Declines in insect numbers have been hitting the headlines, leading to fears that we are experiencing an ‘insect apocalypse’. Two research papers generated particularly alarming news stories. First came a widely reported study from Germany, which concluded that there had been a decline of more than 75% in the total flying insect biomass over a 27-year period [4]. Given that the researchers had been looking at protected areas rather than the wider countryside, their finding was all the more troubling.
This was followed up with what purported to be a global
review [9] concluding that almost half of the world’s insect species are
rapidly declining and a third are threatened with extinction. Even worse, the authors argue that unless we
change our ways of producing our food ‘insects as a whole will go down the path
of extinction in a few decades.’
This is an alarming claim.
It is also a bold claim. And, as
such, it has attracted academic criticism [10].
For example, the researchers’ method of selecting studies to review was
to use search terms including the keyword ‘decline’. It is therefore unsurprising that the
resulting picture was overwhelmingly negative. Even worse, the geographical basis of the
review was, to say the least, biased. The vast majority of reviewed studies were
from Europe and the USA, while only a handful had considered the tropics. Although this reflects the distribution of
most of the world’s insect studies, it does not reflect the distribution of
most of the world’s insects.
No doubt anticipating criticism on the latter point, the
study’s authors argue that the factors causing decline applied to all countries
in the world and, as a result, ‘insects
are not expected to fare differently in tropical and developing countries.’ This statement may be true, but no evidence
is presented to justify it. For example,
much depends upon whether substantial areas of tropical forest can be
retained. If they can (and that’s a big
if) then much insect diversity may well be conserved. Also, it is uncertain to
what extent other possible factors, such as light pollution, apply equally in
the tropics.
All of which is a shame, as conservation needs to be
evidence-based, and there are obvious dangers of ‘crying wolf’ in terms of lack
of credibility and adding fuel to those who would seek to undermine
conservation efforts. But all is not
lost. There is evidence out there,
although it’s rather more nuanced than the headlines would suggest.
Turning to the German study, closer examination reveals
that most of the sites had only been visited once or twice; the pattern of
decline derives from the application of a series of models based on
combinations of different variables rather than a trend experienced in any one
location. So the seasonal estimates of
daily biomass for any given year are generated by a model rather than
representing actual measurements. In
addition, biomass is not the same as abundance; for example, a decline in
larger insects could mask an increase in smaller species.
However, it is clear that some insect species and
populations are in trouble. Back in
2006, a study examining the long-running UK Rothamsted Insect Survey (which
kills its catches, but that’s a story for another day) 2006 revealed that the
annual total number of macro-moths decreased by almost a third between 1968 and
2002 [2]. Two thirds of the sampled moth
species declined, the most significant effects being in the south of England –
a result that suggests that agricultural intensification may be an important
cause, although other factors (such as the effects of light pollution or the
effect of changes of phenology on the availability of food resources) cannot be
ruled out [1, 3]. Indeed, recent work by the late, and much missed, Douglas Boyes, has demonstrated for the first time that street lighting significantly reduces moth caterpillar abundance. Even worse, such adverse effects are more marked in the case of (increasingly popular) LED lighting [12].
A major development has been the publication of the Atlas of Britain and Ireland’s Larger Moths [8]. If you’re into moths then it’s a ‘must have’
purchase. In terms of moth abundance,
the Atlas shows that 248 macro-moth species declined and 149 species increased
over the period 1970-2016 – a similar picture to the earlier study. When statistically significant changes are
considered the numbers are less spectacular, although the underlying pattern
remains – 136 species decreased (34% of the total) while 45 species (11%)
increased. Effects on certain species
have been dramatic; for example, the familiar Garden Tiger Arctia caja [photo above] has shown an 88% decrease in abundance since 1970. Changes in abundance were found to be correlated
with changes in distribution, meaning that those species that declined in
abundance also tended to have reduced distributions. So – some species are doing better than
others. This argues against a simplistic
view of an ‘insect apocalypse’.
However, perhaps predictably, the headline trends are more
complicated than they initially appear.
Returning to the Rothamsted data, a 2019 study [5] showed that a
decline in moth biomass after 1982 was preceded by an increase between
1970 and 1982. What is less certain is
whether this represented a true long-term trend or whether it was the
consequence of particular climatic or ecosystem factors at that time – such as the
long hot summer of 1976.
So, moths are clearly responding to a range of factors. Habitat loss and climate change seem to be the most significant. But these responses are not consistent between species – or indeed localities. A recent study of moths in Finland [1] reached the surprising conclusion that, heading northwards, species richness increased while total abundance declined. Species richness was not specifically assessed in the British and Irish Atlas; however, a number of species (25), including the Black Rustic Aporophyla nigra, increased in distribution but decreased in abundance. This pattern has also been found closer to home, with Butterfly Conservation's The State of Britain's Larger Moths 2021 finding declines in the abundance of many (but not all) species, while the distribution range of many actually increased.
