Insectageddon ?

Le déclin des insectes dans le monde est bien documenté. Mais il n’est pas homogène, et les causes possibles de ce déclin sont multiples. L’importance globale de ce déclin est elle aussi discutée. Certains auteurs n’hésitent pas à parler d’insectageddon ou d’insect armageddon, d’autres auteurs, généralement sans nier le déclin, soulignent la faiblesse de certaines études et les contre-exemples.

  1. Publications concluant à un déclin massif des populations d’insectes.
  2. Publications relativisant ce déclin.
  3. Causes possibles du déclin.

Publications concluant à un déclin massif des populations d’insectes

Insect Declines in the Anthropocene
David L. Wagner
Annual review of entomology, 2020
https://www.annualreviews.org/doi/abs/10.1146/annurev-ento-011019-025151

Insect declines are being reported worldwide for flying, ground, and aquatic lineages. Most reports come from western and northern Europe, where the insect fauna is well-studied and there are considerable demographic data for many taxonomically disparate lineages. Additional cases of faunal losses have been noted from Asia, North America, the Arctic, the Neotropics, and elsewhere.

Declines in insect abundance and diversity : We know enough to act now
Forister et al.
Conservation science and practice, 2019
https://conbio.onlinelibrary.wiley.com/doi/pdf/10.1111/csp2.80

Recent regional reports and trends in biomonitoring suggest that insects areexperiencing a multicontinental crisis that is apparent as reductions in abundance,diversity, and biomass. Given the centrality of insects to terrestrial ecosystems andthe food chain that supports humans, the importance of addressing these declinescannot be overstated. The scientific community has understandably been focusedon establishing the breadth and depth of the phenomenon and on documenting fac-tors causing insect declines. In parallel with ongoing research, it is now time forthe development of a policy consensus that will allow for a swift societal response.We point out that this response need not wait for full resolution of the many physi-ological, behavioral, and demographic aspects of declining insect populations. Tothese ends, we suggest primary policy goals summarized at scales from nations tofarms to homes.

Arthropod decline in grasslands and forests is associated with landscape-level drivers
Seibold et al.
Nature, 2019
https://www.nature.com/articles/s41586-019-1684-3

In annually sampled grasslands, biomass, abundance and number of species declined by 67%, 78% and 34%, respectively. The decline was consistent across trophic levels and mainly affected rare species; its magnitude was independent of local land-use intensity. However, sites embedded in landscapes with a higher cover of agricultural land showed a stronger temporal decline. In 30 forest sites with annual inventories, biomass and species number—but not abundance—decreased by 41% and 36%, respectively. This was supported by analyses of all forest sites sampled in three-year intervals. The decline affected rare and abundant species, and trends differed across trophic levels. Our results show that there are widespread declines in arthropod biomass, abundance and the number of species across trophic levels. Arthropod declines in forests demonstrate that loss is not restricted to open habitats. Our results suggest that major drivers of arthropod decline act at larger spatial scales, and are (at least for grasslands) associated with agriculture at the landscape level. This implies that policies need to address the landscape scale to mitigate the negative effects of land-use practices.

(Commentaire)
Evidence for Recent Decline of Arthropods in Germany is Not Yet Robust
Hambler & Henderson, 2019
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3481752

What has been demonstrated is a decline in the relative activity-abundance of a limited selected set of taxa in a selected period of years. Use of a small number of years to detect trends is statistically problematic, and highly dependent on start and end dates of the time series.

Climate-driven declines in arthropod abundance restructure a rainforest food   Bradford C. Lister & Andres Garcia
PNAS, 2018
https://www.pnas.org/content/pnas/115/44/E10397.full.pdf(non cliquable)

Analysis of long-term data on canopy arthro-pods and walking sticks taken as part of the Luquillo Long-TermEcological Research program revealed sustained declines in abun-dance over two decades, as well as negative regressions of abun-dance on mean maximum temperatures. We also document paralleldecreases in Luquillo’s insectivorous lizards, frogs, and birds. While ElNiño/Southern Oscillation influences the abundance of forest arthro-pods, climate warming is the major driver of reductions in arthropodabundance, indirectly precipitating a bottom-up trophic cascade andconsequent collapse of the forest food web.

