• Eline van Bloois

The pollen reveal: pollinator communities on dikes

With a mixture of sadness and disgust, I press my cell phone screen to black. The article concerned an insensitive slob, a mower and a rare Lizard Orchid on a dike in Zeeland. As you can understand, the latter drew the short straw. The slob didn't really care. My heart sank into my boots again. You never get used to the umpteenth person stating, without blinking an eye, that he/she doesn't give a damn that rare flora/fauna has been locally eradicated.

A reaction like this is quite extreme and perhaps does not deserve attention. But the fact remains that we often see nature as something inconvenient. Gardens should be as neat as possible (or made entirely of stone) and road verges 'neatly' mown. Except for the flowerbeds full of exotic species, that is. In addition, we spray litres of poison into the soil and air. Everything to keep nature as far away as possible. As such, it may not come as a surprise when it turned out that we have lost ~75% of all our insects over the last decades [1]. Still, the shock was enormous.

An unprecedented problem Then you're stuck with a problem of which the consequences are hard to predict. After all, we have no examples of such a drastic change. What we do understand is that it is bad. Insects fulfil numerous functions in ecosystems and preserve the balance in nature: predators eat insect pests, herbivores ensure that certain plants do not become too dominant and pollinators ensure the survival of flowers. And they are also a fantastic source of food for many other animals, such as birds. Besides their role in ecosystems, their importance for us humans can’t be underestimated. Without insects, there are no apples or pears. And if you’re not convinced already that we should protect our insects, I leave you with a sentence of zoologist Mark Carwardine that I always think of: ‘There is one last reason [to preserve animals] ... And it is simply this: the world would be a poorer, darker and lonelier place without them'. So how can we help insects?

Flower power The most common way is by "enhancing" the local vegetation to provide food for insects. You are probably familiar with the famous flower strips in agricultural areas. But also by managing roadsides, nature development and all kinds of other changes to the vegetation we try to help our insects. Often this works well, sometimes not. But also ‘working well’ comes in different gradations. So gradually we have to admit that we do not yet understand when and how we can best help insects by changing the vegetation.

In my research I look at pollinator communities on dikes. Dikes (and road verges) are interesting objects in the context of insect protection. They are often unused pieces of land where we can do a lot of good for insects with relatively little effort. We have seen this on dikes around Nijmegen, where over 100 species of bees habit [2] (almost a third of all Dutch species)! In addition, dikes and verges form a kind of 'capillary network' through the country. In this way, they connect many nature areas with each other. A beautiful flowery dike can therefore be a habitat in itself for insects, but also a kind of insect highway!

The pollen reveal At least, that’s the theory. Now in practice. In my research I determine what species diversity we can find on dikes, what this means (are these rare or common species) and finally what determines patterns in their occurrence. I do this by monitoring bee populations on dikes for multiple years. This monitoring is a complex procedure that requires precise timing (a stopwatch), sharp vision, fitness (to walk 150m), the right outfit (you do want to look like an ecologist after all) and finally a piece of high-tech equipment (sometimes called a ‘butterfly net’). Yes, I run around with a butterfly net catching bees and that’s called research. By doing so, I eventually know exactly what bees occur where. As such we’ve already identified over 100 species, among which over 15 red listed (threatened) bee species!

Then the hard and most important part - what determines where bees can thrive? To figure this out, I look at the structure of pollinator communities on dikes and the role that vegetation plays in the formation of these communities. My hypothesis is that ‘niche-based processes’ are crucial; each species has its own preferred food source, nesting place etc. (= its niche) and multiple species can only co-occur when they can all realise their own niche without getting outcompeted. I expect that food sources (flowers), through this idea of niches and competition, determine to a large extent what pollinator species can live together. This can happen in two ways: 1) a higher diversity of flowers provides more preferred food sources for more different bees, thus allowing more species to live together; and 2) there could also be flower species that simply provide food to a great number of species that can as such not be missed. To study how this mechanism works, I collect pollen from the bees and use DNA analysis to exactly determine what flower species the individual bees have collected pollen from and how much of each species. This allows me to determine whether different species indeed use different food sources (to avoid each other) and to see whether there are certain flower species of which the pollen are very commonly collected. This information is highly valuable for nature conservation.

A look at the future If we don’t understand by what factors pollinator communities are shaped, we cannot possibly protect them in a targeted way. The knowledge we are currently gaining is therefore crucial to protect pollinators effectively in the future. By knowing which types of plants are needed to help certain groups of pollinators, we can better plan how to ‘enhance’ vegetation (e.g. flower strips) to optimally provide pollinator habitat. Then all that is left to do is to change the mindset of those that believe species extinction is not a big deal…

1. Hallmann, C. A. et al. More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS One 12, e0185809 (2017).

2. Swinkels, C., Liebrand, C., van Rooijen, N., Visser, E. & De Kroon, H. De dijk als habitat voor bloemen en wilde bijen. Levende Nat. 3, 96–101 (2020).

Constant Swinkels

University: Radboud University Nijmegen Department: Plant Ecology Favourite beverage: depends on the situation. Pairing food and wine is a hobby of mine. However, in the bar I would gladly have a pint!

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