These are difficult trends to interpret. One suggestion for the Finnish result is that
species may be retracting into smaller areas of the landscape – restricted
pockets of favourable habitat, perhaps – causing a decrease in abundance
without registering a reduction in their larger-scale distribution pattern. The key message is that while some moth
species are expanding their ranges in response to climate change this is not
necessarily good news. Although particular
species may be winners or losers, overall moth numbers could indeed be
reducing.
Why are some species doing better than others? One answer suggested by a recent study of
British moths and butterflies [6] is that range expansions are favoured for
those species that can change their phenology (the timing of various life
stages during the year) and voltinism (the number of generations each year). For example, those species with multiple
generations in any one year can make the most of warmer springs or autumns,
thereby increasing their reproductive success with the potential for increases
in abundance and/or distribution. One
such species is the Swallow-tailed Moth Ourapteryx sambucaria, where a
smaller second brood (in September/October)has become more regular since the
1970s in the southern parts of its range.
Species that are both restricted to specialist habitats and that have a
single generation each year (univoltine species) appear to suffer from climatic
change. Put simply, they do not appear
to have the flexibility to survive.
Interestingly, such problems do not seem to affect univoltine habitat
generalists.
What does this all mean? Well, the take-home message is that insect
numbers and distributions are changing.
While a number of moth species are on the increase, others are doing
very badly indeed. Overall numbers may
well be in significant decline – even if total extinction appears to be an
unlikely outcome. It appears that a
range of factors is at work, producing effects that are sometimes complementary
but at other times pull in different ways. This means that studies need to look at a
variety of measures; it cannot be assumed that relying upon one measure (such
as distribution) will necessarily give the whole picture. And, as ever, more work is needed [7]. Keep running those moth traps and submitting
those records!
Mike Hetherington
References
1. Antao, L.H. et al. (2020) ‘Contrasting
latitudinal patterns in diversity and stability in a high-latitude species-rich
moth community.’ Global Ecology and Biogeography.
Link to abstract - https://doi.org/10.1111/geb.13073
2. Conrad, K.F., Warren, M.S., Fox, R., Parsons,
M.S. & Woiwod, I.P. (2006) ‘Rapid declines of common, widespread British
moths provide evidence of an insect biodiversity crisis.’ Biological Conservation, 132, pp. 279–291.
Link to abstract - https://doi.org/10.1016/j.biocon.2006.04.020
3. Fox, R. (2013) ‘The decline of moths in Great
Britain: a review of possible causes.’ Insect
Conservation & Diversity 6, pp. 5-19.
Link to full paper - http://www.highlandbiodiversity.com/userfiles/file/newsletters/Decline-of-moths-in-GB-a-review-of-possible-causes.pdf
4. Hallmann, C. A. et al (2017) ‘More than 75
percent decline over 27 years in total flying insect biomass in protected
areas.’ PLoS One 12, e0185809.
Link to open access paper - https://doi.org/10.1371/journal.pone.0185809
5. McGregor, C.J. et al. (2019) ‘Moth biomass
increases and decreases over 50 years in Britain.’ Nature Ecology and
Evolution, 3 pp. 1645-1649.
Link to abstract - https://www.nature.com/articles/s41559-019-1028-6
6. McGregor, C.J. et al. (2019) ‘Climate-induced
phenology shifts linked to range expansions in species with multiple
reproductive cycles per year.’ Nature Communications (2019) 10:4445 https://doi.org/10.1038/s41467-019-12479-w
Link to open access paper - https://www.nature.com/articles/s41467-019-12479-w.pdf?origin=ppub
7. Montgomery, G.A, et al (2019) ‘Is the insect
apocalypse upon us? How to find out.’ Biological Conservation 241:
108327.
Link to open access paper - https://www.sciencedirect.com/science/article/pii/S0006320719313497
8. Randle, Z. et al. (2020) Atlas of Britain
& Ireland’s Larger Moths. Newbury: Pisces Publications.
Link to purchase - https://www.atroposbooks.co.uk/atlas-of-britain-and-ireland-s-larger-moths
9. Sánchez-Bayo, F. & Wyckhuys, K.A.G. (2019)
‘Worldwide decline of the entomofauna: A review of its drivers.’ Biological
Conservation, 232 pp. 8-27.
Link to full paper - https://farmlandbirds.net/sites/default/files/2019-02/Sanchez-Bayo%20%26%20Wyckhuys%202019_2.pdf
10. Simmons, B. I. et al. (2019) ‘Worldwide insect
declines: An important message, but interpret with caution.’ Ecology & Evolution, 9, pp. 3678–3680.
Link to open access paper - https://doi.org/10.1002/ece3.5153
Link to document - https://butterfly-conservation.org/moths/the-state-of-britains-moths
Link to open access paper - https://www.science.org/doi/pdf/10.1126/sciadv.abi8322