Une réponse à l’article :

Populations are not declining and food webs are not collapsing at the Luquillo Experimental Forest
Willig et al.
PNAS, 2019
https://www.pnas.org/content/116/25/12143

Canopy arthropod density does not decline between 1994 and 2009 but does increase significantly with increasing temperature (figures 5 and 6 of Supplementary Materials), even for the 10 most abundant taxa (tables 1 and 2 of Supplementary Materials), which Lister and Garcia claimed to have used (3).

Long-term data do not suggest a simple decline in adult frogs from 1987 to 2017 (figure 7 of Supplementary Materials) but do document an increase in numbers with increasing temperature (figure 8 of Supplementary Materials). Numbers vary in a consistent and nondirectional manner, except for short-term increases after Hurricanes Hugo and Georges, which modified habitat structure

Et une réponse à la réponse :

Reply to Willig et al.: Long-term population trends in the Luquillo Rainforest
Brad Lister and Andres Garcia
PNAS, 2019
https://www.pnas.org/content/116/25/12145

More than 75 percent decline over 27 years in total flying insect biomass in protected areas
Hallmann et al.
Plos One, 2017
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0185809

Global declines in insects have sparked wide interest among scientists, politicians, and the general public. Loss of insect diversity and abundance is expected to provoke cascading effects on food webs and to jeopardize ecosystem services. Our understanding of the extent and underlying causes of this decline is based on the abundance of single species or taxonomic groups only, rather than changes in insect biomass which is more relevant for ecological functioning. Here, we used a standardized protocol to measure total insect biomass using Malaise traps, deployed over 27 years in 63 nature protection areas in Germany (96 unique location-year combinations) to infer on the status and trend of local entomofauna. Our analysis estimates a seasonal decline of 76%, and mid-summer decline of 82% in flying insect biomass over the 27 years of study. We show that this decline is apparent regardless of habitat type, while changes in weather, land use, and habitat characteristics cannot explain this overall decline. This yet unrecognized loss of insect biomass must be taken into account in evaluating declines in abundance of species depending on insects as a food source, and ecosystem functioning in the European landscape.

Defaunation in the Anthropocene
Rodolfo Dirzo et al.
Science, 2014
http://discovery.ucl.ac.uk/1436030/1/Collen_Dirzo%20etal%202014%20Science%20Accepted.pdf
https://www.researchgate.net/publication/264247848_Defaunation_in_the_Anthropocene

Among terrestrial vertebrates 322 species have become extinct since 1500, while populations of the remaining species show 25% average decline in abundance. Invertebrate patterns are equally dire: 67% of monitored populations show 45% mean abundance decline. Such animal declines will cascade onto ecosystem functioning and human well-being. Much remains unknown about this “Anthropocene defaunation”; these knowledge gaps hinder our capacity to predict and limit defaunation impacts. Clearly, however, defaunation is both a pervasive component of the planet’s sixth mass extinction, and also a major driver of global ecological change.

Diminution invertébrés depuis 1970

 


Publications relativisant le déclin des populations d’insectes

Nuanced changes in insect abundance
Dornelas & Daskalova
Science, 2020
https://science.sciencemag.org/content/368/6489/368.summary

Drastic declines in insect biomass, abundance, and diversity reported in the literature have raised concerns among scientists and the public (13). If extrapolated across Earth, biomass losses of ∼25% per decade (1) project a potential catastrophe developing unnoticed under our noses. The phrase “insect Armageddon” has captured the collective attention and shined a spotlight on one of the most numerous and diverse groups of organisms on the planet. Yet, insects are critically understudied. For example, the BioTIME database (4)—a compilation of biodiversity time series—contains records for 22% of known bird species but only 3% of arthropods (the phylum that includes insects and spiders).

