Insects are declining – but talk of “insectageddon” is premature

I originally posted this blog on the Nature Ecology & Evolution community forum; you can read the original version here.

Recent research showing severe declines in biomass of flying insects has been much-discussed in the literature and the global media. In a new study of a long-term insect population dataset, we found that the biomass of moths increased before it declined, and remains higher now than in the 1960s.

Insects play a number of vital roles in our ecosystems, and as a consequence, several studies in recent years that have reported sharp declines in their “biomass” (combined weight) have been greeted with alarm by the media and public.

However, the conclusions of these papers have been met with scepticism by the scientific community. There is a widely-held opinion that studies on insect biomass have so far been based on too little data to be certain of their conclusions, variously having too few separate sampling sites, data from too short a time period, or from only the beginning and end of a sequence (rather than continuous data), or data collected with a non-standardized method over time.

To try and understand patterns in insect biomass change over time in a more robust way, overcoming some of these criticisms, my colleagues and I turned to the long-running network of moth-traps of the Rothamsted Insect Survey (RIS). Rothamsted currently operate around 80 identical moth-traps across the UK and Ireland, each of which collects moths (which are then identified and counted) on a nightly basis using a standardized methodology. From among these, we identified 34 traps that had operated continuously for at least 30 years in the 51-year period since the trap network commenced in 1967.

Peppered Moth (Biston betularia) has declined in abundance by 81% since 1967, according to the Rothamsted Insect Survey’s data.

To translate the abundance records collected by the RIS into estimates of biomass, we first needed to know how much each moth weighed. Rothamsted don’t retain every moth they capture, and catching and weighing a representative sample of every species in their database would be prohibitively labour-intensive. Existing dry body mass data weren’t available for most British moth species either, but field guides do list typical forewing lengths for every species as an identification tool, so we resolved to model the relationship between these two variables.

Thanks to the hard work of summer project student Becci Kinsella, we collected an empirical dataset on the forewing length and body mass of 600 individual moths from 94 species and, fitting a model to this data, estimated the body mass of over 1000 species of larger moths. Amazingly, we found this allowed us to predict around 90% of the total variation in the biomass of mixed-species samples of moths.

Next, Masters student Jonny Williams applied these estimates to data from the 34 long-running traps from the RIS, generating annual estimates of moth biomass from each trap. The results were astonishing. We found that the average biomass of moths sampled annually by each trap had approximately doubled over the 50-year duration of our dataset. This was not a simple increasing trend, either: biomass had increased steeply between 1967 and 1982, but gradually declined thereafter. The 10% per decade rate of these more recent declines actually matches up well to the findings of other recent studies of insect abundance and biomass (none of which commenced earlier than 1976), but the overall longer-term pattern of steep increase, then gradual decline does not support the widely-suggested scenario of “insect Armageddon”.

Abundance of the Large Emerald (Geometra papilionaria) reached a peak in 1983, matching the trend for moth biomass as a whole.

To understand drivers of these recent declines, we turned our attention to land-use, categorising sites into four groups: arable, grassland, woodland and urban. Two drivers of environmental change that are often mooted as potential causes of insect decline, especially for moths, are agricultural intensification (most relevant in arable sites) and light pollution (most relevant in urban sites); but we found that the steepest post-1982 biomass declines were in grassland and woodland.

The third of the “usual suspects” is climate change. A strong degree of synchronisation in both year-to-year biomass change and longer-term trends between sites, land-use types, and taxonomic groups of moths, pointed the finger of blame squarely towards factors that operate across all land-use types, such as temperature and rainfall. Yet surprisingly, we found no relationship between either of these variables and biomass change. However, two of the biggest periods of biomass change – a strong increase in the late 1970s and a decline in the late 1990s – directly followed the heatwave/drought years of 1976 and 1996. It seems possible, therefore, that extreme climatic events can perturb populations and communities, with resultant ecosystem feedbacks causing subsequent biomass change – an avenue for further research, perhaps.

Finally, we resampled the full RIS dataset to investigate the influence of data structure on estimates of insect biomass change, taking every possible subset of data of over 5 years’ duration, both for single study sites and the full 34 sites combined. It’s well-known that insect populations can fluctuate wildly from year to year, so unsurprisingly, longer spans of data were less likely to estimate massive increases or declines. Likewise, biomass changes were larger at single study sites than across the full dataset of 34 sites. Many studies rely on single return visits to previously-sampled locations, generating an estimate of biomass change between two points in time. Unfortunately, when compared to the trend fitted by a linear model through annual estimates over the same time periods, we found the two-sample approach incorrectly estimates the direction of biomass change in a quarter of cases. Lastly, it’s unwise to hang your hat on biomass change since an arbitrary start date, since patterns change over time: within our study, biomass increased since 1967, decreased since 1982, but has been roughly stable since 1997.

