Warming temperatures at night combined with habitat destruction are the cause of large population declines in wild native bee populations.
The world’s insect population is collapsing, with more than 40% of all insect species facing extinction (ref). The most affected insect groups include our old friends, the butterflies and moths (Lepidoptera), bees and wasps (Hymenoptera) and dung beetles (Coleoptera). This impending collapse poses grave threats to global food security, to the proper functioning of global ecosystems and to global biodiversity, and is particularly severe for solitary bee species, which are especially sensitive indicators of ecosystem health.
A recently published study reports that the complex interplay between climate change and human land use – agriculture and urbanisation – is the main problem underlying the rapid decline of insect populations. Already, scientists report that intensive agriculture is the main driver of population declines in unrelated animals such as birds and insectivorous mammals. But how do land use changes and climate change interact with each other to impact insect communities in different habitats?
To better answer this question, ecologist Cristine Ganuza, a postdoc at Julius-Maximilians-Universität Würzburg, and a large team of scientists from the University of Bayreuth, the Technical University of Munich, and Weihenstephan-Triesdorf University of Applied Sciences, collaborated to investigate how the effects of warmer temperatures act in concert with intensified land use practices.
Using several field survey methods, the researchers examined 179 plots in 60 study regions across Germany. These plots comprised one of four habitat types: forest, grassland, arable land and human settlements (Figure 2, also see Figure 3). As they predicted, Dr Ganuza and collaborators found that pollinator diversity increased as human land-use intensity declined (also ref). The researchers also found that bee populations living in forest environments were extraordinarily resilient to the stresses of warmer days; surprisingly, increased daytime temperatures in these natural landscapes even correlated with higher bee abundance and diversity.
Unfortunately, this beneficial climate warming effect was not seen for bees living in urban settings, where their numbers plunged by 65%. This finding highlights the detrimental effects of excessive heat combined with habitat destruction. But what specifically was harming the insects?
Although bees are active during the day when it’s generally warmer, Dr Ganuza and collaborators found that warmer nighttime temperatures, particularly when combined with urbanization, were harmful, creating “climate traps” where bees and other insects struggle to survive despite environmental protections. Further, warmer nighttime temperatures consistently diminished both the richness and abundance of bees across all habitat types investigated. This is especially worrying because global climate change is increasing nighttime temperatures faster than daytime temperatures.
“The fact that night-time temperatures have such an impact on diurnal insects is significant. Precisely because average night-time temperatures rise even faster than daytime temperatures,” Dr Ganuza said.
Do hot nights affect bees in the same way they affect humans? Are they, like humans, unable to sleep on a hot night, making them unable to function at their best during the day? Or are they dying during the night because of the heat?
“We do not find it likely that they die from excessive heat during the night,” Dr Ganuza replied in email. “However, the problem is worse for bees than for humans. This is because bees are ectothermic organisms, which means that they cannot regulate their body temperatures as we humans do.”
Increased body temperatures affects insect physiology
“Bee metabolism is determined by external temperature conditions,” Dr Ganuza explained in email. “Therefore, warm nights might keep bees metabolically active when they should be resting. This could stop their bodies from recovering properly, and a high metabolism can also consume fat (i.e. ‘energy’) reserves.”
Dr Ganuza emphasized the importance of this insight because it helps scientists understand how physiological stresses during rest periods may undermine daytime activity and survival.
“While this is just a hypothesis, we do know that flying takes a lot of energy for bees,” Dr Ganuza continued. “If they are tired and weak after a hot night, they might have trouble foraging for food and could be more likely to get eaten by birds or other animals.”
In short, understanding how these climate change and land use changes operate in concert will be pivotal for developing conservation strategies that buffer insect populations against rapid climatic changes.
Different insect species are affected differently by warming temperatures and by agriculture
Dr Ganuza and collaborators found that the combined stresses of climate warming and land use intensity did vary across insect trophic levels. On one hand, insects higher in the food chain – predatory insects – showed a greater tolerance to elevated temperatures but, on the other hand, they were harmed by simplified landscapes that are devoid of natural vegetation – agricultural fields.
This study’s findings have important implications for agriculture and land management practices because many predatory insects play a critical role in natural pest control. This damaging effect can be somewhat mitigated by maintaining a mosaic of farmland interspersed with natural habitats that are suitable for supporting insects that are crucial to sustainable food production. Additionally, the study’s findings advocate strongly for the protection and restoration of heterogeneous landscapes that maintain connectivity between forest and grasslands, thereby allowing native insects to move around between these natural habitats. Therefore, the preservation and creation of interconnected natural habitats within agricultural and urban areas is of great importance.
Such measures are critically important because pollinators and predators respond differently to environmental stresses and to increasing nighttime temperatures, and these differences may upset the balance between these groups of insects that are crucial for proper ecosystem function. Such an imbalance could end up threatening vital ecosystem functions such as pollination and natural pest regulation, which are the foundation for both biodiversity and human agriculture.
Probably the most worrying finding is the implication that even subtle climatic shifts can amplify existing human pressures on ecosystems in complex and unexpected ways. For example, as environmental temperatures continue their upward trajectory, understanding the intricacies of insect responses – especially among critical pollinators like bees – will be essential for safeguarding global food security and, indeed, for protecting life on Earth.
Dr Ganuza and collaborators meticulously documented how the interaction between climate warming and land use forms a complex web of threats that cannot be addressed in isolation. Dr Ganuza and collaborators’ meticulous study calls for urgent, nuanced responses to insect declines that take into consideration the synergistic effects of climate warming and land-use change, to prevent more irreversible losses to biodiversity and ecosystem services.
Sources:
Cristina Ganuza, Sarah Redlich, Sandra Rojas-Botero, Cynthia Tobisch, Jie Zhang, Caryl Benjamin, Jana Englmeier, Jörg Ewald Ute Fricke, Maria Haensel, Johannes Kollmann, Rebekka Riebl, Susanne Schiele, Johannes Uhler, Lars Uphus, Jörg Müller and Ingolf Steffan-Dewenter (2025). Warmer temperatures reinforce negative land-use impacts on bees, but not on higher insect trophic levels, Proceedings of the Royal Society B: Biological Sciences 292(2046):20243053 | doi:10.1098/rspb.2024.3053
Cristina Ganuza, Sarah Redlich, Johannes Uhler, Cynthia Tobisch, Sandra Rojas-Botero, Marcell K. Peters, Jie Zhang, Caryl S. Benjamin, Jana Englmeier, Jörg Ewald, Ute Fricke, Maria Haensel, Johannes Kollmann, Rebekka Riebl, Lars Uphus, Jörg Müller, and Ingolf Steffan-Dewenter (2022). Interactive effects of climate and land use on pollinator diversity differ among taxa and scales, Science Advances 8:18 | doi:10.1126/sciadv.abm9359
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