No net insect abundance and diversity declines across US Long Term Ecological Research sites
Crossley et al.
Nature ecology & evolution, 2020
https://www.nature.com/articles/s41559-020-1269-4

Some taxa and sites showed decreases in abundance and diversity while others increased or were unchanged, yielding net abundance and biodiversity trends generally indistinguishable from zero. This lack of overall increase or decline was consistent across arthropod feeding groups and was similar for heavily disturbed versus relatively natural sites. The apparent robustness of US arthropod populations is reassuring. Yet, this result does not diminish the need for continued monitoring and could mask subtler changes in species composition that nonetheless endanger insect-provided ecosystem services.

Critique de l’étude :

Meta-analyses of insect temporal trends must account for the complex sampling histories inherent to many long-term monitoring efforts
Welti et al. (preprint)
https://ecoevorxiv.org/v3sr2/

In an article recently published in Nature Ecology & Evolution (Crossley et al. 2020 “No net insect abundance and diversity declines across US Long Term Ecological Research sites”), sampling effort within Long-Term Ecological Research (LTER) datasets was assumed to be consistent across years. Given the complex history of many long-term datasets at LTER sites, this assumption does not often hold and we believe this assumption led to errors in Crossley et al.’s analysis. Here we first use the Konza Prairie grasshopper dataset as an example of how changes in sampling locations and effort can cause errors when data are assumed to be collected with invariant sampling. Second, we describe similar and additional errors in data use from 7 of the 13 LTER sites included in Crossley et al. (2020)’s analysis.

Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances
Van Klink et al.
Science, 2020
https://science.sciencemag.org/content/368/6489/417/tab-pdf

Recent case studies showing substantial declines of insect abundances have raised alarm, but how widespread such patterns are remains unclear. We compiled data from 166 long-term surveys of insect assemblages across 1676 sites to investigate trends in insect abundances over time. Overall, we found considerable variation in trends even among adjacent sites but an average decline of terrestrial insect abundance by ~9% per decade and an increase of freshwater insect abundance by ~11% per decade. Both patterns were largely driven by strong trends in North America and some European regions. We found some associations with potential drivers (e.g., land-use drivers), and trends in protected areas tended to be weaker. Our findings provide a more nuanced view of spatiotemporal patterns of insect abundance trends than previously suggested.

“Insectageddon”: A call for more robust data and rigorous analyses
Thomas et al.
Global change biology, 2019
https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.14608?casa_token=VK7IL3EzYX4AAAAA%3ABWEPPMzf3XhaBDRni7DuKmZPtWF0kmNkZuW1JFr0Wz1AdM4N4ZbpcZ6ZRvspyQH9z-48Sli78YOYn8U

As members of that subset of the human population who love insects, we have been alarmed by a recent publication reporting their global decline and impending extinction (Sánchez‐Bayo & Wyckhuys, 2019), and the accompanying media furore. Indeed, there has been a growing tide of concern over the magnitude and potential consequences of diminishing insect populations (e.g., Hallmann et al., 2017; Lister & Garcia, 2018). However, we respectfully suggest that accounts of the demise of insects may be slightly exaggerated. Bad things are happening—we agree—but this is not the whole story. We call for hard‐nosed, balanced, and numerical analysis of the changes taking place, and for calm and even‐handed interpretation of the changes, rather than rushing headlong into the hyperbole of impending apocalypse.

No Simple Answers for Insect Conservation: Media hype has missed the biggest concern that ecologists and entomologists have about six-legged life: how little we know about it
Manu E. Saunders
American Scientist, 2019
https://go.galegroup.com/ps/anonymous?id=GALE%7CA585577272&sid=googleScholar&v=2.1&it=r&linkaccess=abs&issn=00030996&p=AONE&sw=w