These findings emphasize the vital importance of long-term, standardized data collection for understanding population change. Several UK initiatives, like the RIS and the UK Butterfly Monitoring Scheme, have been doing this for decades on home soil, but globally very few such datasets exist, particularly in the tropics. Establishing long-term monitoring at a much broader scale is a challenging prospect, but a worthy goal.

Three steps to save Britain’s butterflies

British populations of butterflies, including some of the most familiar countryside species, will begin disappearing within decades unless we take action. This is the alarming conclusion of new research published in Nature Climate Change by a group of British scientists.

Butterflies are naturally sun-loving creatures, and with the UK sat on the northern edge of many species’ ranges, previous studies have forecast possible benefits to UK populations from a warming climate. However, as the climate changes, extreme weather events including droughts are expected to become more common. Droughts can be a problem for butterflies, especially if they harm the plants upon which caterpillars rely for food. With less food around, populations can crash, and may take several years to recover to pre-drought levels.

The new study used models to predict the frequency of droughts like that of 1995 under different scenarios of greenhouse gas emissions, and examined factors affecting the likelihood and speed of recovery for populations of six species of butterflies that experienced population collapses after the 1995 drought.

While droughts as severe as 1995 have previously only occurred as little as once in 200 years, allowing plenty of time for butterfly populations to recover, the study found that they may become far more frequent. If greenhouse gas emissions continue to increase at current rates, they might even occur on average once every 1.29 years (effectively every summer).

The red admiral is one of the UK’s most common butterflies.
Kenneth Dwain Harrelson, CC BY-SA

Under “business as usual” scenarios, the research forecasts the widespread extinction of local colonies of butterflies as soon as 2050. So, what can be done to conserve our butterflies? Here is my simple, three-step guide:

Step 1: stop global warming in its tracks

Butterflies don’t have to be colourful.
Soebe, CC BY

Clearly, reducing the impacts of climate change will be important. Delegates from around the globe will meet in Paris later this year for the 2015 UN Climate Change Conference, hoping to reach the first deal on reducing emissions since Kyoto 1992. Under the study’s best case scenario for emissions, 1995-like droughts might occur only every six to seven years, giving butterfly populations much more opportunity to recover in between.

Step 2: protect butterfly habitats

Ensuring the availability of suitable habitats for butterflies can also make a big contribution. The researchers found butterfly populations were more likely to persist through droughts and recovered more rapidly if situated in areas with larger, less fragmented patches of semi-natural habitat, such as grassland. Larger areas are likely to contain more abundant and diverse food-plants, helping more species of butterfly, and can also better resist edge effects associated with drought, such as moisture loss from woodland.

Highly fragmented habitats have more edge relative to their area, and therefore experience more severe edge effects. Well connected habitats, through which butterflies can easily mingle and locate breeding sites, could add decades on to the survival of certain populations as the climate warms.

Buddleia, also known as the butterfly bush, is one of the UK’s best plants for encouraging butterflies.
Andy Fogg, CC BY

Step 3: create more butterfly-friendly gardens

While large-scale habitat management programmes, such as the establishment of nature reserves, are an important means to preserve semi-natural habitat, the restoration of connectivity is where butterfly enthusiasts can help at home.

According to Richard Fox from the charity Butterfly Conservation, many drought-prone species can be encouraged to breed in gardens by leaving grass to grow long. “You don’t have to let your prize lawn go to rack and ruin, you can just leave a strip along the fence”, Fox told me. Depending on how much is left, this could provide breeding habitat for species including the speckled wood, ringlet, meadow brown and large skipper.

A female speckled wood butterfly
Charles J Sharp, CC BY

Meanwhile, other species can be helped by choosing garden flowers with care, or letting them choose themselves. “Large and small white will breed on Nasturtiums and love to nectar on flowers like buddleia and perennial wallflower,” advises Fox, while “green-veined white caterpillars can feed on lots of weeds, so not being too tidy can help”. If you have a garden, why not plant some butterfly-friendly plants of your own?

So while butterfly lovers will be among those waiting with bated breath for the outcome of the Paris summit, they may also be able to help closer to home. Habitat availability will be vital to the survival of butterflies when drought strikes, and by providing such refuges in back gardens anybody can help them survive and flourish.

The Conversation

Callum Macgregor, PhD student in Ecology, University of Hull

This article was originally published on The Conversation. Read the original article.