[Pollinator] Floral Network - What Determines Who Pollinates Whom

[Ladadams at aol.com]

Ecology :: The floral network -- What determines who pollinates whom
>From SpiritIndia.com

The topology of plant-pollinator networks can be explained by relatively 
simple rules incorporating both "complementarity" and "barrier" traits, thus 
providing insights into the possible evolutionary and ecological processes driving 
the pattern.
A field of spring wildflowers, abuzz with busy insects seeking nectar and 
spreading pollen, may look like a perfect model of random interaction. But 
ecologists have discovered order within this anarchy. For instance, as the number of 
species grows, the number of interactions does too, while the connectivity 
(the fraction of possible interactions that actually occur) and the nestedness 
(the relative importance of generalist species as mutualistic partners of 
specialist species) shrinks. Study of such networks of species is still in its 
youth, and the rules that generate these patterns of interaction are still being 
worked out. In a new study, Luis Santamaría and Miguel Rodríguez-Gironés 
propose that two key mechanisms, trait complementarity and barriers to exploitation, 
go a long way in explaining the structure of actual networks of plants and 
their many pollinators.

The two mechanisms each arise from fundamental aspects of the interaction 
between species. An insect will be unable to reach nectar in floral tubes longer 
than its proboscis; the tube length sets up a barrier to some species, but not 
to others. Each plant species also has a given flowering period. The specific 
activity period of each insect species will complement the flowering of some 
plant species more than others. Other barriers and other complementary traits 
have been described for a variety of plant–pollinator pairs. To explore the 
significance of these mechanisms, the authors modeled plant–pollinator 
interaction networks using a few simple rules, and compared their results to data from 
real networks in real plant communities. The models incorporated from one to 
four barrier or complementary traits, or a combination of two of each. They 
also tested two variations of a "neutral" interaction model, in which species 
interact randomly, based simply on their relative abundance. 

Different models did better at mimicking different aspects of real networks, 
but the two that performed best overall were the combination model and one of 
the neutral models. The authors argue that the neutral model, despite its 
appealing simplicity, can be discounted because it requires key assumptions 
regarding species abundances and random interaction that conflict with empirical 
observations of real communities. In contrast, the model combining barriers and 
complementary traits matches well with observed plant–pollinator interactions. 
Barriers alone would mean that pollinators with the longest proboscis would be 
supreme generalists, able to feed on any flower, causing perfect network 
nestedness; while complementarity alone would mean that specialist pollinators do 
not interact primarily with generalist plants, causing unrealistically low 
network nestedness. Instead, the authors suggest, a combination of barriers and 
complementary traits accounts for the pattern of specialists and generalists 
seen in real pollination networks. 

The superiority of the combination model also has implications for 
understanding floral evolution. A common principle has been that plants coevolve with 
their most-efficient pollinator to strengthen the complementarity of their 
matching adaptations. Barriers, however, while reducing exploitation by inefficient 
pollinators, may also interfere with pollination by efficient ones. 
Nonetheless, the results of the present study indicate that barriers are likely to play 
an important role in pollinator networks, suggesting that coevolution with 
the most-efficient pollinator is not the sole factor governing floral evolution.

Laurie Davies Adams
Executive Director
Coevolution Institute
423 Washington St. 5th
San Francisco, CA 94111
415 362 1137
LDA at coevolution.org
http://www.coevolution.org/
http://www.pollinator.org/
http://www.nappc.org/

Bee Ready for National Pollinator Week:  June 24-30, 2007.  Contact us 
for more information at www.pollinator.org 

Our future flies on the wings of pollinators.
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