The question of whether bumblebees are invasive requires a nuanced answer. Most bumblebees encountered by the public are native species established in their local ecosystems and are currently facing population declines. However, specific bumblebee species, typically those commercially transported for agricultural pollination outside their natural geographic range, have escaped and become invasive in new territories, causing significant ecological harm.
Understanding the Criteria for Invasive Species
A species is classified as non-native, or introduced, when human activity translocates it outside its historical range. Not all non-native species are harmful; some integrate into a new environment with neutral or positive effects, sometimes becoming “naturalized.” The distinction between a non-native and an invasive species is defined by the negative consequences it imposes on the new ecosystem.
To be labeled invasive, a non-native species must cause significant environmental, economic, or human health harm in its new location, typically manifesting as the displacement or reduction of native species through competition or predation. Invasive species often succeed because they lack the natural predators, competitors, or diseases that regulated their populations in their native range. Rapid reproduction and generalist survival abilities also contribute to an organism’s invasive potential.
The Status of Native Bumblebee Populations
Bumblebees are experiencing widespread population declines due to multiple stressors. The United States, for example, is home to 49 native bumblebee species, many of which show severe reductions in geographic range and overall numbers. These species are integral to local biodiversity, having co-evolved with native plants and other wildlife.
Threats to these populations include habitat loss from agricultural intensification and urbanization, and exposure to pesticides. Climate change is also having a direct effect, with rising temperatures potentially contributing to a 50% drop in some North American bumblebee numbers since 1974. Furthermore, the spread of pathogens, or “disease spillover,” from commercially managed bees poses a risk to wild native bumblebees. This combination of factors places many native species, such as the Rusty-patched bumblebee (Bombus affinis) and the American bumblebee (Bombus pensylvanicus), on threatened or endangered lists.
Case Studies of Non-Native and Invasive Bumblebees
The buff-tailed bumblebee, Bombus terrestris, provides a clear example of an invasive species outside its native temperate Eurasian range. This species is commercially mass-reared for greenhouse pollination, particularly for crops like tomatoes, facilitating its global introduction. When these managed bees escape, they often establish wild populations that meet the criteria for invasiveness.
In South America, the introduction of B. terrestris into Chile in 1998 resulted in a rapid invasion that spread across the continent, reaching Patagonia and the Atlantic coast of Argentina. This invasive species is directly linked to the disappearance of the native giant bumblebee, Bombus dahlbomii. B. terrestris meets the harm criteria by outcompeting native bees for floral resources and nesting sites, and by having superior reproductive ability.
Beyond direct competition, a significant mechanism of harm is the transmission of parasites and pathogens, such as the microsporidian Nosema bombi, from commercially bred bees to native wild populations. In regions like Japan and Tasmania, researchers have documented its potential to displace native bumblebees and reduce the reproductive capacity of native plants.
The Ecological Role of Bumblebees
All bumblebees play an important role as pollinators. Their dense, fuzzy bodies make them highly effective at collecting and transferring pollen between flowers. Bumblebees are generalist foragers, meaning they visit a wide range of plant species, which benefits diverse ecosystems and supports various crops.
A unique trait of bumblebees is their ability to perform sonication, or “buzz pollination.” By vibrating their flight muscles while grasping a flower’s anthers, they dislodge trapped pollen. This behavior is required for effective pollination of several economically significant crops, including tomatoes, peppers, and cranberries. Their ability to fly and forage in cooler temperatures and lower light levels than many other bees, including the European honey bee, underscores their importance to agriculture and wild plant communities.