Global insect decline: What are the causes and consequences?

The global decline of insect populations has become increasingly evident in recent years, with not just a decrease in individual insect numbers but also a collapse in insect diversity. A special issue of the journal Biology Letters has been published to provide a comprehensive understanding of the causes and consequences of this alarming trend.

The special issue was edited by Dr. Florian Menzel from the Institute of Organismic and Molecular Evolution at Johannes Gutenberg University Mainz (JGU), Professor Martin Gossner of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), and Dr. Nadja Simons of TU Darmstadt.

The experts concluded that the primary drivers of the worldwide insect decline are land-use intensification, climate change, and the spread of invasive animal species due to human trade. 

“Our aim was not to document insect population declines but to better understand their causes and consequences,” said Dr. Menzel. The researchers also aimed to prompt new studies on the subject by collaborating with international researchers to gather information on insect declines.

The results suggest that not only do land-use intensification, global warming, and the dispersal of invasive species contribute individually to the disappearance of insects, but they also interact with one another. 

“Ecosystems deteriorated by humans are more susceptible to climate change and so are their insect communities,” explained Dr. Menzel.

Additionally, invasive species can more easily establish themselves in habitats damaged by human activity, displacing native species. As a result, while many insect species decline or become extinct, a few others, including invasive species, thrive and increase. This decreases diversity among insect communities across habitats. 

Specialized insect species suffer the most from these changes, while more generalized species tend to survive. “This is why we are now finding more insects capable of living nearly anywhere while those species that need specific habitats are on the wane,” said Dr. Menzel.

The consequences of this development are far-reaching and generally detrimental for ecosystems. A decline in insect diversity threatens the stability of ecosystems, as fewer species are available to pollinate plants and control pests. This, in turn, means there is less food available for insect-eating birds and other animals, placing their continued existence at risk due to the decline in insect numbers.

To address this issue, Menzel, Gossner, and Simons propose standardized techniques to monitor insect diversity across multiple habitats and countries, particularly in regions where little is known about insect populations. 

The experts also advocate for the creation of a network of interconnected nature reserves to allow species to move from one habitat to another. This would enable less heat-tolerant insects to migrate from areas affected by global warming to higher elevations or cooler regions in the north.

Moreover, the researchers emphasize the importance of implementing measures to reduce the dispersal of invasive animal and plant species through globalized trade and tourism. 

“This is another problem that has become extremely serious in the last decades,” concluded Dr. Menzel. The special issue highlights the invasion of non-native insectivorous fishes in Brazil as an example, which has led to a significant decline in freshwater insects.

Which insects are disappearing?

As of September 2021, the most rapidly declining insects include bees, butterflies, and moths, largely due to habitat loss, pesticide exposure, and climate change.

Bees

Bees, particularly honeybees and bumblebees, are facing significant declines across the globe. These pollinators are essential for many ecosystems and for agricultural production. Reasons for their decline include loss of flower-rich habitats, exposure to harmful pesticides (neonicotinoids, in particular), diseases, parasites, and climate change.

Butterflies and Moths

These are also experiencing rapid declines, with many species becoming increasingly rare. Habitat loss, intensive agriculture, pesticides, light pollution, and climate change all contribute to their dwindling numbers.

Beetles

Some species of beetles, such as certain types of dung beetles, are also facing significant declines. These insects play a crucial role in decomposition and nutrient recycling, and their loss can have profound effects on ecosystem functioning.

Aquatic insects

Freshwater ecosystems are under great pressure from pollution, habitat destruction, invasive species, and climate change. This has led to declines in many species of aquatic insects, such as dragonflies and mayflies.

Insectivorous insects

Certain species that rely on other insects for food, like certain types of ants and ladybugs, are also seeing their numbers decline as their food sources become less abundant.

It’s important to note that while some insect species are declining, others may be increasing, potentially leading to a shift in community composition and ecosystem function. Also, data on insect declines is much stronger for some regions (like Europe and North America) compared to others, so global patterns may vary.

Climate change and global insect decline

Climate change affects insects in several ways, primarily through alterations in temperature, precipitation patterns, and the timing of seasonal events. Here are some key impacts:

Temperature Changes

Many insects are ectotherms, meaning their body temperature and metabolic rates are directly influenced by the environment. Warmer temperatures can accelerate their life cycles, but extreme heat can also be lethal. Conversely, some insects may expand their range into previously cooler regions as those areas warm up.

Altered Seasonal Timing

Many insects synchronize their life cycles with specific seasonal events, like the blooming of certain plants or the availability of prey. Changes in the timing of these events due to climate change, a phenomenon known as phenological mismatch, can have detrimental effects. For example, if insects hatch after their primary food sources are available, it can lead to high mortality rates.

Habitat Loss

Climate change can lead to habitat loss for certain insects. For example, insects adapted to cold mountain environments may “run out of mountain” as their habitable zones shift upward with warming temperatures. Similarly, insects living in coastal areas may lose habitat due to sea-level rise.

Drought and Precipitation Changes

Changes in rainfall patterns can affect insect populations. Drought can reduce food availability and breeding sites, while increased rainfall can drown insects or lead to disease outbreaks.

Changes in Plant Communities

Climate change can lead to shifts in plant communities, which can affect plant-dependent insects. For example, changes in plant nutritional quality due to elevated CO2 levels can impact herbivorous insects.

Increased Extreme Events

More frequent and intense extreme weather events, such as storms, floods, and heatwaves, can directly kill insects or destroy their habitats.

Interactions with Other Stressors

Climate change can exacerbate the impacts of other stressors like habitat loss, pollution, and invasive species, leading to additive or multiplicative effects on insect populations.

The specific impacts of climate change on insects can vary widely depending on the species and its particular ecology, life history, and geographical location.

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