Since various studies have suggested that insect biomass has declined about 75% in recent decades, one could say that there were four times as many insects. Large biomass losses also occurred in the time earlier (agricultural revolution, as e.g. described in Rachel Carlson's book 'Silent Spring'). The 75% loss refers to the latest decades only. How many insects were there around 1900 or before? Why is there so little data?
It is becoming easier to find historical literature in digitized library collections. A longer time ago I discovered that around 1900 cockchafer could be collected on an afternoon bucket-wise to cook cockchafer soup. Amazing! Unthinkable.
Sometimes you can find interesting things through reference lists, as it was in this case. I was looking for an essay by Franz Leopold Friedrich Sintenis (1835-1911). He observed pollinators in the Baltic States in the nineteenth century and published about it, focusing mainly on the fly fauna (Diptera). After a long search I found a digital copy:
Sintenis,
F. Bericht über die Ergebnisse und Beobachtungen an Hymenopteren,
Lepidopteren und Dipteren im Frühling u. Sommer 1896. Sitzungsber. Ges.
Dorpat XI p. 189—199.
Optimized version here (b&w, ocr, 0,3 Mb)
More on this later on. The decline of insects in 1896 is addressed here, referring to an essay from 15 years earlier. In its time, this must have been an important source:
Adolf
Karl Rössler (1880-1881): Die Schuppenflügler (Lepidoptoren) des Kgl.
Regierungsbezirks Wiesbaden und ihre Entwicklungsgeschichte – Jahrbücher
des Nassauischen Vereins für Naturkunde – 33-34: 1 - 393.
Optimized version here (b&w, ocr, 8,6 Mb)
And when you read these publications, it does make you feel a little uneasy. I was most surprised by the mention of the changing climate.
(translated) "...for any deterioration (improvement I hardly dare to hope) of the climate..."
"...hot year series from 1851 to 1861..."
which Rössler associated with butterflies having more yearly generations than usual and with species migrating north from the south.
Rössler is even very forward-looking:
(translated) "In order to make the gradual change of our fauna recognizable for the researchers of a later age, I have also often indicated the time of appearance in the field with day and year. One will
then have a measure (...) In this respect faunal inventories can also have a historical value."
Compared to many ecologists, he seems a century ahead.
I have wondered if this can really be, the global warming starting so early and being noticeable to the scientists of the time?
A NATURE article from 2016 leaves little doubt (although the interpretation of the data is still under discussion, it fits the observations described):
Figure is modified to fit here. The original can be found here. It shows the relative deviation from a 0 value worldwide: Static air temperature (SAT) & Sea surface temperature (SST). See article for more details.
Rössler writes more troubling things, such as changes in landscape maintenance:
(translated) "Aspen, ash, lime, elm, birch, hazel, etc. are along with the so-called forest weeds, such as blueberries, broom, clematis and other beautiful plants, on the list to be destroyed. Hedges are eradicated everywhere without exception, not only in the field, but also in the forest, even at the forest edges, where they could prevent soil evaporation and the blowing away of leaf litter as the greatest benefit of the forest. (...)
Trees at the forest edge have been stripped of their branches that were protruding outward up to the top, offending every eye receptive to beauty. The birds living on berries and insects no longer find sufficient food or nesting places outdoors; they settle in parks where it is still possible. The fruit trees are increasingly attacked by harmful insects as a result of the absence of the hedges that serve as distraction. The exposed plots are dried out by the wind, hit harder by frost in winter, and during heavy rain the sediment soil, which used to be held back by the hedges, is washed down into the depths.
Scholarly farmers and land surveyors bother owners of meadows that still are in a natural state until they agree the stream in it to be stretched. The stream is given a bed that is as straight as possible and barely sufficient for the usual water level. In our mostly steeply sloping valleys, the water then rushes through as fast as an arrow. These artificial irrigation ditches which require constant care are soon neglected, and the subterranean moisture that used to keep the soil fertile is missing."
