Pollination syndrome – plants can be picky, too
Pollination syndrome sounds a bit like a disease. But what comes to my mind hearing it, is the garden of my aunt in Chile. In the very back of it, there was a fig tree. When I studied in Concepción for a year, I regularly took the bus to Chillán for the weekend. And in summer, I spent much time crawling into this tree to get the fruit (and eat far too many of them). Which, to be honest, aren’t fruit. They’re flowers. Or, to be precise, an inflorescence, i.e. a group of flowers that make a whole. A dandelion is a more common example for an inflorescence, with many little flowers that give the impression of a big one. These two also mark two opposite cases: a generalist flower, which attracts many different pollinators like the dandelion in opposite to the extremely specialized fig.
Pollination syndrome – generalist flowers
To start with the definitions: “pollination syndrome” means that flowers have certain traits, that are “suitable” for specific pollinators. According to this concept, there are bee flowers, fly flowers, moth flowers etc. I went into more detail with some examples last summer in this post. What I’m thinking about a lot lately, is the specialization of both pollinators and flowers. That bees can be quite picky was already part of the nutrition series last year. But also plants are picky, they need successful pollination to reproduce. It’s better not to leave this important task to anyone just passing by: what if they come from a flower of another species, avoid passing over the the stamens or stigma (i.e. male and female parts) or do anything else that doesn’t help plant reproduction?
However, many plants attract a wide range of pollinators. Dandelions are a good example for this. In a survey I did some years ago in cider apple orchards, we also observed the pollinators in other flowers next to the trees. Dandelions were the dominant species.
On the flowers, we observed a huge variety of different pollinators: different bee species, but also bee flies, hoverflies and, sometimes, also butterflies. Apple flowers are also generalists and we observed nearly the same set of pollinators on them. That’s why I have this impression that dandelion is like a “highway to the apple”: the pollinators are already there, when the apples start to bloom. But, as I say, it’s an impression, we didn’t assess the orchards in the right way to prove this.
These generalist flowers don’t really fit into the pollination syndrome concept. As I mentioned last week, bee flowers like strawberries are also successfully pollinated by hoverflies. Depending on the conditions, different pollinators may be the most important ones. So what are strawberries – bee flowers or hoverfly flowers?
The opposite – extremely specialized flowers
You may wonder why I started this post talking about figs. It’s not only that I miss summer (I do), or my aunt’s garden (dito), but because figs are the complete opposite of dandelions when it comes to pollination. Remember: they’re flowers, not fruit. Though the actual flower is inside the structure we name fig. They have a one-to-one relationship with their pollinators, the fig wasps. These enter the fig through a tiny hole which you can see quite well here:
Pollination of figs is actually quite complicated. When the female flowers of a fig are ready for pollination, a female enters the flower and lays eggs. By doing this, it spreads the pollen of the flower in which it was born. The males hatch first and mate with the females within the flower, who then fly out searching for the flower where they can lay eggs taking pollen with them. It’s an incredibly fine-tuned mechanism. There are many fig species, and each of them has it’s own fig wasp pollinator. And there are fig wasps that don’t pollinate but are parasites. For more details, you can check this article from the USDA.
These highly specialized flowers totally fit into the scheme of pollination syndromes: a fig is a fig wasp flower, no doubt. There may be also less specialized plants that fit nicely into the concept. Most flowers are in between these two extremes of generalist and highly specialized flower. And as I already mentioned in my post on floral ecology, the classifications in bee, fly or moth flowers helps, but isn’t static. For instance, there is a specialized, oligolectic bee on the fly flower ivy: the Ivy Bee, Colletes hederae (check Steven Falk‘s amazing Flickr account for pics). A critical review of the pollination syndrome concept couldn’t predict the most common pollinator in almost two thirds of the plant species reviewed. Reading this made me laugh. I love it when established systems and classifications are challenged. Though, after this, it left me deeply puzzled.