Friday, August 6, 2010

PS 103-115: The invasive Argentine ant, Linepithema humile, reduces nectar resources for the giant hummingbird, Patagona gigas, on the coast of Chile

Liam M. Stacey, University of Washington


The Argentine ant, (Linepithema humile) is a well known tramp ant that is displacing native ants in some remnant patches of Chile’s sclerophyllous forests. The invading ant occurs at higher densities than did native ants, but some native ants could drink over ten times as much nectar at artificial nectar stations.

I conducted this study to test the hypothesis that invading Argentine ants remove significantly more nectar than did the assemblage of native ants in sclerophyllous forest remnants. I quantified nectar theft from Lobilia excelsa, whose flowers are defended by the giant humming bird Patagona gigas on the coast of Chile.

In order to quantify nectar theft, I compared nectar levels of ant excluded and ant exposed flowers in both argentine ant invaded and uninvaded areas. I bagged a total of 53 pairs of protoandrous flowers to keep out hummingbirds and bees, and, one of each pair was sealed form ants. After 24 hours, I quantified nectar levels using a 100 micro liter pipette. I used an independent sample t-test to compare the differences in nectar volume between pairs of ant exposed and ant excluded flowers as a percentage of nectar in control flowers.


Exposure to Argentine ants resulted in a significantly greater percentage difference between closed and open bagged flowers (P < .000). The mean difference in nectar removed by Argentine ants was an order of magnitude higher than it was for native ants (2.7μl vs. 0.2μl). Flowers exposed to native ants had significantly more nectar left in them than those exposed to Argentine ants, (P < .000).

This is the first study to examine the impact of invasive nectar thieving ants on nectar abundance. Lower nectar volumes in L. excelsa should limit populations of P. gigas. Butterflies and the giant orange bumble bee,  Megabombus dahlbomi, may be more threatened by diminished nectar at L. excelsa. since they drink only nectar brimming  in the floral tube and can not reach the nectar chamber.

Other invasive ants may have similar impacts on floral nectar, thereby impacting native flower visitors. However, impacts of Argentine ants might not differ greatly from those of native ants when flowers lack narrow floral tubes. Further studies on other invasive ants and exploited flowers may indicate which flower visitors could be negatively impacted by invasive ants, and which plants may suffer reduced seed set.