This new study was
funded by the Deutsche
Forschungsgemeinschaft
( DFG ).
An international team
of scientists compile data
from 166 long-term surveys
performed at 1,676 sites
worldwide, between 1925
and 2018, to investigate
trends in insect
abundances
( number of individual
insects, not species ).
This was the largest study
of insect change to date,
and was published
in the journal Science.
The study was led by
researchers from the
German Centre for
Integrative Biodiversity
Research (iDiv),
Leipzig University (UL) and
Martin Luther University
Halle-Wittenberg (MLU).
It fills key knowledge gaps
of the much-discussed
“insect declines“.
In countries where many
insect surveys have taken
place, such as Germany,
the UK, and the US,
some places experienced
declines while others
close by indicated
no changes, or even
increases.
When all of the trends
across the world
were combined,
the researchers found
that for terrestrial insects
(insects that spend
their whole lives
on land, like butterflies,
grasshoppers and ants),
there was an average
decrease of 0.92% per year.
First author Dr Roel van Klink,
a scientist at iDiv and UL, said:
“0.92% may not sound like much, but in fact it means 24% fewer insects in 30 years’ time and 50% fewer over 75 years. Insect declines happen in a quiet way and we don’t take notice from one year to the next. It’s like going back to the place where you grew up. It’s only because you haven’t been there for years that you suddenly realize how much has changed, and all too often not for the better.”
Insect declines
were strongest
in some parts
of the US
(West and Midwest)
and in Europe,
particularly
in Germany.
For Europe in general,
trends became on average
more negative over time,
with the strongest
declines since 2005.
At the same time,
the number of insects
living in freshwater,
such as midges
and mayflies,
has increased
on average
by 1.08%
each year.
This is possibly due
to effective water
protection policies.
Local trends are
highly variable, and areas
that have been less impacted
by humans appear
to have weaker trends.
Last author Jonathan Chase,
professor at iDiv and MLU, said:
“Many insects can fly, and it’s those that get smashed by car windshields. Our analysis shows that flying insects have indeed decreased on average.
However, the majority of insects are less conspicuous and live out of sight – in the soil, in tree canopies or in the water.”
On average, there are fewer
insects living in the grass
and on the ground today
than in the past –
similar to the flying insects.
By contrast, the number
of insects living
in tree canopies has,
on average, remained
largely unchanged.
At the same time,
studies of insects
that live (part of)
their lives under water,
like midges and mayflies,
showed an average
annual increase
of 1.08%.
This corresponds to
a 38% increase
over 30 years.
This positive trend
was particularly strong
in Northern Europe,
in the Western US,
and since the early
1990s, in Russia.
For Jonathan Chase
this is a good sign.
He said:
“Over the past 50 years, several measures have been taken to clean up our polluted rivers and lakes in many places across the world. This may have allowed the recovery of many freshwater insect populations. It makes us hopeful that we can reverse the trend for populations that are currently declining.”
Ann Swengel,
co-author of the study,
has spent the last 34 years
studying butterfly populations
across hundreds of sites
in Wisconsin and nearby
states in the US.
She stresses how complex
the observed abundance
trends are and what
they mean for effective
conservation management:
“We’ve seen so much decline, including on many protected sites. But we’ve also observed some sites where butterflies are continuing to do well. It takes lots of years and lots of data to understand both the failures and the successes, species by species and site by site. A lot is beyond the control of any one person, but the choices we each make in each individual site really do matter.”
Although the scientists
were unable to say
exactly why such trends
– both negative and positive –
emerged, they were able
to point to a few possibilities.
They found
that destruction
of natural habitats –
particularly through
urbanization –
is associated with
the declines of
terrestrial insects.
Other reports,
such as the IPBES§
Global Assessment,
also noted that land-use
change and habitat
destruction are a
main cause of
global biodiversity
change.