Plant-pollinator relationships at two altitudes in the Andes of Mendoza, Argentina

The assemblages of visitors to angiosperms flowering at a montane and at a high alpine site in the Andes of Mendoza, Argentina (33-34degreesS) were described and the plant-flower visitor matrices were analyzed and compared to other systems, in particular those located at a similar latitude on the western slope of the Andes. In the low-attitude (montane) habitat, 23 plant species had a total of 126 interactions with 71 taxa of insects and one hummingbird, and at the higher site 21 plants and 45 insect species had 83 interactions. Connectances of the visitor matrices were 7.6 and 8.7, respectively. Diptera and Hymenoptera dominated the visitor assemblages at both sites without change of proportions with attitude, while Lepidoptera significantly increased at the higher site. Flies were more species-rich than expected at the sites' latitude and more constant across altitudes than is usually observed. Lack of a significant decrease with altitude of (1) the frequency of Hymenoptera and (2) the number of interactions per plant were the main differences with a comparable gradient in the Chilean Andes. The proportion of self-compatible species increased with altitude, however. use of phylogenetically-independent contrasts showed that the prevalence of selfers at higher altitudes does not reflect a generalized reaction pattern but results from two speciose families (Asteraceae and Fabaceae) showing more self-compatibility at high altitudes.