When invasive plants move in, where do native pollinators go?

Pollinators are intimately entwined with the plants they pollinate, and invasive species can disrupt their plant communities. While these new species may prove popular with pollinators in the short term, they can reduce the diversity of the animals they support in the longer term. Therefore conservationists should consider the community-wide effects of invasive species when planning the management of habitats, argue Anikó Kovács-Hostyánszkia and colleagues in a paper published in Basic and Applied Ecology.

For botanists, an invasive alien plant is automatically a problem, as it will drive out native plants in a location. In the past, not all ecologists have shared this perspective. While invasive species may drive out other plants, they may prove attractive to pollinators, and new species might even offer a better food source than the native plants for some insects.

The study by Kovács-Hostyánszkia and colleagues combines observations of twelve invasive plant species across Hungary and Romania. The team examined the effect of the plants on pollinators by sampling wild bees, honey bees and hoverflies. The pollinators were sampled before the peak flowering of the invasive species and then again at the peak.

The scientists found that when they were flowering, the invasive plants significantly affected the pollinators. They write: “The combined analyses of the 12 invasive plant species showed in most cases higher abundance, species richness and diversity of pollinators during the flowering of the invasive plant species in the invaded sites. Although the availability of native flowering resources remained lower, the flower boom of the invasive plants attracted some of the pollinators. Such controversial effects of invasive plants before and during flowering have already been suggested by former single-species studies… and got confirmed by our multi-species approach. During their flowering, invasive plants integrate into plant-pollinator communities and are utilized as a resource by many native pollinators…” 

So it might seem that invasive plants are bad news for plants but good news for pollinators. The reality is more complicated.

The invasive plants out-compete the native plants for light, water and nutrients, which harms reproduction. Ultimately, fewer native plants means there’s less variety of plants for pollinators to feed on. This loss of native plants can have a couple of effects on pollinators in the longer term. Wild bees specialising on specific native plants lose out on possible food sources. 

The invasive plants also tend to offer more opportunities for more generalist pollinators, changing the diversity of pollinators in a landscape. Honeybees and Hoverflies did remarkably well out of plant invasions. The flowering can also increase the opportunities for pollinators in their flowering period. But these interactions are complicated and require looking at more than one species at a time – which the authors say is essential.

Kovács-Hostyánszkia and colleagues argue that their study is important as a lot of knowledge is based on single species studies. In their article, they write: “Our results suggest both threats and benefits of invasive plants to pollinator communities, highlights the importance of multi-species studies and the inclusion of certain life traits of the studied invasive species. Although we found that some invasive plant species can be beneficial for some pollinators during their flowering, this is usually only a shorter period than the whole vegetation season and could not compensate for losses in native floral resources over the year….”

READ THE ARTICLE

Kovács-Hostyánszki, A., Szigeti, V., Miholcsa, Z., Sándor, D., Soltész, Z., Török, E. and Fenesi, A. (2022) “Threats and benefits of invasive alien plant species on pollinators,” Basic and Applied Ecology. https://doi.org/10.1016/j.baae.2022.07.003