Widespread, consistent insect declines are a real concern. Yet there is little published evidence that worldwide decline of all insects is happening. The aforementioned studies were localized and skewed toward particular taxa. The authors of the Krefeld and Luquillo studies tried not to overextend their results in their journal articles, but the press releases from the lead authors’ institutions, media coverage, and the recent review paper were less cautious. Sanchez-Bayo and Wyckhuys claimed that their review showed that « almost half of the [world’s insect] species are rapidly declining. » Yet their data show declines for only about 2,900 species–a tiny fraction of the estimated 5 million insect species on Earth.
[…]
The Krefeld study is based on trap samples from 63 nature reserve sites in Germany, collected over a 27-year period. But more than half the sites were only surveyed once during the study period, and only 26 sites were surveyed in multiple, though not consecutive, years. The lack of repeated sampling at exactly the same locations over a long period of time limits understanding of the true extent of declines across Europe, let alone in the rest of the world. Moreover, biomass is a poor proxy for abundance or species richness.

Mitigating the precipitous decline of terrestrial European insects: Requirements for a new strategy
Habel et al.
Biodiversity and conservation, 2019
https://link.springer.com/article/10.1007/s10531-019-01741-8

Generality of recent scientific findings on insect decline have shortcomings, as results have been based on irregular time series of insect inventories, and have been carried out on restricted species sets, or have been undertaken only in a particular geographical area.

Moth biomass increases and decreases over 50 years in Britain
MacGreggor et al.
Nature Ecology & Evolution, 2019
https://www.nature.com/articles/s41559-019-1028-6

Steep insect biomass declines (‘insectageddon’) have been widely reported, despite a lack of continuously collected biomass data from replicated long-term monitoring sites. Such severe declines are not supported by the world’s longest running insect population database: annual moth biomass estimates from British fixed monitoring sites revealed increasing biomass between 1967 and 1982, followed by gradual decline from 1982 to 2017, with a 2.2-fold net gain in mean biomass between the first (1967–1976) and last decades (2008–2017) of monitoring. High between-year variability and multi-year periodicity in biomass emphasize the need for long-term data to detect trends and identify their causes robustly.

Long‐term changes in the abundance of flying insects
Shortall et al.
Insect conservation and diversity, 2009
https://onlinelibrary.wiley.com/doi/full/10.1111/j.1752-4598.2009.00062.x

1. For the first time, long‐term changes in total aerial insect biomass have been estimated for a wide area of Southern Britain.

2. Various indices of biomass were created for standardised samples from four of the Rothamsted Insect Survey 12.2 m tall suction traps for the 30 years from 1973 to 2002.

3. There was a significant decline in total biomass at Hereford but not at three other sites: Rothamsted, Starcross and Wye.

4. For the Hereford samples, many insects were identified at least to order level, some to family or species level. These samples were then used to investigate the taxa involved in the decline in biomass at Hereford.

5. The Hereford samples were dominated by large Diptera, particularly Dilophus febrilis, which showed a significant decline in abundance.

6. Changes in agricultural practice that could have contributed to the observed declines are discussed, as are potential implications for farmland birds, with suggestions for further work to investigate both cause and effect.


Causes possibles du déclin des insectes

Sans préjuger de leur importance relative, on trouve :
Pesticides ; perte des habitats du fait de l’intensification de l’agriculture et de l’urbanisation ; lumière artificielle ; intensification du trafic routier ; pathogènes ; espèces invasives ; concurrence entre abeilles domestiques et pollinisateurs sauvages ; changement climatique.

Gradual replacement of wild bees by honeybees in flowers of the Mediterranean Basin over the last 50 years
Carlos M. Herrra
Proceedings of the royal society B, 2020
https://royalsocietypublishing.org/doi/10.1098/rspb.2019.2657

The proportion of wild bees at flowers was four times greater than that of honeybees at the beginning of the period, the proportions of both groups becoming roughly similar 50 years later. The Mediterranean Basin is a world biodiversity hotspot for wild bees and wild bee-pollinated plants, and the ubiquitous rise of honeybees to dominance as pollinators could in the long run undermine the diversity of plants and wild bees in the region.

