When Birches Become Wasps

Wasps are highly visible, with their black and yellow stripes serving as a strong warning that eating them is a bad idea. In a new article in Plant Ecology and Diversity, Hamish Ireland and Graeme Ruxton argue that many birch species’ white bark and dark branches warn herbivores against bark stripping in a similar way.

Birch doesn’t have a sting. Instead, it has Betulin. Betulin is a chemical plant defence that inhibits microbial action and can disrupt a herbivore’s ability to digest material. Birches load their bark with Betulin to such an extent that it gets a distinctive white colour. The white bark probably has many advantages but one, Ireland and Ruxton argue, is that it can act as a signal to warn away herbivores.

Ireland and Ruxton list features a signal needs to work as a warning. It needs to be detectable, obviously, and discriminable so that a potential attacker knows to avoid this target but not that one. It needs to be memorable, so animals can learn from their experience and have redundancy, so you’re not relying on just one feature. The botanists give an example: Birch is white over all its vertical height, not just at nibble height. It also has degeneracy. A degenerate signal is a signal that looks different but carries a similar meaning. In this case, the branches and leaves of a Birch. Finally, they argue the signal deters multiple species of bark-strippers, it is pluripotent.

The pair tested their idea by surveying two woodlands in East Lothian, Scotland. They wanted to know if deer avoided white birch compared to adjacent tree species. They also wanted to see if deer preferred juvenile brown birch bark over mature white birch bark. Additionally, they examined published literature to search for evidence that, across the Northern Hemisphere, white birch bark is avoided by known bark-stripping mammals. The results supported their hypothesis.

“In our assessment, roe deer bark-stripping of white birch was rare. In these woodlands, deer prefer to strip relatively small stems (<5cm dbh [diameter at breast height]) including brown-barked silver birch. Silver birch bark starts to transition to white at about 5cm diameter and stripping was less on transitional coloured trees and then rare on white stems. When species were compared in the greater than 5cm dbh category, white birch was stripped less than rowan, ash, hazel and oak,” write Ireland and Ruxton.

They concede their study does not prove that white bark is a signal to potential predators, but their results are consistent with this being the case. They also caution that white bark probably did not evolve exclusively for one reason and that signalling may be just a contributing factor to the development of white bark. To focus in on the signal element, they suggest an experiment for the future would be to find a way of marking trunks as white without odour or other side effects to see if this makes a difference to herbivores.

READ THE ARTICLE

Ireland, H.M. and Ruxton, G.D. (2022) “White bark in birch species as a warning signal for bark-stripping mammals,” Plant Ecology & Diversity, https://doi.org/10.1080/17550874.2022.2122754