CONICET | Buscador de Institutos y Recursos Humanos

Eichhornia azurea and Eichhornia crassipes (Pontederiaceae) form cohesive and dense floating meadows that may cover great part of the water surface in many lakes and wetlands of South America. The Acridoidea associated with aquatic environments represent a small percentage of Orthoptera but, within this group some species are important components of food webs because they consume some floating plants, while others live on the margins of flooding zones and shores of running waters. In South America the relationship between the Orthoptera assemblages and macrophytes in floodplain environments is scarcely known. The aim of this study was 1) to determine the species composition and to evaluate the seasonal variations in the abundance of Acridoidea that live in E. crassipes and E. azurea floating meadows in two lakes, one connected to the Paraná River and the other isolated, 2)to determine the relationship between grasshopper abundance and vegetation biomass of each lake. The study area includes two lakes, one of which, El Puente (27°26 S, 58° 51 W), has a connection with the Paraná River during flooding and the other, Pampín (27°30 S, 58°45 O) which is isolated. In El Puente, E. crassipes is the prevailing plant whereas in Pampín there is a plurispecific plant cover, with E. azurea prevailing andassociated with the floating mats of Oxycaryum. cubense f. paraguayense (Cyperaceae). Grasshoppers were sampled seasonally (fall 2006 to summer 2007). Individuals were captured from a boat using a sweep net. Abundance was expressed as the number of individuals captured per minute.Plant biomass was sampled seasonally using an aluminum ring with a 0.30 m2 area.All green leaves and roots within the boundaries of the ring were cut, separated and dried. The leaves were separated as to whether they were emerged or submerged. The emerged leaves were considered as refuge and food for the Acridoidea. Grasshopper species richness (S), diversity index (H´) and total abundance in each lake at each sampling time was calculated. The similarity of the grasshopper communities present in both lakes was evaluated using the Jaccard Index. Grasshopper species were classified according to their frequency of occurrence as constant (>50%), accessory (25-50%), or accidental ( 0.05). Comparing both floating meadows, there were significant differences in grasshopper total abundance (p< 0.0001) and C. aquaticum abundance (p= 0.0001), being higher in E. crassipes. The remaining species did not show significant differences (p>0.05). C. aquaticum was the dominant species, representing 94.5% and 62.5% of total individuals collected in E. crassipes and E. azurea, respectively. This insect showed a significant correlation with the grasshopper total abundance in E. crassipes (rS= 1.00; p= 0.0009) and in E. azurea (rS= 0.84; p= 0.0006). Eichhornia crassipes and E. azurea showed the highest biomass values in summer. E. azurea showed a significant difference in leaf biomass between seasons (Kruskal Wallis test, p= 0.03). The emerged leaves represented 52-62% and 16.5-36.6% of the total plant biomass in E. crassipes and E. azurea, respectively. In E. crassipes there was a significant correlation between the total grasshopper abundance and the total plant biomass (rS= 0.70; p= 0.020) and leaves ofE. crassipes (rS= 0.77; p= 0.011). Cornops aquaticum showed a significant correlation with total plant biomass (rS = 0.70; p= 0.020) and leaves (rS= 0.77; p= 0.011) of the same plant. In E. azurea there was a significant correlation between grasshopper abundance and the total biomass (rS= 0.58; p= 0.048) and leaves (rS= 0.64; p= 0.026) of a given plant. No significant correlation was recorded between grasshopper abundance and O. cubense leaves (rS= 0.41; p= 0.180). The diversity index H was different between sites (p