Wild pollinator activity negatively related to honey bee colony densities in urban context
Ropars et al.
PLOS One, 2019
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0222316

Here, we show that in the city of Paris (France), wild pollinator visitation rates are negatively correlated to honey bee colony densities present in the surrounding landscape

Light pollution is a driver of insect declines
Owens et al.
Biological conservation, 2019
https://www.sciencedirect.com/science/article/abs/pii/S0006320719307797

Habitat loss, pesticide use, invasive species, and climate change all have likely played a role, but we posit here that artificial light at night (ALAN) is another important—but often overlooked—bringer of the insect apocalypse.

Arthropod decline in grasslands and forests is associated with landscape-level drivers
Seibold et al.
Nature, 2019
https://www.nature.com/articles/s41586-019-1684-3

In annually sampled grasslands, biomass, abundance and number of species declined by 67%, 78% and 34%, respectively. The decline was consistent across trophic levels and mainly affected rare species; its magnitude was independent of local land-use intensity. However, sites embedded in landscapes with a higher cover of agricultural land showed a stronger temporal decline. In 30 forest sites with annual inventories, biomass and species number—but not abundance—decreased by 41% and 36%, respectively. This was supported by analyses of all forest sites sampled in three-year intervals. The decline affected rare and abundant species, and trends differed across trophic levels. Our results show that there are widespread declines in arthropod biomass, abundance and the number of species across trophic levels. Arthropod declines in forests demonstrate that loss is not restricted to open habitats. Our results suggest that major drivers of arthropod decline act at larger spatial scales, and are (at least for grasslands) associated with agriculture at the landscape level. This implies that policies need to address the landscape scale to mitigate the negative effects of land-use practices.

(Commentaire)
Evidence for Recent Decline of Arthropods in Germany is Not Yet Robust
Hambler & Henderson, 2019
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3481752

What has been demonstrated is a decline in the relative activity-abundance of a limited selected set of taxa in a selected period of years. Use of a small number of years to detect trends is statistically problematic, and highly dependent on start and end dates of the time series.

Mitigating the precipitous decline of terrestrial European insects: Requirements for a new strategy
Habel et al.
Biodiversity and conservation, 2019
https://link.springer.com/article/10.1007/s10531-019-01741-8

Agricultural intensification is the main driver of recent terrestrial insect decline, through habitat loss, reduced functional connectivity, overly intense management, nitrogen influx, and use of other fertilisers, as well as application of harmful pesticides. However, there are also supplementary and adversely synergistic factors especially climate change, increasingly intense urbanisation, and associated increase in traffic volume, artificial lighting and environmental pollution.

Cette publication lie le déclin des coléoptères dans des zones préservées de l’activité humaine du New Hampshire au réchauffement climatique.

Decline in beetle abundance and diversity in an intact temperate forest linked to climate warming
Jennifer E. Harris et al.
Biological conservation, 2019
https://www.sciencedirect.com/science/article/abs/pii/S0006320719310572

Beetle capture rate was least when and where climate was warmest. Capture rate was significantly lower in the 2010s when mean daily temperature was about 1.8 °C warmer, and sampling during 2016–2017 at low, mid and high elevations (320, 540, and 810 m asl, respectively) revealed lowest beetle captures at low elevation where climate was warmest. Most importantly, beetle capture rate was significantly lower after winters with less snow cover during the previous winter, indicating that snow cover in northern hardwood forest is essential for sustaining the beetle community. These results imply that additional climate warming might further reduce the abundance and diversity of beetles and other arthropods inhabiting the forest-floor, potentially affecting critical ecosystem processes such as decomposition and carbon storage.

Worldwide decline of the entomofauna: A review of its drivers
Sanchez-Bayo & Wyckhuys

Biological conservation, 2019
https://www.sciencedirect.com/science/article/pii/S0006320718313636

The main drivers of species declines appear to be in order of importance: i) habitat loss and conversion to intensive agriculture and urbanisation; ii) pollution, mainly that by synthetic pesticides and fertilisers; iii) biological factors, including pathogens and introduced species; and iv) climate change. The latter factor is particularly important in tropical regions, but only affects a minority of species in colder climes and mountain settings of temperate zones. A rethinking of current agricultural practices, in particular a serious reduction in pesticide usage and its substitution with more sustainable, ecologically-based practices, is urgently needed to slow or reverse current trends, allow the recovery of declining insect populations and safeguard the vital ecosystem services they provide. In addition, effective remediation technologies should be applied to clean polluted waters in both agricultural and urban environments.

