Xylocopa (Koptortosoma) sp. female Kit PrendergastPollinators – including bees, flies, wasps, moths, butterflies and some nectar-loving birds – are a cornerstone of our natural environment. By helping plants reproduce, they keep our ecosystems healthy and ensure we can grow vital food crops.But climate change threatens the very survival of these hard-working animals.Previous research has focused on how temperature changes affect bees, finding that certain groups are more vulnerable to hotter temperatures.But our new study shows rising carbon dioxide (CO₂) levels may also be putting pollinators – such as bees and hoverflies – at risk. Larger bees are the most vulnerable. We found populations of big bees – including Bombus asiaticus and Xylocopa pubescens – were smaller and less genetically diverse in areas with high CO₂. Small-bodied pollinators, however, may actually do better in higher-CO₂ environments. A dire situationIn Australia and around the world, we are facing a pollinator crisis. There are several reasons for this, including the loss and degradation of wild pollinator habitat, the introduction of the European honey bee and other invasive species, and the use of broad-spectrum pesticides.But climate change – driven by human-made greenhouse gases including CO₂ emissions – is another key factor. Australia contributes to higher CO₂ levels as the second-largest fossil fuel exporter in the world.Elevated CO₂ levels can affect pollinators by reducing how much protein is in flower pollen. Having more CO₂ in our atmosphere may also change the chemical make-up of nectar, for example reducing how much sugar it contains. Both factors influence how pollinators develop and survive.Other research suggests higher CO₂ concentrations could impact how pollinators’ bodies function, for example by accelerating how quickly they break down fats. Read more: Native pollinators need more support than honeybees in Australia – here’s why What we studiedIn our latest study, we examined how bees and hoverflies coped with different levels of CO₂ across 25 sites. This is the first time researchers have investigated how natural changes in CO₂ levels affect pollinators. While our study was conducted in Pakistan, it is relevant to pollinator networks in Australia and around the world, given that CO₂ levels are rising globally. Importantly, we controlled for other factors that may affect the number and distribution of bees and hoverflies, including altitude, temperature, humidity and rainfall.Our results suggest pollinators respond to variations in CO₂ in different ways. Specifically, smaller pollinators may actually do better in higher-CO₂ environments. In contrast, larger-bodied pollinators were less abundant in areas with more CO₂.We identified Xylocopa and Amegilla bees, two genera found in Australia, as being particularly vulnerable to increased CO₂ levels. Ceratina and Lasioglossum bees are two examples of smaller-bodied Australian genera that did well in higher-CO₂ environments. The pubescent carpenter bee is a larger-bodied species that is vulnerable to rising CO₂ levels. Waseem Akram Why big bees matterThese findings are concerning for several reasons.Existing evidence shows larger-bodied bees such as Bombus are already more vulnerable to climate change. This is because they tend to retain more heat and don’t cope as well in dry conditions. Big bees also have higher metabolic demands than smaller pollinators, meaning they need more resources to keep their bodies functioning.Large bees are among our most effective pollinators. They typically carry and deposit more pollen than smaller-bodied pollinator species. They also fly longer distances, meaning they can transport seeds and pollen to help plants reproduce and spread their genes to new places. Other research suggests flowers have even evolved to match pollinators with particular body sizes. We see this in Australia with Melastoma. This plant’s impressive flowers are most effectively pollinated by large pollinators such as Xylocopa bees, which use vibrations to release pollen from flowers. Read more: How will Australian native bees cope with climate change? Their nests hold a clue So, what can we do?Here are four practical ways we can help our larger pollinators thrive in a warming world. protect their habitat by preventing further land clearing, for example to make room for more livestock farmsensure pollinators have access to wildlife corridors to help them move to areas that are naturally lower in CO₂, such as dense forestsplant more bee-friendly trees, with some Australian examples being Eucalyptus, Corymbia, Angophora, Melaleuca, Banksia and Brachychitonmaintain populations of larger-bodied bees by reducing other threats such as competition from introduced honey bees, to ensure they have the genetic diversity to adapt to rising CO₂ levels.To protect our pollinators in a rapidly changing climate, we must act now. Ensuring their habitat remains intact and curbing our greenhouse gas emissions are vital first steps. Read more: Australia’s native bees struggled after the Black Summer fires – but a world-first solution brought them buzzing back Kit Prendergast is affiliated with the University of Southern Queensland. She is the Vice-Chair of the International Union for Conservation of Nature's Wild Bee Specialist Group, and a member of the Australian Entomological Society.