This is not how I learned it in my studies. The systematic and large-scale clearing and straightening of streams was taught to take place much later. This makes clear that there have been several phases.
He can also tell a lot about invasive or frequently imported species:
(translated) "As a rule, the plants originally belonging to a region are consequently the ones most infested by caterpillars. Plants introduced from abroad, however suitable, are discovered by insects after longer periods. (...) Acer platanoides and Pseudoplatanus serve as food for many caterpillars in southern Germany, where it is native; here, where it has been imported only since this century, it is almost insect-free."
And then the realization that humans convert the Earth's biomass into livestock and crops, as pointed out in Dave Goulson's book 'Silent Earth' and in the publication by Yinon M. Bar-On (here), is already old:
(translated) "Thus we are approaching a state on earth, with the extreme end in an inpredictable time, in which only humanity would remain with his domestic animals and cultivated plants."
For 2018, Bar-On managed an convincing estimate:
"Today, the biomass of humans (≈0.06 Gt C) and the biomass of livestock (≈0.1 Gt C, dominated by cattle and pigs) far surpass that of wild mammals, which has a mass of ≈0.007 Gt C. This is also true for wild and domesticated birds, for which the biomass of domesticated poultry (≈0.005 Gt C, dominated by chickens) is about threefold higher than that of wild birds (≈0.002 Gt C)."
Rössler wrote about the period 1850-1880 in Germany. Friedrich Sintenis then reported about his experiences in Estonia in 1896.
He writes how numerous insects were on flowers:
(translated) "When the row of alder buckthorn (a row of bird cherry?) was in bloom, it was an incredible treasure of diptera for me. In the sunshine at midday, leaves and flowers were covered with flies of all groups. But also the abundant overgrown ground of the slope was swarmed with insects, especially shade-loving ones, which were raised from the scrubs and ferns. Since all these shrubs and trees bloomed in May, a very rich insect life unfolded on this slope."
and
(translated) "There, dense rows of Pastinaca sativa L. grew along field margins and an incredible number of dipterans gathered at their umbel flowers."
Well, how many insects are that?
(translated) "For hours I have been busy here, striding from one group of umbellifers to another, picking off dipterans and ichtneumons from the bushes of umbels; the flowers were always crowded. Here I caught beautiful dipteran species in large numbers, including some apparently quite unknown."
How many? Hours of collecting on a dozen square meters? Unfortunately, we won't know more precisely. Maybe the collection of Mr. Sintenis will reveal something (the diptera collection is said to have been brought to the Kaiser-Friedrich-Museum in Posen in 1940). If he collected all catches and marked them with date and 'On umbellifers', we know that he collected them within several hours. But most likely, like other entomologists, he collected only a few specimens of a 'morphospecies'.
Nevertheless, the way Sintenis describes the situation, you won't find it anymore. His descriptions are dated in 1896 before various extinction waves, long before 'Silent Spring', and 'Silent Earth'. I would like to calculate briefly. In the latest extinction wave, measured roughly from 1990 to 2015, 75% of the biomass disappeared. So before that, there were roughly 4 times as many insects. But 75% of what? Not of 100%. How much disappeared before 1990 in the other phases together, 75%, 95%? If we assume another 75%, it means that in the time of Sintenis and Rössler there were 16 times as many insects as now. But if you consider the agricultural revolution and read the records of Rössler and Sintenis, which also report large-scale habitat loss, then the 95% might be more realistic? If you would take the 95% estimate, that means 4x20 = 100 times as many insects as now. If I, as an entomologist (who has also been to Estonia and Lithuania observing pollinators), visualize these descriptions, I think that the factor is closer to 100 than to 16. Hundred times as many insects (or even as much as 200?!) - these are numbers that we can hardly imagine.
Is that exaggerated? A very rough look in the past provides this artcle by Jeff Ollerton.
This curve of extinction events of bee species in the pre-1990 period does not suggest a more positive or moderate value. How the world and its insect abundance looked in 1850 has become a little more vivid with the above examples, giving food for thoughts on how to shape our future landscapes.
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