Country-specific effects of neonicotinoid pesticides on honey bees and wild bees
Woodcock et al.
Science, 2017
https://science.sciencemag.org/content/356/6345/1393.full

For honey bees, we found both negative (Hungary and United Kingdom) and positive (Germany) effects during crop flowering. In Hungary, negative effects on honey bees (associated with clothianidin) persisted over winter and resulted in smaller colonies in the following spring (24% declines). In wild bees (Bombus terrestris and Osmia bicornis), reproduction was negatively correlated with neonicotinoid residues. These findings point to neonicotinoids causing a reduced capacity of bee species to establish new populations in the year following exposure.

Cette publication suggère que les parasites et les pathogènes sont les responsables directs de la mort d’abeilles domestiques et sauvages, mais que les pesticides, particulièrement les néonicotinoïdes, sont responsables de leur affaiblissement, qui les rend plus sensibles aux parasites.

Are bee diseases linked to pesticides? — A brief review
Sanchez-Bayo et al.
Environment international, 2016
https://www.sciencedirect.com/science/article/pii/S0160412016300095

Wild and managed bees have been declining in recent years.
Parasites and pathogens are the main cause of the current bee demise.
Stress agents acting on bees can promote pathogen spread and virulence.
Sublethal doses of immunosuppressive pesticides favour the diffusion of bee diseases.
Pesticides and their interactions contribute to stress-induced colony losses.

Monarchs in decline: a collateral landscape‐level effect of modern agriculture
Stenoien et al.
Insect science, 2016
https://onlinelibrary.wiley.com/doi/full/10.1111/1744-7917.12404

Glyphosate‐tolerant soybean and maize have enabled the extensive use of this herbicide, generating widespread losses of milkweed (Asclepias spp.), the only host plants for monarch larvae. Modeling studies that simulate lifetime realized fecundity at a landscape scale, direct counts of milkweeds, and extensive citizen science data across the breeding range suggest that a herbicide‐induced, landscape‐level reduction in milkweed has precipitated the decline in monarchs. A recovery will likely require a monumental effort for the re‐establishment of milkweed resources at a commensurate landscape scale.

The new world atlas of artificial night sky brightness
Falchi et al.
Science advances, 2016
https://advances.sciencemag.org/content/2/6/e1600377

Artificial lights raise night sky luminance, creating the most visible effect of light pollution—artificial skyglow. Despite the increasing interest among scientists in fields such as ecology, astronomy, health care, and land-use planning, light pollution lacks a current quantification of its magnitude on a global scale. To overcome this, we present the world atlas of artificial sky luminance, computed with our light pollution propagation software using new high-resolution satellite data and new precision sky brightness measurements. This atlas shows that more than 80% of the world and more than 99% of the U.S. and European populations live under light-polluted skies. The Milky Way is hidden from more than one-third of humanity, including 60% of Europeans and nearly 80% of North Americans. Moreover, 23% of the world’s land surfaces between 75°N and 60°S, 88% of Europe, and almost half of the United States experience light-polluted nights.

Effects of roads on insects: a review
Muñoz et al.
Biodiversity and conservation, 2015
https://link.springer.com/article/10.1007/s10531-014-0831-2

We found that roads negatively affect the abundance and diversity of insects due to two main factors: (1) the high mortality of some groups when crossing the road, with more impact at higher traffic volumes. (2) The unwillingness of many species to cross a road or live close to it. Roads are major barriers for small or flightless species, although the response varied for flying species. Finally, both experimental and observational evidence support the idea that air pollutants and de-icing salt used for the road maintenance negatively affect insects.