BIO: Painter, writer, and ecologist. Susan Elliott studied botany and French at Humboldt
State University and has a Ph.D. in biology from Dartmouth College. She is a student
of Chinese watercolor techniques.
Reciprocal benefits in a plant-pollinator mutualism
From 2003-2008, I studied the interdependence between a long-tongued bumble bee,
Bombus appositus, and a perennial wildflower, Delphinium barbeyi (Ranunculaceae),
at the Rocky Mountain Biological Lab in Gothic, CO. Bombus appositus acquires most
of its food from D. barbeyi, and in return, most of D. barbeyi's pollination services
come from B. appositus. My research tests how sensitive these species are to changes
in the abundance of their mutualist partner.
I tested how seed production varies with natural and experimental levels of pollen
receipt and pollinator abundance.
I found that when D. barbeyi flowers have very high pollination, they also experience
high seed predation by fly seed predators. As a result, seed predation dampens the
benefits of high pollination for seed production. If pollinators are scarce, then
flowers can still produce a few seeds autogamously, without pollinators. Therefore,
the plants are buffered from responding to both extreme high and extreme low pollinator
abundances, and in most years, I find that flowers are saturated with pollinator
I tested how bee reproduction varies with natural and experimental variation in flower
abundance (or food supply).
I found that bee reproduction was constant across meadows that vary 10-fold in flower
abundance, likely because they spread their colonies out evenly so that flower availability
per colony remains constant. Bees may also be unresponsive to flower abundance because
they are not always food-limited. In 2006, colonies did very well and reproduction
was food-limited (i.e., fed colonies had higher reproduction than control colonies).
However, in 2007, colonies did very poorly and never got large enough to exhaust
their floral resources. As a result, there was a surplus of flowers for bee reproduction
and fed colonies did not produce significantly more offspring than control colonies.
Since there was a surplus of flowers for bee reproduction in 2007, this meant that
there was shortage of bees to pollinate D. barbeyi flowers. Consequently, in contrast
to previous years when flowers were saturated with pollinator visits, in 2007, there
was a strong relationship between bee abundance and seed production.
The bees and flowers do respond to changes in each others abundance. However, since
these responses are separated in time; if one species changes in abundance, it may
be 10 years or 10 miles down the road, before we see an effect in the partner population.
This work supports the growing evidence that to understand bee-plant interdependence,
we need to incorporate a broader geographic and temporal perspectives.