How diverse do you need biodiversity to be?

Intensification of agriculture has led to a decline in pollinators. One reason is the loss of key species to support bees and flies, but how diverse a mix of plants to you need to support an ecosystem?

In the past century, agriculture has intensified hugely. Along with this we seem to be seeing a drop in the number of insects in the countryside. Even if you’re driven by purely selfish motives, this is a problem as it means trouble for pollination. One solution is to add wildflowers around the margins, but which wildflowers are most effective? Daniela Warzecha and her colleagues have been researching which flowers attract pollinators in Giessen, Germany.

For the study, investigators tested four recommended seed mixtures for their attractiveness to wild bees and hoverflies. They kept test plots for two years to see what they could attract. Of 94 plant species in the seed mixtures, 14 key plant species were crucial for the whole flower-visiting bee and hoverfly community, with the four top plants supporting 80% of flower visitors. This seems to suggest that you could get by with relatively few species, but the results are a bit more complicated than that.

Chenopodium album
Chenopodium album. Often considered a weed, but also an important plant for pollinator communities. Photo: Andreas Rockstein / Flickr.

While some plants supported a wide range of pollinators, Warzecha et al. found that different mixes were more attractive to different pollinators. Another factor is that, usually, plant diversity correlates with pollinator diversity. The authors noted that the mixtures with both the highest and lowest number of plants species were successful. They put this down to the mixtures having the right key species in the mix. Diversity alone would therefore not be enough in a seed mix.

They spend some time discussing this importance of seed mixture. The authors note that a selection of efficient key plant species, targeted at different pollinator groups throughout the flowering season, seems a promising tool for future development. However they also add that any pollination scheme still needs to balance for other species, so that you’re not replacing a disaster area for one species with a similar problem for another.

“Since wildflowers typical to agricultural habitats were among the most effective plant species, our study shows that the protection of pollinators will be most successful when Agri-environmental schemes are directed at advancing the sustainable use of arable landscapes,” said Daniela Warzecha, lead author of the study.

While this will help more generalist pollinators, the paper also mentions that the plots were not successful in attracting rare pollinator species. This would suggest that in some locations, where conservation is possible then early action to provide specialist plants would be a good idea. When these rare pollinators are lost, it’s not simply a matter of restoring the habitat. Other factors like habitat fragmentation and local extinction should also be taken into account.

Source: AlphaGalileo

Reference List

Warzecha, D., Diekötter, T., Wolters, V., & Jauker, F. (2017). Attractiveness of wildflower mixtures for wild bees and hoverflies depends on some key plant species. Insect Conservation and Diversity. https://doi.org/10.1111/icad.12264