
• The resistance of agrosystems to disturbances increases with the diversity and age of interactions between species and decreases with the intensity of agricultural practices.
• The massive disruption of ecosystems by and for agriculture hinders their functioning and productivity in the medium term, to the detriment of human societies and many species.
• The agricultural challenge of the 21st century is huge: to achieve sustainable agriculture, with sufficient production, while preserving biodiversity and mitigating climate change.

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This article is a slightly modified version of the author’s “A look at biodiversity” (R103), published in June 2022 by the French Society of Ecology and Evolution (SFE2).

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Notes and references

Cover image. Corn field. [Source: Photo © A. Teyssèdre]

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