Hummingbird foraging behaviour and plant reproductive output

A shining sunbeam (Aglaeactis cupripennis) feeding on
a Puya hamata inflorescence. (Photo: Paul Ramsay)

A new study on plant-pollinator interactions by Paola García-Meneses and Paul Ramsay is about to be published in Basic and Applied Ecology. It is the first to show that the relationship of a plant to its neighbours within patches affects pollinator visits and reproductive output of the plants.

It is well known that insect and bird pollinators are attracted to patches of flowers where rewards are greater. In fact, some birds set up territories around the plants and chase away others. Typically, the idea goes, plants in big patches get more visits than plants in smaller ones, and individual, isolated plants would be expected to get the fewest visits. This sometimes leads to better reproductive output of the plants in larger patches. But this may not always be the case...

Patches of plants are sometimes made up of close relatives (offspring or clones of the same parent plant) and territorial pollinators forage mostly from plants in the patch, preventing other birds from feeding there. And while plants in the centre of the patches would be well-defended by the owner of the territory, the plants around the edge of patches might get an occasional visit from a passer-by.

A patch of Puya hamata plants in
the Páramo of Volcán Chiles,
Ecuador. (Photo: Paul Ramsay)

In the centre of larger patches, pollen from close relatives or clones would be commonly transferred by the pollinators, but isolated plants would tend to get pollen from more distant relatives. The inbreeding resulting from reproduction of close relatives could lead to reduced reproductive output in those plants in the middle of large patches. So, a plant’s spatial context might be crucial to plant reproductive success.

The new study is the first to demonstrate that this situation does happen. The study considered the reproductive output of a species of giant rosette plants in the Ecuadorian Andes: Puya hamata. It produces large inflorescences at the end of its life. Its seeds only disperse a few metres away from the parent plant and germinate much, much better after fires, which open up the dense vegetation and let in more light and warmth. Without fires, only very lucky seeds get to germinate and there tends to be a scattering of individual plants around the landscape. These plants are visited by hummingbirds which fly from plant to plant, feeding as they go—known as trapliners. On the other hand, if a lot of seeds fall in a burned area, many of them germinate and a patch forms. Years later, these plants flower together to form a very attractive resource to territorial hummingbirds that exclude the traplining species. 

Ripe fruits with seeds
falling out.
(Photo: Paul Ramsay)

Hummingbird delight: flowers
with lots of nectar.
(Photo: Paul Ramsay)

The new paper presents shows that isolated plants are visited by a wider variety of hummingbirds but, in the patches, an aggressive, territorial hummingbird almost excludes the other birds. Although the numbers of flowers, fruits and seeds was similar, regardless of the spatial context of the plants, the viability of the seeds and their germination success was reduced in situations where greater levels of inbreeding would be expected. Reproductive output was highest in isolated plants, and plants on the edges of patches, but lowest in plants at the centre of large patches.

Paola (centre, right) with Ecuadorian field assistants Maggy,
Mayra and Santiago (left to right). (Photo: Paul Ramsay)

Although this is the only case where this effect has been demonstrated so far, similar results are likely where patches are made up of related plants and where pollinators become territorial in response to the patches. These plant-pollinator relationships are not just of ecological interest, but could have implications for managing biodiversity at species and genetic levels.

You can find the paper on the journal website here: http://dx.doi.org/10.1016/j.baae.2012.08.011.