Climate change triggers global collapse in insect numbers

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By Tim Newbold&Charlie Outhwaite
INSECTS are critical to the future of our planet. They help to keep pest species under control and break down dead material to release nutrients into the soil. Flying insects are also key pollinators of many major food crops, including fruits, spices and — importantly for chocolate lovers — cocoa.

The growing number of reports suggesting insect numbers are in steep decline is, therefore, of, urgent concern. Loss of insect biodiversity could put these vital ecological functions at risk, threatening human livelihoods and food security in the process. Yet across large swathes of the world, there are gaps in our knowledge about the true scale and nature of insect declines.

Habitat loss is known to be a key threat to biodiversity, yet its impact on insects is still under-studied, and assessments of tropical species tend to be very rare.

One study found that forest-dependent orchid bees in Brazil have declined in abundance by around 50% (although it only sampled their numbers at two-time points). Orchid bees, found only in the Americas, are important pollinators of orchid flowers, with some plants being entirely dependent on this insect for their pollination.

Adding to the challenges of deforestation and other, long term habitat changes, is climate change.

This fast-emerging threat to insect biodiversity has already been implicated in  the decline of moths in Costa Rica and bumblebees in Europe and North America. Rising temperatures and increasing frequency of extreme weather events, such as droughts, are just two manifestations known to be having a harmful impact on many insect species.

It is predicted that climate change will have a particularly big impact in the planet’s tropical regions. Temperatures in the tropics are naturally quite stable, so species are not used to coping with the fast changes in temperature we are seeing with climate change. Again, though, our ability to understand how this is affecting tropical insects is hampered by a lack of data for these regions.

Almost all of the available data comes from only a few very well-studied groups of insects — in particular, butterflies, moths and bees — while many other groups receive very little attention. Despite a big increase in studies of insect biodiversity change, there is still much we do not know.

To help address this knowledge gap, our study has assessed three-quarters of a million samples of insects from all over the world. Of the 6 000 sites included, almost one-third is from tropical locations. Our samples of nearly 20 000 different insect species include beetles, bees, wasps, ants, butterflies, moths, flies, bugs, dragonflies and other, less well-known groups.

This was made possible through the use of predicts, a biodiversity database which brings together millions of samples collected by researchers all over the world. Predicts records biodiversity in natural habitats and also in areas used by humans for growing crops, among other purposes. It is one of very few global databases that allow us to study biodiversity changes across the whole world.

While our 20 000 strong sample represents only a fraction of the vast diversity of insect species, it is still a sample from more sites that have ever been studied before. We were particularly interested in using it to understand how habitat loss and climate change play off each other to affect insect biodiversity, and were able to include these interactions in our models for the first time.

These twin conditions are found most profoundly on farmland in tropical countries and our results demonstrate that farmland in these regions has typically lost a lot of insect biodiversity, relative to areas of primary vegetation. This highlights that climate change may present a major threat to food security not only by directly impacting crops, but also through loss of pollinators and other important insects.

As climate change accelerates, the ability to grow cocoa and other crops in their current geographical ranges is already becoming more uncertain, threatening local livelihoods and reducing the availability of these crops for consumers all over the world.

The insect losses our study highlights are only likely to add to this risk. Indeed, threats to food security due to the loss of insect biodiversity are already being seen in both temperate and tropical regions: for example, evidence of reduced yields due to a lack of pollinators has been reported for cherry, apple and blueberry production in the US.

In some parts of the world, farmers are resorting to hand-pollination techniques, where the flowers of crops are pollinated using a brush. Hand pollination is used for cocoa in a number of countries, including Ghana and Indonesia. These techniques can help to maintain or increase yield, but come at a high labour cost.

Our study also highlights changes that could help to reduce insect declines. Lowering the intensity of farming for example, by using fewer chemicals and having a greater diversity of crops —  mitigates some of the negative effects of habitat loss and climate change. In particular, we show that preserving natural habitat within farmed landscapes really helps insects. Where farmland in climate-stressed areas with its natural habitat largely removed shows insect reductions of 63%, on average, this number drops to as little as 7% where three-quarters of the nearby natural habitat has been preserved.

Making a difference

Meanwhile, governments and other public and private organisations should consider more carefully the impact their actions and policies are having on insects. This could range from the proper consideration of biodiversity within trade policies and agreements, to ensuring that products are not sourced from areas associated with high deforestation rates.

Understanding what is causing insect declines is key for preventing even greater losses in the future, and for safeguarding the valuable functions that insects perform. Climate change and biodiversity loss are major global crises that are two sides of the same coin. Their combined effects on food production mean the health, wellbeing and livelihoods of many people in the tropics and beyond are hanging in the balance. Insect biodiversity losses are a crucial, but as yet understudied, part of this story.

  •  Tim Newbold is a senior research fellow with the Centre for Biodiversity and Environment, UCL
  •  Charlie Outhwaite is a post-doctoral researcher in biodiversity change, UCL.