Hysteria over disappearing bees is widespread: declines in domesticated and wild insect pollinator populations have been documented worldwide. Insect pollinators, particularly bees, are responsible for the vast majority of plant pollination services in wild landscapes and agricultural systems. Without effective and sufficient pollination, the persistence of native plants and various fruit and vegetable crops is in jeopardy. The suspected drivers of pollinator decline include habitat loss, habitat fragmentation, agrochemicals (e.g. pesticides and fertilizers), invasive species, emerging pathogens, and climate change. Herein, the threats posed to insect pollinators by invasive plants are explored.
Invasive species are those transported to novel habitats, often across great distances, which demonstrate negative impacts on recipient communities. Invasive plants often dominate vast contiguous areas of land, in effect generating a form of habitat loss or fragmentation. This feature of invasive plant ‘behavior’ has severe consequences for native insect pollinators and their access to food. Bees and other insect pollinators feed on nectar and pollen harvested from flowers. Invasive plants with large, showy and rewarding flowers are often very attractive to bees – they tend to draw in more species, and more individual pollinators than neighboring native plant species. At first glance, it might appear that invasive plants are in some way “better” than their native competitors – the invasive plants are associated with greater insect diversity and abundance. However, closer inspection reveals quite the opposite story. Invasive plants tend to reduce the number of pollinator species and the abundance of pollinators in a landscape, and they also reduce the number of pollinator visits that native plants receive.
How and why does this happen? What is it about invasive plant that is so destructive to native insect pollinator communities? Scientists are still figuring this out, and there is a lot of work left to be done, but here’s a bit of what we know so far:
Invasive plants are visited a lot by native pollinators, especially by native generalist pollinators who visit many species and types of flowers. This weakens the relationships between generalist pollinators and native plants. When invasive plants are present, neighboring native plants generally receive fewer pollinator visits, and are thus less likely to be pollinated! In addition, the pollen transported by visiting pollinators is overwhelmingly that of the invasive plant, native plants can’t use this to create their own seeds, and it may even be harmful to early seed production. This double-whammy leaves native plants with reduced reproductive success: they produce fewer fruits and fewer seeds. These circumstances present a dismal prognosis for the native plant community: the few flowers present produce few seeds, so fewer native plants grow the following year. As this continues season after season, the native plants might disappear altogether, becoming locally extinct. Clearly, this is a threat to the native plant community; it also has cascading consequences for entire communities of native pollinators. Along with the generalist pollinators already described, many types of wild bees and other insects are specialist pollinators that rely on specific native plant species for their survival. When those native plant species are difficult to find, or even non-existent, specialist pollinators are faced with starvation.
The scenario described above is bleak. Likewise, there are a lot more ways that ecologists know invasive plants could be harming native pollinator communities, they just haven’t been investigated thoroughly yet. Ecologists around the world are working to understand how invasive plants affect pollinators, and how to mitigate negative impacts to support biodiversity conservation, agricultural productivity and ecosystem stability. This research inspires hope for the future of bees and other insect pollinators, as well as all our favorite insect-pollinated foods.
A List of Some Important References
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