Tuesday, July 24, 2012

It takes two to pollinate

Networking is not just a thing that computers and college counselors are into; birds, bees and trees do it too.  Pollination networking, that is. The pollination of all the world’s flowers is not the sole job of the European honey bee; many other insects, birds, and even little furry mammals carry pollen from flower to flower. But, not every pollinator can visit every type of flower. A hummingbird’s long bill is just not ideal for sucking nectar out of a pansy and you won’t see a bumble bee fitting his bumble into a heliconia. Some plant pollinator partners get so close that they don’t share the job with anyone else, like the Yucca moth or the fig wasp.

If you go out to a field (or forest, or mountaintop) and sit for many many hours watching which pollinators visit which species of plants then you can draw what is called a pollination network. This is just a set of dots, one for each plant and pollinator species, with lines connecting the plants and pollinators that do the deed.

Dots and lines are pretty simple, but with a little bit of math, can tell a detailed story about how plants and animals interact with each other.  Want to know whether each pollinator tends to specialize on a particular plant? Calculate the average number of pollinators that are linked up with a single plant. If it is low, then pollinators are specialists, and if it is high they are generalists, visiting any kind of plant. Another calculation, “modularity”, tells whether specific groups of pollinators tend to visit specific groups of plants, with little socialization across cliques. This might happen if similar pollinators are better suited to pollinating similar flowers.

These calculations, called ‘network properties’ can be used to compare pollination networks from different places, even if the species that live in these places are completely different. And finally, all this biology culminates in a fascinating geographical question:  Do plants and pollinators interact in similar ways around the globe, or is there a pattern in the way theses interactions vary?

Two researchers at Aarhus University in Denmark compiled a database of more than fifty pollination networks from all around the world to find out how interactions between plants and pollinators change from the tropics to the temperates, from mountain tops to mountain bottoms, on islands versus mainlands, and among hot, cold, wet and dry places.

In the tropics, plants and pollinators tended to specialize more than at temperate latitudes. Circular reasoning would argue that the specialization between pollinators and plants has helped generate the high diversity seen in the tropics and that this high diversity has forced pollinators and plants to become more specialized to avoid competing with their neighbors. We can leave it to the chicken to sort it out.

Another interesting pattern is that, once above 500 meters in altitude as you go up there tend to be fewer pollinators for each plant and pollinators become more generalists, visiting many different plants. This could be because it is colder at higher altitudes and fewer animals (especially cold-blooded insects) are available to act as pollinators. Also, the higher elevations tend to have more variable environments, so that a pollinator that depends on one type of flower for food may find itself out of luck in a year that that plant produces few flowers.

Networks don’t just happen between plants and pollinators- they occur among  all living things that interact, such as predators and prey or diseases and hosts.  Ecologists have used these tools for many years to understand how particular ecosystems work. This research shows how biogeographers can take the reins and learn how ecosystems compare. 

You can find this article at:

ResearchBlogging.orgKristian Trojelsgaard, & Jens M. Olesen (2012). Macroecology of pollination networks Global Ecology and Biogeography : 10.1111/j.1466-8238.2012.00777.x